Emendation of the Genus Issatchenkia Kudriavzev and Comparison of Species by Deoxyribonucleic Acid Reassociation, Mating Reaction, and Ascospore Ultrastructure

Abstract

The genus Issatchenkia Kudriavzev is 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 (I. orientalis), P. terricola, P. scutulata and P. scutulata var. exigua are assigned to this genus as I. orientalis Kudriavzev, I. terricola (van der Walt) comb. nov., I. scutulata var. scutulata (Phaff et al.) comb. nov. and I. scutulata var. exigua (Phaf et al.) comb. nov., respectively. I. occidentalis is also described. The type strain of I. occidentalis is NRRL Y-7552 (= CBS 5459). EM demonstrated that the ascospore walls of I. terricola have thick inner and outer layers and 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. DNA reassociation studies and mating tests confirmed the recognition of 4 spp. in the genus Issatchenkia and showed Candida krusei to be the imperfect state of I. orientalis; C. sorbosa was identified as the imperfect form of I. occidentalis. I. scutulata var. scutulata, and I. scutulata var. exigua showed only 25% DNA complementarity, yet intervarietal matings formed viable ascospores. This was the lowest DNA relatedness even shown between strains capable of genetic hybridization; the implications of this finding were discussed.