Cell-cell interactions and later developmental events during the Chlamydomonas sexual cycle are known (or presumed) to be controlled by alleles at the mt, or "mating-type" locus. mt exemplifies a master regulatory gene orchestrating the expression of groups of unlinked cell- type-specific genes. This proposal describes work designed to determine which aspects of sexual development are under direct mt regulation, and to define the circuitry of genetic pathways that underlies progression of the sexual cycle. Intensive searches for mutant phenotypes showing mating-type-limited expression will be conducted. Altered chloroplast gene transmission (under mt control) will be used to identify mating- type-limited (mtl) mutants. Through the characterization of sexual development in mutant strains (microscopy), analysis of chloroplast inheritance (marker transmission genetics and DNA-specific flourescence microscopy), measurement of linkage between mt1 gene loci, and definition of epistatic interactions between these genes in constructed double mutant strains, the mt locus will be identified and distinguished from the genes it regulates. Induction of second-site suppressor mutations and characterization of the suppressed strains will be used to further clarify the primary functions of the mtl loci. Genetic analysis of mating mutants will also be used to evaluate alternative models for the mating-type "switching" typical of homothallic species. The proposed experiments will lay a genetic foundation for the eventual understanding of the molecular basis of differentiation of the sexual stage of the organism. Chlamydmonas is a simple one-celled plant and an understanding of the mechanisms involved in the simple differentiation of mating types may give insight into the more complex mechanisms used in differentiation in higher plants.
