Abstract
Negative frequency-dependent selection should result in equal sex ratios in large populations of dioecious flowering plants, but deviations from equality are commonly reported. A variety of ecological and genetic factors can explain biased sex ratios, although the mechanisms involved are not well understood. Most dioecious species are long-lived and/or clonal complicating efforts to identify stages during the life cycle when biases develop. We investigated the demographic correlates of sex-ratio variation in two chromosome races of Rumex hastatulus, an annual, wind-pollinated colonizer of open habitats from the southern USA. We examined sex ratios in 46 populations and evaluated the hypothesis that the proximity of males in the local mating environment, through its influence on gametophytic selection, is the primary cause of female-biased sex ratios. Female-biased sex ratios characterized most populations of R. hastatulus (mean sex ratio = 0.62), with significant female bias in 89% of populations. Large, high-density populations had the highest proportion of females, whereas smaller, low-density populations had sex ratios closer to equality. Progeny sex ratios were more female biased when males were in closer proximity to females, a result consistent with the gametophytic selection hypothesis. Our results suggest that interactions between demographic and genetic factors are probably the main cause of female-biased sex ratios in R. hastatulus. The annual life cycle of this species may limit the scope for selection against males and may account for the weaker degree of bias in comparison with perennial Rumex species.
Highlights
Sex ratios in equilibrium populations of dioecious organisms are expected to be close to equality (1:1 ratio of females and males) as a result of negative frequencydependent selection (Fisher 1930; Edwards 2000; Hardy 2002)
GP is the pooled G-test, G-heterogeneity test statistics (GH) is G-heterogeneity test, and GT is the Total G-test. df is the degrees of freedom for each test, with the total number of families indicated by the df for GT
There was no significant difference in the mean sex ratio of populations of the Texas and North Carolina races of R. hastatulus
Summary
Sex ratios in equilibrium populations of dioecious organisms are expected to be close to equality (1:1 ratio of females and males) as a result of negative frequencydependent selection (Fisher 1930; Edwards 2000; Hardy 2002). Surveys of the sex ratio in dioecious plants commonly report significant deviations from equality (Delph 1999; Barrett et al 2010; Sinclair et al 2012). The frequent occurrence of biased sex ratios in plant populations raises the question of what ecological and genetic factors might cause deviations from equality and when during the life cycle these become evident. Sex ratios are usually determined from surveys of flowering ramets and the extent to which ramet sex ratios reflect a 2013 The Authors.
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