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Detail of topology of ITS-based parsimony tree for the Platanthera bifolia aggregate, highlighting the position and nature of the small number of base-pair changes (including autapomorphies, and optimised via Acctran). The arrowed branch subtending the entire clade is 18 steps long, with bootstrap support of 100% and a decay index exceeding three. The seven closely similar ITS alleles (I-VII) differ by a maximum of five base-pair changes (for the broader phylogenetic context see Bateman et al., 2009, fig. 1).
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We consider the conceptual relationship between pollinator specfificity, genetic isolation and species delimitation, critiquing Darwin’s (1877) hypothesis that differential placement of pollinia on the probosces and eyes of sphingid moths explained the divergence of the Eurasian orchids Platanthera bifolia and P. chlorantha — species that have simi...
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... sively Asiatic species of the section (i.e. P. mandar- inorum, P. florentia, P. bakeriana). The P. bifolia aggregate proved robustly monophyletic but, to our surprise, the 50 accessions of seven putative species Table 3), and these alleles differed among each other by a maximum of only five base-pair changes (i.e. less than 0.8% divergence: Fig. 3). Also, three of the seven alleles (including the two most common) were found in more than one putative species, and the apparently plesiomorphic allele I was found in no less than five of the seven putative species analysed (Appendix 2). Moreover, the sequence chromatograms of several individual accessions of P. bifolia and P. ...
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... together representing seven putative species (including five of the six putative species of section Platanthera occurring in Europe: cf. Delforge, 2006). The aggregate is well supported as monophyletic in the ITS tree ( . However, even with this greatly expanded sampling, levels of sequence divergence detected by us were exceptionally low (Fig. 3); a maximum of five base-pairs separated two Chinese accessions attributed to P. chlorantha from one of two sequences obtained from the Azorean endemic P. micrantha. The only apparent phyloge- netic structure that we detected within the P. bifolia aggregate was a single-step branch (a single C.T transition at site 172, attracting ...
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... the P. bifolia aggregate was a single-step branch (a single C.T transition at site 172, attracting predictably low statistical support) that separated from the remaining accessions of the P. bifolia aggregate the sequences obtained from the Azorean endemics P. micrantha and P. azorica plus that found in the accession of P. holmboei from Cyprus (Fig. ...
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... data that emerged from our morphometric study in southern England with spur- length measurements taken from populations in south- ern and eastern Sweden by Nilsson (1983). Both datasets revealed classic bell-curve distributions that appeared consistent with a polygenic feature under selection, and showed limited overlap between the two species (Fig. 13). Superficially, there appeared to be only small offsets in the peaks of the two curves between southern England and southern Sweden, but we soon realised that the two bell-curves were in fact transposed between the two counties: P. bifolia actually has substantially shorter spurs than P. chlorantha in south- ern England but ...
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... the figure deviated upward to values of 32-38 mm in the majority of populations occurring in southern England and 38 mm in both populations measured in the Alps (Fig. 9). The lower r 2 value for P. chlorantha relative to that for P. bifolia primarily reflects a relatively large spread of mean spur lengths in southern England (most of the data being derived from the present morphometric study) and especially in Continental Europe, culminating in a mean value in the Andorra population of 26 mm, 11 mm shorter than the length predicted at that latitude by the Figure 13 Spur lengths (mm) of Platanthera bifolia and P. chlorantha from southern England (top, present study) compared with data for these species from southeast Sweden (bottom, Swedish data abstracted from Nilsson, 1983 (Fig. 9) several curve-fitting algorithms more complex than simple linear regression failed to improve the statistical fit of line to data points for either species of Platanthera. The obvious temptation is to interpret these latitu- dinal trends as reflecting adaptation of the spur to proboscis length in the respective moth pollinators attributed to these two orchid species, at least within the British Isles, where the relationship between spur length and latitude is strongest. ...
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... of variation for spur length within our study populations range 2-18% for P. bifolia (typically ca. 11%) and 6-14% for P. chlorantha (typically ca. 9%). Thus, there is no obvious contrast between the two species in presumed selective pressure affecting spur length; their respective bell-curves exhibit similar dispersions about the mean (e.g. Fig. ...
