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The evolution of apertures in the spores and pollen grains of embryophytes
... As has been revealed, the cytokinesis of G. quadriradiata is of the simultaneous type and in this way represents the one that is typical of the eudicot clade of Angiosperms, which is a kind of cytokinesis (Furness, 2008). In this type of cytokinesis, the nuclei and spindles of the second meiotic division can interact and the nuclei usually take up positions as far away from each other as possible, thus leading to the formation of tetrahedral tetrads (Blackmore and Crane, 1998), which is similar to our observations in G. quadriradiata. Furthermore, this type of cell division is correlated with the formation of tetrahedral tetrads (Blackmore and Crane, 1998) and related to their tricolpate characteristic (Furness and Rudall, 2004), which was also visible in G. quadriradiata. ...
... In this type of cytokinesis, the nuclei and spindles of the second meiotic division can interact and the nuclei usually take up positions as far away from each other as possible, thus leading to the formation of tetrahedral tetrads (Blackmore and Crane, 1998), which is similar to our observations in G. quadriradiata. Furthermore, this type of cell division is correlated with the formation of tetrahedral tetrads (Blackmore and Crane, 1998) and related to their tricolpate characteristic (Furness and Rudall, 2004), which was also visible in G. quadriradiata. The position of the apertures during development suggests the existence of a close relationship between the aperture pattern definition and the cytoplasmic compartment that completes meiosis (Blackmore and Crane, 1998). ...
... Furthermore, this type of cell division is correlated with the formation of tetrahedral tetrads (Blackmore and Crane, 1998) and related to their tricolpate characteristic (Furness and Rudall, 2004), which was also visible in G. quadriradiata. The position of the apertures during development suggests the existence of a close relationship between the aperture pattern definition and the cytoplasmic compartment that completes meiosis (Blackmore and Crane, 1998). ...
Galinsoga quadriradiata Ruiz & Pav. is an annual weedy plant that can be found all over the world. It belongs to the Asteraceae family and is recognised as one of the invasive foreign plants in Poland, which are native to Central and South America. The aim of this study was to describe the reproductive features of Galinsoga quadriradiata focusing on the changes that occur during microsporogenesis and microgametogenesis along with the morphology of its pollen. As it is typical of the eudicot clade of Angiosperms, cytokinesis of G. quadriradiata is simultaneous. The pollen grains are tricolporate with spiny outer walls and the course of the microsporogenetic process is fairly typical of the Echinatae group of weed plants. The high viability of the pollen grains, which mature unequally in the inflorescences, and the proper course of meiosis determine whether a plant has the invasive character of Galinsoga quadriradiata.
... In the eudicot clade (an old and speciose clade with c. 165 000 species) the huge majority of species have the same microsporogenesis type (no. 17 on Fig. 3) (Wodehouse, 1935;Blackmore & Crane, 1998;Nadot et al., 2008;Matamoro-Vidal et al., 2012). By contrast, in the monocot clade and in early-divergent angiosperms, microsporogenesis is quite variable (Furness & Rudall, 1999b;Furness et al., 2002;Penet et al., 2005;Sannier et al., 2006). ...
... For this, looking at microsporogenesis in species with nonequatorial pollen aperture patterns (e.g. lacking apertures (inaperturate), or with irregular or global aperture patterns) ( Fig. 1i-l) is appropriate because in these, a partial or total disconnection of aperture position from microsporogenesis occurs (Wodehouse, 1935;Blackmore & Crane, 1998;Ressayre et al., 2002a;Furness, 2007;Albert et al., 2009;Matamoro-Vidal et al., 2012). Because of this disconnection, the effects of the selective pressures, if any, acting on microsporogenesis due to aperture positioning should be released or even absent in these species. ...
... The finding that in most of our species with atypical aperture patterns pollen morphology could not be associated with any microsporogenesis pattern, and that microsporogenesis was variable at the intra-individual level without any change in pollen morphology, is consistent with previous works (Wodehouse, Blackmore & Crane, 1998;Ressayre et al., 2002a;Furness, 2007) proposing that in these species microsporogenesis is disconnected from pollen aperture pattern determination. The fact that under these conditions microsporogenesis was highly variable also supports the view that microsporogenesis is not strongly constrained by the genotype-phenotype map or by other unidentified selective pressures. ...
The contribution of developmental constraints and selective forces to the determination of evolutionary patterns is an important and unsolved question. We test whether the long-term evolutionary stasis observed for pollen morphogenesis (microsporogenesis) in eudicots is due to developmental constraints or to selection on a morphological trait shaped by microsporogenesis: the equatorial aperture pattern. Most eudicots have three equatorial apertures but several taxa have independently lost the equatorial pattern and have microsporogenesis decoupled from aperture pattern determination. If selection on the equatorial pattern limits variation, we expect to see increased variation in microsporogenesis in the nonequatorial clades. Variation of microsporogenesis was studied using phylogenetic comparative analyses in 83 species dispersed throughout eudicots including species with and without equatorial apertures. The species that have lost the equatorial pattern have highly variable microsporogenesis at the intra-individual and inter-specific levels regardless of their pollen morphology, whereas microsporogenesis remains stable in species with the equatorial pattern. The observed burst of variation upon loss of equatorial apertures shows that there are no strong developmental constraints precluding variation in microsporogenesis, and that the stasis is likely to be due principally to selective pressure acting on pollen morphogenesis because of its implication in the determination of the equatorial aperture pattern.
© 2015 The Authors. New Phytologist © 2015 New Phytologist Trust.
... In pteridophytes, the transition from trilete spores associated with simultaneous division to monolete spores associated with successive division has occurred within lycopods and several times within ferns. Mixtures of both spore types are found in sporangia of some fern species, indicating that the relative timing of spindle formation and cytokinesis may be more labile in these taxa than in other ferns (Blackmore and Crane 1998). ...
... The extent to which the spindles of the first and second divisions can interact and the relative orientations of the spindle axes affect tetrad configuration so that in some cases it may be difficult to predict microsporogenesis type from tetrad type. Intermediate forms are known (reviews by Murty 1964;Bhandari 1984;Blackmore and Crane 1998;Furness and Rudall 1999b), such as the "modified simultaneous" type, where ephemeral cell plates are formed after the first meiotic division but then disperse, and simultaneous cleavage follows the second meiotic division. Ephemeral cell plates may isolate the spindles of the second meiotic division, resulting in tetragonal, rhomboidal, linear, decussate, or T-shaped tetrads, as in a successive division (Blackmore and Crane 1998). ...
... Intermediate forms are known (reviews by Murty 1964;Bhandari 1984;Blackmore and Crane 1998;Furness and Rudall 1999b), such as the "modified simultaneous" type, where ephemeral cell plates are formed after the first meiotic division but then disperse, and simultaneous cleavage follows the second meiotic division. Ephemeral cell plates may isolate the spindles of the second meiotic division, resulting in tetragonal, rhomboidal, linear, decussate, or T-shaped tetrads, as in a successive division (Blackmore and Crane 1998). Microsporogenesis may also be modified by the shape of the microsporocytes and their arrangement in the anther locule. ...
Microsporogenesis is highly labile in early-branching angiosperms, i.e., those with mostly sulcate pollen, compared with the tricolpate and tricolpate-derived eudicots. New records of microsporogenesis in basal angiosperms (19 taxa were examined), together with a review of the literature, demonstrate that the existing typology has been too strictly applied; several basal angiosperms have apparently intermediate forms and therefore do not fit easily into simultaneous or successive categories. Intermediate forms include the "modified simultaneous" type, where ephemeral cell plates are formed after the first meiotic division but then disperse, and simultaneous cleavage follows the second meiotic division. This relative diversity reflects a range of variation in number and position of pollen apertures in basal angiosperms, although both monosulcate and inaperturate pollen may occur in conjunction with either simultaneous or successive microsporogenesis. However, many taxa with inaperturate pollen have successive microsporogenesis, whereas many monosulcate taxa have the simultaneous type (although successive and monosulcate is common in monocotyledons). The predominance of simultaneous microsporogenesis in extant basal angiosperms and in land plants in general (including gymnosperms) indicates that simultaneous microsporogenesis is plesiomorphic in angiosperms, despite the occurrence of the successive type in the putative first-branching extant angiosperm, Amborella. This conclusion contradicts earlier views on the evolutionary polarity of this character.
... Our study found a range of between about 2-10% of the grains to have aberrant apertures with the proportion higher in the cultivated form (Table I). This is consistent with Blackmore's unpublished observations of ca. 5 000 grains of Nelumbo nucifera that found about 5% of the grains to be nontricolpate (Blackmore & Crane, 1998). To investigate whether this proportion of aberrant grains is unusual in the context of other angiosperms, we examined pollen from two species of Platanus (P. ...
... The developmental basis of the aberrant aperture forms seen in Nelumbo and other eudicots remains uncertain. In general, the formation of equatorial triaperturate pollen almost certainly reflects the duplication and repositioning of developmental processes that are usually restricted to the distal surface in monosulcate pollen (Blackmore & Crane, 1998). This repositioning may be linked to the development of a quadripolar spindle in tetrahedral tetrads (Blackmore & Crane, 1998), perhaps as a result of a shift to new kind of simultaneous microsporogenesis at the base of the eudicot clade. ...
... In general, the formation of equatorial triaperturate pollen almost certainly reflects the duplication and repositioning of developmental processes that are usually restricted to the distal surface in monosulcate pollen (Blackmore & Crane, 1998). This repositioning may be linked to the development of a quadripolar spindle in tetrahedral tetrads (Blackmore & Crane, 1998), perhaps as a result of a shift to new kind of simultaneous microsporogenesis at the base of the eudicot clade. The very widespread occurrence of tricolpate or tricolpate-derived pollen in eudicots (more than 175 000 species) indicates that the developmental mechanisms by which it is controlled have remained fixed and remarkably stable over a long period of time and through a massive process of diversification. ...
