Selfing Rate in an Alfalfa Seed Production Field Pollinated with Leafcutter Bees

Abstract

A utotetraploid alfalfa (2n = 4x = 32) is the third largest crop by acreage in the United States with an estimated 28 million kg of seed produced per year (NASS, 2009). Although alfalfa tends to outcross (i.e., xenogamous), it is a self-compatible species meaning that self-pollination or "selfing" is possible. Selfing in alfalfa is more likely to occur in the absence of pollen from another genotype, suggesting that, when present, outcross pollen may outcompete self pollen (Viands et al., 1988). Selfing has been shown to occur in insect-pollinated alfalfa seed production fields. Reported field-wide selfing rate estimates range from 9 to 53% (Burkart, 1937, 15% selfing;Johansen, 1963, multiple studies 9 to 26% selfing; Bradner and Frakes, 1964, 34% selfing; Pedersen, 1968, 53% selfing; Knapp and Teuber, 1993, 24% selfing; and Brown and Bingham, 1994, 28% selfing). Most of the pre-1990s studies used white flowered "sentinel" plants in fields of purple flowered alfalfa plants to estimate selfing rates. The pollinators used in these studies varied widely or were not mentioned and ABSTRACT Alfalfa (Medicago sativa L.) self-pollination (i.e., selfing) causes inbreeding depression. Determining factors influencing alfalfa seed production selfing rates could inform potential mitigation strategies to reduce selfing. We measured in situ selfing rates from seed sampled from random plants in a commercial alfalfa seed production field pollinated by leafcutter bees (Megachile rotundata F.). Alfalfa selfing rates were estimated by genotyping ~24 progeny from each of 38 maternal plants. Maternal plant distance to pollinator domicile, pod position on racemes, raceme position on stems, and seeds per pod were noted during seed and tissue collection. Selfing rates averaged 11.8% with individual selfing rates ranging from 0% to 52.2%. Seed from pods collected from upper parts of racemes had lower selfing rates (9.1%) compared to pods from lower parts of racemes (15.1%). When "low" self-compatible (<15% selfing rate in 3+ seeded pods) and "high" self-compatible (≥15% selfing rate in 3+ seeded pods) plants were examined separately, however, this pattern remained significant only for low self-compatible plants (upper raceme selfing rates 3.1% vs. lower raceme 8.3%). Low self-compatible plants had higher selfing rates in 1-2 seeded pods (12.9%) compared to 3+ seeded pods (3.8%) while high self-compatible plants showed no differences in selfing rates based on seed number per pod. Genetic differences for self-pollen's ability to outcompete outcross pollen when growing down the style best explained observed differences between low and high self-compatible plants. Best management practices and selection could help reduce but not eliminate selfing in alfalfa seed production fields.

Marker‐assisted parentage analysis reveals high individual selfing rates in tetraploid red clover genotypes selected for seed yield

“…The uneven pollen‐parent fecundity distribution in our trial corresponds to Riday, Reisen, Raasch, Santa‐Martinez, and Brunet () and Riday () who found that pollen‐parent fecundity distributions are uneven in polycrosses with most forage Legumes. The distribution in the present study could be somewhat biased due to the lower success rate of assigning parentage thus making certain pollen‐parents more likely to be identified based on rare SSR alleles.…”

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

“…The other strategy is to minimize inbreeding by selecting enough parents to make "broad"based synthetics (Busbice, 1969;Hill et al, 1988). Regardless if a narrow-or broad-based synthetic mating strategy is used, with associated inbreeding due to mating among relatives in both cases, a proportion of seed is expected to result from selfing (Riday et al, 2015). On the other hand, a significant number of breeders continue to use only a few parents to create so-called "narrow"based synthetics, despite theoretical work showing that this strategy should lead to inbreeding depression (Busbice, 1969).…”

A simple model for pollen-parent fecundity distributions in bee-pollinated forage legume polycrosses

“…The 63 plants were completely randomized prior to each pollination. Self-pollination occurs at low frequencies in bee-pollinated alfalfa (Riday et al 2015). Therefore, polycross size (N) was determined based on paternal parents detected during paternity testing of progeny and seed-parents which produced seed c Dimensions include immediate size of polycross; actual indoor room size was larger Table 1).…”

“…The parents and majority of progeny of C584Y, WI21, and C276 were extracted as described in Riday (2011); additional progeny DNA samples analyzed since the 2011 paper were extracted using methodologies described in Riday et al (2013). Finally, parent DNA from the KU09B polycross was extracted as described in Riday et al (2013), while progeny DNA extracted directly from seed was accomplished as described in Riday et al (2015).…”

“…During multiplex development, each primer pair was amplified individually to determine its pattern and verify its products when multiplexed. Raw image files were imported into the GeneMarker 1.91 software program (SoftGenetics LLC, State College, PA), and for 5-dye runs a manual color matrix adjustment was made to reduce pull-up between colors as presented in Riday et al (2015). Trial and error was used to determine amount of each primer pair utilized in the multiplex in order to adjusted PCR amplicon production amounts to be more uniform across all primer pairs.…”