Phylogenetic relationships between two species from the genera
Saccharomyces (
S. cerevisiae, S. kluyveri) and
Kluyveromyces (
K. aestuarii, K.
marxianus), a plant parasitic dimorphic yeast (
Nematospora coryli), and two phytopathogenic filamentous fungi (
Ashbya gossypii, Eremothecium ashbyi) were investigated by comparing partial nucleotide sequences of ribosomal DNA (rDNA). The three types of sequenced regions evolve with different speed and allow the analysis at distinct taxonomic levels: conserved regions (18S rDNA, small ribosomal subunit), variable regions (25S rDNA, large ribosomal subunit), and highly variable regions ITS1 and ITS2 (internal transcribed spacers). Segments homologous to positions 619 to 1035 and 1205 to 1617 of the 18S rDNA and positions 470 to 890 and 1535 to 1984 of the 25S rDNA of
S. cerevisiae were sequenced. The maximal resolving power of rDNA sequence analysis is provided by including the rapidly evolving ITS1 and ITS2 regions. In the phylogenetic sequence analysis of all regions by the genera
Kluyveromyces and
Saccharomyces cluster together with two filamentous pathogens on cotton,
A.
gossypii and
E. ashby, as well as the plant pathogenic yeast
N. coryli. The molecular evidence from ribosomal sequences suggests that morphology and ornamentation of ascospores, the persistence of ascus walls as well as mycelium formation should not be used as differentiating characters in yeast taxonomy, especially in family delimitation. Our sequence data support the inclusion of plant pathogenic, predominantly filamentous genera like
Ashbya or
Eremothecium or dimorphic genera like
Nematospora with falcate ascospores within a new family of the Saccharomycetaceae. Similarly, the saprophytic genus
Kluyveromyces with reniform ascospores and deliquescent ascus walls unequivocally belongs to the family of Saccharomycetaceae.
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