Adaptative walk or random walk?

by Martin Lascoux
Department of Ecology and Genetics, EBC, Uppsala University, Uppsala University, Sweden

The search for the genetic factors controlling phenological traits in Arabidopsis thaliana, in particular flowering time, started in a candid and optimistic mood, with the rapid identification of FRI and FLC as two of the main culprits. Further studies, however, have consistently led to a far more complex picture, with more genes involved and sometimes either FRI, FLC, or both missing. Given what is today known about the genetic basis of complex traits this is perhaps hardly surprising. The new emerging picture is also a consequence of a shift in the type of experiments that are carried out. There are three main differences between recent studies and earlier ones. First, in contrast to early studies that used a single individual per population, recent studies often incorporate local genetic variation. Second, phenology tends today to be measured under more natural conditions. Finally, earlier studies were based on unstructured worldwide samples while more recent studies have considered different geographical scales. In a recent and very interesting study, Brachi and collaborators focus on the latter. Using a hierarchical sampling of French Arabidopsis thaliana populations together with a worldwide sample, they investigated the association of phenology with environmental ecological variables, on the one hand, and variation at 135 SNPs, on the other hand, at different geographical scales. Three main conclusions emerge, none of which is particularly reassuring with regards to our ability to one day reach a generic understanding of the control of phenology in Arabidopsis. First, history matters and it matters a lot: most of phenological variation can simply be explained by neutral genetic variation. It is worth pointing out that, in this respect, A. thaliana might be a particularly difficult species. Other species might not have been disturbed to the same extent. Second, local selective agents such as edaphic conditions or interspecific competition, play an important role in shaping adaptive variation. Third, different genes control phenology in different places and at different geographical scale. I would tend to be a bit more pessimistic than the authors and think that all this would seem to seriously complicate the reconstruction of “the adaptive walks that natural populations follow towards local phenotypic optima”. Hopefully I am wrong and they are right.

Brachi B, Villoutreix R, Faure N, Hautekèete N, Piquot Y, Pauwels M, Roby D, Cuguen J, Bergelson J, Roux F. (2013) Investigation of the geographical scale of adaptive phenological variation and its underlying genetics in Arabidopsis thaliana. Mol Ecol 22: 4222-4240

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Flowering Newsletter published by the Journal of Experimental Botany
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