The evolutionary and developmental origin of tricolpate pollen is of great interest because pollen of this kind defines a major clade of angiosperms (eudicots), a clade that is also well supported by molecular data. We examined evidence that tricolpate and monosulcate pollen types are produced alongside each other in the anthers of Nelumbo flowers, as has previously been reported. Observations of pollen in situ within individual anthers revealed mainly tricolpate pollen produced in tetrahedral tetrads, but also a small percentage of clearly aberrant pollen grains that have a great variety of aperture configurations. Previously published evidence for tetragonal tetrads is not supported, and previously reported monosulcate grains are part of a continuum of variation among the aberrant grains in aperture number, position and form. Other eudicots show similar variability in their pollen apertures. The variation in the pollen of Nelumbo is not exceptional, and may not be more significant than variation seen in the other taxa with regard to the origin of the tricolpate and tricolpate-derived pollen characteristic of eudicots. Nevertheless further studies of aberrant pollen in Nelumbo and other eudicots, together with comparisons of pollen development in "normal" eudicots and closely related species that show radical, and developmentally fixed, reorganization of apertures and pollen polarity, may be helpful in understanding the processes that controlled the transition from the monosulcate to the tricolpate condition.
... tetrads are also associated with simultaneous microsporogenesis , since the centres of the daughter cells (in two pairs) are arranged in a tetrahedral orientation (Muller, 1970). Actually, intermediates exist between the two microsporogenesis types (see reviews by Murty, 1964 ; Bhandari, 1984 ; Blackmore and Crane, 1998). In the ' modified simultaneous ' type, an ephemeral cell plate is laid down after the first meiotic division, which then disappears and simultaneous cytokinesis occurs, or sometimes the second division quickly follows the first, before the cell plate is completely formed. ...
... Three basic microsporogenesis types are recognized inTable 1 (see Introduction) : (1) successive, where dyads and\or tetragonal , T-shaped, linear, rhomboidal or decussate tetrads are described ; (2) simultaneous, where dyads are not described and tetrads are tetrahedral or decussate ; and (3) intermediate or irregular, where there are ephemeral cell plates or the second division occurs quickly after the first, before the cell plate is completely formed, and tetrads may be described as irregular. Actually, as in many developmental processes, a continuum may exist between all three categories, dependent upon the exact mode and timing of meiosis (Blackmore and Crane, 1998), however, the types recognized above do appear to have systematic significance (see below). ...
A seco ndary thickening meristem is record ed for the first time in some herbaceous taxa of Asparagales (Herreria montevide nsis and Thysanotus sp inige r), and the new records are assessed in a systematic co ntext. All monocotyl edons lack a vascular cambium, which is typicall y a single per sistent row of cells producing phloem centrifugally and xylem ce ntripetally. However, some monocotyled ons achieve stem thicken ing by means of a different type of later al meristern, either a primary thickening meristem (PTM) near the apex, together with diffuse secondary growth (as in palm s). or a secondary thicken ing meristern (STM) further away from the apex (as in some Asparagales; see Rudall 1991, for review). The PTM and STM are probabl y developmenlally related, although there is complex tissue involvement. In taxa with an STM, the PTM and STM may someti mes be longitudi nally co ntinuous, at least at some stage in the life cycle (Steven son 1980). Virtually all monocotyledons have a PTM , but among tree-forming or woody taxa this has developed along different lines, probabl y more than once, either as an exten sive apical PTM, as in palms, or as an STM, in some Asparagales (see below). The PTM and STM are not homologous with the vascular cambium, as they are tiered (etagen) men stems which produce distinct vascular bundles (of both xylem and phloem) in a parench ymatou s ground tis sue (Fig. 1-3), mainly ce ntripe tally. There are record s of a PTM in some dicotyledon s and other plant groups (DeMaso n 1983), although the hom ology of these requ ires testing . All unequi vocal records of an STM are in the asparagoid clade (sensn Cha se et al. 1995), which compri ses Dahlgren et al.'s (1985) order Asparagales, together with a few membe rs of their Liliales, such as Iridaceae (Rudall 1991). A STM has been re ported in several tree-forming and shru bby asparagoid s: Aga ve, Aloe, Beaucarnea, Calibanus, Cordyiine, Dasylirion, Dra caena, Furcra ea, Klatti a, Nive nia, No lina, Pleomel e, Sansevieria, Witsenia, Xanth orrh oea and Yucca, and also in some more her baceou s taxa with a thick ' woody' rhizome or stem, such as Aphyllanthes, Gasteria.
... Examples of the co-occurrence of pollen with differing aperture configurations representing both polar and equatorial orientation are unknown. The influence of microsporogenesis type on tetrad configuration and aperture type and position is well recognised (Blackmore & Crane 1998), and has been documented for palms (Harley 1999, Harley & Baker 2001. According to Blackmore & Crane (1998) spindle orientation of the meiotic cytoskeleton, the relative timing of cytokinesis, and the mode of cytoplasmic partitioning, exert a decisive influence on tetrad configuration and ultimately the outcome of aperture position in relation to the poles. ...
... The influence of microsporogenesis type on tetrad configuration and aperture type and position is well recognised (Blackmore & Crane 1998), and has been documented for palms (Harley 1999, Harley & Baker 2001. According to Blackmore & Crane (1998) spindle orientation of the meiotic cytoskeleton, the relative timing of cytokinesis, and the mode of cytoplasmic partitioning, exert a decisive influence on tetrad configuration and ultimately the outcome of aperture position in relation to the poles. ...
Triapertury is rare in monocotyledons. The well-defined, regularly spaced, circular porate apertures that occur in Arecaceae: Areca klingkangensis from Borneo, and species of the West African genus Sclerosperma, appear to be unique in monocotyledons. There is evidence to suggest that tripory in Arecaceae has been derived from trichotomosulcy, although in Areca equatorial zonosulcy may have an important role. The apical triporate, and zonosulcate pollen of Areca are described, as well as examples of mono- and trichotomosulcate pollen within the genus. The sub-apical distal triporate pollen of Sclerosperma gilletii and S. mannii are described. Notably, in Sclerosperma pollen, aperture position at post-meiotic tetrad stage follows the rare 'Garside's rule' (four groups of three apertures), previously only demonstrated for Proteaceae and Olacaceae. Possible reasons for the occurrence of these rare triporate pollen phenomena in palms are considered. The bearing this may have on the transition from the distal polar position of the single sulcus, to the radial symmetry of the triaperturate condition in many dicotyledons is discussed in comparison with other examples of triapertury in monocotyledons.
... Water loss may be reduced in breviaxe pollen grains, and could be bene®cial to the length of time the pollen remains viable, for example allowing more opportunity to effect outcrossing. Apertures are one of the most important features of pollen with regard to their function and in providing valuable characters for systematic analysis at a variety of levels in the taxonomic hierarchy (Erdtman 1952, Van Campo 1976, Blackmore & Crane 1998). The morphological diversity of apertures is correlated with the process of meiosis and a variety of developmental processes (Blackmore & Crane 1998). ...
... Apertures are one of the most important features of pollen with regard to their function and in providing valuable characters for systematic analysis at a variety of levels in the taxonomic hierarchy (Erdtman 1952, Van Campo 1976, Blackmore & Crane 1998). The morphological diversity of apertures is correlated with the process of meiosis and a variety of developmental processes (Blackmore & Crane 1998). Research by Pozhidaev (in press) suggests that aperture positioning does not occur in patterns correlated with phylogenetic evolution, but are varieties of an aperture pattern series determined by developmental processes. ...
The diverse pollen morphology of fourteen species of the genus Eperua is described and illustrated using light, scanning electron and transmission electron microscopy. Six pollen types are described and a key for their identification is provided. A cladistic analysis was carried out using macromorphological and palynological characters to form a hypothesis of relationships between taxa. The pollen morphology is discussed with regard to systematic relationships, function and phylogenetic significance of certain pollen morphological structures within the genus.
... The indication that simultaneous cytokinesis is plesiomorphic in Piperales contrasts with most existing opinion (e.g. Sampson, 1969; Blackmore &Crane, 1998) that successive cytokinesis with one centrifugal cell plate is plesiomorphic in angiosperms. If the successive type is plesiomorphic in angiosperms, an early transformation into simultaneous cytokinesis took place in flowers of Piperales, followed by a reversal back to successive within Aristolochia. ...
... The functional and genetic significance of microsporogenesis type requires further investigation. Heslop-Harrison (1966) suggested that the continuity of cytoplasm between the four nuclei during simultaneous microsporogenesis enables a better co-ordination of pollen development, which in turn could be related to the acceleration of development and morphological reduction of the male gametophyte in land plants, as also pointed out by Blackmore & Crane (1998). Asynchrony and the frequent independence of the two pairs of nuclei observed in the second meiotic division is uncommon in simultaneous microsporogenesis , but seems to be of frequent occurrenceHeslop-Harrison, 1966), in which cytoplasmic connections are lacking between the two halves of a dyad. ...
Within Aristolochiaceae, a secretory tapetum and orbicules are ubiquitous, but both simultaneous and successive types of microsporogenesis occur. Simultaneous cytokinesis is apparently plesiomorphic within the order Piperales, in which Aristolochiaceae are now placed. Successive microsporogenesis was found only in species of Aristolochia confined to a crown clade in the proposed phylogeny of this genus. In contrast to many other taxa, within Aristolochiaceae there is no strict relationship between microsporogenesis type and tetrad configuration, which is strongly influenced by spindle orientation, especially during meiosis II. There is also no direct correlation between microsporogenesis type and the aperture of mature pollen grains.
