Gene flow and local adaptation in two endemic plant species

Abstract

In order to detect the evolutionary potential of two endangered species, Brassica insularis (Brassicaceae) and Centaurea corymbosa (Asteraceae), within and among-population genetic variation for both quantitative traits and allozymic markers was examined. Four populations of each species were studied, representing a large proportion of extant populations. High values of θST (0.213 and 0.364 for B. insularis and C. corymbosa respectively) suggested that low amounts of gene flow occur among the study populations. In each species, the genetic distance based on allozymes (estimated by the ratio (θST/1−θST)) was positively correlated with the geographical distance, indicating isolation by distance. In contrast to previous studies in either outcrossing or selfing plant species, and especially for B. insularis, population differentiation for quantitative traits (QST) was generally found lower than differentiation for allozymes (θST), suggesting that the populations studied were experiencing similar selective forces acting upon the quantitative traits measured. Such forces would be strong enough to counteract local genetic drift. Interestingly, for both species QST's were statistically independent of geographical distance, in contrast to the marginally significant positive isolation by distance shown by θST. Altogether, these results suggest that θST's might not always be used as conservative estimates of QST's, and might instead overestimate the evolutionary potential of endangered species. This would be especially expected in narrow-endemic species, whose ecological niche is often so restricted that indeed homogeneous selective forces are likely to occur, whereas small population sizes and restricted dispersal are likely to produce strong differentiation for neutral variation. In fact, knowledge of both neutral and quantitative diversity patterns allows identification of those traits undergoing natural selection, and could be useful in designing reinforcement or reintroduction programs. However, this approach might have limitations too, in the presence of outbreeding depression due to locally coevolved gene complexes.