No abstract

The circumscription of mycelial genera among the ascomycetous yeasts has been controversial because of widely different interpretations of the taxonomic significance of their phenotypic characters. Relationships among species assigned to mycelial genera were determined from extent of divergence in a ca. 600-nucleotide region near the 5′ end of the large subunit (26S) ribosomal DNA gene. Phylogenetic analyses showed that Stephanoascus is distinct from Zygoascus and that Blastobotrys, Sympodiomyces, and Arxula represent anamorphs of the Stephanoascus clade. The analyses demonstrated the following teleomorphic taxa to be congeneric: Ambrosiozyma/Hormoascus, Saccharomycopsis/Guilliermondella/Botryoascus/Arthroascus, Dipodascus/Galactomyces, and Eremothecium/Ashbya/Nematospora/Holleya. Species assigned to Dipodascus comprise two separate clades. New taxonomic combinations are proposed that reflect the phylogenetic relationships determined. Key words: mycelial yeasts, ribosomal rRNA/rDNA, molecular systematics.

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 (Issatchenkia orientalis), Pichia terricola, Pichia scutulata var. scutulata, and Pichia scutulata var. exigua are assigned to this genus as Issatchenkia orientalis Kudriavzev 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. The genus Issatchenkia was described by Kudriavzev (11) in 1960 for yeasts which were isolated from fruit juices and berries and which had unconjugated asci and spheroidal ascospores. The single species originally included in the genus, Issatchenkia orientalis Kudriavzev Pichia terricola van der Walt and Pichia scutulata PhafT, Miller et Miranda are two other taxa phenotypically similar to P. kudriauzeuii. Scanning electron microscopy has shown that all three have warty ascospore walls (14,23), and in this respect, they differ from other roundspored species of Pichia, which have essentially smooth spore walls. Although these three species are similar in ascospore surface structure to species of Torulaspora (van der Walt [31] group 111, Saccharomyces species) and Debaryomyces (17), they do not form the asci with tapered projections that are associated with Torulaspora or exhibit the mother-daughter conjugation typical of Debaryomyces. Consequently, when ascus morphology and ascospore ultrastructure are considered, these species appear to represent a natural grouping that is different from other yeast genera, and we propose to place them in the genus Issatchenkia. On the basis of deoxyribonucleic acid (DNA) base sequence complementarity, mating reaction, and ascospore ultrastructure, we describe one new species, and several suggested perfect-imperfect relationships are verif'ied. Additionally, our multifaceted approach allowed an examination of species parameters that heretofore was not possible and suggested that, as is the case among some higher eucaryotes, the boundaries of individual yeast species may be less precisely definable than previously supposed. 503

A new ascosporogenous yeast, Zygosaccharomyces kombuchaensis sp. n. (type strain NRRL YB-4811, CBS 8849), is described; it was isolated from Kombucha tea, a popular fermented tea-based beverage. The four known strains of the new species have identical nucleotide sequences in domain D1/D2 of 26S rDNA. Phylogenetic analysis of D1/D2 and 18S rDNA sequences places Z. kombuchaensis near Zygosaccharomyces lentus. The two species are indistinguishable on standard physiological tests used for yeast identification, but can be recognized from differences in restriction fragment length polymorphism patterns obtained by digestion of 18S-ITS1 amplicons with the restriction enzymes DdeI and MboI.