In seed plants, populations located at the margins of species distributions are often characterized by reduced sexual reproduction and an increased reliance asexual reproduction. The degree to which this pattern applies to reproductive traits in haploid–dominant, non–seed plants was assessed in the Syrrhopodon involutus Schwaeg complex, a group of closely related tropical mosses with unisexual gametophytes. The second internal transcribed spacer (ITS2) of the nuclear ribosomal DNA was sequenced for samples from throughout the range of the Syrrhopodon involutus Schwaeg complex and a maximum likelihood analysis was performed on the sequence data to infer phylogenetic history. Reproductive data were collected from 270 herbarium vouchers representing at least 337 individuals, and analyzed to test for geographic or phylogenetic differences in sex expression and sexual reproduction. Female sex expression was significantly lower among marginally distributed individuals compared to individuals collected from areas central to the distribution of the complex, and the marginal group contained a higher proportion of non–expressing individuals. The proportion of male–expressing gametophytes was not significantly different between marginal and central areas. Despite the lower proportion of female–expressing gametophytes in marginally distributed individuals, the percentage of female–expressing gametophytes undergoing successful fertilization and producing sporophytes was not significantly different in marginal and central distributional areas. These results suggest that the availability of males constrains the production of sporophytes, not only at the margins of species distributions, but in central areas as well, and that geography exerts a disproportionate effect on male and female sex expression.
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Vol. 114 • No. 4