The Polycomb (Pc) group of genes are required for maintenance of cell determination in Drosophila melanogaster. At least 11 Pc group genes have been described and there may be up to 40; all are required for normal regulation of homeotic genes, but as a group, their phenotypes are rather diverse. It has been suggested that the products of Pc group genes might be members of a heteromeric complex that acts to regulate the chromatin structure of target loci. We examined the phenotypes of adult flies heterozygous for every pairwise combination of Pc group genes in an attempt to subdivide the Pc group functionally. The results support the idea that Additional sex combs (Asx), Pc, Polycomblike (Pcl), Posterior sex combs (Psc), Sex combs on midleg (Scm), and Sex combs extra (Sce) have similar functions in some imaginal tissues. We show genetic interactions among extra sex combs (esc) and Asx, Enhancer of Pc, Pcl, Enhancer of zeste E(z), and super sex combs and reassess the idea that most Pc group genes function independently of esc. Most duplications of Pc group genes neither exhibit anterior transformations nor suppress the extra sex comb phenotype of Pc group mutations, suggesting that not all Pc group genes behave as predicted by the mass-action model. Surprisingly, duplications of E(z) enhance homeotic phenotypes of esc mutants. Flies with increasing doses of esc+ exhibit anterior transformations, but these are not enhanced by mutations in trithorax group genes. The results are discussed with respect to current models of Pc group function.
Mating and pathogenesis of the corn smut fungus, Ustilago maydis, are controlled by two unlinked mating-type loci, a and b. Yeast-like haploids that differ at both loci are compatible and fuse to establish a pathogenic dikaryon. Mating is assayed in vitro by co-inoculation on culture medium containing activated charcoal; compatible combinations have a characteristic "fuzzy" appearance caused by the growth of aerial hyphae. In general, this test has not been useful for assaying the mating ability of strains that are already mycelial (e.g., those heterozygous at b or at both mating-type loci). Using an assay for cytoduction involving transfer of a mitochondrial marker during transient cell fusion, and engineered strains with defined genotypes, we examined the mating abilities of strains heterozygous or hemizygous at the mating-type loci. The data (which have not been available from conventional pathogenicity or plate mating tests) show that heterozygosity at b attenuates fusion in haploid and diploid strains, whereas strains heterozygous at a retain the ability to fuse with a compatible haploid partner. It appears, therefore, that subsequent fusion events are attenuated once fusion has occurred to establish the U. maydis dikaryon.
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