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... P. bifolia aggregate outside northern Europe Thus far, our account has focused on the relatively well-researched relationship between P. bifolia and P. chlorantha in northern Europe. However, our DNA- based analyses (Appendix 2, Fig. 3) included a few representatives of other named taxa that are assigned to the P. bifolia aggregate but occur beyond its northern European heartland. To the east in Asia lies a poorly understood mélange of subtly varying morphology that includes P. metabifolia. To the south occurs the green-flowered but otherwise P. chlorantha-like ...
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... revealed the presence of ITS allele I, which dominates P. bifolia elsewhere (Appendix 2, Table 3). One of our three accessions of P. chlorantha from China also yielded allele I, but the two remaining accessions were the only samples to yield allele III, which differs from the apparently plesio- morphic allele I in two base substitutions (Fig. 3). This suggests that cryptic species may occur within P. chlorantha s.l. in eastern ...
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... populations of P. algeriensis from populations of P. chlorantha in mainland Italy (Pavarese et al., 2011). In contrast, both of the accessions attributed to P. holmboei that we analysed for ITS yielded interesting results. The accession from Lesvos contained the plesiomorphic allele I, typical of P. bifolia and P. chlorantha (Appendix 2, Fig. 3), tending to support assignment of the controversial Platanthera populations from Lesvos to P. chlorantha rather than to P. holmboei (as per Gö lz & Reinhard, 1990). However, the plant from Cyprus (the type region of P. holmboei: Lindberg, 1942) yielded the rare allele IV, which differs from the plesiomorphic allele I in just one ...
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... 3), tending to support assignment of the controversial Platanthera populations from Lesvos to P. chlorantha rather than to P. holmboei (as per Gö lz & Reinhard, 1990). However, the plant from Cyprus (the type region of P. holmboei: Lindberg, 1942) yielded the rare allele IV, which differs from the plesiomorphic allele I in just one substitution (Fig. 3). Possession of this genotype offers a tentative indica- tion that bona fide P. holmboei did not originate recently from P. bifolia or P. chlorantha, or via hybridisation between these taxa. European taxa segregated from P. bifolia s.s. A more recent and more energetic debate has surrounded the possibility of dividing the P. bifolia ...
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... this was not the case. Analyses of our two DNA samples of P. micrantha yielded two sequences that differed by only a single substitution. One of these sequences was allele IV, interestingly otherwise found only in the one unequi- vocal accession of P. holmboei (Appendix 2). The second allele, V, simply differed by a single additional substitution (Fig. 3). The two accessions of P. azorica appeared derived relative to P. micrantha due to the putative synapomorphy of an additional substitution in alleles VI and VII; allele VII also exhibited a one base-pair insertion relative to allele VI (Fig. 3). Thus, despite their substantial morphological divergence from other members of the P. ...
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... of P. holmboei (Appendix 2). The second allele, V, simply differed by a single additional substitution (Fig. 3). The two accessions of P. azorica appeared derived relative to P. micrantha due to the putative synapomorphy of an additional substitution in alleles VI and VII; allele VII also exhibited a one base-pair insertion relative to allele VI (Fig. 3). Thus, despite their substantial morphological divergence from other members of the P. bifolia aggregate, the Azorean species appear to have originated from within that group only recently. The only member of the aggregate that currently occurs in northwest Africa and Iberia is P. algeriensis, which therefore constitutes the most ...
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... we consider the evolution of our own interpretations of available spur-length data through the last few years. Our detailed morphometric survey of both species in southern England was completed in summer 2004. When spur-length data were abstracted from the matrix they showed two classic bell-curves with limited overlap between the two species (Fig. 13). Any idea that these curves might be diagnostic of the two species was rapidly abandoned when we realised that the two curves were transposed relative to the Swedish spur-length data of Nilsson (1983). Our detailed, but UK-dominated, spur-length survey of 2007 suggested that latitude was the single most important factor controlling ...
