We used spatial and genotypic data of parent and offspring cohorts to jointly estimate individual male and female effective fecundity on the one hand and pollen and seed dispersal kernels on the other hand. In particular, we tested for the effect of density on male and female effective fecundity. We here investigated sex-specific differences in selection of sexually dimorphic traits and in the spatial distribution of effective fecundity (our fitness proxy) in a highly dimorphic dioecious wind-pollinated shrub, Leucadendron rubrum. Sexual dimorphism in plants may emerge as a result of sex-specific selection on traits enhancing access to nutritive resources and/or to sexual partners. This study exposes a hitherto unidentified association between sexual dimorphism and maternal care in plants and points to a possible additional reproductive burden carried by serotinous plants compared with those that release their seeds at maturity.
We found that, in Leucadendron, the evolution of increased serotiny corresponds with the evolution of more marked sexual dimorphism. Serotiny is a strategy of maternal care that contributes to offspring survival after seed maturation. This association is consistent with the idea that the costs of maintaining cones may select females to be less highly branched than males.ĥ. There was a strong positive association between the degree of serotiny and sexual dimorphism. We tested the effect of serotiny and several other factors on the degree of sexual dimorphism in Leucadendron by conducting a comparative analysis of variation across 49 species using phylogenetic regression.Ĥ. Given that branching architecture is known to affect water conductance, with less resistance in less highly branched plants, we predicted a positive correlation between the degree of serotiny in a species and sexual dimorphism in branching.ģ. Since cones must be kept alive, serotinous females presumably incur higher costs in terms of water loss than non-serotinous females and, particularly, males. Many Leucadendron species are serotinous, with females of serotinous species maintaining seeds in transpiring cones for a number of years. Species of the dioecious South African plant genus Leucadendron vary strikingly in their degree of sexual dimorphism, with males of dimorphic species typically possessing more and thinner branches and smaller leaves than the corresponding females.Ģ.
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