Self-fertilization, inbreeding and yield in alfalfa seed production

Authors: Dieterich Mabin, Molly; Brunet, Johanne; Riday, Heathcliffe; Lehmann, Lauren

Submitted to: Frontiers in Plant Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/4/2021
Publication Date: 7/6/2021
Citation: Dieterich Mabin, M.E., Brunet, J., Riday, H., Lehmann, L.0. 2021. Self-fertilization, inbreeding and yield in alfalfa seed production. Frontiers in Plant Science. 12. Article 700708. https://doi.org/10.3389/fpls.2021.700708.
DOI: https://doi.org/10.3389/fpls.2021.700708

Interpretive Summary: In plants, self-fertilization, or selfing, occurs when a flower is pollinated with pollen from that same plant. The level of similarity between alleles (homozygosity) increases under selfing and this can lead to inbreeding depression, or a reduction in the survival and reproduction of inbred relative to outbred offspring. In alfalfa, the strong inbreeding depression observed, suggests a potentially strong negative impact of selfing on both hay and seed yields. This study quantifies the selfing rate of 32 seed production fields across three major alfalfa seed production regions in the western United States. Selfing rates varied between 5.3 and 30% among fields, with an average selfing rate of 12.2%. Interestingly, field selfing rate did not negatively impact seed yield, mainly because the selfed stems in these fields had more racemes and stems with more racemes set more seeds. The relationship between a stem selfing rate and the number of racemes on a stem strongly supports the presence of geitonogamous or among flower selfing, a form of selfing that requires pollinators. Based on available evidence, we conclude that the great majority of selfing in alfalfa is pollinator-mediated. This implies that pollinators create selfing, selfing will not decrease with greater pollinator abundance, and to eliminate selfing in alfalfa seed production we need to select for self-incompatible varieties to ensure outbred seed production. We did, however, quantify a 15% reduction in seed yield between selfed and outcrossed stems and potential future impact on hay yield. Eliminating selfing in alfalfa through selection of self-incompatible varieties is expected to increase seed yield by 15%. Identifying the mating system of a crop and its prevailing mode of selfing can guide the development of effective strategies to reduce selfing and increase yield. This information will be of great benefit to farmers, plant breeders and the alfalfa seed and hay industry.

Technical Abstract: Selfing (or self-pollination) is the ultimate form of inbreeding, or mating among close relatives. Selfing can create yield loss when inbreeding depression, defined as a lower survival and reproduction of inbred relative to outbred progeny, is present. To determine the impact of selfing in alfalfa (Medicago sativa L.), we quantified the selfing rate of 32 alfalfa seed-production fields located in three regions, the Pacific Northwest, the Central Valley and the Imperial Valley of California. Selfing rates (the proportion of selfed seeds) varied between 5.3 and 30% with an average of 12.2% over the 32 seed production fields. In both the parents and their progeny, we observed an excess of heterozygotes relative to Hardy-Weinberg expectations. We detected notable levels of inbreeding in parents (0.231 ± 0.007 parental inbreeding coefficient) and progeny (0.229 ± 0.005). There was a 15% decrease in the number of seeds per stem (seed set) and a 13% decline in the number of seeds per pod in selfed relative to outcrossed stems, but negligible inbreeding depression for pods per raceme and seed weight. The number of racemes on selfed stems increased significantly in fields with greater selfing rates, supporting the presence of geitonogamous or among flower selfing. Despite the significant level of inbreeding depression, seed set did not decrease in fields with higher selfing rates, where the greater number of racemes on the selfed stems increased seed set. The effects of field selfing rate on seed yield metrics were mostly indirect with direct effects of the number of racemes per stem. Available data indicate that the majority of selfing in alfalfa is pollinator-mediated and thus eliminating selfing in alfalfa seed production would require the selection of self-incompatible varieties while providing a 15% potential increase in seed yield and an increase in future hay yield.