Systematics of North American Petasites (Asteraceae: Senecioneae). II. Isozyme analysis and population genetic structure

Abstract

Petasites Mill. (Asteraceae: Senecioneae) is a genus of diploid, perennial, clonal herbs with a widespread distribution across North America. Because of the low variability in floral morphology and high diversity in leaf morphology, considerable taxonomic confusion surrounds the genus. Petasites is generally recognized in North America by five ill-defined native taxa and two introduced species. Enzyme electrophoresis was conducted to assess the genetic variation in Petasites and obtain an understanding of the evolutionary relationships within the genus. Results show divergence at isozyme loci is not associated with morphological divergence. The mean number of alleles per locus and proportion of polymorphic loci are comparable with plants with similar life-history traits; however, levels of heterozygosity ranged substantially. Although there is a weak, negative association between geographical distance and genetic identity, this is not concordant with morphological differences. Typical of clonal species, genetic identities of Petasites are more comparable with selfing plants than with other diploid, outcrossing perennials. Petasites also maintains a level of genetic diversity that is similar to other clonal plants and exhibits high levels of differentiation among its populations. This study provides the first contribution to the population genetic structure of Petasites. The relatively high values of genetic identities between the different taxa of Petasites and the similarity in isozymes and chromosome number indicates a rapid and recent origin in North America. These data, in accordance with the close morphological associations of the four taxa in Petasites, suggest the recognition of one species only, Petasites frigidus.Key words: Petasites, Asteraceae, North America, clonal, systematics, genetic variation.