Citations
... Managed by the National Trust, the Hill forms a salient along the South Downs chalk escarpment of East Sussex, 10 km North of Brighton, England (N 50°54', W 0°10'; ca 103 m asl). The Wolstonbury population was first utilised scientifically in 2007, when it contributed to a citizen science project designed to explore variation across Europe in spur length in both P. chlorantha and P. bifolia (Bateman and Sexton, 2008;Bateman and Sexton, 2009;Bateman et al., 2012). In 2008, the initial short-term project was expanded to include a further three morphological variables plus annual counts of the number of flowering plants (Table 1). ...
... Many prior morphometric studies performed by RB and colleagues on populations of a wide range of European orchid species, typically scoring approximately 50 characters per plant (e.g. Bateman and Rudall, 2011;Bateman et al., 2013), had already identified consistent patterns of variation among contrasting traits in several European orchid genera, including Platanthera (Bateman et al., 2012;Bateman et al., 2013). The strongly nocturnally scented white flowers of P. chlorantha are typical of orchids pollinated predominantly by moths ( Figures 1A-C). ...
... Fortunately, we have access to directly comparable morphometric data gathered by Bateman et al. (2012) (Table 1). It is also important to rule out the possibility that the Wolstonbury population could have been rendered atypical of the species through past hybridization. ...
A large English population of the temperate tuberous Greater Butterfly-orchid, Platanthera chlorantha, was monitored through a 16-year period. Each June the number of flowering plants was counted and 60 flowering plants were measured in situ for four morphological traits, selected for both ease of measurement and their contrasting contributions to the life history of the species. Trait data were tested annually in pairwise combinations for individual plants, before mean values throughout the study period were regressed and cross-correlated against each other and against local data for four meteorological parameters. Labellar spur length proved to be more constrained than either flower number or stem height, and rarely yielded statistically significant correlations with other traits, whereas the three remaining traits reliably showed modest but significant correlations. Mean values and coefficients of variation differed only modestly among years and showed few of any meaningful trends. Spring rainfall and insolation had no detectable effect on traits of plants flowering that June; instead, they impacted on trait expression during the following year, presumably as a result of differential resourcing of replacement tubers formed during the previous year. High spring rainfall in year t–1 increased leaf area and stem height in year t, whereas the widely fluctuating number of flowering plants was highest in years immediately following those characterised by relatively dry and/or sunny springs. The “decision” to flower is taken during the previous summer, though it may be modified through winter/spring abortion of above-ground organs. The proportion of the population electing to flower is the only measured parameter that impacts significantly on annual reproductive output, emphasising the under-rated difficulty of evolving through directional selection. Any attempt to predict the behaviour of plant species in response to climate change must integrate information on demography with that on life history, habitat preference and intimate symbioses.
... The terrestrial orchid genus Platanthera (Rich.) [27] is a powerful system for studying plant hybridization because it maintains extensive diversity and species with overlapping spatial boundaries [19,25,28]. Platanthera orchids span most of the temperate Northern Hemisphere and have undergone extensive floral radiation [10,29,30]. ...
... Sequences are available on NCBI GenBank (accession numbers OQ474558-OQ474566 and OQ507373-OQ507385). The genetic similarity across the hybrid complex for these markers likely reflects recent species divergence that is better resolved with multilocus analyses of genetic divergence [28,44,45]. ...
Natural hybridization between closely related species in sympatry is an evolutionary process that is common in orchids. Once seen as a threat to parent species, interspecific genetic change is increasingly viewed as a source of novel variation in some ecological contexts. Terrestrial fringed orchids in the genus Platanthera contain several clades with high genetic compatibility among species and many putative hybrids. We used biallelic SNPs generated with 3RAD sequencing to study the hybrid complex formed from the parent species P. blephariglottis, P. ciliaris, and P. cristata with high resolution. The genetic structure and phylogenetic relationship of the hybrid complex revealed site-dependent gene flow between species. We documented extensive hybridization and cryptic hybrids in sympatric sites. Interspecific genetic exchange is particularly common between P. blephariglottis and P. ciliaris, with cryptic hybrids among putative P. ciliaris samples being more common than parental assignments in sympatric sites. Hybridization across the triad species complex can reticulate lineages and introduce adaptive alleles. Conversely, it can reduce diversification rates and introduce maladaptive alleles. Investigation into whether anthropogenic forces are eroding species boundaries, particularly the permeable P. blephariglottis and P. ciliaris boundary, is appropriate for conservation efforts.
