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... Thus, host specificity is a useful trait to distinguish P. rubicundulus but species recognition within the P. involutus complex remains very difficult. Mycologists have long suspected that the name P. involutus encompasses different species (Fries 1985;Hahn & Agerer 1999;Jarosch & Bresinsky 1999;Bresinsky 2006). Thus, mating tests performed with Swedish isolates revealed three intersterility groups (Fries 1985). ...
... Mycologists have long suspected that the name P. involutus encompasses different species (Fries 1985;Hahn & Agerer 1999;Jarosch & Bresinsky 1999;Bresinsky 2006). Thus, mating tests performed with Swedish isolates revealed three intersterility groups (Fries 1985). Although these groups were not correlated with clear-cut morphological characters, ecological traits distinguished them: group 1 was found in forest habitats whereas groups 2 and 3 were found in city parks and gardens (Fries 1985). ...
... Thus, mating tests performed with Swedish isolates revealed three intersterility groups (Fries 1985). Although these groups were not correlated with clear-cut morphological characters, ecological traits distinguished them: group 1 was found in forest habitats whereas groups 2 and 3 were found in city parks and gardens (Fries 1985). On the basis of a morphological study, Hahn & Agerer (1999) recognized four European species in the P. involutus complex: P. involutus, Paxillus validus, Paxillus albidulus, and Paxillus obscurisporus. ...
Paxillus involutus is a model species for ecological or physiological studies of ectomycorrhizal agaricomycetes. Three to six groups or species linked to it have been ecologically and morphologically distinguished. Phylogenetic studies have revealed the existence of four species in Europe: Paxillus ammoniavirescens, Paxillus obscurisporus, P. involutus, and a fourth as yet not described species. We studied 47 collections from 24 French and Italian locations, supplemented with GenBank data, in order to genetically and taxonomically delineate these species. Phylogenetic analyses of three nuclear DNA regions (rDNA internal transcribed spacer (ITS), tef1-α, and gpd) confirmed the four European species. Morphology, culture, and ecology features allowed us to delineate species boundaries and to describe the fourth species we named Paxillus cuprinus since it turns coppery with age. As there is no existing original herbarium specimen for P. involutus, one of our collections was chosen as the epitype. The low genetic diversity found in P. cuprinus correlates with stable morphological traits (basidiome colour, ovoid-amygdaliform spores with an apical constriction) and with ecological preferences (association with Betulaceae in open and temperate areas). In contrast, P. ammoniavirescens is characterized by a high genetic diversity and a high variation of its morphological and ecological features.
... It has also long been recognised that P. involutus in its traditional sense is a species complex. Thirty years ago Nils Fries, a great-grandson of Elias Fries, showed (Fries, 1985) that in Sweden it comprised at least three intersterile biological taxa, one mainly from woodland, the other two mainly from parks and gardens, but he did not give them names. There have since been several sometimes conflicting accounts of new species all broadly supporting his observations. ...
... A brief history of segregates from P. involutus in Europe * Fries (1985), as noted above, showed that there were two further taxa in Sweden, mainly from parks and gardens, distinct from P. involutus. * Sutara (1992) published a nearly white form from what is now the Czech Republic as P. albidulus. ...
... pi.), New Zealand (McNabb, 1969) and South America (Singer, 1964). It is common and widespread in Europe and North America although recently it has been shown to be composed of at least three inter-sterility races (Fries, 1985). To date these races have not been linked with specific characters, although gross morphology and certain ecological features are possibly correlated. ...
... The Australian collections noted above have slightly shorter and more abundant pleurocystidia than those described by Watling (1970) for British material, although in this character they are still considered to come within the variation of the taxon. Perhaps the speciation noted by Fries (1985) is operating within these introductions and populations with slightly different spore sizes are evolving. 7. P. muelleri (Berk.) ...
The lamellate boletes found in the Cooloola Sand-mass, Queensland are described and discussed in the light of earlier Australian records of paxilloid fungi. Nine species of Paxillus are discussed, two in Tapinella, which is maintained as a separate entity, and 11 are assigned to Phylloporus. Of the last, eight are new records to Australia and two are new species. Three appendices cover various aspects of studies on Australian boletes which have come to light since the publication of this series, including notes on the host-range of the parasitic hyphomycete Sepedonium.
... It is recognized that P. involutus in the traditional sense is a complex of species, including at least four taxa in Europe (Fries, 1985;Hahn, Agerer, 1999;Jarosch, Bresins ky, 1999;Bresinsky, 2006). A thorough molecular study of the P. involutus complex in Europe, recently conducted by Jargeat et al. (2014) confirmed and extended the results previously obtained by Hedh et al. (2008) and Vellinga et al. (2012). ...
Specimens of Paxillus cuprinus, a species recently described from P. involutus complex and characterized by less massive, dully coloured basidiomes with more slender stipes, as well as longer basidiospores, were collected by authors during an expedition to the Southern Urals (Republic of Bashkortostan). The studied collection represents the first record of the species in Russia based on basidiomes. The identity of the studied collection was confirmed by molecular data (nrITS and tef1 sequence analyses) and morphology. A full description, illustration of specimens, and results of phylogenetic analyses are provided.
... One clear example is the species Paxillus involutus is one of the best studied and a model species for ecological and physiological studies, and it was worldwide report to have a relevant role in polluted areas ecosystem. Several authors have reported a large variability in the morphology and physiology between different isolates of this fungus, which suggests that P. involutus encompasses multiple species (Laiho 1970;Fries 1985;Hahn and Agerer 1999;Bresinsky, 2006). Phylogenetic analyses of nuclear DNA performed by different authors (Jageat et al. 2014;Hedh et al. 2008;Vellinga et al. 2012) confirmed the presence of five different species in the P. involutus complex: P. involutus, P. ammoniavirescens, P. cuprinus, P. obscurosporus, and P. vernalis; among which it is extremely difficult to differentiate based on morphological characters or differential ammonia reactions. ...
... Some EM morphospecies also reveal CBSs. At least three CBSs occur within Paxillus involutus that partly fit into the sexually incompatible groups previously described in the 80s by pairing monokaryons in vitro (Fries 1985;Hedh et al. 2008). CBSs occur, sometimes in sympatry, within P. tinctorius Hitchcock et al. 2003;Jourand et al. 2010), R. vinicolor sensu lato distinguished R. vinicolor and R. vesiculosus), C. formosus (Dunham et al. 2003), T. scalpturatum complex (Gryta et al. 2006;Carriconde et al. 2008aCarriconde et al. , 2008bJargeat et al. 2010), Strobilomyces (Sato et al. 2007;Sato & Murakami 2008) and Scleroderma species from Africa (Sanon et al. 2009). ...
