Should Petasospora BOIDIN et ABADIE (Saccharomycetaceae) be retained?--the phylogeny based on the partial sequences of 18S and 26S ribosomal RNAs.

The six strains of the Pichia species, once classified in the genus Petasospora, were examined for their 18S (positions 1451-1618, 168 bases) and 26S (positions 1611-1835, 225 bases and 493-622, 130 bases) rRNA partial base sequencings. In the 18S rRNA partial base sequencings, the type species of the genus Petasospora (Petasospora rhodanensis) was found to be closely related to Pichia anomala (identical to Hansenula anomala, type species of genus Hansenula) with base differences of two, but not to Pichia membranaefaciens, the type species of the genus Pichia, with base differences of ten. However, the species was not so closely related to P. anomala in the 26S rRNA partial base sequencings with base differences of twelve and 68 percent similarity. The genus Petasospora was known to be a very heterogeneous taxon phylogenetically with base differences of thirty-one to three and ninety-two to eight and with 40 to 83 percent similarities. The sequence data obtained here and the phenotypic features described previously indicate that the type species of the genus Petasospora (Pet. rhodanensis, identical to Pichia rhodanensis) is adequate to be accommodated temporarily in the genus Pichia, a heterogeneous taxon, until the precise taxonomic position of the type species is defined.