Genetic introgression as a potential to widen a species' niche: insights from alpine Carex curvula

Abstract

Understanding what causes the decreasing abundance of species at the margins of their distributions along environmental gradients has drawn considerable interest, especially because of the recent need to predict shifts in species distribution patterns in response to climatic changes. Here, we address the ecological range limit problem by focusing on the sedge, Carex curvula, a dominant plant of high-elevation grasslands in Europe, for which two ecologically differentiated but crosscompatible taxa have been described in the Alps. Our study heuristically combines an extensive phytoecological survey of alpine plant communities to set the niche attributes of each taxon and a population genetic study to assess the multilocus genotypes of 177 individuals sampled in typical and marginal habitats. We found that ecological variation strongly correlates with genetic differentiation. Our data strongly suggest that ecologically marginal populations of each taxon are mainly composed of individuals with genotypes resulting from introgressive hybridization. Conversely, no hybrids were found in typical habitats, even though the two taxa were close enough to crossbreed. Thus, our results indicate that genotype integrity is maintained in optimal habitats, whereas introgressed individuals are favored in marginal habitats. We conclude that gene flow between closely related taxa might be an important, although underestimated, mechanism shaping species distribution along gradients.

Fig. 1.

(A and B) Overall distribution range in Europe, growth habit, and transverse leaf sections of Cr and Cc. Drawings are adapted from ref. . Distribution of Cc or Cr is shown in black. (C) Location of the populations studied with AFLP in the Alps. Shaded areas, >1,000 m. Crm and Ccm indicate marginal populations of Cr and Cc, respectively. ○, Cc; •, Cr; □, Ccm; ▪, Crm.

Fig. 2.

Coinertia analysis of species–environment relationships in alpine grasslands of the Alps. (A–C) Display of all 1,300 vegetation relevés along the first two axes of variation with superimposition of relevés containing Cr (A) and Cc (B) or relevés corresponding to the sampling sites of AFLP investigated populations (C). Ellipses were drawn to include 80% of Cc or Cr relevés. The ellipse is centered on the mean of the ordination scores; its width and height are given by the variances, and its slope is given by the covariance of ordination scores. (D) Display of the environmental variables along the first two axes of variation. Original sources for these data are available in Supporting Bibliography. Symbols are as in Fig. 1.

Fig. 3.

MCA of the AFLP phenotypes. (A) Display of the 177 individuals AFLP phenotypes along the first two axes of MCA that accounted for 16.3% and 4.5% of the total variance. Eigenvalues are shown (Inset). (B) Population score (mean and SE) along the first MCA axis. A one-way ANOVA followed by a post hoc Tukey test was run to test for differences among populations. Different letters indicate significant differences at P < 0.05. ▿, Cc from the Pyrenees; other symbols are as in Fig. 1.

Fig. 4.

The relation between geographical distance and genetic distance for pairs of populations of the Cc core group (open circles), Cr core group (filled circles), and between Cc and Cr core-group populations (shaded circles). The Pyrenean population is not included.