Abstract
Adaptation of morphological, physiological, or life‐history traits of a plant species to heterogeneous habitats through the process of natural selection is a paramount process in evolutionary biology. We have used a population genomic approach to disentangle selection‐based and demography‐based variation in morphological and life‐history traits in the crucifer Diplotaxis harra (Forssk.) Boiss. (Brassicaceae) encountered in populations along aridity gradients in S Tunisia. We have genotyped 182 individuals from 12 populations of the species ranging from coastal to semidesert habitats using amplified fragment length polymorphism (AFLP) fingerprinting and assessed a range of morphological and life‐history traits from their progeny cultivated under common‐garden conditions. Application of three different statistical approaches for searching AFLP loci under selection allowed us to characterize candidate loci, for which their association with the traits assessed was tested for statistical significance and correlation with climate data. As a key result of this study, we find that only the shape of cauline leaves seems to be under differential selection along the aridity gradient in S Tunisian populations of Diplotaxis harra, while for all other traits studied neutral biogeographical and/or random factors could not be excluded as explanation for the variation observed. The counter‐intuitive finding that plants from populations with more arid habitats produce broader leaves under optimal conditions of cultivation than those from more mesic habitats is interpreted as being ascribable to selection for a higher plasticity in this trait under more unpredictable semidesert conditions compared to the more predictable ones in coastal habitats.
Highlights
The study of adaptation to abiotic and biotic factors of a species’ environment through the process of natural selection is paramount to our understanding of evolutionary change in morphological, physiological, and ecological traits
While in the two former method groups, the phenotypes are the starting point for selection studies and an assessment of fitness is necessary, the latter is less straightforward and starts with variation at the DNA level as a footprint of local adaptation, infers loci that are under selection, and tests for statistically significant correlations of those loci with functional traits (Storz, 2005; Vasemägi & Primmer, 2005)
Studies on local adaptation are especially informative when clinal variation in functional traits is studied along environmental gradients, and alternative explanations for phenotypic clines are excluded (Keller et al, 2009; Keller & Taylor, 2008; Savolainen et al, 2013)
Summary
The study of adaptation to abiotic and biotic factors of a species’ environment through the process of natural selection is paramount to our understanding of evolutionary change in morphological, physiological, and ecological traits. As a consequence of these steep gradients, changes among vegetation types are pronounced and range between Mediterranean coastal vegetation (Djerba), over different steppe types in the coastal strip, to plant associations substituting Juniperus phoenicea woodland (Matmata Mts) and the semidesertic Anthyllis sericea steppe type at the eastern boundary of the Sahara desert (Frankenberg & Klaus, 1987). This constellation provides a suitable setting for studying climate-dependent differential selection of morphological and life-h istory traits along an aridity gradient. Following the strategy of Herrera and Bazaga (2008), who pinpointed floral traits being under adaptive divergence in the Spanish hawk moth-pollinated violet Viola cazorlensis Gand. (Violaceae), this study aims at the detection of amplified fragment length polymorphism (AFLP) loci under selection and asks whether these are correlated with phenotypic traits of interest, assuming that environment-dependent selection in these morphological and life-history traits has led to a selection signal in some of the AFLP loci via genetic hitchhiking
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