... Gametophyte cells are separated by thin walls, and from outside, as in flowering plants, are protected by inner cellulosic wall (the intine) and massive outer sporopollenin wall (exine) [11]. Apertures are specialized areas, where the exine is thinner and the intine is usually thickened, channeled, or forms multiple layers [12]. Pollen wall in many cases has polar structure, and the polarity of microspores of gymnosperms, such as angiosperms, is determined during meiosis [8]. ...
Pollen germination and pollen tube growth are common to all seed plants, but these processes first developed in gymnosperms and still serve for their successful sexual reproduction. The main body of data on the reproductive physiology, however, was obtained on flowering plants, and one should be careful to extrapolate the discovered patterns to gymnosperms. In recent years, physiological studies of coniferous pollen have been increasing, and both the features of this group and the similarities with flowering plants have already been identified. The main part of the review is devoted to physiological studies carried out on conifer pollen. The main properties and diversity of pollen grains and pollination strategies in gymnosperms are described.
... The exine is a chemically and physically robust outer wall made of 34 sporopollenin, a complex, highly resistant chemical whose structure and composition are not 35 fully described 3 . Apart from the structure of the exine itself, pollen can be patterned with a 36 varying number and geometric arrangement of apertures, which are regions of the extracellular 37 material that have a reduced or absent exine and are the sites where the pollen tube emerges 38 during germination 4 . Apertures also allow the pollen grain to reversibly fold during desiccation 39 and rehydration 5 . ...
Pollen grains are known for their impressive variety of species-specific, microscale surface patterning. Despite having similar biological developmental steps, pollen grain surface features are remarkably geometrically varied. Previous work suggests that a physical process may drive this pattern formation and that the observed diversity of patterns can be explained by viewing pollen pattern development as a phase transition to a spatially modulated phase. Several studies have shown that the polysaccharide material of plant cell walls undergoes phase separation in the absence of cross-linking stabilizers of the mixed phase. Here we show experimental evidence that phase separation of the extracellular polysaccharide material (primexine) during pollen cell development leads to a spatially modulated phase. The spatial pattern of this phase-separated primexine is also mechanically coupled to the undulation of the pollen cell membrane. The resulting patterned pools of denser primexine form the negative template of the ultimate sites of sporopollenin deposition, leading to the final micropattern observed in the mature pollen. We then present a general physical model of pattern formation via modulated phases. Using analytical and numerical techniques, we find that most of the pollen micropatterns observed in biological evolution could result from a physical process of modulated phases. However, an analysis of the relative rates of transitions from states that are equilibrated to or from states that are not equilibrated suggests that while equilibrium states of this process have occurred throughout evolutionary history, there has been no particular evolutionary selection for symmetric, equilibrated states.
... Precladistic workers assumed that tricolpate pollen evolved independently in several lines (e.g., Cronquist 1968), but molecular studies confirmed preliminary indications from morphological analyses (Dahlgren & Bremer 1985;) that eudicots are monophyletic and tricolpate pollen originated only once (except for a superficial convergence in Illicium and Schisandra, in the Austrobaileyales, where the three furrows are oriented differently : Huynh 1976;Doyle 2005). Phylogenetic analyses provide little evidence on the mode of origin of tricolpate pollen, since the potential outgroups are basically monosulcate (Doyle 2005), but some of the oldest Early Cretaceous tricolpate pollen has obliquely oriented and interconnected colpi, suggesting an origin by elongation, spiralization, and fragmentation of a sulcus (Doyle & Hotton 1991;Blackmore & Crane 1998). ...
An increasingly robust phylogenetic framework based on molecular and fossil data clarifies the sequence of evolutionary innovations in land plants. Oogamy and cellular novelties (phragmoplast, plasmodesmata, incipient meristems) evolved in aquatic streptophytes. Invasion of the land entailed interpolation of the sporophyte, jacketed gametangia and sporangia, and air‐dispersed spores, followed by stomata. Origin of vascular plants involved branching of the sporophyte and stepwise evolution of vascular tissue. Leaves originated independently in lycophytes and euphyllophytes; in some euphyllophytes leaves were derived from single dichotomous branches, in others from whole branch systems. In seed plants, secondary growth evolved before the seed. Pinnately compound leaves were replaced by simple leaves in coniferophytes. The origin of the angiosperm flower remains unresolved, but bitegmic ovules may be derived from cupules, and the ancestral carpel can be reconstructed as ascidiate. Evolution of double fertilization was a stepwise process that continued within angiosperms; vessels also evolved within the group. Monocots show major reorganization tied to loss of secondary growth, while pentamerous flowers evolved from dimerous within eudicots.
... Aperture pattern is determined during microsporogenesis and can already be observed in mature tetrads. Some key features of aperture pattern have been identified, including the role played by meiotic spindles (Sheldon and Dickinson, 1986;Brown and Lemmon, 1992;Ressayre et al., 2002b) and the shape of the tetrad that results from the combination of meiosis and cytokinesis, as discussed throughout the pollen literature (Rudall et al., 1997;Blackmore and Crane, 1998;Ressayre et al., 2003). An ontogenic model of aperture pattern that explicitly takes into account consequences of tetrad shapes and early microsporogenesis events to predict aperture type (Ressayre et al., 2002a) was used as the basis of the present work. ...
... Pollen grains have openings from which the genetic material exits to fertilize the eggs: colpus (elongated) and pores (circular). A combination of porus and colpus is termed a colporus (Knox & McConchie, 1986;Blackmore & Crane, 1998;Banks, 2003). ...
... After the microspores are released from the tetrad stage, our results show the presence of remnants of the special cell wall with pectins, AGPs and extensins at the aperture region and they can also be slightly observed in the young intine. These remnants of the special cell wall determine the aperture pattern of M. indica that becomes tricolpate at anther dehiscence, a common aperture type found in the pollen of eudicots (Blackmore and Crane 1998), including Anacardiaceae (Kelkar 1958;Copeland 1959;Ibe and Leis 1979;de Oliveira and de Mariath 2001;Pell et al. 2010). Similarly, JIM8 and JIM13-specific labelling were observed at the aperture sites in Trithuria (Costa et al. 2013), a basal angiosperm. ...
The mango (Mangifera indica) is a woody perennial crop currently cultivated worldwide in regions with tropical and subtropical climates. Despite its importance, an essential process such as pollen development, and, specifically, cell wall composition that influences crosstalk between somatic cells and the male germline, is still poorly understood in this species and in the Anacardiaceae as a whole. A detailed understanding of this process is particularly important to know the effect of low temperatures during flowering on pollen development that can be a limiting factor for fertilization and fruit set. To fill this gap, we performed a thorough study on the cell wall composition during pollen development in mango. The results obtained reveal a clear differentiation of the cell wall composition of the male germline by pectins, AGPs and extensins from the early developmental stages during microsporogenesis and microgametogenesis reflecting a restricted communication between the male germline and the surrounding somatic cells that is very sensitive to low temperatures. The combination of the results obtained provides an integrated study on cell wall composition of the male germline in mango that reveals the crucial role of the sporophyte and the gametophyte and the vulnerability of the process to low temperatures.
... and a few ontogenetic stages from Echinops sphaerocephala L. observed using SEM. Wodehouse (1935) was the first to apply Thompson's (1917) ideas on pattern determination to pollen morphological diversity, with observations on the relationship between meiosis in the microsporocyte and pollen symmetry and aperture disposition (for reviews, see Blackmore and Crane 1988;Blackmore et al. 2007). As Heslop-Harrison (1968) observed, building on this primary level of organization in pollen, two further levels of organization determine the stratification of the exine and its surface ornamentation. ...
We report on pollen development in two species of Echinops (Asteraceae, Cynareae) studied using transmission and scanning electron microscopy with an emphasis on the organisation and development of the massive sporoderm (maximum thickness 18 micrometres). The major events of exine deposition during the tetrad stage follow the now familiar sequence of self-assembling micellar mesophases and the subsequent incorporation of sporopollenin, observed here as: (1) spherical units with light cores; (2) columns of spherical units with dark cores; (3) large branched macromolecules arranged in a dendritic, three-dimensional network of long alveoli; and (4) alveoli with electron-transparent cores and dense walls. Later, (5) the primexine exhibits an elongated-alveolate pattern in which the alveoli have electron-dense cores and lighter exteriors. When (6) the thick inner columellae make contact with the outer primexine, sporopollenin accumulation in the cores of the primexine alveolae establishes continuity between the inner and outer columellae. In the free microspore stage (7) the foot layer and first lamellae of the endexine appear (8). The endexine lamellae then increase in number and massive accumulation of sporopollenin occurs on all exine elements, making individual elements such as tectal spines, more pronounced. These and earlier findings, as well as experimental simulations of exine development, show that pollen wall morphogenesis involves a subtle interplay of gene-driven biological processes and physico-chemical factors offering abundant opportunities for the generation of complex, taxon-specific patterns.
Key words: exine ontogeny, pattern formation, micelle self-assembly
... and a few ontogenetic stages from Echinops sphaerocephala L. observed using SEM. Wodehouse (1935) was the first to apply Thompson's (1917) ideas on pattern determination to pollen morphological diversity, with observations on the relationship between meiosis in the microsporocyte and pollen symmetry and aperture disposition (for reviews, see Blackmore and Crane 1988;Blackmore et al. 2007). As Heslop-Harrison (1968) observed, building on this primary level of organization in pollen, two further levels of organization determine the stratification of the exine and its surface ornamentation. ...
