Mapping the self-incompatibility (S) locus in white clover (Trifolium repens L.)

White clover (Trifolium repens L.) is a member of the Leguminosae flowering plant family and is used as a high quality forage crop capable of fixing atmospheric nitrogen. It is an amphidiploid (2n = 4x = 32) species constructed of two ancestral genomes, T. palescens and T. occidentale. T. repens possesses a self-incompatibility mechanism which prevents self-seed setting. The self-incompatibility reaction is contolled by a single poly-allelic locus expressed gametophytically in the male, where common allele specificity in pollen and stylar expressed genes results in late-acting, pre-fertilisation arrest of pollentube growth (Atwood, 1940). Based on the assumption that both parents were heterozygous for unique S-alleles, ninety four individuals of a bi-parental mapping family were classified into four intra-incompatible groupings from partial diallel crosses and pod-filling assessments six to nine days post-pollination of emasculated florets.

Initial analysis of the two parental maps based on AFLP and SSR markers has identified marker associations with the S-locus attributing it to linkage group E2 as classified by Barrett et al (2004). To date, 12, 8, 6 and 16 plants have been assigned the S genotypes S13, S14, S23 and S24 respectively. Further work will place more markers on this linkage group, and polymorphisms of candidate genes/proteins will be compared to the S-locus genotypes of the characterised plants.

References
Atwood, S.S. (1940). Genetics of cross-incompatibility among self-incompatible plants of Trifolium repens. Journal of American Society of Agronomy 32: 955-968.
Barrett, B., Griffiths, A., Schreiber, M., Ellison, N., Mercer, C., Bouton, J., Ong, B., Forster, J., Sawbridge, T., Spangenberg, G., Bryan, G. And Woodfield, D. (2004). A microsatellite map of
white clover. Theoretical and Applied Genetics 109: 596-608.