Mating Type and Sporulation in Yeast. 11. Meiosis, Recombination, and Radiation Sensitivity in an (yly Diploid with Altered Sporulation Control

In wild-type S. cerevisiae, diploid cells must be heterozygous at the matingtype locus in order to sporulate. In the preceding paper, we described a number of mutants (CSP mutants), isolated from nonsporulating aa and aa parent strains, in which sporulation appeared to be uncoupled from control by mating type. The characterization of one of these mutants (CSPI) is now extended to other processes controlled by mating type. This mutant is indistinguishable from aa cells and unlike aa cells for mating factor production and response, zygote formation, intragenic mitotic recombination, and for X-ray sensitivity. The mutant apparently undergoes a full round of DNA synthesis in sporulation medium, but with delayed kinetics. Only 20% of the cells complete sporulation. Among spores in completed asci, the frequency of both intraand intergenic recombination is the same as it is for spores produced by aa cells. However, experiments in which cells were shifted from sporulation medium back to minimal growth medium gave a frequency of meiotic recombination between d e 2 or leu2 heteroalleles only 25% to 29% as high for CSPI aol diploid or CSPI aa disomic cells as for aol diploid or disomic cells. Because the latter result, indicating recombination defectiveness, measured recombinant production in the entire cell population, whereas the result indicating normal recombination sampled only completed spores, we infer that all meiotic recombination events occurring in the population of CSPl aa cells are concentrated in those few cells which complete sporulation. This high degree of correlation between meiotic recombination and the completion of meiosis and sporulation suggests that recombination may be required for proper meiotic chromosome segregation in yeast just as it appears to be in maize and in Drosophila N heterothallic yeast strains, sex is determined by the alleles of mating type I present in a given cell (LINDEGREN and LJNDEGREN 1943). Cells of matingtype a mate with cells of mating-type a! and Dice versa. Cells containing both mating-type alleles, aa! diploids, do not mate but are able to undergo meiosis and sporulation. Diploid yeast cells homozygous for the mating-type alleles can mate, but cannot sporulate (ROMAN and SANDS 1953). We have isolated yeast mutants with aberrant control of the sexual cycle. As a consequence of mutation in a gene unlinked to mating type, diploid aa and aa strains became able to sporulate (HOPPER and HALL 1975). In this paper, we report an extensive characterization of one such CSP (control of sporulation) Genetics 80: 61-76 May, 1975. 62 A. K. HOPPER, J. KIRSCH A N D B. D. HALL mutant which was isolated from a wild-type (nonsporulating) aa diploid parent, The focus of this characterization is upon those processes, all related to genetic exchange and transmission, which distinguish aa from homozygous mating-type diploid strains. The mating reaction of homozygous mating-type cells appears to have as its basis the production of specific mating factors by both a and a mating types and a complementary response to these substances by cells of the opposite mating type (DUNTZE et al. 1973; MACKAY and MANNEY 1974a, b; WILKINSON and PRINGLE 1974). aa diploids can undergo sporulation and also the related meiotic processes of DNA replication (ROTH and LUSNAK 1970), genetic recombination (ROTH and FOGEL 1971; HOPPER et al. 1974) and chromosome segregation while a01 cells cannot undergo any of these processes. For induced intragenic recombination and X-ray radioresistance in mitotic cells, there is a quantitative difference; aa cells perform these processes more efficiently than aa or aa cells (LASKOWSKI 1960; FRIIS and ROMAN 1968). For each of the above processes we have compared the behavior of wild-type aa and aa strains to that of A-l69-aa, a CSPl mutant which appears to undergo a full round of meiotic DNA replication, but sporulates with only 20% efficiency. Particular attention has been given to the extent and nature of the recombination defectiveness (as compared to aa cells) of the CSPl mutant. Mitotic DNA repair and recombination are defective (or aa-like) and the frequency of intragenic meiotic recombination measured for the whole population of sporulating CSPl mutant cells is correspondingly low. However, a normal (aa-like) extent of recombination occurs in those cells (20%) which produce ascospores. These results suggest that meiotic recombination and chromosome segregation, which necessarily depend upon the prior occurrence of meiotic DNA synthesis, also are coordinately dependent upon some process which aa cells do efficiently and CSPl mutant cells do inefficiently. AP-1 -ma. MATERIALS A N D METHODS Yeast strains: Parental strains AP-1-aa and AP-I-aa and the CSPl mutant A-169-aa (all heteroallelic for ade2) were described in the previous communication (HOPPER and HALL 1975). All of the strains have a common genetic background. Construction of disomic strains: The disomic strains used to study meiotic recombination were made by crosses between Z4521-96C, and a CSPl or a csp+ haploid segregants of A-169-aa. 24521-96C, obtained from R. ROTH (ROTH 1973), is disomic for chromosome 111, homozygous for a mating type, and heteroallelic for leu2: 4leu2-27 a leu2-l + n a c) ade2-I. CSPl mutant haploid segregants from A-169-aa were identified by their ability to synthesize DNA in sporulation medium (HOPPER and HALL 1975). A-169-aa-4a (a, gaZl, lys2, tyr l , csp’) A-169-aa-4d (a , his7, cyh2, CSPI) each were mated with the disomic strain. Following sporulation of the 2n+l aaa hybrids, asci containing two disomic (aa or aa) leucine-requiring segregants were identified by mating-type analysis of the segregants. aa segregants were determined to be either CSPl or csp+ by scoring for the ability to synthesize DNA during “sporulation”. All disomic strains were further confirmed to be heteroallelic at leu2 by the ability to give rise MATING TYPE A N D SPORULATION IN YEAST