Genetic investigations of heterosis and inbreeding depression in diploid and tetraploid alfalfa
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Hybrids between Medicago sativa subsp. sativa and M. sativa subsp. 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 intra-subspecific crosses due to 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 inter-subspecies 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 two years. Sativa x falcata crosses (SFC) showed greater F1 fertility than intra-subspecies crosses suggesting that the two subspecies have similar chromosome structure. The SFC did not show high-parent F1 heterosis probably due to an intensive harvest schedule favoring sativa x sativa hybrids. However, SFC showed greater inbreeding depression in both F2 and S1 generations than intra-subspecific 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.