... Paxillus involutus is a basidiomycete mycorrhizal fungus with a wide host range and world-wide distribution that has recently been shown to be a complex of several cryptic biological species that are specialized on different hosts and habitats (Fries 1985;Hahn & Agerer, 1999). Only recently have these distinct biological species been taxonomically and genetically separated within the P. involutus complex in Europe (Hedh, Samson, Erland, & Tunlid, 2008;Jargeat, Chaumeton, Navaud, Vizzini, & Gryta, 2014) and, to a lesser extent, in North America (Vellinga, Blanchard, Kelly, & Contu, 2012). ...
A growth-inhibitory polysaccharide (GIPinv) was purified using size-exclusion and ion-exchange chromatography from the fourth sodium hydroxide extraction step of a fungus found in British Columbia. The fungus was genetically identified as a member of the Paxillus involutus complex. GIPinv has an average molecular weight of 229. kDa and is a heteroglycan composed of glucose (65.9%), galactose (20.8%), mannose (7.8%), fucose (3.2%) and xylose (2.3%). GC-MS methylation analysis suggests that GIPinv has mixed linkages in the backbone containing (1. →. 6)-Gal (25.5%), (1. →. 4)-Glc (18.3%), (1. →. 6)-Glc (8.3%), (1. →. 3)-Glc (5.3%) and (1. →. 2)-Xyl (4.5%). GIPinv has branching points at (1. →. 2, 6)-Man (8.6%) and (1. →. 3, 6)-Man (4.9%) having unsubstituted fucose (8.3%) and glucose (16.3%) as terminal sugars. GIPinv had growth-inhibitory activity against several cancer cell lines and triggered apoptosis. GIPinv should be further explored as a potential anti-cancer agent and a unique polysaccharide.
... involutus encompasses multiple species [3][4][5]7]. ...
The genus Paxillus is characterized by the difficulty of species identification, which results in reproducibility problems, as well as the need for large quantities of fungal inoculum. In particular, studies of Paxillus ammoniavirescens have reported divergent results in the in vitro growth while little is known of its capacity to degrade organic matter. For all the above, and assuming that this variability could be due to an inappropriate culture media, the aim of this study was to analyse growth in different culture media (MMN, MS, and 1/2 MS) and in the case of MMN in presence/absence of two types of organic matter (fresh litter and senescence litter) to probe the saprophytic ability of P. ammoniavirescens. We also evaluated the effects of pH changes in the culture media. Growth kinetics was assessed by weekly quantification of the area of growth in solid culture media over 5 wk, calculating the growth curves and inflection points of each culture media. In addition, final biomass after 5 wk in the different culture media was calculated. Results showed that best culture media are MS and 1/2 MS. Moreover, an improvement in growth in culture media containing decomposing fall litter was observed, leading to confirm differences in the culture media of this species with others of the same genus. Further, we established that all growth media suffered a significant acidification after fungal growth.
... 678), Paxillus filamentosus (Kotlaba & Pouzar 1960;K€ uhner 1962) or Paxillus rubicundulus (Orton 1969;Hahn 2000). As a first step towards the recognition of the real diversity within this genus, Fries (1985) recognized intersterility groups within P. involutus that formed the basis for the genetic species concepts of later authors. Then, both morphological (Hahn & Agerer 1999) and molecular studies (Hedh et al. 2008, Vellinga et al. 2012Jargeat et al. 2014) identified four species in the P. involutus complex in Europe: P. involutus, Paxillus ammoniavirescens, Paxillus obscurisporus, and the more recently described Paxillus cuprinus. ...
Paxillus rubicundulus P.D. Orton has been shown to be a complex of at least 3 ectomycorrhizal taxa strictly associated with alders (Alnus) in Europe, P. rubicundulus s. str., and two undescribed clades. To assess the taxonomic status of these three clades and their phylogenetic relationships, phylogenetic analyses of two independent gene regions (ITS and gpd), combined with macro- and micromorphological comparisons of genetically identified specimens, were carried out. A total of 85 sequences were successfully obtained from basidiomata and alder mycorrhizae collected in France and Algeria and combined with GenBank and UNITE sequences. The phylogenetic results and estimates of genetic diversity confirmed that the three clades are distinct species, often found in sympatry. As a result, P. rubicundulus s. str. was redefined based on the revision of type material, and Paxillus adelphus and Paxillus olivellus are introduced as new Linnaean names. The often used name Paxillus filamentosus is rejected since it could not be applied to any of the new species. The three species are distinguished micromorphologically by spore size and shape. They are widely distributed in Europe, North Africa and western Asia; P. rubicundulus is rare, and all species have a limited host range.
... The remaining ICGs could not be uniquely characterised, implying that cryptic biological species are the rule within the H. crustuliniforme complex. Similar observations have been made in other species groups such as the Corticiaceae (Hallenberg, 1991) and the genera Paxillus (Fries, 1985) and Laccaria (Mueller, 1991;Mueller & Gardes, 1991), although in some cases morphological differences could be found between ICGs. The biological species of the H. crustuliniforme complex are also not meaningful ecological entities. ...
A method is presented to derive an operational phenetic species concept for the Hebeloma crustuliniforme complex in northwestern Europe. The complex was found to consist of at least 22 biological species (intercompatibility groups; ICGs). Almost none of these biological species could be recognised unambiguously by morphological criteria. It is therefore necessary to base a phenetic species concept on combinations of biological species. However, such species delimitation must be performed within an explicitly phylogenetic context. It is crucial therefore to have a reliable estimate of the phylogeny of 22 biological species in that complex. Based on two nuclear sequences, we present a best estimate of the phylogeny of biological species within the complex. Using this phylogeny, on the basis of strict monophyly only two species can be morphologically recognised among 22 biological species. Relaxing the criterion of monophyly and allowing paraphyletic groupings of biological species as phenetic species would result in the recognition of three phenetic species. A tree, with the five ICGs of the previously defined morphospecies H. crustuliniforme (1, 2, 3, 4 and 5) constrained as a monophyletic group, can not be rejected. This constrained tree, together with the relaxed criterioon that allows for paraphyletic groupings of biological species, leads to the recognition of four phenetic species, viz. H. crustuliniforme, H. helodes, H. incarnatulum and H. velutipes. These phenetic species are described and a key is provided. Other taxon names are briefly discussed. The very limited ability to translate a biological species concept into an operational phenetic species concept is explained by the lack of qualitative characters and the plasticity of quantitative characters. Recency of common evolutionary history is also a major factor. Intercompatibility tests and DNA based phylogenies indicate that most biological species are very closely related and hence provide support for the claim that correspondence between a biological species concept and a phenetic species concept in the H. crustuliniforme complex is not likely to be forthcoming. In an Appendix morphological descriptions are provided of the 22 ICGs.
