Genetic variation in Trillium erectum (Melanthiaceae), a widespread forest herb in eastern North America

Trillium erectum L. is an insect-pollinated understory herb widespread in forests of eastern North America. Marker gene studies indicate that the species has a mixed mating system, but aspects of population genetic structure have not been investigated. Using 10 allozyme loci, we measured genetic variation within and among 23 populations sampled from throughout the species' range. Overall, T. erectum displayed moderate levels of genetic diversity in comparison with other herbaceous plants. The percentage of loci that were polymorphic was 52%, with average values (+/-SE) of 1.20 +/- 0.02, 0.08 +/- 0.01, and 0.13 +/- 0.01 for the number of alleles per locus (A), observed heterozygosity (H-o), and expected heterozygosity (H-e), respectively. There was evidence of inbreeding within populations (F-is = 0.39, 95% CI 0.26-0.55) and significant population differentiation (F-st = 0.16, 0.05-0.24). Analysis of genetic data provided no evidence of isolation by distance, and together with the occurrence of population subdivision, this suggests that there is relatively limited contemporary gene flow among populations. Northern populations of T. erectum tended to have less genetic variability than southern populations, probably as a result of historical factors associated with post glacial migration. Limited opportunities for gene dispersal as a result of low plant densities, the capacity for self-fertilization, and local seed dispersal by ants are likely to be the main factors maintaining contemporary patterns of genetic variation in T. erectum.