Synthetic cultivar parent number impacts on genetic drift and disease resistance in alfalfa (Medicago sativa L.)

Authors: STEINMETZ, OLIVIA - University Of Wisconsin; HUSET, DAVID - Legacy Seeds; ROUSE, DOUGLAS - University Of Wisconsin; Raasch, John; GUTIERREZ, LUCIA - University Of Wisconsin; Riday, Heathcliffe

Submitted to: Crop Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/6/2020
Publication Date: N/A
Citation: N/A

Interpretive Summary: The number of parents used to create alfalfa varieties is an important consideration during breeding. The number of parents used to create an alfalfa variety can impact a variety’s environmental stability and performance due to factors such as inbreeding. The goal of this study was to test if there was more genetic drift in alfalfa cultivars created with fewer parents (i.e., narrow-based synthetics) or more parents (i.e., broad-based synthetics). A more common term associated with genetic drift is genetic bottleneck or the tendency to lose genetic diversity in breeding populations after a restriction in the number of mating individuals. Evidence of genetic drift was explored using DNA markers and four alfalfa diseases (Anthracnose, Phytophthora root rot, Aphanomyces root rot, and Fusarium wilt). We found strong evidence of genetic drift in the narrow-based synthetics based on the DNA markers and limited genetic drift for changes in disease resistance. This work demonstrates to alfalfa breeders the effects of genetic drift for disease resistance when they create narrow based synthetics in alfalfa.

Technical Abstract: Cultivated alfalfa (Medicago sativa L.) is a highly heterozygous allogamous autotetraploid (2n = 4x = 32), and is generally bred to become synthetic varieties. The number of parents selected to create a variety is a critical consideration during breeding, as alfalfa suffers from severe inbreeding depression and plant breeders improve multiple traits simultaneously. The goal of this study was to test if there was more genetic drift in synthetic populations with fewer parents (i.e., narrow-based synthetics) as measured by a loss of allelic richness and disease resistance. This study utilized five alfalfa populations as base germplasm to select parents based on plant field vigor and size to create derived narrow-based (six parent) and broad-based (56-86 parent) synthetics in order to test the new synthetics compared to their base populations for resistance to four diseases: Anthracnose, Phytophthora root rot (PRR), Aphanomyces root rot (ARR), and Fusarium wilt (FW). Using SSR DNA markers we also calculated allele frequency changes and allele richness for the original populations and the selected parents of the derived synthetics. Based on the DNA markers there was strong evidence of greater genetic drift in the narrow-based selections compared to the broad-based selections. The narrow selections had significantly greater allele frequency changes, and had significantly fewer (11.5-33.9% less) alleles compared to the broad selections and original populations. There was only moderate evidence of genetic drift in resulting disease resistance. Parent number had a significant effect on resistance within some populations for Anthracnose and FW, but not for PRR or ARR. Disease resistance is a complex, often multi-genic trait in alfalfa, and appears to be less influenced by genetic drift than expected.