Taxonomic And Molecular Phylogenetic Studies On The Basidiomycetous Yeasts From China

This thesis is mainly on the conventional taxonomy and molecular phylogeny of the basidiomycetous yeasts isolated from representative geographic and ecological regions in China. The application of single-stranded conformation polymorphism (SSCP) in molecular taxonomy of yeasts was also evaluated. Based on conventional characteristics, approximately 3000 strains isolated from eleven provinces or regions in China, including Beijing, Hebei, Hubei, Gansu, Jilin, Liaoning, Qinghai, Shanxi, Shan'xi, Shandong and Xinjiang were classified to different genera. More than 170 representative strains were selected for molecular taxonomic studies. A total of 76 species belonging to 14 genera were identified, among which 24 species were new to science, they are distributed in the genera Bensingtonia, Bullera, Cryptococcus, Dioszegia, Pseudozyma, Sporobolomyces and Tilletiopsis. The high frequency of new species discovery indicates that species diversity of basidiomyctous yeasts in China is very rich, and considerable number of undescribed species remain to be isolated from natural sources. Phylogenetic analysis based on the sequences of 18S rDNA, ITS (including 5.8S rDNA) and 26S rDNA D1/D2 regions showed that the species identified in the present study clustered in more ten lineages in the Hymenomycetes, Urediniomycetes and Ustilaginomycetes of Basidiomycota. The phylogenetic relationships some clades of basidiomycetous yeasts became clearer when the new species were added into the phylogenetic trees. Two new phylogenetic clades, Vicotoriae and Huiaensis, were recognized in the Tremellales of the Hymenomycetes. The distinctions of the Humicola clade in the Trichosporonales lineage of the Hymenomycetes, and the Ciliata and Ruber clades in the Agaricostilbum lineage of the Urediniomycetes, were also confirmed. The recognitions of the new clades will be valuable contributions to the adjustment of taxonomic system of basidiomycetous yeasts in the future. The molecular phylogenetic analysis in this study demonstrated that, though the ability to produce ballistoconidia does not reflect phylogenetic relationships of basidiomycetous yeasts, the morphology of ballistoconidia is a valuable criterion for genus or higher taxa demarcation. For example, the species clustered in a distinct clade together with the new symmetric ballistoconidium-forming species Sporobolomyces symmetricus sp. nov. in phylogenetic trees, also form symmetric or nearly symmetric ballistoconidia. These species are Sporobolomyces gracilis, S. vermiculatus, S. coprosmae and S. oryzicola. The other species including the type species of the genus Sporobolomyces produce typically asymmetric ballistoconida. This result provides a significant basis for the reclassification of the polyphyletic genus Sporobolomyces. The present study showed that even one base change in the internal transcribed spacer (ITS) sequences of yeast strains could be detected by single-stranded conformation polymorphism (SSCP) technique. By using SSCP analysis, the strains with different sequences could be easily recognized from a group of yeasts with similar morphological and physiological characters. When type strains were used as controls, SSCP could also be used for rapid identifications of physiologically similar species and to confirmed the identities of the strains assigned to a species by conventional criteria.