Self-Fertilization, Inbreeding, and Yield in Alfalfa Seed Production.

Johanne Brunet,Molly E Dieterich Mabin,Heathcliffe Riday,Lauren Lehmann

doi:10.3389/fpls.2021.700708

Abstract

Selfing (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, namely, the Pacific Northwest (PNW), the Central Valley of California (CEV), and the Imperial Valley of California (IMP). 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 were 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 the seed set. The effects of the field selfing rate on the 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, which, by eliminating inbreeding depression, would provide a 15% potential increase in seed yield and an increase in future hay yield.

Highlights

Selfing or self-pollination, where ovules are pollinated by the plant’s own pollen, is the ultimate form of inbreeding or mating between relatives
The polymorphic information content (PIC) varied among loci but there was no difference in the mean PIC per locus between parents (0.603 ± 0.056) and progeny (0.607 ± 0.059; t15 = −0.75, p = 0.46), with a PIC range of 0.035–0.822 for the parents and 0.035–0.818 for the progeny
If we examine the relationships between field selfing rate, the number of racemes per stem, and the four seed yield metrics, overall, the number of racemes per stem had a stronger effect on the distinct seed yield metrics relative to the field selfing rate

Summary

Introduction

Selfing or self-pollination, where ovules are pollinated by the plant’s own pollen, is the ultimate form of inbreeding or mating between relatives. Studies on inbreeding depression of autotetraploid wild plants (Galloway et al, 2003; Galloway and Etterson, 2007) and crops (Jones and Bingham, 1995; Li and Brummer, 2009) often reveal a substantially higher-than-expected level of inbreeding depression, approaching the level found in diploid species (Busbice and Wilsie, 1966; Galloway and Etterson, 2007) This higher level of inbreeding depression cannot be explained solely by the expression of deleterious recessive alleles with increased inbreeding and has been partly ascribed to the loss of complementary gene interactions (Bingham et al, 1994; Jones and Bingham, 1995). Because selfing can lead to inbreeding depression and affect crop yield, it is important to examine the extent of selfing in seed production fields

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Conclusion