Genetic diversity and structure in Pelvetia fastigiata (Phaeophyta: Fucales): does a small effective neighborhood size explain fine-scale genetic structure?

Abstract

We investigated the genetic diversity and genetic structure of southern California populations of the common intertidal fucoid seaweed Pelvetia fastigiata, (J. Ag.) De Toni by means of allozyme electrophoresis and estimates of genetic neighborhood area and size, which are the first for seaweeds. We predicted that P. fastigiata populations would exhibit relatively low genetic diversity and high genetic structure because the seaweed is monoecious and has limited dispersal of gametes and zygotes. This prediction was supported; genetic diversity indices were all low compared to other seaweeds studied, but high genetic structure was evident particularly within individual reefs. Geospatial statistical analyses (second-order analyses) revealed clustered distribution of glucose-6-phosphate isomerase (GPI) alleles at the scale of 1 to 6 m within three reefs. The rare alleles were distributed only at the landward third of the reefs. Genetic neighborhood area (2.3 m2) and size (133 individuals) were estimated from parent-offspring dispersal distributions of gametes and zygotes from attached thalli and also detached reproductive fragments, which contributed very little to the effective neighborhood size. The neighborhood size was in the small theoretical range in which genetic drift could be responsible for the within-reef genetic structure. This result was equivocal, because the stereotyped distribution of rare alleles on the tips of each reef was highly unlikely to be due to random events (6.9×10-24). These results emphasize (1) the importance of allele mapping in addition to spatial statistics to elucidate genetic structure, and (2) that interpretation of genetic-structure statistics as evidence for gene flow can be complicated, even when supported with independent estimates of gene flow, if data are lacking on selection and sporadic migration events. The emerging pattern of low levels of polymorphisms in brown seaweeds will limit the use of Wright's F-statistics and will require alternative, more direct techniques for the analysis of mechanisms responsible for population genetic structure.