Direct measures of sexual selection in plants are rare and complicated by modular growth. Because of modularity, instantaneous measures of fitness scale with size, but size variation in plants is largely non-heritable, obscuring patterns of selection on heritable variation. We measured the magnitude of sexual selection in a monoecious and a dioecious population of the clonal plant Sagittaria latifolia using Bateman gradients (ß_ss). These gradients were calculated using parentage analysis and residual regression to account for the effects of shoot and clone size on mating and reproductive success. In both populations: (i) mating via male function was associated with greater promiscuity; and (ii) ß_ss were positive, with significant associations between mating and reproductive success for male but not female function. Moreover, estimated ß_ss were similar for the monoecious and dioecious populations, possibly because non-overlapping female and male sex phases in hermaphroditic S. latifolia reduces the scope for interference between sex functions during mating. This study builds on previous studies of selection on plant mating traits, and of sexual selection under experimental conditions by showing that sexual selection can operate in natural populations of plants, including populations of hermaphrodites.

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