Increasing Population Hybridity by Restricting Self-Incompatibility Alleles in Red Clover Populations

Current red clover (Trifolium pratense L.) breeding methods for quantitative traits consist almost entirely of recurrent phenotypic selection. In many species, improved cultivars have been achieved through hybrid breeding methods, especially for highly nonadditive traits. Hybrid methods have previously been studied and developed in red clover; inbreeding depression was a hindrance to the utility of these breeding systems. In other forage legumes, population hybrids have been proposed as a way to achieve 50% hybrid seed while eliminating problems associated with inbreeding. This study proposes using gametophytic self-incompatibility S-locus in red clover to achieve population hybrids with 75% interpopulation hybrid seed; this would be accomplished by restricting S-alleles to three per population. To test this concept, five S-allele restricted populations were developed. These five restricted populations were randomly mated with a unrestricted population using bumble bees (Bombus impatiens Cresson) in five separate screened enclosures. Halfsib seed was harvested off plants for paternity testing. On average, 75% of the progeny of the five S-allele restricted population plants were hybrids, while 48% of the progeny of the unrestricted population plants were hybrids. Observed hybridity corresponded almost perfectly with expected hybridity. This study demonstrates the feasibility of increasing hybridity by restricting S-alleles within red clover populations.