Genetic structure, reproductive biology and ecology of isolated populations of Asplenium csikii (Aspleniaceae, Pteridophyta)

The potential for environmental heterogeneity to generate spatial structuri ng of genotypes in seed-plant populations that occupy patchy habitats has b een demonstrated by several studies, but little is known about the populati on 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 obs erved in this taxon was partitioned between localities; no allozyme variati on was found within a site and each site was colonized by a single multiloc us phenotype (MLP). In total, five different MLPs were recorded from the ni ne localities, with two MLPs present at more than one site. Previous examin ation of population structure and genetic diversity in another rock fern, A . ruta-muraria, showed that the genetic diversity increases through multipl e colonization over time. However, we cannot find any such correlation for A. csikii. All populations are genetically uniform, despite the probably co nsiderable age of the populations and sites. Earlier studies concluded that the ample production of wind-borne propagules would lead to multiple colon ization of sites and that reproductive features, such as single-spore colon ization and subsequent intragametophytic selfing, would lead to very little genetic structuring of fern populations. In contrast to this prediction, i t appears that ecological specialization and the scarcity of the narrowly d efined niche contribute strongly to the pronounced partitioning of genetic variability observed in populations of A. csikii.