Isozyme Studies on the Dryopteris "spinulosa" Complex, I: The Origin of the Log Fern Dryopteris celsa

Most of the species in the genus Dryopteris of eastern North America, as well as several European species, are implicated in an intricate hybrid/polyploid complex referred to here as the D. spinulosa complex. Conflicting hypotheses have been proposed to account for the ancestry of the allopolyploid species of this complex: 1) the semicristata scheme invokes a hypothetical diploid, putatatively extinct, as an ancestor of certain alloploids; 2) the reinterpretation scheme interprets all all alloploids as derived from extant diploids. To test these hypotheses, a comparative study of isozymes among the species comprising the D. spinulosa complex has been undertaken. The present paper reports evidence relative to the origin of the allotetraploid D. celsa, a widespread but locally occurring wetland species. The parentage of D. celsa has been alternatively hypothesized as either D. ludoviciana x goldiana (semicristata scheme) or D. ludoviciana x marginalis (reinterpre- tation scheme). Isozyme patterns were compared for 12 enzymes coded by 19 loci resolved and interpreted across the four relevant species. The inferred isozyme genotype of all D. celsa individuals examined was identical, with rare exceptions, and was homozygous for 13 loci and fixed hetero- zygous for seven loci. Under the assumption that D. ludoviciana is one ancestor of D. celsa (as both hypotheses agree), all loci were consistent with the hypothesis that D. goldiana was the other ancestor, whereas only 12 were in any way consistent with ancestry by D. marginalis. The assumption of ancestry by D. ludoviciana was supported by all loci except Pgi-2, where D. celsa is fixed for an allele not detected in any of the putative parents. While the origin of this orphan allele is uncertain, its fixation in D. celsa suggests that this allopolyploid species has had a unique origin, in contrast to the multiple origins inferred for other allopolyploid species. This study has provided unequivocal evidence that the ancestry of D. celsa is D. goldiana x ludoviciana, a result that substantially strengthens the plausibility of the semicristata scheme.