... Hedh et al. (2008), who recently addressed species concepts of the European taxa in the P. involutus complex, recognized four species based on five gene genealogies and nrITS sequence analyses. Three of the four phylogenetic species correspond with the intercompatibility groups identified by Fries (1985). Hedh et al. (2008 recognized Paxillus involutus (Batsch) Fr., P. validus, P. obscurosporus C. Hahn, and a fourth species, sister to P. involutus, which they refrained from naming. ...
Comparison of nrITS sequences of the type specimens of Paxillus ammoniavirescens and P. validus show them to be identical. The name Paxillus ammoniavirescens was published in 'Spring 1999' [northern hemisphere] and that of Paxillus validus in August 1999,
making P. ammoniavirescens the correct name for this species. A variant without pigments fits into P. involutus, suggesting that P. albidulus might be an albino variant of any species in the P. involutus complex. Paxillus ammoniavirescens is widespread in
Europe and has been introduced in the southern hemisphere; P. obscurosporus is known from China and Europe; and the two taxa in the P. involutus clade are reported from Europe and from North America.
... Hedh et al. (2008), who recently addressed species concepts of the European taxa in the P. involutus complex, recognized four species based on five gene genealogies and nrITS sequence analyses. Three of the four phylogenetic species correspond with the intercompatibility groups identified by Fries (1985). Hedh et al. (2008) recognized Paxillus involutus (Batsch) Fr., P. validus, P. obscurosporus C. Hahn, and a fourth species, sister to P. involutus, which they refrained from naming. ...
Paxillus is a small Northern Hemisphere genus in the Boletales; all species are ectomycorrhizal with a broad range of host trees. Paxillus involutus is widely used as a model organism in ectomycorrhizal experiments, as it is one of the few species easily grown in culture. Despite the importance in the forest ecosystem and the abundance of the species, the North American species are not well known. These species, their relationships to the European species and their ecology are now under investigation; the first step is a phylogenetic analysis of the nrITS and the elongation factor 1alpha gene sequence data to recognize and circumscribe the species. Recently, four phylogenetic species were recognized in Europe in the P. involutus complex: P. involutus s. str., P. obscurosporus, P. validus, and one undescribed species. So far, at least four species can be recognized in North America: the occasionally occurring Paxillus involutus s. str., P. vernalis, the most common and widespread species, associating with aspen and oaks in a wide range of habitats, an undescribed conifer associated species with native and with introduced tree species, and a fourth species, also known from Europe, but still nameless, growing with planted birch in the Pacific states. The alder associated species P. filamentosus is a sister taxon to the group of P. involutus, and Paragyrodon sphaerosporus is the closest relative outside the genus.
... (see also Vellinga et al. 2012) According to our data the P. rubicundulus complex shows a high molecular divergence (P%IV = 95.0), probably suggesting that a cryptic speciation process has taken place (which means lack of really clear-cut morphological divergence in spite of genetic isolation), as already reported for the P. involutus complex on the basis of intercompatibility tests (Fries 1985), morphology (Hahn and Agerer, 1999a) and molecular data Jarosch 2001;Hedh et al. 2008;Gelardi et al. 2011;Vellinga et al. 2009Vellinga et al. , 2012. ...
The new species Paxillus orientalis is reported from Yunnan Province (south-western China). Based on morphological and molecular characters the novel taxon belongs to the sibling, holarctic alder-associated P. rubicundulus complex. Colour pictures of fresh basidiomes and line drawings are provided, accompanied by notes concerning its taxonomic and phylogenetic relationships within the genus. The new subgenus Alnopaxillus is established to accommodate P. rubicundulus and allied undescribed taxa characterized by basidiomes usually associated with Alnus and with a distinctly areolate–squamulose pileal surface.
... Paxillus involutus was not observed on root tips and as mycelium in the Swedish podzol (Rosling et al. 2003; Landeweert et al. 2003a) but occurred in the middle part of a productivity gradient in Sweden (Toljander et al. 2006). In general, the species [or possibly species complex (Fries 1980)] occurs more frequently in organic horizons than in mineral horizons (Laiho 1970) and its ability to produce LMWOA could also be beneficial in the organic layers (see above). Hyphae of Piloderma croceum with the characteristic wall incrustation of calcium oxalate were observed on mineral grains (Fig. 4). ...
A decade ago, tunnels inside mineral grains were found that were likely formed by hyphae of ectomycorrhizal (EcM) fungi. This
observation implied that EcM fungi can dissolve mineral grains. The observation raised several questions on the ecology of
these “rock-eating” fungi. This review addresses the roles of these rock-eating EcM associations in plant nutrition, biogeochemical
cycles and pedogenesis. Research approaches ranged from molecular to ecosystem level scales. Nutrient deficiencies change
EcM seedling exudation patterns of organic anions and thus their potential to mobilise base cations from minerals. This response
was fungal species-specific. Some EcM fungi accelerated mineral weathering. While mineral weathering could also increase the
concentrations of phytotoxic aluminium in the soil solution, some EcM fungi increase Al tolerance through an enhanced exudation
of oxalate. Through their contribution to Al transport, EcM hyphae could be agents in pedogenesis, especially podzolisation.
A modelling study indicated that mineral tunnelling is less important than surface weathering by EcM fungi. With both processes
taken together, the contribution of EcM fungi to weathering may be significant. In the field vertical niche differentiation
of EcM fungi was shown for EcM root tips and extraradical mycelium. In the field EcM fungi and tunnel densities were correlated.
Our results support a role of rock-eating EcM fungi in plant nutrition and biogeochemical cycles. EcM fungal species-specific
differences indicate the need for further research with regard to this variation in functional traits.
... Bref. (Worrall, Parmeter & Cobb, 1983), Puxillus involutus (Batsch: Fr.) Fr. (Fries, 1985) and Peniophora cinerea (Pers: Fr.) Cooke (Chamuris, 1991). Recently, Vilgalys & Sun (1994) reported that some isolates assigned to P. eryngii from Europe and P. fossulatus (Cooke) Sacc. ...
