Population Genetics of the Grass Self-incompatibility System—Practical Implications for Grass Breeding Programmes

Self-incompatibility (SI) is a mechanism that prevents plants from self-pollination through molecular recognition between pollen and pistil; but neither the genes nor the proteins involved are known. Determining haplotype diversity of the SI locus region is an indirect way of predicting incompatibility genotype which would enable plant breeders to develop strategies to ensure precision crossing for exploiting heterotic combinations. In the case of ryegrass (Lolium perenne), SI is known to be a gametophytic reaction involving two loci, S and Z. This paper reports on an assay of marker allele diversity around the Z locus for a population produced from twelve generations of mixed and half-sib family selection. Using conventional genotyping of marker length polymorphisms, as well as high resolution melt (HRM) curve analysis, the 55 plants of the population were classified into thirteen groups according to their genotypes for each marker. HRM genotyping proved to be more discriminating than STS markers and could enhance the precision of Z genotype prediction. Although half-sib family selection is expected to lead to allele fixing, this initial analysis was encouraging in that it indicated high haplotype diversity around the Z-locus suggesting maintenance of a high degree of cross-compatibility in the breeding population. This would be expected as SI alleles are subjected to frequency dependent selection.