Inbreeding Depression for Fertility and Biomass in Advanced Generations of Inter‐ and Intrasubspecific Hybrids of Tetraploid Alfalfa

Abstract

Hybrids between Medicago sativa subsp. sativa L. (hereafter “sativa”) and M. sativa subsp. falcata (L.) Arcang. (hereafter “falcata”) often express heterosis for yield. Yield generally declines when hybrids are inbred due to the loss of desirable dominant alleles or epistatic combinations. We hypothesized that inbreeding sativa–falcata hybrids may show more extensive yield loss than intrasubspecific crosses because of the large genetic difference between subspecies. Our objective was to compare fertility in the greenhouse and biomass production in the field of F1 hybrids and their F2 and S1 generations in both intra‐ and intersubspecific crosses. Field experiments including F1, F2, S1, and double‐cross populations were planted in two locations in Iowa in 2003 and evaluated for yield in 2 yr. Sativa × falcata crosses (SFC) showed greater F1 fertility than intrasubspecific crosses, suggesting that the two subspecies have similar chromosome structure. The SFC did not show high‐parent F1 heterosis, probably because of an intensive harvest schedule favoring sativa × sativa hybrids. However, SFC showed greater inbreeding depression in both F2 and S1 generations than intrasubspecific crosses, possibly reflecting a greater loss of favorable complementary gene interactions. These results suggest that selection within each parental population followed by intercrossing to produce population hybrids may be a better way to improve biomass with this germplasm than advancing hybrids into a recurrent selection program.

Applied Genetics and Genomics in Alfalfa Breeding

“…Natural hybrids between the two subspecies have been found at both ploidy levels [4]. Inter-subspecies hybrids show normal meiosis [5] and equivalent or superior fertility compared to intra-subspecies hybrids [6]. Describing the full complexity of the M. sativa taxon is outside the purview of this paper; the interested reader is directed to the monograph by Small [1], which comprehensively discusses this and other Medicago species.…”

Implementation of Molecular Markers Diversity in Parental Selection of Alfalfa (Medicago SativaL.) Germplasm

“…While for the past 30 years the maize yield has an average yield increase of 2% per year, the alfalfa yields have increased by only 0.15% to 0.30% or less per year over the same time period (28). there are several possible reasons for this lagging in the alfalfa yield improvement: (i) genetic complexity of alfalfa at both, individual and population level, due to its autotetraploidy and allogamy that is characterized by a high level of heterozygosity and severe inbreeding depression (6,9,17), (ii) perennial growth habit which requires evaluation of experimental strains for several years before making any decision in selection programs (21), (iii) existing limitations to capitalize heterosis through commercial cultivars, and (iv) focus of breeding programs on a wide range of economically important traits, such as diseases or insect resistance and forage quality, many of which are not specifically correlated or may be negatively correlated with the forage yield (3,15).…”

Development of Genomic Resources in the Species of Trifolium L. and Its Application in Forage Legume Breeding

“…Such approaches are likely to be complementary to QTL analysis and will allow the results of previous studies to be applied more effectively in breeding programmes. The potential of heterosis in outbreeding forage legumes has been explored by Brummer [30], and some progress towards this end has been made, particularly in alfalfa [31]. Heterotic combinations of populations, derived from germplasm of different geographic origins are also underway.…”