Comparison of neutral and adaptive differentiation in the Mediterranean grass Brachypodium retusum

Abstract The Mediterranean grass Brachypodium retusum is the dominant species of a vulnerable steppe habitat. Differentiation in phenotypic traits has been found in a previous study, but scales and drivers are largely unknown. In this study, we compared molecular and phenotypic variation between populations to (1) analyse spatial patterns of neutral genetic variation; (2) test for effects of selection on differentiation and (3) identify major drivers of adaptive differentiation. We collected plant material of 17 populations in the western Mediterranean covering a large part of the species range. Neutral population differentiation was estimated using AFLP markers. A regional-scale subset of pairs of French populations was sampled in close proximity from calcareous and red Mediterranean soils as major habitat types. Sampling sites differed in climate at a regional scale and in soil characteristics such as pH at a local scale. These populations were grown in a common garden experiment to measure phenotypic traits. To test for the effects of selection on phenotypic differentiation, we calculated pairwise θST values based on neutral AFLP markers and compared them to pairwise PST values using phenotypic traits. Global dataset θST indicated significant neutral genetic differentiation between western Mediterranean populations. In the French populations, PST of vegetative and reproductive traits were higher than θST, suggesting that directional selection contributed to phenotypic population differentiation. We also found significant local-scale differentiation between soil types, but differentiation was substantially higher at the regional scale, pointing towards climate as a stronger selective factor than soil type. Mean temperature of the hottest month and winter frost frequency were identified as major drivers of adaptive differentiation. The study demonstrated the importance of combining neutral marker and phenotypic trait analysis at different spatial scales to evaluate genetic structure. Despite relatively low differentiation in AFLP markers, environmental pressure was sufficient to maintain phenotypic differentiation at regional scales.