... Before drawing specific conclusions regarding phenotypic variation across the genus Ophrys, we will first critically appraise the morphometric approach that we have employed here, in order to establish limits to the strength of the conclusions that can reasonably be drawn from this study. We should begin by noting that this is the 22nd such empirical study of European native orchids that one of us (R.B.) has published using this morphometric approach, together spanning seven genera: Dactylorhiza (6 papers [66,77]), Gymnadenia (2 papers [69,78]), Platanthera (4 papers [79,80]), Pseudorchis (1 paper), Orchis (4 papers [76,81]), Himantoglossum (3 papers [82]) and Ophrys (1 paper [45]). The strengths and weaknesses of our approach have therefore been amply demonstrated empirically. ...
... In previous morphometric studies of orchid genera such as Dactylorhiza [66,77] and Platanthera [79,80,83], we have found vegetative characters to be relatively problematic in that they are on average considerably more variable than floral characters [76], collectively reflecting the vigour of the plant in question. The life history of most terrestrial orchids involves the annual replacement of their all-important stem-tuber; the relative success of this process of somatic replacement strongly influences the likelihood that a plant will flower during the following spring/summer and, if so, how vigorously. ...
... Characterising morphological variation in bee orchids as near-continuous and exceptionally multidimensional encourages us to compare our study of Ophrys with those conducted by us previously on other genera of European orchids [45,66,69,76,77,[79][80][81][82][83]. ...
Simple Summary: Our frequently deployed approach to optimally circumscribing species requires large-scale field sampling within and between populations for large numbers of morphometric characters, followed by multivariate ordinations to objectively seek discontinuities (or, failing that, zones of limited overlap) among sets of populations considered to represent species. Corresponding boundaries are sought in DNA-based outputs, either phylogenies or preferably ordinations based on population genetic data. Herein, we analyse within a molecular phylogenetic framework detailed morphometric data for the charismatic bee orchids (Ophrys), seeking a 'mesospecies' species concept that might provide a compromise between the nine 'macrospecies' recognised primarily through DNA barcoding and the several hundred 'microspecies' recognised primarily through perceived pollinator specificity. Our analyses failed to find robust groupings that could be regarded as credible mesospecies, instead implying that each macrospecies constitutes a morphological continuum. This problematic result encouraged us to reappraise both our morphometric approach and the relative merits of the contrasting macrospecies and microspecies concepts, and to reiterate the key role played by genetics in species circumscription. Abstract: Despite (or perhaps because of) intensive multidisciplinary research, opinions on the optimal number of species recognised within the Eurasian orchid genus Ophrys range from nine to at least 400. The lower figure of nine macrospecies is based primarily on seeking small but reliable discontinuities in DNA 'barcode' regions, an approach subsequently reinforced and finessed via high-throughput sequencing studies. The upper figure of ca. 400 microspecies reflects the morphological authoritarianism of traditional taxonomy combined with belief in extreme pollinator specificity caused by reliance on pollination through pseudo-copulation, enacted by bees and wasps. Groupings of microspecies that are less inclusive than macrospecies are termed mesospecies. Herein, we present multivariate morphometric analyses based on 51 characters scored for 457 individual plants that together span the full morphological and molecular diversity within the genus Ophrys, encompassing 113 named microspecies that collectively represent all 29 mesospecies and all nine macrospecies. We critique our preferred morphometric approach of accumulating heterogeneous data and analysing