THE ORIGIN OF STERILE CYTODUCTANTS OBTAINED IN THE ALPHA-TEST Shiriaeva A.A.1, Andreychuck Y.V.1, Zhuk A.S.1,2, Stepchenkova E.I.1,2, Inge-Vechtomov S.G.1,2 1 Saint-Petersburg State University, St.Petersburg 2 Saint-Petersburg Branch of Vavilov Institute of General Genetics, RAS, St.Petersburg, Russia The alpha-test is a sensitive system for genetic toxicology. The system utilizes heterotallic strains of yeast Saccharomyces cerevisiae. It is based on genetic system controlling the mating type in yeast. The mating type of yeast strains is controlled by the MAT locus which is located on the right arm of chromosome III. The MAT locus determines “a” or “α” cell type of haploid cells. Normally haploids of the opposite cell types can mate and form diploid cells. Primary lesions, mutations and recombination events in MATα as well as loss of the chromosome III or its right arm impair the MATα expression. It causes mating type switch from α to a and mating of two initially α-type cells. The frequency of the "illegitimate" hybridization is a criterion of genome instability in the alpha-test. Different phenotypes of hybrids in the alpha-test correspond to different molecular events of the mating type switch. It was predicted that sterile cells obtained in "illegitimate" cytoduction (the modification of the alpha-test) contain mutations in the MATα locus. Our new data indicate that two thirds of sterile cytoductants do hold mutations in the MATα. We demonstrate that the other sterile cells are different in their ploidy. One of the reasons for sterility is forming of triploid cells α/a/α. We show that overexpression of STE12 transcription factor restores mating type “α” to triploid cells α/a/α as well as to diploid cells α/a. Our findings open new opportunities for further improvement of the alpha-test. Acknowledgements: Grant RFBR # 15-04-08625 and Research Grant of St. Petersburg State University #1.38.426.2015.