Patterns of genetic variation within and among populations in Arbutus unedo and its relation with selection and evolvability

The study of plant adaptation is especially relevant in the context of expected global change, which will modify key environmental factors, such as temperature and precipitation. Here, we study the strawberry tree (Arbutus unedo L.) throughout its climatically variable range: predominantly circum-Mediterranean, but also along the Atlantic coasts from Morocco to Ireland. In a common garden experiment, we studied 12 plant traits to assess the hypothetical effects of natural selection on the population genetic structure. These traits were selected because it is well known that they are highly affected by climatic factors such as temperature and precipitation. To this aim, we used the QST–FST approach. Firstly we calculated the additive genetic variance (VA), heritability (h2) and evolvability. Then, we obtain QST, which is a standardized measure of genetic differentiation among populations displayed by quantitative traits, and compare it with FST, a measure of the genetic differentiation of putatively neutral loci. Our results showed that QST was significantly lower than FST in 6 of the 12 traits, and 11 of the traits had demonstrable VA and evolvability. Low QST values are typically explained by stabilizing selection. We propose that stabilizing selection may select for a norm of reaction that produces a similar phenotype even from diverse genotypes from different localities. Further, the evolvability that we found in key morpho-structural traits predicts that Arbutus unedo retains genetic variation that will allow it to continue to adapt to the diverse environments throughout its species range.