them primarily using principal coordinates, noting that our conclusions would have been strengthened by even greater sampling and the inclusion of data describing pseudo-pheromone cocktails. Morphological variation within Ophrys proved to be exceptionally multidimensional, lacking strong directional trends. Multivariate clustering of plants according to prior taxonomy was typically weak, irrespective of whether it was assessed at the level of macrospecies, mesospecies or microspecies; considerable morphological overlap was evident even between subsets of the molecularly differentiable macrospecies. Characters supporting genuine taxonomic distinctions were often sufficiently subtle that they were masked by greater and more positively correlated variation that reflected strong contrasts in flower size, tepal colour or, less often, plant size. Individual macrospecies appear to represent morphological continua, within which taxonomic divisions are likely to prove arbitrary if based exclusively on morphological criteria and adequately sampled across their geographic range. It remains unclear how much of the mosaic of subtle character variation among the microspecies reflects genetic versus epigenetic or non-genetic influences and what proportion of any contrasts observed in gene frequencies can be attributed to the adaptive microevolution that is widely considered to dictate speciation in the genus. Moreover, supplementing Biology 2023, 12, 136. https://doi.org/10.3390/biology12010136 https://www.mdpi.com/journal/biology Biology 2023, 12, 136 2 of 52 weak morphological criteria with extrinsic criteria, typically by imposing constraints on geographic location and/or supposed pollinator preference, assumes rather than demonstrates the presence of even the weakest of species boundaries. Overall, it is clear that entities in Ophrys below the level of macrospecies have insufficiently structured variation, either phenotypic or genotypic, to be resolved into discrete, self-circumscribing ("natural") entities that can legitimately be equated with species as delimited within other less specialised plant genera. Our search for a non-arbitrary (meso)species concept competent to circumscribe an intermediate number of species has so far proven unsuccessful.
... An example of a suitable evolutionary model system, offering a unique opportunity to study the above-mentioned problems are P. bifolia and P. chlorantha. They have been the objects of studies on species biology (especially pollination biology) and demography (Nilsson, 1983(Nilsson, , 1985Maad, 2000;Brzosko, 2003;Maad & Alexandersson, 2004;Maad & Nilsson, 2004;Maad & Reinhammar, 2004;Bateman & Sexton, 2008;Claessens & Kleynen, 2011;Bateman, James & Rudall, 2012;Steen, 2012;Boberg et al., 2014;Sexton, 2014;Mõtlep et al., 2018), genetic diversity (Brzosko et al., 2009;Brzosko & Wróblewska, 2013) and hybridisation (Durka et al., 2017;Esposito, Merckx & Tyteca, 2017;Esposito et al., 2018;Mõtlep et al., 2021). Because populations from NE Poland are located more centrally in the geographical range (under distinct climate and habitat conditions), plant-pollinator interactions, and in effect reproduction may be modified. ...
... Our results show an opposite pattern than Maad's (2000) observations in most populations, especially in BC and POB, where MRS was few times greater than FRS. The imbalance between MRS and FRS could be explained by the low efficiency of accidental visitors, which remove pollinaria but are not able to transfer them onto the stigmas due to a mismatch in partner morphology (Bateman, James & Rudall, 2012). Another explanation for this imbalance is high pollen discounting; the highest MRS: FRS ratio was found in meadow populations more exposed to wind. ...
... Are reproductive isolation mechanisms enough for maintaining species integrity? P. bifolia and P. chlorantha can produce hybrids (Nilsson, 1983;Maad & Nilsson, 2004;Bateman, James & Rudall, 2012;Esposito et al., 2018;Mõtlep et al., 2021). Our PCA analysis demonstrated the presence of intermediate morphotypes, localised mainly in mixed POB1+POB2 and in the BON P. chlorantha populations. ...
