Genetic control of resistance to Kabatiella caulivora in Trifolium subterraneum subspecies yanninicum II. Variation in segregation populations

Abstract

Variation in resistance to K. caulivora in F, and backcross populations was examined in six crosses amongst lines of T. subterraneum ssp. yanninicum representative of the known range in resistance. Frequency distribution patterns indicated that resistant lines were differentiated from the other lines by a single major gene, resistance being completely dominant to susceptibility. Populations derived from a cross between two moderately resistant lines and a cross between two susceptible lines exhibited marked transgressive segregation towards susceptibility in the former and higher resistance in the latter. At least four genes of relatively minor effect were presumed to be contributing to differences in resistance amongst the moderately resistant and susceptible lines. Analyses of population means for each of the six crosses showed that gene interactions were not significant in any of the crosses. Significant additive gene effects were detected only in crosses between the resistant line on the one hand and a moderately resistant and a susceptible line. The results of these analyses were generally in agreement with a diallel analysis of resistance involving lines used in this study. Genetic information obtained from this study is discussed with respect to the prospects of transferring effective resistance to agronomically superior ssp. yanninicum cultivars by backcrossing.