Gene flow and local adaptation in two endemic plant species

In order to detect the evolutionary potential of two endangered species, Br assica insularis (Brassicaceae) and Centaurea corymbosa (Asteraceae), withi n and among-population genetic variation for both quantitative traits and a llozymic markers was examined. Four populations of each species were studie d, representing a large proportion of extant populations. High values of th eta (ST) (0.213 and 0.364 for B. insularis and C. corymbosa respectively) s uggested that low amounts of gene flow occur among the study populations. I n each species, the genetic distance based on allozymes (estimated by the r atio (theta (ST)/1-theta (ST))) was positively correlated with the geograph ical distance, indicating isolation by distance. In contrast to previous st udies in either outcrossing or selfing plant species, and especially for B. insularis, population differentiation for quantitative traits (theta (ST)) was generally found lower than differentiation for allozymes (theta (ST)), suggesting that the populations studied were experiencing similar selectiv e forces acting upon the quantitative traits measured. Such forces would be strong enough to counteract local genetic drift. Interestingly, for both s pecies theta (ST)'s were statistically independent of geographical distance , in contrast to the marginally significant positive isolation by distance shown by theta (ST). Altogether, these results suggest that theta (ST)'s mi ght not always be used as conservative estimates of Q(ST')s, and might inst ead overestimate the evolutionary potential of endangered species. This wou ld 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 like ly to produce strong differentiation for :neutral variation. In fact, knowl edge of both neutral and quantitative diversity patterns allows identificat ion of those traits undergoing natural selection, and could be useful in de signing reinforcement or reintroduction programs. However, this approach mi ght have limitations too, in the presence of outbreeding depression due to locally coevolved gene complexes, (C) 2001 Elsevier Science Ltd. All rights reserved.