Athyrium filix-femina (L.) Roth (Lady Fern) comprises a complex array of homoploid (n=40) taxa, distributed over much of the northern hemisphere and extending into South America, whose relationships and evolutionary history are poorly understood. The present study addressed genetic relatedness between two morphologically differentiated taxa of eastern North America, most recently treated at varietal rank as A. filix-femina var. angustum (Northern Lady Fern) and var. asplenioides (Southern Lady Fern). Originally based on features of growth form, frond shape, and spore color, distinction of these taxa was supported by a recent study that showed consistent differences in perispore sculpturing and allele frequencies for two enzyme-coding loci (Idh-1 and Pgm-2). Here we report data from an expanded isozyme survey that increased the sampling of enzymes (16 loci), populations and individuals, and geographic range. Most alleles were shared among all populations. However, at the four most polymorphic loci (Idh-1, Pgi-2, Pgm-2, and Tpi-2) allele frequencies were significantly divergent between populations of angustum sampled from Vermont and Pennsylvania and asplenioides sampled from New Jersey, Virginia, and South Carolina. The most diagnostic locus was Idh-1, which approached fixation for alternate alleles between the putative taxa. A population of asplenioides sampled from 1300m elevation in southwestern Virginia possessed higher frequencies than other asplenioides populations for the predominantly angustum alleles at Pgm-2 and Tpi-2, but not at Idh-2 or Pgi-2. These data indicate that the morphological divergence between the putative taxa is accompanied by divergence at isozyme genes, reflecting a history of genetic isolation and justifying their continued distinction . Whether they should be recognized as distinct species or as infraspecific taxa remains an open question, awaiting additional isozyme data from their region of overlap as well as from the more western portion of their range.

Key words: allozyme, Athyrium filix-femina, divergence, genetic variation, isozyme, speciation