GENETIC CONSEQUENCES OF SEED DISPERSAL IN 3 SYMPATRIC FOREST HERBS .1. HIERARCHICAL POPULATION-GENETIC STRUCTURE

To examine the effects of seed dispersal on spatial genetic structure, we compare three sympatric species of forest herbs in the family Apia ceae whose fruits differ widely in morphological adaptations for anima l-attached dispersal. Cryptotaenia canadensis has smooth fruits that a re gravity dispersed, whereas Osmorhiza claytonii and Sanicula odorata fruits have appendages that facilitate their attachment to animals. T he relative seed-dispersal ability among species, measured as their ab ility to remain attached to mammal fur, is ranked Sanicula, Osmorhiza > Cryptotaenia. We use a nested hierarchical sampling design to analyz e genetic structure at spatial scales ranging from a few meters to hun dreds of kilometers. Genetic differentiation among population subdivis ions, estimated by average genetic distance and hierarchical F-statist ics, has an inverse relationship with dispersal ability such that Cryp totaenia > Osmorhiza > Sanicula. In each species, genetic differentiat ion increases with distance among population subdivisions. Stochastic variation in gene flow, arising from seed dispersal by attachment to a nimals, may partly explain the weak relationship between pairwise spat ial and genetic distance among populations and heterogeneity in estima tes of single locus F-statistics. A hierarchical island model of gene how is invoked to describe the effects of seed dispersal on population genetic structure. Seed dispersal is the predominant factor affecting variation in gene flow among these ecologically similar, taxonomicall y related species.