The genus Issatchenkia Kudriavzev has been emended to include all nitrate-negative, multilateral budding yeast species that form unconjugated persistent asci with roughened spheroidal ascospores and have Q-7 ubiquinone in the electron transport system. Pichia kudriavzevii (Issatchenkia orientalis), Pichiaterricola, Pichia scutulata var. scutulata, and Pichia scutulata var. exigua are assigned to this genus as Issatchenkia orientalis Kudriavzev, Issatchenkia terricola (van der Walt) comb. nov., Issatchenkia scutulata var. scutulata (Phaff et al.) comb. nov., and Issatchenkia scutulata var. exigua (Phaff et al.) comb. nov., respectively. Additionally, one new species, Issatchenkia occidentalis, is described. The type strain of I. occidentalis is NRRL Y-7552 (=CBS 5459). Electron microscopy demonstrated that the ascospore walls of I. terricola have thick inner and outer layers and that the ascospores of the other species have walls with a thick inner layer and a thin, dense outer layer. With the exception of I. scutulata var. scutulata, ascospore surface ornamentation arises from the dense outer layer. Deoxyribonucleic acid reassociation studies and mating tests confirmed the recognition of four species in the genus Issatchenkia and showed Candida krusei to be the imperfect state of I. orientalis; Candida sorbosa was identified as the imperfect form of I. occidentalis. I. scutulata var. scutulata and I. scutulata var. exigua showed only 25% deoxyribonucleic acid complementarity, yet intervarietal matings formed viable ascospores. This is the lowest deoxyribonucleic acid relatedness ever shown between strains capable of genetic hybridization, and the implications of this finding are discussed.
In the course of a taxonomic study of methanol-assimilating yeasts, a number of isolates were obtained from various natural habitats in Japan, and from these three new species, Pichia cellobiosa, Candida cariosilignicola, and Candida succiphila, have been described. P. cellobiosa differs from the related species Pichia castillae with respect to fermentation and assimilation of sugars, biotin requirement for growth, and lower maximum temperature for growth. C. cariosilignicola differs from the related species Torulopsis apicola and Torulopsis bombicola with respect to assimilation of sugars and proton magnetic resonance spectra of cell wall mannans. C. succiphila is similar to C. cariosilignicola but differs from it with respect to fermentation and assimilation of sugars, lower maximum temperature for growth, higher deoxyribonucleic acid base composition, and proton magnetic resonance spectra of cell wall mannans. The type strains of these new species are KL 1 (=IAM 12481), KL 24 (=IAM 12484), and KL 30 (=IAM 12489), respectively. Cultures of the type strains have been deposited in the Institute of Applied Microbiology Culture Collection Center, University of Tokyo, Tokyo, Japan, and the Institute for Fermentation, Osaka, Japan.