Main conclusion:
The exceptionally complex exine of Echinops, representing a significant investment of energy, develops from an elaborate glycocalyx which establishes, by self-assembly, a multi-layered system of micelles upon which sporopollenin polymerizes. We report on pollen development in two species of Echinops (Asteraceae, Cynareae) studied using transmission and scanning electron microscopy with an emphasis on the organisation and development of the massive sporoderm (maximum thickness 18 μm). The major events of exine deposition during the tetrad stage follow the now familiar sequence of self-assembling micellar mesophases and the subsequent incorporation of sporopollenin, observed here as: (1) spherical units with light cores; (2) columns of spherical units with dark cores; (3) large branched macromolecules arranged in a dendritic, three-dimensional network of long alveoli; and (4) alveoli with electron-transparent cores and dense walls. Later, (5) the primexine exhibits an elongated-alveolate pattern in which the alveoli have electron-dense cores and lighter exteriors. When (6) the thick inner columellae make contact with the outer primexine, sporopollenin accumulation in the cores of the primexine alveolae establishes continuity between the inner and outer columellae. In the free microspore stage, (7) the foot layer and first lamellae of the endexine appear (8). The endexine lamellae then increase in number and massive accumulation of sporopollenin occurs on all exine elements, making individual elements such as tectal spines, more pronounced. These and earlier findings, as well as experimental simulations of exine development, show that pollen wall morphogenesis involves a subtle interplay of gene-driven biological processes and physico-chemical factors offering abundant opportunities for the generation of complex, taxon-specific patterns.
... Endoapertures form in the endexine and may involve the development of onci and assembly of pectocellulosic materials (Blackmore & Crane, 1998;Pozhidaev, 1998). Within eudicots, simple apertures have been considered a plesiomorphic state (Blackmore et al., 1995;Zhang et al., 2017), and this is also supported in our study. ...
This study, the fifth in a series investigating palynological characters in angiosperms, aims to explore the distribution of states for 19 pollen characters on five early diverging orders of Superasteridae (Berberidopsidales, Caryophyllales, Cornales, Ericales, and Santalales) plus Dilleniales. To illustrate the character states found in the pollen of this group, we examined pollen grains of 15 species exemplifying 15 families across all studied orders using light, scanning, and transmission electron microscopy. We reconstructed the phylogeny of the early diverging Superasteridae and related taxa with eight genetic markers for 172 genera, using maximum likelihood (ML) analysis. Nineteen pollen characters were coded for the genera used in this phylogeny and compiled into two morphological matrices using two coding strategies. The characters were then optimized on the newly generated ML tree plus two constrained trees differing in the position of Dilleniales, using three methods of inference. Taxa in this grade show a striking diversity of pollen morphologies, particularly in certain characters such as size, tectum sculpture, and aperture number. The plesiomorphic condition for the early diverging Superasteridae is unambiguously and consistently inferred to comprise monad-dispersed, isopolar, spheroidal, circular in outline, equatorially arranged, tricolpate pollen grains with granular aperture membranes, a smooth tectum, and endexine present. We identify diagnostic character states and synapomorphies for several monophyletic groups, and explore the palynological evidence that may shed light on some unresolved relationships. For example, the hypothesis that Dilleniales is sister to Superrosidae is better supported than alternative hypotheses, being consistent with a number of shared palynological state changes including transitions to presence of costae, reticulate tectum, and columellar infratectum structure. Across this part of the angiosperm phylogeny, most state transitions occur repeatedly, and their frequency varies among both clades and characters. We discuss the impact of optimization method, tree topology, and coding strategy upon ancestral state reconstruction.
... In Bryophyta, Pteridophyta, and dicotyledonous angiosperms, simultaneous cytokinesis predominates-the cell wall is formed already at the end of meiosis after both stages of karyokinesis (Davis 1966;Kapil and Bhatnagar 1991;Shimamura et al. 2003;Brown et al. 2010;Brown and Lemmon 2013). Besides these two main types, intermediate cytokinesis types are distinguished (Murty 1964;Bhandari 1984;Blackmore and Crane 1998). Finally, four haploid cells with half the chromosome number of the mother cell are formed through meiosis. ...
Main conclusion:
Chondriokinesis represents a highly orchestrated process of organelle rearrangement in all dividing plant and animal cells, ensuring a proper course of karyokinesis and cytokinesis. This process plays a key role in male gametophyte formation. Chondriokinesis is a regular rearrangement of cell organelles, assuring their regular inheritance, during both mitotic and meiotic divisions in plant and animal cells. The universal occurrence of the process implies its high conservatism and its probable origin at an early stage of plant evolution. The role of chondriokinesis is not only limited to segregation of cell organelles into daughter cells, but also prevention of fusion of karyokinetic spindles and delineation of the cell division plane. Thus, chondriokinesis plays an indispensable role in mitosis and meiosis as one of the various factors in harmonised cell division, being a key process in the formation of viable cells. Therefore, disturbances in this process often result in development of abnormal daughter cells. This has far-reaching consequences for the meiotic division, as emergence of abnormal generative cells impedes sexual reproduction in plants. This review is focused on microsporogenesis, because various plants exhibit a problem with sexual reproduction caused by male sterility. In this paper for the first time in almost 100 years, it is presented a compilation of data on chondriokinesis proceeding during microsporogenesis in plants, and providing view of the role, mechanism, and classification of this process in male gametophyte formation.
... Nonetheless, not all pollen and spore walls have apertures. Inaperturate angiosperm pollen grains, whose walls can only be escaped by wall rupture or disintegration are found in 18 orders containing 54 families ( Kress and Stone, 1983 ;Th anikaimoni, 1986 ;Zavada and Anderson, 1997 ;Blackmore and Crane, 1998 ;Furness and Rudall, 1999 ;Stone, 1987 ;Dobritsa and Coerper, 2012 ). Several examples are known of pollen wall disintegration aft er pollination; spines may melt, or walls become amorphous and porous ( Gherardini and Healey, 1969 ;Dickinson and Lewis, 1974 ;Rowley and Rowley, 1983 ;Dulberger, 1992 ); similarly, the spore wall disintegrates during germination for the protist Chlamydomonas monoica ( Malmberg and VanWinkle-Swift , 2001 ). ...
Premise of the study:
Most pollen walls are interrupted by apertures, thin areas providing access to stigmatic fluids and exit points for pollen tubes. Unexpectedly, pollen tubes of Arabidopsis thaliana are not obligated to pass through apertures and can instead take the shortest route into the stigma, passing directly through a nonaperturate wall.
Methods:
We used stains and confocal microscopy to follow early pollen tube formation in A. thaliana and 200+ other species. We germinated pollen in vitro and in situ (at control and high humidities) and also used atomic force microscopy to assay material properties of nonaperture and aperture walls.
Key results:
Pollen tubes of A. thaliana breached nonaperture walls despite these being an order of magnitude stiffer than aperture walls. Breakout was associated with localized swelling of the pectin-rich (alcian blue positive) intine. The precision of pollen tube exit at the pollen-stigma interface was lost at high humidity. Pollen from ∼4% of the species surveyed exhibited breakout germination behavior; all nine breakout species identified so far are in the Brassicaceae family (∼25% of the Brassicaceae sampled) and are scattered across seven tribes.
Conclusions:
The polarity of pollen germination in A. thaliana is externally induced, not linked to aperture location. The biomechanical force for breaking nonaperture walls is found in localized swelling of intine pectins. As such, the pollen from A. thaliana, and likely many Brassicaceae family members, are functionally omniaperturate. This new mechanism for germination between extant apertures raises questions about exine porosity and the diversity of mechanisms across taxa.
... Precladistic workers assumed that tricolpate pollen evolved independently in several lines (e.g., Cronquist 1968), but molecular studies confirmed preliminary indications from morphological analyses (Dahlgren & Bremer 1985;) that eudicots are monophyletic and tricolpate pollen originated only once (except for a superficial convergence in Illicium and Schisandra, in the Austrobaileyales, where the three furrows are oriented differently : Huynh 1976;Doyle 2005). Phylogenetic analyses provide little evidence on the mode of origin of tricolpate pollen, since the potential outgroups are basically monosulcate (Doyle 2005), but some of the oldest Early Cretaceous tricolpate pollen has obliquely oriented and interconnected colpi, suggesting an origin by elongation, spiralization, and fragmentation of a sulcus (Doyle & Hotton 1991;Blackmore & Crane 1998). ...
An increasingly robust phylogenetic framework based on molecular and fossil data clarifies the sequence of evolutionary innovations in land plants. Oogamy and cellular novelties (phragmoplast, plasmodesmata, incipient meristems) evolved in aquatic streptophytes. Invasion of the land entailed interpolation of the sporophyte, jacketed gametangia and sporangia, and air-dispersed spores, followed by stornata. Origin of vascular plants involved branching of the sporophyte and stepwise evolution of vascular tissue. Leaves originated independently in lycophytes and euphyllophytes; in some euphyllophytes leaves were derived from single dichotomous branches, in others from whole branch systems. In seed plants, secondary growth evolved before the seed. Pinnately compound leaves were replaced by simple leaves in coniferophytes. The origin of the angiosperm flower remains unresolved, but bitegmic ovules may be derived from cupules, and the ancestral carpel can be reconstructed as ascidiate. Evolution of double fertilization was a stepwise process that continued within angiosperms; vessels also evolved within the group. Monocots show major reorganization tied to loss of secondary growth, while pentamerous flowers evolved from dimerous within eudicots.
... According to Atlagic et al. (2009), the pollen grain appearance and the number of pores are species specific. Dajoz et al. (1991), Blackmore and Crane (1998) and Furness and Rudall (2004) had the opinion that pollen evolution has been resulted by an increase in aperture number in angiosperms. ...
