Clonal genetic structure and diversity in populations of an aquatic plant with combined versus separate sexes

Clonality is often implicated in models of the evolution of dioecy, but few studies have explicitly compared clonal structure between plant sexual systems, or between the sexes in dioecious populations. Here, we exploit the occurrence of monoecy and dioecy in clonal Sagittaria latifola (Alismataceae) to evaluate two main hypotheses: (1) clone sizes are smaller in monoecious than dioecious populations, because of constraints imposed on clone size by costs associated with geitonogamy; (2) in dioecious populations, male clones are larger and flower more often than female clones because of sex-differential reproductive costs. Differences in clone size and flowering could result in discordance between ramet- and genet-based sex ratios. We used spatially explicit sampling to address these hypotheses in 10 monoecious and 11 dioecious populations of S. latifolia at the northern range limit in eastern N. America. In contrast to our predictions, monoecious clones were significantly larger than di...