Genetic structure of a widely dispersed beach annual, Cakile edentula (Brassicaceae)

Genetic structure of coastal plant species is influenced by the homogenizing effects of long-distance seed dispersal and the isolating effects of fragmented geographic distribution. Although dispersed coastal annuals play a significant role in establishing healthy dune communities, relatively little is known about their population genetic structure.

We investigated the population genetic structure of Cakile edentula, an annual colonizer of beaches of coastal North America and the Great Lakes. We surveyed 22 populations for 24 allozyme loci across a geographic range encompassing three infraspecific taxa: two subspecies (C. edentula subsp. edentula and subsp. harperi) and two varieties of subsp. edentula (var. edentula and var. lacustris).

Genetic diversity in C. edentula (H(es) = 0.118) is equivalent to mean estimates for other annual mixed-mating species, but is much lower (H(ep)=0.041) within its populations. UPGMA identified three groups that are consistent with previously described subspecies and possibly consistent with the two varieties. An AMOVA indicates that variation among taxonomic groups explains 54% of total genetic diversity within the species. Although pairwise F(ST) estimates indicate moderate rates of gene flow between populations within groups, there was no evidence of significant isolation by distance.

Genetic structure of C. edentula is strongly related to previously described phenotypically and geographically based infraspecific taxa. Within taxa, the effects of long-distance dispersal of its buoyant propagules are balanced by the isolating effects of fragmented geographic distribution and a mixed-mating system.