Population Genetic Structure in Temperate and Tropical Species of Viola (Violaceae) with a Mixed Breeding System

Inter–simple sequence repeat (ISSR) molecular markers were used to characterize inter- and intrapopulation genetic diversity in populations of two contrasting species of Viola, Viola striata of the eastern United States and Canada and Viola grahamii of central Mexico. Both species show a mixed breeding system with the production of both chasmogamous and cleistogamous flowers, although the temporal pattern of flowering varies between the two species. In both species the majority of the seeds produced in a given season are the result of cleistogamous flowers; additionally, V. grahamii reproduces asexually by means of runners. Our results indicate similar high levels of genetic diversity, low levels of population subdivision, and weak interpopulation differentiation in both species. In particular V. grahamii showed a slightly higher level of genetic diversity than V. striata, which does not reproduce vegetatively. Modest chasmogamous reproduction is proposed to maintain levels of outcrossing sufficient to ameliorate population subdivision and retard interpopulational divergence that would otherwise result from cleistogamy. These findings reiterate a recurring pattern in Viola in which high population genetic diversity is maintained even with the presence of high levels of cleistogamy and vegetative reproduction.