Molecular confirmation of hybrid swarms in the fern genus Polystichum (Dryopteridaceae)

We present molecular evidence for later-generation hybridization between two western North American ferns, Polystichum munitum and P. imbricans. Several plants in two mixed-species populations were found to have the chloroplastic haplotype of one of the species and the single-locus allozymic genotype of the other. Such individuals can only have arisen via matings between F-1 hybrids or by later-generation hybridization. The pattern of morphological variation at the two sites is consistent with the molecular data, and together the two data sets suggest that the populations are hybrid swarms. Our data cofirm the only known case involving temperate-zone pteridophytes where hybridization has gone beyond the F-1 generation at the diploid level.