Plant species evolution is driven by many factors that have different roles in space and time. Using different field and laboratory methods, we studied reproductive patterns and their determinants in pure and mixed P. bifolia and P. chlorantha populations in different habitats. We also considered the probability of hybridisation between these two species and the role of intra-population processes in maintaining species integrity. Generally, we found a high level of reproductive success in both Platantherans. In both species, male (MRS) and female (FRS) reproductive success depended on floral display, and male reproductive success additionally on population structure. The flower traits were only weakly related to reproductive success. Moths' assemblages varied spatially and temporally, and their diversity and numbers were correlated with MRS in the year, when their abundance was markedly lower. Analysis of patterns of pollen transfer showed that pollen was transported up to 25 m (average 8.2 ± 4.83 m) and showed gene exchange between these two Platanthera species. The germination level of both species was significantly lower than seed viability, although P. bifolia seed germinated with higher frequency than P. chlorantha seeds. We noted differences in viability and germination of seeds developed as an effect of experimental interspecies crossings and those developed from natural pollination. The presence of intermediate ecotypes together with observations of spontaneous interspecies crosses in the field and viability of seeds produced in interspecies crossing suggest that both pre-and postzygotic reproductive barriers are not complete and do not prevent hybrid production.
... Since then, there has been consensus to separate P. bifolia and P. chlorantha at species level. Both species are diploid with 2n = 42 chromosomes (Heusser 1938, Kliphuis 1963, Averyanov 1979, Alba et al. 2003, recent molecular studies have shown them to be highly genetically similar (Pavarese et al. 2011, Bateman et al. 2012, and both have nectar-producing flowers that are almost exclusively pollinated by nocturnal Lepidoptera of the families Noctuidae, Sphingidae and Geometridae (Nilsson 1978, Claessens and Kleynen 2011, Steen 2012, Steen and Mundal 2013, Boberg et al. 2014, Sexton 2014, Esposito et al. 2017, Steen et al. 2019. ...
... Hybrids between P. bifolia and P. chlorantha are found sporadically where the species grow in sympatry (Nilsson 1983, 1985, Bateman et al. 2012, Durka et al. 2017. The morphologically intermediate hybrid is fully fertile, but under conditions of open pollination it has much lower fruit set than the parental taxa, at least in the Nordic region (Nilsson 1983). ...
... Comparing populations of P. bifolia s.l. from mainly the British Isles, but also France and the Alps, Bateman and Sexton (2008) and Bateman et al. (2012) demonstrated a reduction in mean spur length of ca 2% per 100 km increasing latitude. They also noted that values for populations from shaded habitats were largely situated on the positive side of the regression line, whereas the opposite trend applied to populations from exposed habitats. ...
Over the years, various authors have (sub)divided the Eurasian moth pollinated Platanthera bifolia into several taxa. Advanced studies using multivariate morphometric analysis and/or genetic fingerprinting have all included regions where the situation appears particularly complex. With the aim to resolve variation patterns in a region where the situation seems less complex, we analysed morphometric and AFLP data from 13 Scandinavian populations using a range of uni‐ and multivariate statistical methods. Variation was largely continuous, though with (individuals from) short‐spurred and long‐spurred populations, respectively, forming loose groups. Provided that successful pollinator shifts usually occur between moth species with small difference in proboscis length, this pattern is congruent with the hypothesis that spur length in P. bifolia s.l. has mainly evolved through pollinator shifts followed by selection in response to proboscis length of the prevailing local pollinator species. Acknowledging an important adaptive role of spur length, observing that spur length was among the main contributors to morphological variation, and noting this pattern to be congruent with both AFLP patterns and habitat requirements, we advocate the formal distinction between a short‐spurred and a long‐spurred taxon. Adopting the operational definitions of species, subspecies and variety provided in Flora Nordica, the two taxa should be recognized as P. bifolia var. bifolia and P. bifolia var. latissima, respectively. A key to the varieties is provided.
... In a study based on plastid DNA sequence variation, Pavarese et al. (2011) found that Platanthera algeriensis was characterized by haplotypes A and B, both of which are shared with P. chlorantha. In another study, Italian Platanthera chlorantha were found to form a single group with P. algeriensis from Tunisia and Sardinia (Bateman et al. 2012). ...
