A transcriptional, genetic, and physical exploration of the S locus in Brassica.

摘要

Self-incompatibility is a mechanism in many plant species that prevents self-pollination and promotes outcrossing. In Brassica, self-incompatibility is controlled by a single, multiallelic, complex locus, the S locus. Two stigma-specific genes, SLG and SRK, are encoded at the S locus and are required for the self-incompatibility response. A gene responsible for imparting pollen identity must also reside at the locus but has not yet been identified. In a novel approach, I conducted a Differential Display screen in conjunction with bulked segregants in order to target anther-expressed genes linked to the S locus. Such genes are candidates for the pollen identity factor. One cDNA segregated perfectly with the S locus and was expressed predominantly in anthers, indicating that the approach was successful. However, further analysis indicated that this cDNA did not meet other criteria expected of the male determinant.; Physical analyses of the S locus to date have not identified the boundaries of the locus nor defined a mechanism for maintaining the genes required for self-incompatibility as a single genetic unit. One possibility is that recombination suppression in the S-locus region is responsible for preserving the integrity of the locus. To determine if the frequency of crossovers is suppressed in the S-locus region, I conducted a segregation analysis using markers that span the region to generate recombination frequencies. Physical distances for each marker interval were determined using pulsed-field gel electrophoresis (PFGE). Genetic and physical distances were compared in order to determine relative recombination frequencies. I discovered that recombination is not suppressed in the S-locus region.; The genetic analysis identified plants with crossover events close to SLG and SRK that can provide valuable functional information about the locus. Preliminary experiments have already demonstrated the existence of a stigma-specific gene involved in the rejection response to one side of the locus. Other experiments have shown that the pollen identity factor must reside in a 50-kb region surrounding SLG and SRK, greatly facilitating the search for the male determinant of self-incompatibility.