The Genetic Control of Self-Incompatibility in Grasses Revisited – Are There Really Just Two Loci?

Outcrossing grasses possess a self-recognition mechanism that prevents pollination of identical genotypes thus ensuring the maintenance of heterozygous populations. The self-incompatibility (SI) system is long-recognised as consisting of two complementary loci (S and Z) that act gametophytically.
Here we present data of proportions of compatible pollen in an incomplete diallel set of pollinations between 50 related Lolium perenne plants. The family showed segregation for 1868 SNPs based on a custom Lolium Illumina Infinium Array.
We were unable to fit the pollination data to the established two-locus SI model. We determined the similarity of pollen compatibilities of all pair-wise plant combinations as the average compatibility score difference of pollinations with all other plants tested. Scores were calculated for pairs of plants used as female and male parents separately. We then performed Principal Component Analysis on the similarity scores. We tested the first and second principal components for significant association with SNPs. The perennial ryegrass GenomeZipper was used to locate the contigs in which SNPs were identified. Significantly associated SNPs were located largely in four distinct regions: two regions syntenic with the S and Zloci regions on chromosomes 1 and 2, a region on chromosome 3, that has been previously shown to be associated with SI and a fourth region on chromosome 6. Haplotype analyses of these regions have identified candidate genes. However, the exact role of S, Z and additional modifier genes in the SI reaction remains to be clarified.