This study examined the chromosome numbers and karyotypes of two taxa of the genus Platanthera (Orchidaceae) from Italy. Cytological analyses showed 2n = 2x = 42 in P. chlorantha and P. algeriensis. Karyotype analysis revealed similarity between the species. The karyotypes are as follows: P. chlorantha consists of 34 metacentric + 8 submetacentric pairs and P. algeriensis consists of 36 metacentric + 6 submetacen- tric pairs. Both species possess a rather symmetrical karyotype. P. chlorantha has a very similar C-banding pattern to P. algeriensis. DAPI bright blocks were observed in P. chl- orantha. These analyses also show the close relationship between the studied species.
... Pollination shifts are important drivers of diversification in the family Iridaceae (Goldblatt and Manning, 2006;Chauveau et al., 2012;Valente et al., 2012), and similar on-off patterns are observed elsewhere in the family (e.g., in Homeria, now sunk in Moraea; Goldblatt, 1998). Given that contrasting floral types attract different pollinator guilds and promote different pollinator behavior, heterochronic events here could lead to rapid reproductive isolation, contributing to increased speciation (e.g., Bateman et al., 2012). ...
Premise:
There is little direct evidence linking floral development and pollination biology in plants. We characterize both aspects in plain and ornamented flowers of Trimezieae (Iridaceae) to investigate how changes in floral ontogeny may affect their interactions with pollinators through time.
Methods:
We examined floral ontogeny in 11 species and documented pollination biology in five species displaying a wide range of floral morphologies. We coded and reconstructed ancestral states of flower types over the tribal phylogeny to estimate the frequency of transition between different floral types.
Results:
All Trimezieae flowers are similar in early floral development, but ornamented flowers have additional ontogenetic steps compared with plain flowers, indicating heterochrony. Ornamented flowers have a hinge pollination mechanism (newly described here) and attract more pollinator guilds, while plain flowers offer less variety of resources for a shorter time. Although the ornamented condition is plesiomorphic in this clade, shifts to plain flowers have occurred frequently and abruptly during the past 5 million years, with some subsequent reversals.
Conclusions:
Heterochrony has resulted in labile morphological changes during flower evolution in Trimezieae. Counterintuitively, species with plain flowers, which are endemic to the campo rupestre, are derived within the tribe and show a higher specialization than the ornamented species, with the former being visited by pollen-collecting bees only.
... Species belonging to the genus Platanthera (Orchidaceae) have been increasingly studied to describe evolution in general and adaptation in particular (Bateman et al. 2012). Platanthera bifolia (L.) Rich. ...
... Platanthera bifolia (L.) Rich. and P. chlorantha (Custer) Rchb. is a sister species pair, and several authors have reported hybridization between these species (Nilsson 1983;Bateman and Sexton 2008;Bateman et al. 2012;Durka et al. 2017;Esposito et al. 2018). For example, hybrids occur in Scandinavia, where the two species grow sympatrically, and they can be identified by intermediate morphological traits (Nilsson 1983). ...
... Bateman et al. (2009) used ITS sequencing to distinguish between species of the whole Platanthera clade, but this did not reveal species in the P. bifolia group. Bateman et al. (2012) detected only one base-pair difference among more than 9000 base pairs in the ITS sequence between P. bifolia and P. chlorantha; thus, the identification of hybrids was not possible. The authors suggest that the sister species are in a relatively early stage of speciation, and extensive gene flow occurs between them. ...
Studies focusing on hybrid zones contribute to a better understanding of reproductive isolation and speciation processes. Reproductive isolation is achieved by various pre- and postmating barriers that act sequentially and together determine whether interspecific mating and hybridization can occur. In this study, we investigated various reproductive barriers acting between Platanthera bifolia and P. chlorantha. These species are known to hybridize in the field, suggesting that reproductive barriers are weak. We conducted a pollen tracking and crossing experiments within and between the species in natural populations in Estonia and germinated seeds in vitro for molecular analysis. We also combined morphological measurements with analysis of 233 AFLP loci to examine variation in P. chlorantha, P. bifolia and their putative hybrids in both sympatric and allopatric populations of these species. Pollen flow occurred from P. bifolia to P. chlorantha but not in the opposite direction. Crossing experiments revealed no postmating isolation between the Platanthera species. Out of nine measured morphological traits, three were intermediate for putative hybrids; for other traits, they resembled either of the parental species. Hybrids with known parents had intermediate molecular traits, as did the putative hybrids. Our results showed weak reproductive isolation between P. bifolia and P. chlorantha and may refer to the early stages of speciation of the species.
