Article 2

MacKay, V. & T.R. Manney (1974b) Mutations affecting sexual conjugation and related processes in Saccharomyces cerevisiae. II. Genetic analysis of nonmating mutants. Genetics 76: 273-288.

In Article 1, MacKay & Manney isolated a series of sterile mutants that they placed into 16 different phenotypic classes. Most of these mutants were not conditional, i.e. they were sterile under all growth conditions. Thus, genetic analysis of these mutants would be difficult. Mating frequency was very low and some diploids did not sporulate. The greatest difficulty was encountered when the mutations were to be placed into complementation groups. It was not possible to do the traditional complementation test because the phenotype, ability to mate, is a characteristic of haploid cells and complementation tests are done in the diploid. So the authors did the next best thing, they mapped the mutations. If two mutations are unlinked, then they must be in different genes. If two mutations are tightly linked, then they may be in the same gene.

1. Ten of the mutants isolated in the MATa strain exhibit tight linkage to the mating-type locus and are probably alterations in MATa, which the authors refer to as MT or STE1. Diagram a cross of a Class 1 mutant (matalstel) to a wild-type MATa/STEl strain. Give the genotype and phenotype of the tetrads resulting from this cross (remember the ste mutation is tightly linked to MA 7a). Are your results consistent with those for mutant VC2 shown in Table 2? Discuss why the authors state that, 'the upper limit for the map distance between the ste mutation in VC2 and MT is 1.2 centimorgans'.

2. Class 4 mutants were isolated in a MATa strain. The five mutants in this class are unlinked to MATa and are «-specific. That is, only MATa strains exhibit the ste phenotype. The authors refer to these mutants as ste3 mutants. Diagram the cross between a Class 4 mutant (MATa ste3) and a wild-type MAT& STE3 strain. Give the genotype and phenotype (mating type and ability to mate) of the spores in the tetrad types that result from such a cross. Are your projected results consistent with those shown in Table 2 for mutant VC3? Discuss. Include a calculation demonstrating that MATa and ste3 mutations in VC3 are unlinked.

3. The results in Table 5 indicate that all five of the Class 4 a-specific mutants are tightly linked. Discuss these results and the authors' conclusion that all the Class 4 mutations are likely to be allelic.

4. Class 5 mutants were nonspecific for mating type. That is, both a and a mating-type strains carrying these ste mutations were sterile. Four mutants fell into Class 5. Based on the results shown in Table 6, the authors place these four mutants into two groups representing two ste genes, ste4 and ste5. Discuss their reasoning.

5. Discuss the authors' conclusion that STE2 and STE3 encode the a-factor and a-factor receptors, respectively.

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