RIBOSOMAL-RNA SEQUENCE DIVERGENCE AMONG SIBLING SPECIES OF YEASTS

The genetic and implicit taxonomic resolution provided by ribosomal RNA sequence divergence was estimated from comparisons of sibling yeast species. Two regions of the large subunit (25S) rRNA and four regions of the small subunit (18S) rRNA comprising a total of 855 nucleotides were examined in all strains. Within these regions, no nucleotide differences were detected between strains of the same species. The sibling species pairs Pichia mississippiensis/P. amylophila, P. americana/P. bimundalis, and Issatchenkia scutulata and its variety exigua could be separated from each other by differences in a ca. 200-base region of the large subunit rRNA. Sequence differences within the sibling species complex composed of Saccharomyces bayanus, S. pastorianius (S. carlsbergensis), and S. cerevisiae were consistent with the prior proposal that S. pastorianus is a partial amphidiploid formed by a rare mating between the other two Saccharomyces species. From these data, the highly variable region identified in the 25S subunit rRNA appears suitable for rapid separation of nearly all species by oligonucleotide probe technology.