... In a study based on plastid DNA sequence variation, Pavarese et al. (2011) found that Platanthera algeriensis was characterized by haplotypes A and B, both of which are shared with P. chlorantha. In another study, Italian Platanthera chlorantha were found to form a single group with P. algeriensis from Tunisia and Sardinia (Bateman et al. 2012). ...
S. (2020). Contribution to the study of wild Orchidaceae, genus Platanthera L.C.M. Richard. Karyotype and C-banding analysis of two species from Italy. Caryologia, Just Accepted. Contribution to the study of wild Orchidaceae, genus Platanthera L.C.M. Richard. Karyotype and C-banding analysis of two species from Italy Abstract. This study examined the chromosome numbers and karyotypes of two taxa of the genus Platanthera (Orchidaceae) from Italy. Cytological analyses showed 2n = 2x = 42 in P. chlorantha and P. algeriensis. Karyotype analysis revealed similarity between the species. The karyotypes are as follows: P. chlorantha consists of 34 metacentric + 8 submetacentric pairs and P. algeriensis consists of 36 metacentric + 6 submetacentric pairs. Both species possess a rather symmetrical karyotype. P. chlorantha has a very similar C-banding pattern to P. algeriensis. DAPI bright blocks were observed in P. chlorantha. These analyses also show the close relationship between the studied species.
... Two rewarding orchid species, Platanthera bifolia and P. chlorantha, have already been considered as a model for understanding the role of floral specialization in the adaptive radiation of the Orchidaceae (Durka, Baum, Michalski, & Baum, 2017;Esposito et al., 2018;Hapeman & Inoue, 1997). The choice of Platanthera as a model for selection studies is due to the fact that components of both female and male fitness are easily estimated (Bateman, James, & Rudal, 2012;Bateman & Sexton, 2008;Efimov, 2011), and the column (i.e., in orchids, the organ uniting the stigma and the pollinaria) shows a high morphological variation within the genus, which can easily shift when it is subject to selection (Hapeman & Inoue, 1997). ...
... However, the selective pressures on those two traits do not contribute to any significant differentiation between the pure and intermediate P. bifolia morphotypes. Bateman et al. (2012) suggested that a single gene might be responsible for variation in column width at the origin of these intermediate individuals. According to this assumption, the expanded column, which characterizes the phenotypic shift from the sympatric P. bifolia to intermediate morphotypes, could be due to a limited number of genes. ...
Pollinators represent one of the main agents of selection on floral traits. Here, we estimated phenotypic selection on floral morphology and phenology in a sympatric population of two orchid species, Platanthera bifolia and P. chlorantha , including enigmatic individuals with intermediate column morphology (as reflected by the distance between viscidia and caudicle length, two traits involved in assortative mating and reproductive isolation among Platanthera species), but genetically indistinguishable from P. bifolia . Our aim was to clarify whether the occurrence of intermediate phenotypes could be explained by the presence of selective pressures exerted by pollinators. Simple linear and quadratic regressions together with univariate and multivariate analyses were used to evaluate the strength of directional, disruptive and stabilizing selection. We found that selection on phenotypic traits varied between groups and sex functions. Contrary to our hypothesis, selection on the viscidia distance and caudicle length appeared to be consistent in the two P. bifolia groups. Interestingly, the viscidia distance was under significant stabilizing selection through female reproductive success in intermediate individuals. Based on these results, we conclude that, despite a significant selective pressure on some phenotypic traits, the presence of individuals with intermediate phenotype is not due to selection. Stabilizing selection on distance between viscidia in intermediate individuals may suggest that assortative mating play a role in the maintenance of this phenotypic polymorphism.
