A biosystematic investigation of the Adiantum pedatum complex in eastern North America

Botanists disagree about the relationship of the serpentine and the woodland members of the Adiantum pedatum complex in eastern North America. Whereas some recognize the two taxa as subspecies, others judge variation to be strictly ecophenic. In the present investigation, starch gel electrophoresis, cytology, and multivariate morphometric analysis were used to elucidate the systematic relationship of these two diploid taxa. Populations of the serpentine and the woodland maidenhair fern are quite divergent genetically. Seven alleles are unique to the serpentine taxon, and genetic identities between populations of the two taxa are low (Ī = 0.495). This value is comparable to those for pairs of congeneric species in other groups of ferns, and particularly low compared to genetic identities for angiosperm species. Patterns of variation in morphological characters of A. pedatum reflect the genetic divergence between the diploid taxa: the taxa are well separated in a discriminant analysis, but no single character is diagnostic. A taxon of tetraploid maidenhair ferns discovered in the study is apparently the polyploid derivative of a hybrid between the serpentine and the woodland maidenhair ferns, as demonstrated by its electrophoretic phenotype. Non-segregation of alleles in gametophyte progeny of the tetraploid indicates that little pairing occurs between the two progenitor genomes. Recent origin of the tetraploid is suggested by several of its attributes: presence of multivalents at meiosis, duplicated gene loci, and restricted geographic range.