The anatomical, physiological, ecological and cultural characters of 11 Pleurotus morphotaxa reported to occur in Europe are described in detail and compared with the outcome of a pairing-analysis performed among monokaryons of 101 dikaryotic strains (including collections from other continents). The results revealed eight intersterility groups in Europe: P. ostreatus, P. pulmonarius, P. cornucopiae, P. eryngii, P. cystidiosus, P. dryinus, P. calyptratus and P. opuntiae. In addition, strains formerly identified as ‘P. sajor-caju’ were found to be intercompatible with P. pulmonarius, while the speciation process currently under way for P. columbinus, P. citrinopileatus and the ecotypes of P. eryngii is discussed. A synthesis of the data obtained so far on Pleurotus systematics is evaluated in the light of recent evidence together with the world distribution of the recognized biological species of the genus.
... Some EM morphospecies also reveal CBSs. At least three CBSs occur within Paxillus involutus that partly fit into the sexually incompatible groups previously described in the 80s by pairing monokaryons in vitro (Fries 1985;Hedh et al. 2008). CBSs occur, sometimes in sympatry, within P. tinctorius Hitchcock et al. 2003;Jourand et al. 2010), R. vinicolor sensu lato distinguished R. vinicolor and R. vesiculosus), C. formosus (Dunham et al. 2003), T. scalpturatum complex (Gryta et al. 2006;Carriconde et al. 2008aCarriconde et al. , 2008bJargeat et al. 2010), Strobilomyces (Sato et al. 2007;Sato & Murakami 2008) and Scleroderma species from Africa (Sanon et al. 2009). ...
Ectomycorrhizal (EM) fungi are major microbial components of boreal, temperate and Mediterranean forests, as well as some tropical forest ecosystems. Nearly two decades of studies have clarified many aspects of their population biology, based on several model species from diverse lineages of fungi where the EM symbiosis evolved, i.e. among Hymenomycetes and, to a lesser extent, among Ascomycetes. In this review, we show how tools for individual recognition have changed, shifting from the use of somatic incompatibility reactions to dominant and non-specific markers (such as random amplified polymorphic DNA (RAPD) and amplified fragment length polymorphism (AFLP)) and, more recently, to co-dominant and specific markers (such as microsatellites and single nucleotide polymorphisms (SNPs)). At the same time, the theoretical focus has also changed. In earlier studies, a major aim was the description of genet size and popul/ation strategy. For example, we show how some studies supported or challenged the simple, classical model of colonization of new forest stands by ruderal (R) species, propagating by spores and forming small genets, progressively replaced in older forests by more competitive
... In other genera, mating studies have shown the existence of such sibling species as well. Sibling species have been found for instance within Xeromphalina campanella (Johnson, 1997), Lentinula boryana (Petersen et al., 1998), the Corticiaceae (Hallenberg, 1991) and the genera Paxillus (Fries, 1985) and ...
This thesis deals with species delimitation and speciation in the ectomycorrhizal Hebeloma crustuliniforme complex. The species concept traditionally used in this complex is based on morphology of basidiocarps. However, species delimitation has been controversial. One of the best known names is H. crustuliniforme (Bull.) Quél., a name regularly encountered in the mycorrhizal literature. However, this name has been used for a species complex of taxa with relatively pale (but sometimes more brown-tinged), veilless carpophores with weeping lamellae. In this thesis the basal criterion to delimit species within the Hebeloma crustuliniforme complex is sexual intercompatibility. Therefore, speciation is here defined as the origin of sexual interincompatibility. The strategy that has been used in this thesis to address the questions of species delimitation and speciation in the Hebeloma crustuliniforme complex can be summarized as follows:Define InterCompatibility Groups (ICGs) and look for examples of partial incompatibility (Chapter 2);Determine phylogenetic relationships between those ICGs (Chapter 3);Focus on closely related, and preferentially partially incompatible, ICGs to study speciation (Chapters 4, 5);Derive an operational species concept for the Hebeloma crustuliniforme complex that is based on morphological recognition of (combinations of) biological species within an explicitly phylogenetic framework (Chapter 6).In Chapter 2 the results are described of intercompatibility tests in this species complex. In a sample of 110 collections 20 InterCompatibility Groups (ICGs) were found. Partial compatibility was found between ICGs 3 and 4, between 2 and 3/4, between 1 and 2 and between 16 and 17. One strain (605) was compatible with all strains of ICGs 3 and 4. In all other cases, however, assignment of isolates to a single ICG was unambiguous. Individual compatible combinations between members of the partially compatible ICG 1 and 2 and between members of the partially compatible ICG 16 and 17 showed signs of reduced compatibility. This was reflected by: (i) no or unidirectional nuclear migration, (ii) reduced growth rate of the dikaryon and (iii) aberrant morphology of hyphae.In Chapter 3 phylogenetic relationships were studied between the ICGs of the H. crustuliniforme complex and between them and the other main groups in the genus Hebeloma based on nuclear ribosomal ITS sequences, using cladistic methods. The 20 ICGs of the H. crustuliniforme complex do not form a monophyletic group, but instead form two distinct clades, one consisting of three ICGs (clade I) and the other of 17 ICGs (clade II). Most of the ICGs in the latter clade were very closely related as suggested by a low sequence divergence. The majority of ICGs of this clade showed a preference for Salicaceae , but the basal ICG (ICG 21) did not. The host tree switch to Salicaceae has probably been followed by extensive and rapid speciation.Several other well supported clades were found in the genus Hebeloma , but the basal relationships between them were not well resolved. It is therefore impossible to propose a new infrageneric division for Hebelama .In Chapter 4 a subclade of clade II (subclade IIa) was studied in detail. In this subclade nine ICGs were found, four of which were partially compatible. This partial intercompatibility was organised hierarchically with an intermediate level of compatibility between ICGs 3 and 4 and very limited compatibility between 2 and 3/4 and between 1 and 2. The single strain (605) that was intercompatible with all strains of ICGs 3 and 4 was a member of subclade IIa as well. A mitochondrial and a nuclear phylogeny of strains belonging to these partially compatible populations were reconstructed. For ICGs 2, 3 and 4 a positive correlation was found between the level of interincompatibility and the relative age of the most recent common ancestor. ICGs generally formed monophyletic groups, ICG 3 and 4 (15% partial intercompatibility) together formed a monophyletic group and the sister group of (3,4) was ICG 2 (0.4% intercompatible with (3,4)). This is consistent with a gradual origin of sexual incompatibility (divergence-first). ICG 1 had a different position in the nuclear and mitochondrial