Investigations into the genetic variation, population structure, and breeding systems of the fern Asplenium trichomanes subsp quadrivalens

The genetic structure of five populations of the tetraploid fern Asplenium trichomanes subsp. quadrivalens in northern Switzerland was analyzed. Asple nium trichomanes subsp, quadrivalens is one of the most common and most wid espread ferns in Europe. In this study we have combined genetic investigati ons, spatial autocorrelation, and breeding experiments to investigate in de tail five populations from natural rock faces. Enzyme electrophoresis revea led very low genetic variability within and among the populations. The smal l amount of variation was partitioned mainly among the localities, indicate d by high F-st values up to 0.764. Overall means of the proportion of polym orphic loci ((P) over bar = 0.076), the mean number of alleles per locus (( A) over bar = 1.086), and the mean expected heterozygosity ((H) over bar = 0.018) were low compared with other ferns (e.g., Kirkpatrick et al. 1990). Very few heterozygous individuals were found. Values of the fixation index (F) were high, ranging between 0.732 and 1.000 and indicating substantial i nbreeding. Spatial autocorrelation showed different patterns of substructur e in populations of A. trichomanes subsp. quadrivalens with a tendency for patches in short distances (up to 1.5 m). The breeding experiments with iso lated prothalli and prothalli pairs showed that a mean of 56.4% of the isol ated prothalli were successful in sporophyte formation. The highest rate in one population was 83.3%. We conclude that genetic load must be low in A. trichomanes subsp. quadrivalens. Sporophyte formation was statistically mor e successful in the experiments with gametophyte pairs than in isolates, in dicating that additional cross-fertilization occurred. The latter agreed wi th the occurrence of few heterozygote samples and the small number of multi locus phenotypes found in natural habitats. Asplenium trichomanes subsp. qu adrivalens is shown to be a highly inbreeding taxon with the capability of single spore colonization and subsequent founding of new populations. Such features can be hypothesized to have contributed to the postglacial coloniz ation and the widespread distribution of this taxon in Europe.