Intraspecific Polymorphism of the Yeast Kluyveromyces lactis: Genetic Populations

— According to the modern classification of yeasts, the species Kluyveromyces lactis includes two taxonomic varieties: cultural dairy yeast K. lactis var. lactis and nonlactose-fermenting environmental strains of K. lactis var. drosophilarum . This subdivision of the species, which is based only on phenotypic and ecological criteria, is formal and does not reflect the existing heterogeneity of K. lactis . Using various molecular methods and genetic hybridization analysis, we studied the genetic relationship of 35 K. lactis strains isolated from dairy products and environmental sources in different regions of the world. The lactose-fermenting yeasts K. lactis , including dairy strains, clinical and soil isolates, had identical molecular karyotypes, did not differ in the nucleotide sequences of a number of molecular markers, and formed fertile hybrids with 84–99% ascospore viability. On the other hand, nonlactose-fermenting yeasts split into three genetically isolated populations: ”krassilnikovii,” “drosophilarum,” and “phaseolosporus,” which differed in molecular karyotypes, had unique SNP substitutions in the ACT1 gene, and formed semisterile hybrids: 6–34% ascospore viability. Despite profound chromosome length polymorphism, K. lactis var. lactis , “krassilnikovii,” “drosophilarum,” and “phaseolosporus” probably have the same haploid number of chromosomes equal to six. The largest range of chromosome sizes was observed in “krassilnikovii” (1000–2900 kb), and the smallest in “drosophilarum” (1600–2200 kb). The biogeography of the yeast K. lactis is noteworthy. Lactose-fermenting strains of K. lactis var. lactis have been isolated in various regions of the world. Genetic “drosophilarum” and “phaseolosporus” populations are typical of North America, while “krassilnikovii” is represented by European and Central Asian isolates. Comparative analysis of the ACT1 sequences should be applied for reliable delineation of all four genetic populations of K. lactis.