Genetic variation and selection response in model breeding populations of Brassica rapa following a diversity bottleneck

Abstract

Domestication and breeding share a common feature of population bottlenecks followed by significant genetic gain. To date, no crop models for investigating the evolution of genetic variance, selection response, and population diversity following bottlenecks have been developed. We developed a model artificial selection system in the laboratory using rapid-cycling Brassica rapa. Responses to 10 cycles of recurrent selection for cotyledon size were compared across a broad population founded with 200 individuals, three bottleneck populations initiated with two individuals each, and unselected controls. Additive genetic variance and heritability were significantly larger in the bottleneck populations prior to selection and this corresponded to a heightened response of bottleneck populations during the first three cycles. However, the overall response was ultimately greater and more sustained in the broad population. AFLP marker analyses revealed the pattern and extent of population subdivision were unaffected by a bottleneck even though the diversity retained in a selection population was significantly limited. Rapid gain in genetically more uniform bottlenecked populations, particularly in the short term, may offer an explanation for why domesticators and breeders have realized significant selection progress over relatively short time periods.

Figure 1.

Schematic of the artificial selection experiment. One broad and three bottleneck populations were founded with random individuals taken from the rapid-cycling B. rapa stock. These population bases were each subdivided into two recurrent selection subpopulations divergent for cotyledon size and one random control subpopulation.

Figure 2.

Cotyledon surface area over 10 cycles of divergent biparental recurrent mass selection for cotyledon size in one broad (200 founders) and three bottleneck (2 founders each) populations of rapid-cycling B. rapa (graphs). Scanned cotyledons from selection cycle 10 progeny (right).

Figure 3.

MDS plots of individuals sampled from the broad and bottleneck populations before (base) and after (large, control, and small) 10 cycles of selection. The MDS positions for both plots were determined using a single matrix of genetic distance estimates based on AFLP similarities. All axes have a tick distance of 0.1 MDS units.