The study deals with the pollen morphological characterization of thirty-seven accessions under six species of Sesamum, including the wild and cultivated taxa, collected from the state
of Kerala. The assessment of species relationship of the taxa forms the pivotal aim of the investigation. The study displayed the significance of palynological characters such as aperture type, size and shape of pollen grains and exine ornamentation in the delimitation of the taxa at inter- and intra-specific levels. The species exhibited uniformity in their aperture morphoform (stephanocolpate), though they differed in the number of apertures. Wide variations observed in the exine ornamentation such as reticulum cristatum with gemmate (S. indicum), baculate (S. radiatum, S. prostratum), verrucate (S.malabaricum), areolate (S. laciniatum) and gemmate with clavate (S. alatum) suggested the utility of the character in the systematics of the genus. Inter-specific variation noticed in the size and shape of the pollen grains ranged from medium (S. laciniatum) to large (S. radiatum and S. malabaricum) and suboblate to prolate, respectively. The study explained the evolutionary relationship of the taxa based on pollen morphology. A taxonomic key is also prepared and presented based on the characters analysed.
... Apertural heteromorphism is common in angiosperms (Erdtman 1966), and is found in several genera of Convolvulaceae, for example in Bonamia, Cuscuta and Merremia (Lewis 1971;Leite et al. 2005;Welsh et al. 2010). This heteromorphic condition can result from the succession of events during meiotic cytokinesis (Blackmore & Crane 1998). Till-Bottraud et al. (1999) discussed heteromorphic pollen of Violaceae: the four-aperturate grains have more chances to germinate, while the three-aperturate grains have faster pollen tube growth and better survival. ...
Jacquemontia is one of the larger genera in Convolvulaceae, with around 120 species, and is considered taxonomically difficult. The family is eurypalynous and pollen morphology has been considered as an important taxonomic character. Pollen morphology of 43 species, representing all morphological groups of Jacquemontia, was analysed with light microscopy and/or scanning electron microscopy. Three pollen types were characterised. These pollen types do not corroborate the current circumscription of sections in Jacquemontia, which is based on inflorescence structure. Conversely, however, some macro-morphological features are discussed that support groups defined on the basis of pollen analysis.
... We have not observed a well discernible proximal fold in the end of the tetrad period, just a long outgrowth of a thin initial spore envelope ( Figure 12C, E). Blackmore and Crane (1998) mentioned that simultaneous cytokinesis, without a dyad stage, generally results in a tetrahedral tetrad, in which all four meiotic products are in direct contact and appear to be reflected in a distinct trilete aperture. Lugardon (1966Lugardon ( , 1969, describing the formation of a proximal fold in the leptosporangiate homospore fern Blechnum spicant, calls the forming layers by numbers. ...
The pre-meiotic, meiotic and tetrad stages of development in microsporangia of Alsophila setosa were studied with particular emphasis on the early establishment of patterning in the microspore wall and the subsequent development of the sporoderm. The data obtained were compared with corresponding ontogenetic stages of Psilotum nudum. Tapetal behaviour was also examined. During the tetrad period, only one layer, a thin undulating sheet, appeared alongside the plasma membrane of the tetraspores, and this was evidently formed on a pre-patterned structure - a fibrillar layer, corresponding to a kind of primexine matrix. The early free microspores had a wavy plasma membrane with a parallel, sinusoidal, thin initial sporoderm layer. The proximal apertural fold was observed to be an extended outgrowth of this initial spore envelope. Sporoderm ontogeny during the tetrad period in Alsophila and Psilotum show some common points, but also fundamental differences, mainly in the relative timing of events: in Alsophila the end of the tetrad period is the starting point for exospore development, whereas in Psilotum the exospore is already complete at this stage. Considerable differences were also observed in the tapetum of the two species.
... Eudicots are not associated with any taxa with previously suggested intermediate states, such as trichotomosulcate (Wilson 1964, Muller 1970 or inaperturate (Meeuse 1965, Walker 1974). This does not rule out origin via one of these types, or some other, such as spiraperturate forms described from the Barremian-Aptian of Gabon (Doyle & Hotton 1991, Doyle 1992, Blackmore & Crane 1998), but it does imply that any intermediates will be found only in fossil record. Walker & Walker (1984) suggested that Chloranthaceae were related to tricolpate ''lower Hamamelididae'' based on similarities between Chloranthaceae and tricolpate taxa such as Euptelea, including reticulate-columellar exine structure, supratectal spinules, and sculptured aperture membranes. ...
Phylogenetic analyses of molecular and morphological data, revised in this study, shed light on long-standing controversies on the early evolution of angiosperm pollen. Although relationships between angiosperms and other seed plants remain uncertain, the robust rooting of the angiosperms among the “ANITA” groups ( Amborella , Nymphaeales, Illiciales, Trimenia , and Austrobaileya ) clarifies their original pollen morphology. Contrary to the view that the first angiosperms had boat-shaped monosulcate pollen with granular or atectate exine structure, the ANITA rooting implies that globose monosulcate pollen with more or less columellar structure and a continuous tectum was ancestral and a foveolate-reticulate tectum arose soon after. The oldest recognized Cretaceous angiosperm pollen may represent the latter grade of evolution. Structure described as granular evolved independently from columellar within Nymphaeales, Magnoliales, and Laurales. In Magnoliales, columellar Myristicaceae and Magnoliaceae diverge below Degeneria , Galbulimima , Eupomatia , and Annonaceae, which shifted to granular structure. Granular monosulcate pollen was ancestral in Annonaceae but gave rise to columellar monosulcates and permanent tetrads. In Laurales, reduction and granularization culminated in the fragile exines of Lauraceae. Although absence of a distinctly staining endexine in Magnoliales has been considered evidence that the laminated endexine of gymnosperms was lost before the origin of angiosperms, presence of a thin endexine now appears to be ancestral. These results refute the view that granular structure supports a relationship between angiosperms and Gnetales, Bennettitales, and Pentoxylon . Relationships with groups with alveolar exines (e.g., Caytonia , glossopterids) and/or reticulate-columellar Triassic Crinopolles pollen now seem equally likely.
... Megaspore characters have been strongly emphasised in the sectional and specific classification of the genus since Pfeiffer (1922) produced the first monographic treatment of Isoetes [for a more recent discussion, see Hickey (1986)]. As Blackmore and Crane (1998) reported, there is a direct correlation between the occurrence of proximal trilete apertures and simultaneous cytokinesis in pteridophytes. Pteridophytes with proximal monolete apertures exhibit successive cytokinesis with a distinct dyad wall present between the first and second divisions of meiosis. ...
Microspore and megaspore development in Isoetes japonica was studied using light, scanning electron and transmission electron microscopy. Differences in cytokinesis following meiosis result in monolete microspores with bilateral symmetry, and trilete megaspores with radial symmetry. Wall development follows essentially the same sequence in both microspores and megaspores, but with the wall of the latter being very much thicker. A significant difference was observed in the tetrad stage, when microspores have an essentially rounded outline with the plasma membrane folded into a crest at the site of the future aperture. Megaspores in the tetrad stage are strongly invaginated over their entire surface, creating numerous crests, and this appears to have a role in determining the morphology of the perispore. The outline shape of the megaspore perispore having been established in the tetrad stage, most of its substance is added later and is derived from the degenerating tapetum.
... The evolution of the unique multiaperturate and heteromorphic pollen conditions in Physarieae has been labile and extremely complex. Increases in pollen aperture number and heteromorphic pollen in the tribe relate to key questions about the origin and evolution of multiple pollen apertures in angiosperms, a topic that remains in great need of comparative studies (Blackmore and Crane 1998;Ressayre et al. 2002Ressayre et al. , 2005Furness and Rudall 2004;Rudall and Bateman 2007). Further studies about meiosis, early stages of pollen development, and molecular evolution of developmental genes will provide us with a better understanding of the origin and evolution of pollen aperture number in Physarieae. ...
Physarieae, a tribe of mustards with multiaperturate pollen, was investigated phylogenetically for the first time using comprehensive sampling and plastid molecular data. Phylogenetic analyses were employed to test the monophyly of the tribe, to investigate relationships of its seven genera, and to study the evolution of selected morphological traits. Character evolution was inferred using parsimony approaches for nine continuous and three discrete traits related to trichomes, fruits, seeds, and pollen. Two monophyletic clades were recovered within Physarieae: the DDNLS clade, which included five genera (Dithyrea, Dimorphocarpa, Nerisyrenia, Lyrocarpa, and Synthlipsis), and the PP clade, which comprised two genera (Paysonia and Physaria). Under this new phylogenetic context, multiaperturate pollen appeared to be a potential synapomorphy of the tribe. In addition, five continuous fruit and seed traits were identified as new potential synapomorphies for Physarieae. Our analyses also indicated that members of the DDNLS clade present more highly branched dendritic trichomes, narrower replums, longer fruits, and wider seeds, when compared to representatives of the PP clade. Moreover, ancestral state reconstructions of continuous traits in Physarieae showed that seeds became wider and fruits, replums, styles, and seed primary grooves became longer over time.
... could be regarded as more specialised evolutionary acquisitions. Wall and aperture structure are developed earlier during the ontogeny (Blackmore and Crane, 1998), and this is possibly why they appear to be more fixed developmentally than surface ornamentation, which is the last structure to be laid down during development. Because of this, it is possible that wall and aperture structure are less open to rapid change, and are therefore more indicative of phylogenetic relationships, than surface ornamentation. ...
