Evolutionary genetics of speciation in Basidiomycetes: Genetic studies of reproductive isolation in Pleurotus djamor/calyptratus complex

Among many types of speciation process, gradual speciation events are perhaps the most common and are strongly influenced by the geographic distribution of the populations involved. Allopatric speciation occurs when two populations of the same species are separated, often by a geographic barrier, and gradually diverge to become two distinct species. Sympatric speciation is divergence of one population into two species in same geographic location and is thought to often involve diversifying selection due to ecological traits such as food and mate preferences and breeding time.; The pantropical oyster mushroom species, Pleurotus djamor, displays considerable variation at both the morphological and molecular level, and it is unclear whether worldwide populations represent one species, or several genetically differing populations. Incipient reproductive isolation is evident between P. djamor and its sister taxon, P. calyptratus, which differs greatly from P. djamor in morphology, geographic distribution and habitat preference. Pleurotus calyptratus is a rare species and is only found in the temperate zone of central Europe. In this thesis I have attempted to find and describe evidence of reproductive isolation between Pleurotus djamor and P. calyptratus and to genetically characterize the speciation process in this species pair.; The first chapter of this dissertation describes a survey of intersterility, defined by the ability of haploid single spore isolates to form dikaryons in the laboratory. Results from numerous pairings show that no mating barriers have evolved among worldwide allopatric populations of P. djamor. Furthermore, no hybrid breakdown was observed in the progeny of several interpopulation outcross dikaryons, as assayed through one generation of intercrossing. Thus, it can be concluded that P. djamor constitutes a single biological species in the sense of Mayr (1963) and Dobzhansky (1970).; In the second study, I determined the number and locations of segregating genetic factors underlying interfertility/intersterility with P. calyptratus . Using molecular marker assisted genotyping methods, I mapped loci contributing to prezygotic isolation in the two different P. djamor populations, one from Malaysia, and the other from Hawaii. At least eight factors, identified as linked sets of markers, were identified as correlated with intersterility in this cross: three factors from the Malaysian population, and five factors from the Hawaiian population. Thus, incipient speciation of P. djamor and P. calyptratus appears to be underway, and that different sets of genetic factors contribute to hybrid sterility in different geographic populations of P. djamor.; In the final study, interspecific hybrids of P. djamor and P. calyptratus were examined for aspects of postzygotic reproductive isolation, such as hybrid inviability, hybrid sterility and hybrid breakdown. No fitness loss was observed in the vegetative growth of hybrid dikaryons and eight of ten hybrid dikaryons were able to make fruiting bodies, which were djamor-like in morphology. However, substantial reductions of spore numbers were observed in hybrid mushrooms. Also vegetative growth rates of germinated hybrid spores were highly variable and frequently displayed abnormal mycelial morphology. Lastly, a great deal of genomic divergence was inferred from the meiotic behavior of species-specific molecular markers in progeny derived from hybrid spores. Thus, P. djamor and P. calyptratus clearly represent two incipient biological species as evidenced from these patterns of postzygotic reproductive isolation.