Measuring the Inefficiency of Allowing Self‐Pollinated Alfalfa Progeny into Breeding Nurseries

Abstract

Although alfalfa (Medicago sativa L.) is considered an outcrossing species, the self‐pollination incidence during insect pollination can be substantial, with an observed 30% average selfing rate. Despite this, alfalfa breeders utilizing space‐plant evaluation nurseries make no attempt to discriminate self‐pollination‐derived progeny (selfs) from outcross progeny. Due to alfalfa's substantial inbreeding depression, the selfed individuals have much lower breeder utility. This study examines an alfalfa space‐plant breeding nursery in which selfed and outcross progeny had been identified using DNA markers for the following phenotypic traits: plant vigor, fall height, fall dormancy, first and second winter survival, plant lodging, and profusion of flowering. Trait means, phenotypic and additive variances, and narrow‐sense heritabilities on a halfsib mean basis were determined for selfed and outcross progeny. For most traits, inbreeding depression was severe, with approximately 80% reductions in performance. Selfed progeny for most traits had reduced phenotypic variances compared with outcross progeny. No differences in estimated additive variance and heritabilities were observed between the two types of progeny. Assuming a 30% selfing rate and a selection intensity less than or equal to 10%, we would expect to select few if any selfed progeny from a space‐plant nursery that had plant vigor phenotypic distributions observed in this study. The estimated maximum loss in genetic gain due to the presence of selfed progeny in breeding nurseries for selection intensities from 1 to 10% was estimated at a 10% genetic gain reduction. We conclude that selfed progeny in breeding nurseries are a hindrance, and if possible, they should be minimized or eliminated.

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

“…Plants with larger floral displays may set more selfed seeds as a result of geitonogamous selfing and appear more self-compatible while they might not be. This pattern will occur in screened polycrosses ( Riday et al, 2017 ) as well as in large multi-acre pollination (this study) as long as bees are used for seed production. Potential metrics to describe floral display size in alfalfa include the number of racemes per flowering stem, or the total number of open flowers per flowering stem ( Brunet et al, 2021 ).…”

“…To fully understand the effect of inbreeding depression in a plant species, multiple traits must therefore be examined across its life cycle. In alfalfa, inbreeding depression has been shown to impact various stages of the life cycle, including seed production and hay yield ( Aycock and Wilsie, 1968 ; Busbice, 1968 ; Melton, 1970 ; Gallais, 1984 ; Riday et al, 2017 ). In addition, selfing has also been shown to reduce the rates of flowering and winter survival ( Riday et al, 2017 ), increase the number of days to bloom ( Aycock and Wilsie, 1968 ; Posler, 1969 ), reduce plant height ( Panella and Lorenzetti, 1966 ; Gallais, 1984 ; Riday et al, 2017 ), and generally reduce disease tolerance ( Koffman and Wilsie, 1961 ).…”

Synthetic cultivar parent number impacts on genetic drift and disease resistance in alfalfa

“…For the two populations that were grown in both locations, we selected half from the Waupaca nursery and the other half of the plants from the Evansville nursery. These 93–130 plants represented a selection intensity of 16.9–26.0% for developing broad‐based synthetics (Riday et al., 2017). Out of the 93–130 plants, a subset of the best six individuals were selected from each population to develop narrow‐based synthetics with a selection intensity of 0.9–1.2% (Riday et al., 2017).…”

“…These 93–130 plants represented a selection intensity of 16.9–26.0% for developing broad‐based synthetics (Riday et al., 2017). Out of the 93–130 plants, a subset of the best six individuals were selected from each population to develop narrow‐based synthetics with a selection intensity of 0.9–1.2% (Riday et al., 2017). The original, unselected populations were also evaluated using seed from the same seed lots as was used for the field nurseries.…”

“…So‐called narrow‐based synthetics have a few parents while so‐called broad‐based synthetics have around 100 parents (Rumbaugh et al., 1988). A study of the Association of Official Seed Certifying Agency (AOSCA) alfalfa variety registrations from 2008 to 2015 shows a clear bimodal distribution for syn 0 parent number, with one group having an average of 14 syn 0 parents (i.e., narrow‐based), and the other group having an average of 105 syn 0 parents (i.e., broad‐based) (Riday, Johnson, Engh, & Darling, 2017). It is expected that the genetic structure of broad‐based synthetics would remain more stable across generations compared with narrow‐based synthetics, and that genetic drift and inbreeding would be less of a concern (Rumbaugh et al., 1988).…”