Pollen grains of 250 samples of taxa in the Cercideae clade have been studied using light microscopy, scanning electron microscopy and transmission electron microscopy. This study examines how pollen morphological structures can be used as taxonomic characters in systematic studies. Pollen grains of the first branching taxa in the Cercideae phylogeny, such as Cercis and Adenolobus, are unspecialised; they are isopolar, tectate, tricolporate, and released in monads. Surface ornamentation may be micro-reticulate or perforate, and psilate to rugulate. Aperture membranes are granular to coarsely granular. More specialised pollen grain structures are found in Schnella, Lasiobema, Phanera, Piliostigma and most of Bauhinia s.s. Pollen morphology is presented in a table for comparative purposes and illustrated, discussed and compared. Six specialised pollen structures described and identified are diagnostic for groups of related species in the Cercideae. These include a granular infratectum, syncolporate apertures, pororate apertures, spiny opercula, tetrads, and non-supratectal spines. Porate apertures occur in Phanera, Piliostigma and Bauhinia picta. Five pollen structures have been identified within the Cercideae clade that is restricted to Bauhinia s.s. These include striate ornamentation, having more than three apertures per grain, apertures that are indistinct, and colpate apertures. Supratectal ornamentation, structures such as gemmae, verrucae and striae, occur in many species in the Cercideae, as well as throughout subfamily Caesalpinioideae, and the functional implications of this are discussed. Pollen morphological structures are discussed with regard to systematic significance, taxonomic utility, and in relation to functional and developmental considerations.
... Thus we include a discussion on the structure and function of the morphological features we have delimited as characters. Pollen walls evolve in a complex system of compromise and constraint (e.g., Heslop-Harrison, 1976;Muller, 1979;Bolick, 1981;Blackmore and Barnes, 1986;Crane, 1986;Guinet and Ferguson, 1989;Blackmore and Crane, 1998). Many inter-relating factors affect the structure of pollen, and this complexity makes defining characters for analysis challenging. ...
... The definitions of operculum and pontoperculum above indicate that they are composed of exine, which covers part of an aperture. Apertures are specialized areas, formed during pollen development, where the outer sporopollenin wall of the pollen grain (the exine) is usually thinner and the inner cellulosic wall (the intine) is usually thickened, channeled, or forms multiple layers (Blackmore and Crane 1998). Sections of fully hydrated operculate pollen viewed using a transmission electron microscope (TEM) clearly show this thickened intine is continuous beneath the exine of the operculum, for example, in Zigadenus and Veratrum (Halbritter and Hesse 1993, fig. ...
In pollen terminology, an operculum is an area of exine covering a pollen aperture like a lid. Monosulcate-operculate, pontoperculate, disulculate, disulcate, zonasulculate, and zonasulcate aperture types are often confused in the monocot pollen literature. Various factors contribute to this confusion, including the existence of intermediate forms. Other factors include the presence of artifacts (such as the collapse of thin-walled pollen examined using SEM) and insufficient data, either on mature pollen (including absence of ultrastructural data using TEM) or on developmental stages, especially the critical tetrad stage. In this article, we review records of monosulcate-operculate pollen in monocots in relation to recent phylogenetic concepts, using data both from the literature and our own observations, including new records of operculate pollen in Doryanthes and Chamaerops. The exine that forms the operculum often has a simpler structure than that of the rest of the grain and is underlain by thick, often channeled, apertural intine. With the exception of monocots, opercula are rare or absent in basal angiosperms with monosulcate pollen. Within monocots, monosulcate-operculate pollen is absent from basal monocots (Acorus and Alismatales) and is relatively infrequent among commelinids, except for some Dasypogonaceae, Arecaceae, and Poales, in which the aperture is reduced to an ulcus. In contrast, it occurs frequently in the lilioid orders Liliales, particularly Liliaceae, Melanthiaceae, and Uvulariaceae; and Asparagales, particularly Doryanthaceae, Iridaceae, Tecophilaeaceae, and Agavaceae. Species with operculate pollen often have relatives with insulae or granules on the aperture membrane or with diaperturate pollen. In some taxa, monosulcate-operculate pollen may represent a route in the evolution of diaperturate pollen from monoaperturate pollen, although this requires further testing. The operculum has evolved several times independently in monocots and may have a variety of related functions, such as protection of the delicate apertural area from pathogens and/or dehydration, particularly in taxa from dry habitats, and a role in harmomegathy.
... Apertures in angiosperms are either distally (in basal angiosperms and monocots), equatorially (in the "tricolpate" eudicots), or globally situated, but not at the proximal pole (Erdtman, 1952;Thanikaimoni, 1986;Blackmore & Crane, 1998;Harley, 2004;Furness & Rudall, 2004;Blackmore et al., 2007). The earliest germination zones ("apertures") in evolutionary history were situated proximally: the rare term "proximalipolar apertures" embraces the tetrad scars (laesurae) of moss and fern spores and in some "prepollen" (microspores of some extinct seed plants), all other forms are situated distal ("distalipolar") or situated otherwise (Erdtman, 1952). ...
Beschorneria yuccoides (Agavaceae) microspores are arranged mostly in planar tetrads. Later on, the pollen grains of the tetrad usually fall apart, but sometimes remain loosely connected by ektexine elements. The ektexine consists of a tectum, of short columellae, and of a thin, discontinuous foot layer. An endexine is absent. The bilayered intine is without any additional thickening that would usually indicate an aperture region. From this point of view the pollen grain might be considered as omniaperturate. The pollen ornamentation is reticulate with wide lumina and robust, smooth muri. The pollen grains show an indistinct sulcus characterised by a loose reticulate ornamentation. The sulcus is not exactly at the distal pole, but shifted towards the equator. No pollen tubes are formed regularly at the sulcus. Instead, pollen tubes are normally formed at the proximal pollen face. The proximal area, indicating a large germination field, is morphologically and functionally clearly an aperture (a germination zone); however, it does not represent a sulcus. The proximal face of all pollen grains appears as ornamented, with some exine lumps. Asimina triloba (Annonaceae) pollen is shed in permanent planar or decussate tetrads. The distal sides are microreticulate to foveolate, and do not show an aperture; the psilate proximal sides are the germination areas of A. triloba. The presence of apertures placed at the proximal pole was reported for distinct taxa of several angiosperm families. For Drosera, Dionaea (Droseraceae) and most probably for the diaperturate Cuphea species (Lythraceae) the existence of polar germination areas can be excluded. However, in some Annonaceae taxa with permanent tetrads (Annona cherimola, Asimina triloba) a situation similar to Beschorneria might be present, and indeed a proximal polar pollen tube is formed. Beschorneria yuccoides, Annona cherimola and Asimina triloba are unequivocal examples of angiosperm pollen with an exactly proximal aperture (germination area).
... P. emblica) (Kö hler 1967). Thanikaimoni (1986) and Blackmore & Crane (1998) elaborate on a number of phylogenetic trends in aperture configurations. Thanikaimoni stated that a colporate aperture with two or more endopores is derived from the colpate aperture. ...
A total of 129 species from the subtribe Flueggeinae of the tribe Phyllantheae (Euphorbiaceae, Phyllanthoideae) were investigated using light and scanning electron microscopy, and 10 species using transmission electron microscopy, in order to evaluate the relationships between the eight constituent genera: Breynia, Flueggea, Glochidion, Margaritaria, Phyllanthus, Reverchonia, Richeriella, and Sauropus. Of these genera, Flueggea, Margaritaria and Richeriella share pollen with a prolate spheroidal meridional outline and a 3-colporate aperture system. Pollen of Reverchonia is also 3-colporate, but differs from that of the Flueggea alliance by its clearly prolate shape, tilioid ornamentation and absence of costae endopori. Breynia and Sauropus have 4-12 and 3-16-colporate pollen, respectively, with diploporate colpi. Two pollen types are recognised in Breynia, and four in Sauropus, one of which supports the recognition of Sect. Hemisauropus. Glochidion pollen is 3-6-colporate, and similar to that of Breynia in having reticulate sculpture with Y-shaped sexine structures, but it has monoporate colpi. Of the genus Phyllanthus, only species with pollen with diploporate colpi have been studied. Seven types are described. Diploporate Phyllanthus pollen can be distinguished from that of Breynia and Sauropus by its distinct colpus margins consisting of parallel muri. Colpal irregularities and endoaperture configurations in the subtribe are discussed, and pollen morphological trends are hypothesised. Placed in the successiform aperture series, the Flueggea alliance and Reverchonia form a basal group. Glochidion is considered intermediate, giving rise to the Breynia-Sauropus group. The relationship with Phyllanthus remains unclear.
... Apertural heteromorphism is common in numerous angiosperms (Erdtman 1966;Van Campo 1976;Mignot et al. 1994), and can be linked ontogenetically to the succession of events that take place during the meiotic cytokinesis (Blackmore and Crane 1998;Ressayre et al. 2002Ressayre et al. , 2005. Experimental results from heteromorphic eudicots have shown that 4-apertured grains germinate faster than 3-aperturate ones, but the latter have a faster pollenic tube growth and a better survival than the former (Dajoz et al. 1991;Till-Bottraud et al. 1999 ). ...
The genus Cuscuta (dodders, Convolvulaceae) is one of the most significant lineages of parasitic plants from economic, conservation, and anthropogenic perspectives. Members of the genus are twining stem parasites with no roots, lacking almost completely chlorophyll or its function, and gather required nutrition from their host via specialized haustorial connections. While there are almost 200 described species, problems with species identification exist because many diagnostic characters are restricted to their tiny flowers. Probably contributing to this identification difficulty is the fact that the group has not received taxonomic attention for over 75 years; even knowledge of its basic reproductive biology is sparse. Together these conditions have had negative consequences for applied research on this group. More recently, Cuscuta have witnessed a scientific rejuvenation. Molecular phylogenies have been published for 2 of the 3 major infrageneric lineages, 5 of the 15 clades of subg. Grammica have received taxonomic revisions, and work on their basic reproductive ecology has gained momentum. Still lacking, however, are reliable morphological characters that are able to support molecular phylogenies, aid in the description of new sections or species, or provide a solid morphological framework within which ecological variables can be compared. This work presents a thorough survey of the structural diversity of two of the most important, and systematically relevant, reproductive structures in Cuscuta: pollen and the gynoecium. While a number of characters were initially considered, a total of 15 qualitative and 14 quantitative characters were included in analysis. A reclassification of pollen and stigma morphology was required to better account for the variation that is present in these structures. Character states were coded using Thiele’s gap-weighting, and many of the characters were optimized onto a phylogenetic supertree that resulted from the combination of three large-scale phylogenies based on plastid trnL-F and nuclear ITS sequences. Key results are discussed in terms of their ecologic, systematic, taxonomic, cytological or developmental significance.
