Self-incompatibility, inbreeding depression, and potential to develop inbred lines in alfalfa

Authors: PARAJULI, ATIT - Washington State University; Yu, Long-Xi; Peel, Michael; See, Deven; WAGER, STEVE - Corteva Agriscience; NORBERG, STEVEN - Washington State University; ZHANG, ZHIWU - Washington State University

Submitted to: Book Chapter
Publication Type: Book / Chapter
Publication Acceptance Date: 6/7/2021
Publication Date: 7/18/2021
Citation: Parajuli, A., Yu, L., Peel, M., See, D.R., Wager, S., Norberg, S., Zhang, Z. 2021. Self-incompatibility, inbreeding depression, and potential to develop inbred lines in alfalfa. In: Yu LX., Kole C., editors. The Alfalfa Genome. Compendium of Plant Genomes. Springer. Cham, Switzerland. p.255-269. https://doi.org/10.1007/978-3-030-74466-3_15.
DOI: https://doi.org/10.1007/978-3-030-74466-3_15

Interpretive Summary: Alfalfa is a major forage crop around the world and is the third most valuable field crop produced in the U.S. Genetic improvement in alfalfa has occurred at a slower pace than in other crops. Conventional breeding approaches have not exploited its full yield potential. Unlike corn, alfalfa varieties are not hybrids, which typically have higher yield potential, primarily because alfalfa does not perform well after inbreeding The ability to produce fertile inbred lines could lead to significant changes in alfalfa breeding that will benefit alfalfa hay producers. This chapter reviews advances in understanding how inbreeding leads to poor performance in alfalfa and efforts to produce useful inbred lines of alfalfa, which is the first step towards the development of hybrid alfalfa varieties.

Technical Abstract: Alfalfa (Medicago sativa L.) is a perennial, outcrossing legume crop predominantly grown for hay, silage, or pasture. Intensive selection has resulted in dramatic improvement in fitness traits, including winter survival and disease resistance. However, there has been minimal improvement in other economically important traits, such as hay yield, which is still comparable to 30 years ago. Intensive phenotyping costs on this type of trait hinder high selection pressure to identify superior outcross individuals. Severe inbreeding depression inhibits the development of inbred lines with accumulated favorable alleles that exhibit heterosis. This highlights the outcomes of inbreeding depression as well as the causes, including unmasking deleterious alleles and triggering selfincompatibility. We tracked the research efforts that unveil the genetic bases underlying deleterious alleles and self-incompatibility. The magnitudes of inbreeding depression were compared with the rate of heterozygous halved time in diploid and tetraploid organisms. To fill in the gaps between the controversy and existing hypotheses, we theorized a dosage dominant model of inheritance. The dosage dominant model is similar to the Mendelian dominance model, in which a genotype exhibits a dominant phenotype if there is a dominant allele (alphabet dominant). The difference is that in the dosage dominant model, a genotype will result in a dominant phenotype if the number of dominant alleles is equal to or greater than the number of recessive alleles. This review also includes a discussion on the development of pseudo inbreds and a hypothesis to identify deleterious alleles using bulked segregant analysis and consequently to purge deleterious alleles using marker-assisted selection, to progress toward the successful development of pure inbred lines in alfalfa.