Comparative allozyme and microsatellite population structure in a narrow endemic plant species, Centaurea corymbosa Pourret (Asteraceae)

Centaurea corymbosa Pourret (Asteraceae) is a narrow endemic species known only from six populations located in a 3-km(2) area in the south of France. Earlier field experiments have suggested that pollen and seed dispersal we re highly restricted within and among populations. Consistent with the fiel d results, populations were highly differentiated for five allozyme loci an d among-population variation fitted an isolation-by-distance model. In the present study, we investigated the genetic structure of C. corymbosa using six microsatellite loci. As with allozymes, microsatellites revealed no wit hin-population structure and a large differentiation among populations. How ever, allozyme loci were less powerful than microsatellites in detecting th e extent of gene flow assessed by assignment tests. The patterns of structu ration greatly varied among loci for both types of marker; we suggest that differences in single-locus pattern could mainly be an effect of stochastic variation for allozymes and an effect of variation in mutation rate for mi crosatellites. In contrast to the multilocus results, the two most polymorp hic microsatellite loci did not show any isolation-by-distance pattern. Our results suggest that highly variable loci might not always be the best sui ted markers to quantify levels of gene flow among populations.