... It has been proposed that pollen aperture number and pattern is related to microsporogenesis (e.g. Blackmore and Crane 1998;Rudall and Bateman 2007) and that simultaneous cytokinesis allows variation in the interaction between nuclei, which is impossible in successive microsporogenesis, thus leading to heteromorphic pollen (Ressayre et al. 2002). Changes in aperture number are thus potentially easy to achieve (Furness and Rudall 2004). ...
... The arrangement of microspores in a tetrad exhibits one of five patterns: tetrahedral, isobilateral, linear, T-shaped or decussate (Davis 1966). The pattern depends on the shape of microspore mother cells, the position of the meiotic spindles and the resultant division planes (Davis 1966, Blackmore and Crane 1998, Rangaswamy et al. 2001). In the Euphorbiaceae, cytokinesis is simultaneous, and the tetrads are tetrahedral, isobilateral or decussate (Davis 1966, Johri et al. 1992). ...
Microsporogenesis and male gametogenesis of Jatropha curcas L. (Euphorbiaceae) was studied in order to provide additional data on this poorly studied family. Male flowers of J. curcas have ten stamens, which each bear four microsporangia. The development of the anther wall is of the dicotyledonous type, and is composed of an epidermis, endothecium, middle layer(s) and glandular tapetum. The cytokinesis following meiosis is simultaneous, producing tetrahedral tetrads. Mature pollen grains are two-celled at anthesis, with a spindle shaped generative cell. A few abnormal microspores were observed following the early stages of microgametophyte development.
On the example of pollen morphological features of 8 species of Nierembergia and 2 species of Bouchetia (family Solanaceae), the properties of individual variability are described. Most of the studied characters (structure of apertures, ultrastructure of the sporoderm, surface sculpture, dimensions) do not have significant differences at the individual and intraspecific levels; taxonomically significant variability of morphological features of pollen is manifested at the level of the genus and suprageneric groups. The genera differ significantly in the sculpture of the pollen grain surface – striate in Nierembergia and tuberculate in Bouchetia. Pollen contained in one bud, anther or tetrad (fully completed gametophytic generation, where there is no death – all descendants of one ancestor without exception), is considered as an extreme model (maximum completeness with minimum complexity) to study the properties of natural morphological variability and the causes of its occurrence. It was found that pollen characters (sculpture, number and location of apertures) have the same pattern of variability (continuous and transitively ordered series), which embodied at different taxonomic levels in different characters. The natural variability of pollen morphological features is ordered not into a genealogical clade, but into a cline – continuous, geometrically ordered and transitive series (taxon-nonspecific and rank-independent). In this system of parallelisms, homological series are inseparable from non-homological ones, and typical forms, from deviations. The origin of typical and deviant forms cannot be explained separately (typical – genealogically, and deviant – as parallelism, convergence, chance or regularity). The individual variability of pollen forms is geometrically ordered and is not the result of random disturbances, failures of the hereditary program, or pathology. The typical form turns out to be a harmonious part of a geometrically ordered series of pure forms, free from functional and historical connotations. The similarity of pollen forms in these series is determined by their geometry and does not depend on affinity, homology, functionality (improvement, exercise, adaptation). The natural system of pollen forms is formed not by the structure of supposed affinity of supposed taxa (universalia, general concepts, the result of speculation that requires confirmation and admits refutation), but the structure of the observed parallelisms of the variability of individual living bodies. Evolutionary novelty (the current state, the observed variability) arises initially ordered in a pre-established form.
We provide an overview of phylogenetically useful pollen structures in five out of the new six subfamily classification of the legume family (Fabaceae). A large and diverse range of pollen structures occurs in the five subfamilies under review. These were, until recently, all part of a much larger subfamily Caesalpinioideae, except for the 'mimosoid clade', previously recognized as subfamily Mimosoideae, but now part of a recircumscribed subfamily Caesalpinioideae. We do not here present any pollen data on subfamily Papilionoideae, and only consider preliminary data for pollen of the 'mimosoid clade'. The most common pollen structures, and those that particularly identify each subfamily, are discussed, compared and illustrated for the five subfamilies: Caesalpinioideae, Cercidoideae, Detarioideae, Dialioideae and Duparquetioideae. Tricolporate pollen with perforate to microreticulate surface ornamentation occurs in at least some species of all the subfamilies under study except Duparquetioideae. The distribution of pollen structural diversity between and within the subfamilies is discussed. The aperture type, pollen-unit, polarity, symmetry, surface ornamentation and grain size of pollen in each subfamily is described and compared. Apertures are found to be more reliable sources of phylogenetic information than surface ornamentation.
Pollen grains are known for their impressive variety of species-specific, microscale surface patterning. Despite having similar biological developmental steps, pollen grain surface features are remarkably geometrically varied. Previous work suggests that a physical process may drive this pattern formation and that the observed diversity of patterns can be explained by viewing pollen pattern development as a phase transition to a spatially modulated phase. Several studies have shown that the polysaccharide material of plant cell walls undergoes phase separation in the absence of cross-linking stabilizers of the mixed phase. Here we show experimental evidence of a change in density of the extracellular polysaccharide material (primexine) during pollen cell development leads to a spatially modulated phase. The spatial pattern of this phase-separated primexine is also mechanically coupled to the undulation of the pollen cell membrane. The resulting patterned pools of denser primexine form the negative template of the ultimate sites of sporopollenin deposition, leading to the final micropattern observed in the mature pollen. We then present a general physical model of pattern formation via modulated phases. Using analytical and numerical techniques, we find that most of the pollen micropatterns observed in biological evolution could result from a physical process of modulated phases. However, an analysis of the relative rates of transitions from states that are equilibrated to or from states that are not equilibrated suggests that while equilibrium states of this process have occurred throughout evolutionary history, there has been no particular evolutionary selection for distinctly patterned equilibrated states.
Premise of the study:
A diverse range of pollen morphologies occurs within the large, paraphyletic legume subfamily Caesalpinioideae, especially among early-branching lineages. Previous studies have hypothesized an association between surface ornamentation and pollination syndrome or other aspects of pollen function such as desiccation tolerance and adaptations to accommodate volume changes.
Methods:
We reviewed caesalpinioid pollen morphology using light microscopy, scanning and transmission electron microscopy, in combination with a literature survey of pollination vectors.
Key results:
Pollen structural diversity is greatest in the early-branching tribes Cercideae and Detarieae, whereas Cassieae and Caesalpinieae are relatively low in pollen diversity. Functional structures to counter desiccation include opercula (lids) covering apertures and reduced aperture size. Structures preventing wall rupture during dehydration and rehydration include different forms of colpi (syncolpi, parasyncolpi, pseudocolpi), striate supratectal ornamentation, and columellate or granular wall structures that resist tensile or compressive forces respectively. Specialized aperture structures (Zwischenkörper) may be advantageous for efficient germination of the pollen tube.
Conclusions:
In Detarieae and Cercideae in particular, there is potential to utilize pollen characters to estimate pollination systems where these are unknown. Supratectal verrucae and gemmae have apparently evolved iteratively in Cercideae and Detarieae. At the species level, there is a potential correlation between striate/verrucate patterns and vertebrate pollination.
This Festschrift for Chris Humphries provides an opportunity to reflect on how much has changed in systematic biology since the 1970s. Humphries, together with his longtime collaborator, Kåre Bremer, pioneered the application of cladistic methods of phylogeny reconstruction in the Compositae and soon influenced the systematics of other groups of living and fossil plants. Today, the classification of the Compositae has been turned literally upside-down thanks to the availability of DNA sequence characters and the almost universally adopted procedures of phylogenetic systematics. There can be little doubt that Humphries's pivotal role in the promotion of cladistics was greatly enhanced by his appointment to the Department of Botany at the (then) British Museum (Natural History). This chapter focuses on Humphries's contribution to the field of evolution and development. It considers how the field of ontogeny and systematics has developed through to the present day, with particular emphasis on pollen ontogeny. It concluded that Humphries played an influential role in the emergence of the discipline now recognized as “evo-devo”: evolutionary developmental biology.
Pollen morphology of the Turkish Romulea Maratti taxa (Iridaceae) was examined using light and scanning electron microscopy. The taxa are similar in some aspects, such as pollen size and spinulate-microperforate exine sculpturing. Most taxa are monosulcate; however, in 2 taxa, R. bulbocodium (L.) Seb. & Mauri var. crocea (Boiss & Heldr.) Baker and var. leichtliniana (Heldr. ex Hal.) Bég., growing sporadically in south-western Turkey, the existence of pollen type variability (monosulcate, disulcate, trisulcate, trisynsulcate, tetrasulcate, penta-aperturate (with longer and shorter sulci), and monoporate) from single pollen sacs was recorded for the first time. In addition, aperture morphology was found to be variable within most taxa; operculate with 2 bands (bands are free or joined), operculate with 1 band (band is straight or curved), operculate with a circular band, or occasionally insulate.