phylogeny. In the nuclear phylogeny it was the sister taxon of ICG 5, and in the mitochondrial the sister group of ICG 2. A possible explanation is that ICG 1 has a hybrid origin, with the ancestor of ICG 5 as the nuclear donor and the ancestor of 2 as the mitochondrial donor.The subject of Chapter 5 is a polymorphism in the ribosomal Internal Transcribed Spacer of ICG 17. Within this ICG of the morphospecies Hebeloma velutipes a dikaryotic strain (d504) was found with two divergent types ITS. These two types segregated in monokaryotic progeny of the same strain, showing that the different ITS types represent different alleles at homologous rDNA loci. RFLP analysis of more strains of ICG 17 showed that the polymorphism is widespread, with both types occurring in Europe as well as in America. Cladistic analyses of the two ITS sequences showed that they did not form a monophyletic group. One of the types belonged to a clade together with the single ITS type found in the partially compatible ICG 16 and the other to a clade together with the single ITS type found in the fully incompatible ICG 18. RFLP analysis of the mitochondrial ribosomal SSU showed that there were fixed differences between the mitochondria of ICG 16 and 17. Several lines of evidence were described that the ITS polymorphism in ICG 17 is not the result of actual hybridisation between 16 and 17. The polymorphism within ICG 17 must therefore be of a different origin. The lack of recombinants, neither within the rDNA locus nor between ITS 1 and 2, suggests that the two types have come together relatively recently. The ITS polymorphism described in this Chapter clearly showed the potential danger of using single ribosomal sequences for reconstructing species phylogenies and the potential problems for molecular identification of species.In Chapter 6 a method is presented to derive an operational species concept for the Hebeloma crustuliniforme complex that is based on (combinations of) biological species within an explicitly phylogenetic framework. Crucial in this analysis is a reliable estimate of the phylogeny of biological species in the H. crustuliniforme complex. Based on two nuclear sequences, we presented a best estimate of the phylogeny of biological species within the H. crustuliniforme complex. Using this phylogeny, on the basis of (strict) monophyly only two species could be recognised among 20 biological species, viz. H. velutipes and H. helodes . An earlier phylogenetic analysis indicated that these two morphological species are not sister taxa. Relaxing the criterion of monophyly and allowing paraphyletic groupings of biological species as a morphospecies resulted in the recognition of three morphospecies, viz. H. velutipes , H. incarnatulum and H. helodes . A tree, with the five ICGs of the previously defined morphospecies Hebeloma crustuliniforme (1, 2, 3, 4 and 5) constrained as a monophyletic group could not be rejected. This constrained tree, together with the relaxed criterion, allowed the recognition of four species, viz. H. helodes , H. crustuliniforme , H. velutipes and H. incarnatulum . The limited ability to translate a biological species concept into an operational species concept was explained by the lack of qualitative characters and the plasticity of quantitative characters. Based on the close relationship between the ICGs in the two clades of the H. crustuliniforme complex, it was shown that a good correspondence between a biological species concept and a morphological species concept is not likely to be forthcoming.In the final Chapter the results found in this study were integrated and discussed in a broader context and directions for future research were suggested. Future phylogenetic studies should consider the possibility of genetic exchange between divergent populations more explicitly.
... The remaining ICGs could not be uniquely characterised, implying that cryptic biological species are the rule within the H. crustuliniforme complex. Similar observations have been made in other species groups such as the Corticiaceae (Hallenberg, 1991) and the genera Paxillus (Fries, 1985) and Laccaria (Mueller, 1991; Mueller & Gardes, 1991), although in some cases morphological differences could be found between ICGs. The biological species of the H. crustuliniforme complex are also not meaningful ecological entities. ...
A method is presented to derive an operational phenetic species concept for the Hebeloma crustuliniforme complex in northwestern Europe. The complex was found to consist of at least 22 biological species (intercompatibility groups; ICGs). Almost none of these biological species could be recognised unambiguously by morphological criteria. It is therefore necessary to base a phenetic species concept on combinations of biological species. However, such species delimitation must be performed within an explicitly phylogenetic context. It is crucial therefore to have a reliable estimate of the phylogeny of 22 biological species in that complex. Based on two nuclear sequences, we present a best estimate of the phylogeny of biological species within the complex. Using this phylogeny, on the basis of strict monophyly only two species can be morphologically recognised among 22 biological species. Relaxing the criterion of monophyly and allowing paraphyletic groupings of biological species as phenetic species would result in the recognition of three phenetic species. A tree, with the five ICGs of the previously defined morphospecies H. crustuliniforme (1, 2, 3, 4 and 5) constrained as a monophyletic group, can not be rejected. This constrained tree, together with the relaxed criterioon that allows for paraphyletic groupings of biological species, leads to the recognition of four phenetic species, viz. H. crustuliniforme, H. helodes , H. incarnatulum and H. velutipes. These phenetic species are described and a key is provided. Other taxon names are briefly discussed. The very limited ability to translate a biological species concept into an operational phenetic species concept is explained by the lack of qualitative characters and the plasticity of quantitative characters. Recency of common evolutionary history is also a major factor. Intercompatibility tests and DNA based phylogenies indicate that most biological species are very closely related and hence provide support for the claim that correspondence between a biological species concept and a phenetic species concept in the H. crustuliniforme complex is not likely to be forthcoming. In an Appendix morphological descriptions are provided of the 22 ICGs.
... Application of the biological species concept in homobasidiomycete taxonomy has proven to be most enlightening, in many cases revealing taxa previously considered to be a single species or representing complexes of species (Clemenyon 1977, Vilgalys and Miller 1983, Fries 1984, Hallenberg 1985 , Vilgalys 1991, Hallenberg et al. 1994, Petersen 1995, Gordon and Petersen 1997, Aanen and Kuyper 1999, Miller and Methven 2000). The biological species concept has also been extensively applied to basidiomycete taxonomy, where its success is attributed to the various characteristics of these fungi that make mating studies relatively easy to conduct (Boidin 1986). ...
Thesis (Ph. D.)(Genetics)--University of Pretoria, 2003. Includes bibliographical references.
