CHROMOSOMAL REORGANIZATION DURING MEIOSIS OF SACCHAROMYCES-CEREVISIAEBAKERS YEASTS

The genomic constitution of two S. cerevisiae baker's yeasts and their meiotic products have been analyzed by pulsed-field gel-electrophores is, hybridization with specific gene probes, marker segregation, and f low cytometry. The parental strains have chromosomal patterns substant ially different from those of laboratory strains used as controls. Thi s pattern is partly the result of there being more than one copy of ho mologous chromosomes of different size, as judged by Southern-blot hyb ridization carried out with specific gene probes. Flow cytometry indic ated that the strains have a 2.7 C DNA content. Tetrad analysis showed disomy for some chromosomes and tetrasomy for others. When two comple te tetrads were subjected to molecular analysis the results confirmed instances of segregation of homologous chromosomes of different size. However, the presence of chromosomal bands absent in the parentals and the disappearance of chromosomal bands present in the parental strain s were frequently seen. This result was attributed to two different ph enomena: (1) the presence of multiple Ty1 and Ty2 transposable element s which seem to undergo interchromosomal translocation together with a mplification, giving rise to differences in chromosomal size; (2) the presence of multiple Y' subtelomeric regions, giving rise to asymmetri cal homologous recombination and, as a consequence, differences betwen the size of the recombinant chromosomes and the non-recombinant paren tal chromosomes. Chromosomal reorganization occurs with a very high fr equency during meiosis. By contrast, mitosis is very stable, as judged by the reproducible electrophoretic karyotype shown by the parental s trains in successive generations.