Unifying selection acts on competitive ability and relative growth rate in Scabiosa columbaria

Q(ST) vs. F(ST) comparisons can reveal diversifying or unifying selection pressures among populations for specific traits. In this study we performed Q(ST)-F(ST) analyses on eleven populations of Scabiosa columbaria from the Swiss Jura to reveal genetic differentiation in two quantitative traits (above-ground biomass and relative growth rate of leaf lengths) and in neutral molecular markers. Above-ground biomass of plants under competition has been shown to correlate with their competitive ability, which is an important fitness-related trait. We hypothesized that strong unifying selection acts on above-ground biomass, since underperformance would result in decreased fitness and overperformance is unlikely due to trade-offs with other plant functions.Overall G(ST) (an F(ST) analogue) was 0.12. Analysis of variance revealed that above-ground biomass and relative growth rate did not differ among populations, but both traits differed among seed families and were heritable (h(2) = 0.31 and h(2) = 0.35, respectively). Q(ST) was close to zero for above-ground biomass and zero for relative growth rate of leaf lengths, and thus Q(ST) was much lower than G(ST), indicating unifying selection on these traits.This conclusion is restricted by the limits of the used methodology. Q(ST) < F(ST) cannot always be considered as a proof for unifying selection, because in complex traits the assumption of purely additive effects of underlying genes may be violated. However, given the large differences between Q(ST) and G(ST), together with substantial heritabilities of the traits under study, we conclude that our findings are not in contradiction with the hypothesis of unifying selection.