Species delimitation is one of the most fundamental processes in biology. Biodiversity undertakings, for instance, require explicit species concepts and criteria for species delimitation in order to be relevant and translatable. However, a perfect species concept does not exist for Fungi. Here, we review the species concepts commonly used in Basidiomycota, the second largest phylum of Fungi that contains some of the best known species of mushrooms, rusts, smuts, and jelly fungi. In general, best practice is to delimitate species, publish new taxa, and conduct taxonomic revisions based on as many independent lines of evidence as possible, that is, by applying a so-called unifying (or integrative) conceptual framework. However, the types of data used vary considerably from group to group. For this reason we discuss the different classes of Basidiomycota, and for each provide: (i) a general introduction with difficulties faced in species recognition, (ii) species concepts and methods for species delimitation, and (iii) community recommendations and conclusions.
Paxillus is an important genus of Basidiomycota and famous as a nonedible poisonous mushroom, comprises of 38 species. Among all represented species, only Paxillus involutus is well-studied for its biochemical and beneficial role in agroecology. Paxillus involutus can easily be found in the ectomycorrhizal relationship with a variety of tree species in the different ecosystems of North America, Europe, and Asia. Paxillus involutus is important for its contribution in cycling nutrients enabling the host plant to obtain important nutrients and tolerate heavy metal. The species also has the ability to promote plant growth by secreting growth-promoting hormones. In this chapter, we discuss in detail about taxonomy, identification, and other beneficial roles of Paxillus involutus in agroecology.
Sexual intercompatibility has been tested for members of the ectomycorrhizal Hebeloma crustuliniforme complex. Intercompatibility tests have been applied to 110 collections from northwestern Europe, for which monospore cultures were obtained. Crossing experiments indicated the existence of at least 20 intercompatibility groups (ICGs). All ICGs were bifactorial heterothallic with multiple mating type alleles. A low level of partial intercompatibility between ICGs was observed in two species clusters. One isolate was completely compatible with members of two ICGs (and partly compatible with one member of a third ICG) and could therefore not be assigned to an ICG. A large majority of these 20 ICGs could not be recognized by morphological characters, supporting claims that many basidiomycete morphospecies contain cryptic biological species. Possible explanations for this large number of ICGs that are not morphologically, ecologically or geographically delineated are discussed. A simplified strategy for executing intercompatibility tests with a high number of isolates is described. This procedure could substantially reduce the number of necessary matings.
Intracollection pairings of single spore isolates of 35 collections from the United States, Australia, Bulgaria, France, Israel, Republic of China and People's Republic of China demonstrated that compatibility in Ustilago cynodontis is controlled at one locus with two alleles. Positive in vitro pairings correlated with infections in the host, Cynodon dactylon, which produced viable teliospores. Intercollection pairings disclosed five partially intersterile groups. Teliospore and colony morphology are not correlated with the intersterile groups. Ustilago striaeformis appears to have a similar system of partially intersterile groups.
Speciation and species concepts were investigated for the Collybia dryophila complex. Mating compatibility studies show that all members of the C. dryophila complex have a heterothallic tetrapolar mating system and that several intercompatible groups are broadly distributed over several continents including North America, Europe and Asia. Intersterility between different groups is associated with isozyme differences. Isozyme variation between some pairs of intercompatible mating groups from different continents suggests that geographic isolation (allopatry) may be important during speciation in these fungi. Combined evidence from mating compatibility, isozymes, DNA hybridization, and morphology were used to assess patterns of divergence associated with speciation. Results of phylogenetic analysis indicate that speciation in these fungi is gradual and involves many steps that are not linked in any particular sequence with the development of prezygotic intersterility barriers. Varying patterns of speciation in these and other fungi have important implications for the species concept. Limitations of the biological species concept are discussed, and instead a phylogenetic based species concept is proposed, which provides information about evolutionary relationships among allopatric mating groups based on combined data from mating compatibility, morphology and macromolecules. New combinations are proposed for C. ocior ssp. brunneola and C. alpina ssp. subsulphurea.
Mating tests were performed in vitro to determine interfertility patterns among eastern North American populations of Peniophora cinera, and two sympatric sibling species of the P. cinerea complex in Europe. No intersterile groups were detected in eastern North America. Reproductive isolation between the two European species reported previously by Hallenberg was verified. Since one sibling species is apparently restricted to the wood of Fagus sylvatica, speciation accompanied by substratum specialization is indicated. Intercontinental pairings showed that American P. cinerea is partially reproductively isolated from the two European sibling species.
Homokaryon-homokaryon and heterokaryon-homokaryon interactions of sibling strains of the ectomycorrhizal fungus Pisolithus arhizus were investigated. Pairings between sexually compatible homokaryons gave rise, almost always, to heterokaryotic mycelium in both of the progenitors, but in some cases heterokaryotic mycelium was present in only one of the progenitors. This unidirectional movement of nuclei and the speed of nuclear movement was followed in six representative pairings of homokaryons and complete heterokaryotization occurred rapidly, within 4 days, in one progenitor of one pairing. In heterokaryon-homokaryon pairings, heterokaryotization of the homokaryon was a consistent feature in all legitimate mating type pairings except in one case. Heterokaryotization did not occur in illegitimate pairings. The nuclear constitution of three analyzed heterokaryotized colonies was found to be a composite genotype containing one nucleus from the donor heterokaryon and one nucleus from the recipient homokaryon. This study is the first report of the homokaryon-homokaryon and homokaryon-heterokaryon nuclear interactions of P. arhizus.
Divergence of breeding system plays an important role in fungal speciation. Ectomycorrhizal fungi, however, pose a challenge for the study of reproductive biology because most cannot be mated under laboratory conditions. To overcome this barrier, we sequenced the draft genomes of the ectomycorrhizal sister species Rhizopogon vinicolor Smith and Zeller and R. vesiculosus Smith and Zeller (Basidiomycota, Boletales), the first genomes available for Basidiomycota truffles, and characterized gene content and organization surrounding their mating type loci. Both species possess a pair of homeodomain transcription factor homologs at the mating type A-locus as well as pheromone receptor and pheromone precursor homologs at the mating type B-locus. Comparison of Rhizopogon genomes with genomes from Boletales, Agaricales, and Polyporales revealed synteny of the A-locus region within Boletales, but several genomic rearrangements across orders. Our findings suggest correlation between gene content at the B-locus region and breeding system in Boletales with tetrapolar species possessing more diverse gene content than bipolar species. Rhizopogon vinicolor possesses a greater number of B-locus pheromone receptor and precursor genes than R. vesiculosus as well as a pair of isoprenyl cysteine methyltransferase genes flanking the B-locus compared to a single copy in R. vesiculosus Examination of dikaryotic single nucleotide polymorphisms within genomes revealed greater heterozygosity in R. vinicolor, consistent with increased rates of outcrossing. Both species possess the components of a heterothallic breeding system with R. vinicolor possessing a B-locus region structure consistent with tetrapolar Boletales and R. vesiculosus possessing a B-locus region structure intermediate between bipolar and tetrapolar Boletales.
