Genetics of self-compatibility in Trifolium repens

The original male parent was selected because it was the only plant out of 615 that averaged over 100 seeds per head when self-pollinated under bag in the field in 1938. Except for descendants from this plant, few others have since shown a similar self-compatibility, whereas this high seed set has been duplicated on clones from the original plant by (a) different technics of selfing in the greenhouse, (b) bagging in the field both with and without manipulation, and (c) bee pollination under a cage in the field. In contrast, the female parent showed low pseudo-self-compatibility. When 14 selected F1 plants were diallely intercrossed and backcrossed to both parents in the greenhouse, two intra-sterile, interfertile groups of five and six plants, respectively, were found, and these 11 plants were reciprocally compatible with both parents. Different degrees of pseudo-self-compatibility were found among these 11 plants. The other three F1 plants were cross-compatible with both of these groups, with both parents, and with each other. These three plants were also self-compatible like their male parent under all conditions tested. These results are explained by postulating that the male parent was heterozygous for a self-compatibility factor (Sf), which is a member of the multiple-allelic series conditioning self- and cross-incompatibility, and that this factor was transmitted to the self-compatible plants. This theory was confirmed and the different self-compatible genotypes in the F1 were identified by backcrossing to the parental and F1 groups, 48 F2 plants resulting from four F1 intercrosses. The segregations into self-compatible and self-incompatible individuals were clear in most cases, although the effect of modifying factors was more evident with some methods of testing than with others. Certain self-compatible individuals were found to be self-pollinating, i.e., autogamous. Different degrees of self-compatibility were found among 110 first-generation inbreds from the original male parent, but representative plants even from the lower end of the range were shown to bear Sf. Some of these plants were effectively male sterile because of hard anthers. By appropriate tests with plants homozygous for the incompatibility alleles, it was shown that Sf was dominant over the second allele carried with it in a heterozygous plant. Its presence in pollen, however, did not stimulate other pollen applied with it when the latter was inhibited by the same factor in the pistil. It is pointed out how self-compatibility may be utilized in a breeding program.