Speciation in spatially structured populations : identifying genes responsible for local adaptation (original) (raw)

The determination of the genetic basis of adaptive traits in natural populations is fundamental to better understand how populations adaptively diverge in heterogeneous environments and eventually give rise to new species. This work describes the efforts to identify candidate loci influenced by selection in ocellated lizards (Lacerta lepida) along an environmental gradient in the Iberian Peninsula, strongly affected by climatic variables. Two subspecies are recognized at the opposite extremes of the gradient, L. l. iberica in the northwest and L. l. nevadensis in the southeast, with morphological differences that suggest their local adaptation. Candidate loci were detected through an AFLP genome scan. Detection of candidate loci with a frequentist method and with Bayesian method resulted in a similar proportion of outliers (3-4%), but only a few loci were detected by both methods, denoting differences in methods' sensitivity. Several outliers were associated with variation in temperature, insolation or precipitation along the gradient, suggesting the importance of these variables as selective pressures for local adaptation. Seven outliers were successfully characterized, being noncoding, with internal indels or repetitive elements as causes of length polymorphism, indicating that they might act as regulatory elements or are in linkage with the actual target of selection. The analysis of Mc1r, a candidate gene for coloration, revealed a nonconserved and derived substitution (T162I) associated with the brownish colour phenotype of L. l. nevadensis, suggesting a putative partial loss of function. Another substitution (S172C) was associated with the presence of black scales in both L. l. lepida and L. l. iberica, but no mutations were associated with the higher melanization of L. l. iberica. Analysis of genetic structure showed that L. l. nevadensis divergence is well supported by both neutral and non-neutral loci, confirming that the subspecies is at the final stages of its speciation process. L. l. iberica divergence from the nominal subspecies is mostly explained by a few adaptive loci, indicating that L. l. iberica might be at the early stages of ecological speciation.