In biology, incompatibility is usually defined as restriction of mating competence controlled by genes other than those determining sexual differentiation. It has been long recognised that incompatibility concerns not only the sexual phase but also the vegetative phase. The latter becomes apparent especially in fungi and was first termed heterokaryon incompatibility. In both sexual and vegetative incompatibility, the action of the genetic traits involved precludes the exchange of genetic material. Thus, inhibition of recombination results by a lack of karyogamy (sexual incompatibility) as well as by an inability of the nuclei to coexist in a common cytoplasm and to undergo somatic recombination (vegetative incompatibility). Since recombination is of paramount importance in evolution, the biological significance of incompatibility as a factor controlling recombination is immediately apparent.
In biology, incompatibility is usually defined as restriction of mating competence controlled by genes other than those determining sexual differentiation. It has been long recognized that incompatibility concerns not only the sexual phase, but also the vegetative phase. The latter becomes apparent especially in fungi and was termed heterokaryon incompatibility. In both sexual and vegetative incompatibility the action of the genetic traits involved precludes the exchange of genetic material. Thus, inhibition of recombination results by a lack of karyogamy (sexual incompatibility) as well as by an inability of the nuclei to coexist in a common cytoplasm and to undergo somatic recombination (vegetative incompatibility). Since recombination is of paramount importance in evolution, the biological significance of incompatibility as a factor controlling recombination is quite apparent.
Paxillus involutus is currently among the ectomycorrhizal (ECM) fungi used most frequently in experimental work in the laboratory. Between 1988 and 1997 more that 270 research articles containing information on this species were published. Most of these papers concern the mycorrhizal characteristics of this fungus. This situation is in striking contrast to that existing a few decades ago, as reflected in the quote by Laiho (1970): “Paxillus involutus has, indeed, been almost ignored by scientists working with mycorrhiza”. Thus, almost all information about this fungus and its mycorrhizal status has been gathered during the last 25 years.
Sexual intercompatibility has been tested for members of the ectomycorrhizal Hebeloma crustuliniforme complex. Intercompatibility tests have been applied to 110 collections from northwestern Europe, for which monospore cultures were obtained. Crossing experiments indicated the existence of at least 20 intercompatibility groups (ICGs). All ICGs were bifactorial heterothallic with multiple mating type alleles. A low level of partial intercompatibility between ICGs was observed in two species clusters. One isolate was completely compatible with members of two ICGs (and partly compatible with one member of a third ICG) and could therefore not be assigned to an ICG. A large majority of these 20 ICGs could not be recognized by morphological characters, supporting claims that many basidiomycete morphospecies contain cryptic biological species. Possible explanations for this large number of ICGs that are not morphologically, ecologically or geographically delineated are discussed. A simplified strategy for executing intercompatibility tests with a high number of isolates is described. This procedure could substantially reduce the number of necessary matings.
Germination induction in the ectomycorrhizal and wood-decomposing groups of these fungi is discussed, in particular the types of activator: germination-inducing substances; and the mode of germination induction as an ecological adaptation. Short- and long-distance dispersal of basidiospores of the 2 groups of fungi are considered. Indusce spore germination as a key to genetic and ecological studies in ectomycorrhizal fungi is discussed. -after Author
Paxillus involutus (Batsch.: Fr.) Fr., an ectomycorrhizal fungus, is capable of forming sclerotia, resistant vegetative propagules. Sclerotia were induced in vitro on agar-solidified modified Melin Norkrans medium by exposure of the mycelium to temperatures between 6 and 14 °C. The number and size of sclerotia varied greatly with temperature. Mycelium exposed to either 6 or 8 °C produced many smaller sclerotia (less than 1.0 mm in diameter) lacking droplets of exudate, whereas mycelium exposed to temperatures between 10 and 14 °C formed fewer, larger sclerotia (1.0 mm or more in diameter) with abundant exudate. Sclerotia were not induced from mycelium grown in liquid medium at 10 or 25 °C or from mycelium on agar-solidified medium maintained at 25 °C under a variety of conditions, including changes in medium composition and volume. Key words: sclerotia, Paxillus involutus, temperature, ectomycorrhiza, in vitro.
Mycorrhizal synthesis studies revealed that allopatric populations of Suillus granulatus differ in host specificity. Isolates of Suillus granulatus originated from Nepal (associated with Pinus wallichiana), Korea (Pinus densiflora), and the U.S.A. (Pinus strobus). Functional compatibility of the mycorrhizal association was used to measure host specificity. Useful characters for measuring mycorrhizal compatibility were defined as those which revealed statistical differences between the uninoculated controls and mycorrhizal treatments and which were stable within one population. Useful characters, as defined by these two tests, were shoot wet weight, total wet weight, percentage mycorrhizal short roots, shoot dry weight, total dry weight, and Hartig net penetration. Data for these characters were subsequently used in anova tests to determine whether allopatric populations of Suillus granulatus differ in functional compatibility. Isolates associated with Pinus strobus were most compatible with Pinus strobus and least compatible with the other trees, whereas isolates from Pinus wallichiana and Pinus densiflora showed variable compatibility with all three trees. Phenetic analyses used to determine how similarly the isolates responded with the hosts confirmed these results. We conclude that isolates associated with Pinus strobus are most specific for this host, whereas isolates from the other two trees are not host specific. The implications of these results are discussed in light of future investigations regarding the relatedness of these tree-associated isolates. Key words: Boletaceae, Suillus, Pinus, ectomycorrhizae.
This study aimed to elucidate the phylogenetic relationships within Paxillus 5. str. Collections of the Paxillus involu-tus complex from different habitats have been compared. DNA sequence data were generated from the nuclear encoded 28S gene and the ITS region. Phylogenetic analyses of the two data sets showed three clearly differentiated groups within Paxillus s. str. Collections from gardens and park areas under Betula, Cory-lus, Populus, Quercus and Tub showed to have a very close relationship to the North American species Paxillus vernalis, but are separated from P. involutus collections found in forests as well as from P. filamentosus. Possible taxonomic conclusions are discussed.