Microsporogenesis or male meiosis in seed plants is the process leading to a tetrad of four haploid microspores separated by callose walls from a diploid mother cell, or microsporocyte. Each microspore then matures into a pollen grain, the male gametophyte of seed plants that produces the gametes necessary to achieve sexual reproduction. The aperture pattern of pollen grains, defined as the form, number and position of apertures on the pollen surface, is determined during microsporogenesis. The features of microsporogenesis vary among angiosperms and studying the evolution of these features would help to understand the evolution of aperture pattern in this group. We have chosen to examine the diversity and evolution of microsporogenesis in palms (Arecaceae), a family of angiosperms. A large diversity of pollen types is found in this family: a comparable diversity in microsporogenesis can therefore reasonably be expected. Well-supported phylogenies of the family have been recently published and provide the historical framework that is necessary to study the evolution of the characters examined. We have sampled palm species that present one major aperture type, the monosulcate type (a single aperture furrow-shaped and located at the distal pole of the pollen grain) but microsporogenesis was found unexpectedly variable: cytokinesis can be of the simultaneous or successive type or even mixed in some species; intersporal wall formation can be either centripetal or centrifugal; tetrads present a wide range of different forms varying in proportion among and within species. Different developmental pathways can then lead to the production of only one pollen type. We have reconstructed the evolution of the characteristics of microsporogenesis likely to play a role in aperture pattern determination using both Maximum Parsimony and Maximum Likelihood methods. In the case of the Maximum Likelihood method, two different models of evolution were tested: the symmetrical model (one transition rate) and the asymmetrical model (forward transition rate different from backward transition rate). Due to the high variability found in the characters examined, the inference at the ancestral nodes was often equivocal. However, we suggest that the ancestral type of microsporogenesis involved a cytokinesis of the successive type and resulted in tetragonal tetrads. Although it is commonly admitted in the literature that successive cytokinesis is associated with centrifugal cell wall formation whereas simultaneous cytokinesis is associated with centripetal wall formation, no such relationship was found in palms. The reasons underlying the remarkable diversity occurring in pollen morphology and pollen early development are still unclear. In Arecaceae, the relationship suggered by Ressayre et al. (2002a) between the aperture pattern and microsporogenesis could not be highlighted, in particular in consequence of the presence of irregular tetrahedral tetrads and of variable and asymmetrical additional callose deposits.
Ambrosia artemisiifolia L. from Ambrosia of the Heliantheae of the Asteraceae family is a recognized harmful weed worldwide and one of the major invasive foreign plants in China. In this study, we investigated its reproductive features, focusing on its microsporogenesis, microgametogenesis, and pollen morphology. The results show that (1) Ambrosia artemisiifolia L. is a dicotyledonous plant and has spherical, tricolpate pollen grains with spiny outer wall; (2) its anther wall comprises four layers, namely epidermis, endothecium, middle layers, and amoeboid tapetum; (3) cytokinesis of microspore mother cells is successive; (4) most of tetrads are tetrahedral; and (5) mature pollen grains are three-celled. In conclusion, although Ambrosia artemisiifolia L. is a dicotyledonous plant with tricolpate pollen, its microsporogenesis is successive, which is different from typical dicots.
It has been found that a succession of deviant pollen forms, in which the one with a circular colpus (sandwich form) gradually transforms to a common 3-radiate form (3-colpate, 3-syncolpate, 3-parasyncolpate, 3-syndemicolpate or 3-colporate and 3-synsulcate ones) is characteristic of very distant flowering plant taxa. The orientation of a sandwich form in the tetrad coincides with that of zonosulcate pollen, which seems to indicate the homology of these forms. The ways of the appearance of additional third ecto- and endoapertures are described. An additional endoaperture was found to appear prior to the ectoaperture in this succession. The following hypothesis has been suggested to explain the succession emergence and the endoaperture behavior. The succession is likely to reflect some processes occurring in the course of the cytoplasm transformation in the meiocyte and the postmeiotic microspore, which are usually completed before the pollen wall is formed, thus predetermining the common 3-colpate condition. The wall formation prior to the completion of this transformation gives rise to various deviant aperture patterns. A possible structure responsible for establishing the initial cytoplasm conformation may be a quadripolar microtubule system appearing in the meiocyte telophase-II.
In eudicot postmeiotic tetrads, apertures are usually
joined in pairs in highly conserved areas.These appear to be located
at the last points of contact persisting at the end of cytokinesis
between the cytoplasm of the future microspores. In order to investigate
the relationship between cytokinesis and aperture formation,
aperture distribution within postmeiotic tetrads and the progression
of meiosis were studied in Nicotiana tabacum cv. Ambalema. This
variety (inbred line) produces about 85% tricolporate pollen and
15% tetracolporate pollen grains. In addition, about 7% of tetrads
are composed of four equal-sized microspores and a supernumerary
pseudomicrospore of small size and an equal proportion of
tetrads exhibit unpaired apertures (these apertures are not joined
in pairs within tetrads). Observation of cytokinesis indicates that
both unpaired apertures and pseudomicrospores could result from
the persistence of late communications between microsporocytes.
Observations of tetrads indicate that an increase in the number of
elements that are separated during cytokinesis is correlated with
an increase in microspore aperture number. All data converge to
support the hypothesis that aperture site determination is partly
controlled by the number of walls formed to separate the different
elements of the tetrad.
In this study we examine the pollen, stigmas and ovaries from 62 collections of herbarium material representing 16 genera, using light and scanning electron microscopy. The caesalpinioid Dimorphandra group (Burkea, Dimorphandra, Erythrophleum, Mora, Pachyelasma, Stachyothyrsus and Sympetalandra) pollen grains are small, tricolporate monads, with perforate or psilate ornamentation. Dinizia, Pentaclethra and Aubrevillea have morphological characters that have suggested either a mimosoid or caesalpinioid placement. Dinizia pollen is in permanent tetrads with clavate ornamentation. Pentaclethra pollen grains are monads, two species have tricolporate pollen and the third is porate. Aubrevillea has tricolporate, finely reticulate monads. All ten genera have variable, non-predictable stigma type and ovule number. The mimosoid Adenanthera group (Adenanthera, Tetrapleura, Amblygonocarpus, Pseudoprosopis, Calpocalyx and Xylia) pollen grains are in 8- to 16-grain polyads. In all Adenanthera group species, the stigmatic cavity is only large enough to accommodate one polyad. In addition, the number of ovules present matches the number of pollen units in one polyad. Polyads have porate, operculate apertures that differ in layout, aperture morphology and development when compared with caesalpinioid and other eudicot pollen. © 2010 The Linnean Society of London, Botanical Journal of the Linnean Society, 2010, 162, 594–615.
Apertures are key characters of pollen grains with systematic importance in angiosperms. They function as sites for pollen tube exit, water uptake, transfer of recognition substances and accommodation of volume changes. Not all pollen has apertures; inaperturate pollen (lacking obvious apertures) characterizes many angiosperm groups, especially in early divergent angiosperms and monocots, but also eudicots. In order to expand our knowledge of the systematic distribution, possible functional significance and development of inaperturate pollen in angiosperms, this review focuses on inaperturate and cryptoaperturate (with hidden apertures) pollen in the large eudicot clade, which comprises about 75% of present-day angiosperm species. It includes new TEM observations of inaperturate pollen from four exemplar taxa selected from different parts of the eudicot phylogeny. Two categories of inaperturate (including cryptoaperturate) pollen occur in eudicots. (1) Sterile attractant or feeding pollen associated with functional dioecy has evolved iteratively at least six times in conjunction with complex breeding systems in the core eudicots. (2) Fertile pollen has evolved numerous times independently throughout eudicots, though generally in a relatively small number of individual taxa. Notable exceptions are the petaliferous crotonoid Euphorbiaceae s.s., in which fertile inaperturate pollen occurs in c. 1500 species, and two subfamilies of Apocynaceae s.l. (Secamonoideae and Asclepiadoideae) with c. 2500 species with fertile inaperturate pollen in pollinia. Fertile inaperturate pollen is sometimes (but not always) associated with an aquatic habit, parasitism, insectivory, heterostyly, anemophily or pollinia. Most fertile inaperturate pollen has a thin exine, or the exine is largely restricted to isolated components (muri, protuberances, subunits) separated by thinner areas which probably function as apertures. In cryptoaperturate pollen, the aperture is covered by continuous exine which probably has a protective function, similar to an operculum. Developmentally, inaperturate pollen is not associated with any particular tetrad type or meiotic spindle orientation (unlike some apertures) due to the absence of a colpal shield of endoplasmic reticulum or other organelles and hence is independent of microsporogenesis type. The lack of a colpal shield during the tetrad stage of development permits complete deposition of first primexine and then exine around each microspore, possibly mediated by the action of the DEX1 protein. © 2007 The Linnean Society of London, Botanical Journal of the Linnean Society, 2007, 155, 29–48.
Pollen and orbicule morphology of 35 Dioscorea L. species is described based on observations with light microscopy, and scanning and transmission electron microscopy. Pollen and orbicule characters are critically evaluated and discussed in the context of existing hypotheses of systematic relationships within the genus. Pollen is mostly bisulcate (sometimes monosulcate) with a perforate, microreticulate or striate sexine. Our results indicate that pollen data may be significant at sectional rank. The close relationship between sections Asterotricha and Enantiophyllum proposed by Burkill and Ayensu is supported by pollen morphology as all species investigated share bisulcate, perforate pollen with small perforations and a high perforation density. Macromorphological differences between the two compound-leaved sections Botryosicyos and Lasiophyton are also supported by pollen morphology; pollens of these two sections have very different perforation patterns. Orbicules in Dioscorea are mostly spherical and possess a smooth or spinulose surface. The latter is often correlated with a striate sexine.
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