The genus Issatchenkia Kudriavzev has been emended to include all nitratenegative, multilateral budding yeast species that form unconjugated persistent asci with roughened spheroidal ascospores and have Q-7 ubiquinone in the electron transport system. Pichia kudriauzeuii (lssatchenkia orientalis), Pichia terricola, 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.,

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... ssatchenkia 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 of1. occidentalis is NRRL Y-7552 (=CBS 5459). Electron microscopy demonstrated that the ascospore walls of 1. 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 1. 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 1. orientalis; Candida sorbosa was identified as the imperfect form of 1. occidentalis. 1. scutulata var. scutulata and 1. 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.