This study aimed to test the monophyly and present a generic concept of Paxillus s. I. Of special interest were the Southern hemisphere representatives of this genus. Using the primer combination LR0R-LR5, DNA sequence data were generated from the nuclear encoded 28S gene. A data set containing 900 bases was created for 30 different species. Phylogenetic analysis showed 3 main monophyletic groups, also supported by morphological, anatomical, ecological and chemical data. Taxonomic interpretation of these clearly differentiated groups corresponded to the genera PaxilIus s. str. and Tapinella and lead to the proposal of a new Southern hemisphere genus, Austro-paxillus, mycorrhizal with Nothofagus. New combinations are proposed.
1 Institut für Systematische Botanik, Section Mykologie, Universität München. Menzinger Str 67, 0–80638 München, Germany.
In the past, sexual incompatibility was defined from a genetic point of view, that genetically like gametes or nuclei could not undergo karyogamy. This basic principle was valid for all different incompatibility mechanisms found in higher plants as well as in fungi. During the last decade, however, it has become evident that this is not the only general principle leading to incompatibility. In the study of different geographical races of the ascomycete Podospora anserina, which belongs to the Sordariaceae, it was found that the prerequisite for incompatibility can also be a heterogenic structure of the gametic nuclei. We have suggested the terms “homogenic incompatibility” and “heterogenic incompatibility”, respectively, to define these two restricting systems for sexual propagation on the basis of their genetic determination
1.
Comparative morphology, isozyme analysis and mating compatibility approaches were used to investigate the relationships between Psilocybe subaeruginosa and three closely related taxa, P. australiana, P. eucalypta and P. tasmaniana. The four names were found to represent one species, and the use of microscopic features such as coloured pleurocystidia or neck length of cheilocystidia were shown to be inappropriate taxonomic discriminators in the separation of the four taxa. Zymograms of extracellular enzymes of laccase, peroxidase, acid phosphatase, pectinesterase and polygalacturonase were useful tools for delineation of taxa within the genus Psilocybe. The name P. subaeruginosa is retained and P. australiana, P. eucalypta and P. tasmaniana are reduced to synonyms. A lectotype is designated.Investigation of isozyme bands previously reported to be pectinlyase activities showed that they were non-enzymic in nature, and interpretation of these pectinlyase activities should be approached with caution.
Mating compatibility experiments and comparative morphology were used to investigate the Agrocybe praecox species complex in North America and Europe. This species complex was found to consist of four morphologically indistinguishable biological species plus an easily recognizable taxon, Agrocybe molesta. The traditionally used characteristics, such as spore dimensions and macroscopic morphology, are not diagnostic for species recognition. However, ecological characteristics such as habitat, substrate, and geographical origin, are more useful for recognizing taxa within the A. praecox species complex.
We examined the population genetic structure of the ectomycorrhizal fungus Laccaria bicolor (Maire) Orton using single spore homokaryotic cultures from 33 basidiomes collected in northern Minnesota in association with red pine (Pinus resinosa Ait.), jack pine (Pinus banksiana Lamb.), and trembling aspen (Populus tremuloides Michx.) of three age-classes (0–20 years, 21–40 years, and > 41 years). Mating competence between cultures of isolates, as determined by the presence of clamp connections, revealed the presence of two subpopulations that were not freely interbreeding, one composed of 29 dikaryons, the other of 3 dikaryons. Phenetic cluster analysis using random amplified polymorphic DNA markers did not reveal differentiation between these subpopulations. Clustering failed to reveal genetically distinct groups based on incompatibility group, tree host species, or geographic origin of isolates. Key words: heterogenic incompatibility, RAPD, population genetics.
Ectomycorrhizal fungi are known to vary in host range. Some fungi can enter into symbiosis with multiple plant species, while others have restricted host ranges. The aim of this study was to examine variation in host specificity among strains from the basidiomycete Paxillus involutus s. lat. Recent studies have shown that this fungus consists of at least four genetically isolated lineages, phylogenetic species (PS) I (which corresponds to the morphological species Paxillus obscurosporus), PS II (P. involutus s. str.), PS III (Paxillus validus), and PS IV (not yet supported by any reference material). Thirty-five Paxillus strains of PS I to IV were examined in microcosms for their capacity to infect birch (Betula pendula) and spruce (Picea abies). Seventeen strains were compatible and formed mycorrhizae with both tree species. Seven strains were incompatible with both birch and spruce. The gene content in three pairs of incompatible and compatible strains PS I, II, and III were compared using microarray-based comparative genomic hybridizations. Of 4,113 P. involutus gene representatives analyzed, 390 varied in copy numbers in at least one of the three pairwise comparisons. Only three reporters showed significant changes in all three pairwise comparisons, and none of these were changed in a similar way in three comparisons. Our data indicate that changes in host range have occurred frequently and independently among strains in P. obscurosporus, P. involutus s. str., and P. validus. No evidence was obtained demonstrating that these changes have been associated with the gain or loss of similar genes in these three species.
To reconstruct the evolution of reproductive isolation in the ectomycorrhizal Hebeloma crustuliniforme aggregate (Basidiomycetes), phylogenetic relationships were determined between strains that belong to a clade consisting of nine intercompatibility groups (ICGs, biological species). Four of these nine ICGs are partially compatible and belong to the H. crustuliniforme aggregate. Different levels of partial compatibility have been found between these four ICGs. Between ICGs 3 and 4, 15% of the combinations were compatible. One strain was compatible with all isolates of both ICGs 3 and 4 and also with one isolate of ICG 2. Both a nuclear phylogeny, based on ribosomal IGS sequence data, and a mitochondrial phylogeny, based on a group-I intron located in the large subunit ribosomal RNA gene (LrRNA), were reconstructed. The level of incompatibility was compared with the phylogenetic history of individuals belonging to this clade.
Different relationships were found between the level of compatibility and the relative age of the most recent common ancestor (MRCA) for different ICGs. On the one hand, the evolution of incompatibility between ICGs 2 and 3/4 is most consistent with the class of “divergence- first” models because a positive correlation was found between the relative age of the MRCA and the level of incompatibility for ICG 2 versus 3/4. On the other hand, the lack of such a correlation for ICGs 3 and 4 shows that (partial) incompatibility between these ICGs has arisen without strong divergence. The ecological (and to a lesser extent geographical) differences found between ICGs 3 and 4 suggest that selection for incompatibility, associated with host tree preference, has been important in the evolution of incompatibility between these two ICGs. The incongruence between the nuclear and mitochondrial trees for ICG 1 could be explained by a hybrid origin of this ICG, with different donors of the mitochondrial and nuclear sequences.
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