Selfing rate in an alfalfa seed production field pollinated with leafcutter bees

Authors: Riday, Heathcliffe; RIESEN, PETER - Forage Genetics International; Raasch, John; SANTA-MARTINEZ, EMMANUEL - University Of Wisconsin; Brunet, Johanne

Submitted to: Crop Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/10/2014
Publication Date: 3/27/2015
Publication URL: http://handle.nal.usda.gov/10113/62852
Citation: Riday, H., Riesen, P., Raasch, J.A., Santa-Martinez, E., Brunet, J. 2015. Selfing rate in an alfalfa seed production field pollinated with leafcutter bees. Crop Science. 55(3):1087-1095.

Interpretive Summary: Alfalfa seed sold to farmers in the form of varieties is produced in seed production fields in the Western United States. Self-pollination in these seed production fields can lead to lower varietal performance from the seed sold to producers. This study measured self-pollination in a commercial seed production field in Idaho using DNA-markers. Position of seed pods on the plant and plant position in the field were noted. We found that the self-pollination rate in the field was 11.8%, which is low compared to previous studies that determined self-pollination using other techniques. We found that seed pod position on the flowering head was related to self-pollination rate. This study shows that DNA-markers can be used to measure alfalfa self-pollination rates in seed production fields. Future studies will look at various seed production field management factors to determine how these may affect self-pollination rates. This information will help in efforts to breed more productive alfalfa seed for forage growers.

Technical Abstract: Self-pollination or “selfing” in autotetraploid alfalfa (Medicago sativa L.) (2n = 4x = 32) leads to severe inbreeding depression. Investigating selfing in alfalfa seed production may allow mitigation strategy development against potential negative impacts of selfing on varietal performance. Using molecular markers permits in situ selfing rate estimation in commercial alfalfa seed production fields. The objective of this study was to measure in situ selfing rates from seed sampled from random plants in a commercial 0.84 ha leafcutter bee (Megachile rotundata F.) pollinated alfalfa seed production field. Selfing rates of maternal plants were estimated by genotyping progeny arrays and their respective maternal plants at 18 SSR markers. The distance of the maternal plant from pollinator housing, the positions of a pod on a raceme and of the raceme on a stem, and the number of seeds per pod were noted during seed and tissue collection. Field-wide selfing rates averaged 11.8%, which is in the lowest range of historically reported field alfalfa selfing rates. Individual plant selfing rates varied between 0% and 52.2%. Seed from pods collected from the upper part of racemes had, on average, lower selfing rates (9.1%) compared to pods from the lower part of the raceme (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 the low self-compatible plants (upper raceme 3.1% vs. lower raceme 8.3%). Low self-compatible plants also 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 number of seeds per pod. Genetic differences in the ability of self-pollen to outcompete outcross pollen when growing down the style best explained the observed differences between low and high self-compatible plants. We discuss how best management practices and selection could help reduce but would not eliminate selfing in alfalfa seed production fields.