ABSTRACT
The potential for environmental heterogeneity to generate spatial structuring of genotypes in seed-plant populations that occupy patchy habitats has been demonstrated by several studies, but little is known about the population structure of pteridophytes occupying patchy environments. In this study we have examined the genetic structure of isolated populations of the rock fern Asplenium csikii, an ecological specialist, growing almost exclusively on perpendicular walls of natural rock outcrops. All genetic variation observed in this taxon was partitioned between localities; no allozyme variation was found within a site and each site was colonized by a single multilocus phenotype (MLP). In total, five different MLPs were recorded from the nine localities, with two MLPs present at more than one site. Previous examination of population structure and genetic diversity in another rock fern, A. ruta-muraria, showed that the genetic diversity increases through multiple colonization over time. However, we cannot find any such correlation for A. csikii. All populations are genetically uniform, despite the probably considerable age of the populations and sites. Earlier studies concluded that the ample production of wind-borne propagules would lead to multiple colonization of sites and that reproductive features, such as single-spore colonization and subsequent intragametophytic selfing, would lead to very little genetic structuring of fern populations. In contrast to this prediction, it appears that ecological specialization and the scarcity of the narrowly defined niche contribute strongly to the pronounced partitioning of genetic variability observed in populations of A. csikii.
