SUMMARYThe first part of the paper provides strong supportive evidence for the previous findings (Cummins & Day, 1973; Day & Cummins, 1973) that the two alleles of the mating-type locus of the basidiomycete Ustilago violacea have different periods of inducibility during a cell cycle, and that the cell cycle characteristics of each allele are maintained in freshly isolated diploids. This difference in temporal properties of the alleles appears to be the basis of the dominance of allele a2 as it is inducible during a phase of the cell cycle when allele a1 is non-inducible. During G1 both alleles appear to be inducible and apparently ‘neutralize’ each other so that the cell cannot mate.The second part of the paper provides evidence for a unique genetic control mechanism. The evidence suggests that the period of cell cycle inducibility of a locus governing a morphogenetic pathway may be regulated by a separate control gene the cc locus, with two known alleles ccstr(a stringent or restricted period of inducibility) and ccrel (a relaxed or non-restricted period of inducibility). This hypothesis stems from analysis of a diploid that was a1· ccstr/a2· ccrel and showed dominance of allele a2 during the S and G2 phases when freshly isolated, but which became incapable of mating after a period of subculturing. Analysis of haploids derived from this diploid strain showed that both mating-type alleles were functional but that it was now homozygous for ccstr, i.e. of genotype a1· ccstr/a2·ccstr· Thus the temporal and functional aspects of the mating type alleles are determined by different loci. It is postulated that cell cycle control loci may be widespread and serve to regulate the action of genes concerned with morphogenesis in relation to other cell cycle events.
A cell cycle regulatory gene contributes to zebrafish somitogenesis
