Uniform selection as a primary force reducing population genetic differentiation of cavitation resistance across a species range

Cavitation resistance to water stress-induced embolism determines plant survival during drought. This adaptive trait has been described as highly variable in a wide range of tree species, but little is known about the extent of genetic and phenotypic variability within species. This information is essential to our understanding of the evolutionary forces that have shaped this trait, and for evaluation of its inclusion in breeding programs. We assessed cavitation resistance (P(50)), growth and carbon isotope composition in six Pinus pinaster populations in a provenance and progeny trial. We estimated the heritability of cavitation resistance and compared the distribution of neutral markers (F(ST)) and quantitative genetic differentiation (Q(ST)), for retrospective identification of the evolutionary forces acting on these traits. In contrast to growth and carbon isotope composition, no population differentiation was found for cavitation resistance. Heritability was higher than for the other traits, with a low additive genetic variance (h(2) (ns) = 0.43±0.18, CV(A) = 4.4%). Q(ST) was significantly lower than F(ST), indicating uniform selection for P(50), rather than genetic drift. Putative mechanisms underlying Q(ST)