ARS | Publication request: Increasing Population Hybridity by Restricting Self-Incompatibility Alleles in Red Clover Populations

Submitted to: Crop Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: September 2, 2009
Publication Date: April 12, 2010
Citation: Riday, H., Krohn, A.L. 2010. Increasing Population Hybridity by Restricting Self-Incompatibility Alleles in Red Clover Populations. Crop Science. 50:853-860.

Interpretive Summary: Red clover is a widely grown forage legume harvested for hay, grown in pasture for grazing, and sown as a companion crop. However, producers need higher yielding and more persistent red clover varieties to enhance productivity. The resources needed to accomplish this task are limited. This study presents a novel, easy to implement, hybrid cultivar development method that would make it easier and less costly to increase red clover’s forage yield and persistence. The method uses red clover’s self-incompatibility system, which prevents plant self-fertilization. Our hybrid method consists of restricting parental populations to three self-incompatibility alleles per population. Using this method, theoretically the amount of seed produced in a population cross would be 75 percent hybrid as opposed to 50 percent hybrid under normal circumstances. In our study five hybrids were created using this novel approach and, indeed, 75 percent of seed was found to be hybrid. Based on this study, red clover breeders can implement this method into their breeding programs with confidence in its efficacy.

Technical Abstract: Red clover (Trifolium pratense L.) is a widely grown forage legume harvested for hay, grown in pasture for grazing, and sown as a companion crop. Current breeding methods in red clover for quantitative traits, such as biomass yield, consist almost entirely of recurrent phenotypic selection (i.e. repeated cycles of mass selection). In many species, improved cultivars have been achieved through hybrid breeding methods, especially for highly non-additive traits. Previously in red clover, hybrid methods using pseudo-self-compatibility have been studied and developed; inbreeding depression was a major hindrance to the utility of this breeding system. In other forage legumes population hybrids, or hybridizing two heterotic randomly mating populations, has been proposed as a way to achieve 50% hybrid seed while eliminating problems associated with inbreeding. This study proposed using gametophytic self-incompatibility (GSI) S-locus in red clover to achieve population hybrids with 75% inter-population 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. Nine plants from each restricted population were randomly mated with nine plants from each S-allele unrestricted population using bumble bees in five separate screened enclosures. Halfsib seed was harvested off each plant for paternity testing. On average, 75% of the progeny of the five S-allele restricted population plants were hybrids (i.e. intra-population seed), while 48% of the progeny of the S-allele unrestricted population plants were hybrids. Observed hybridity corresponded well with expected hybridity. This study demonstrates the feasibility of increasing hybridity by restricting S-alleles within red clover populations. In conjunction with molecular markers for the S-locus or markers linked closely thereto, genotypes could be selected with restricted S-allele from any population, thus allowing maximum flexibility in increasing intra-population cross hybridity. Applications for this novel hybrid mating approach included hybrid breeding and more rapid intra-population quantitative trait introgression.