HETEROSPECIFIC POLLEN TRANSFER, SELF-ASSURANCE, AND THE DISTRIBUTION OF DIOECY AND HERMAPHRODITISM IN A SINGLE SPECIES: THE IMPORTANCE OF POLLEN LANDSCAPES

Premise of research. The rarity of dioecy in plants remains a challenging issue in evolutionary biology. Two risks are likely exacerbated with dioecy: the difficulties of finding a pollen donor or recipient and heterospecific pollen transfer. These risks should be worsened in unisexual plants, relative to hermaphroditic plants, by the higher demands expected of pollinator services to compensate for their sterility. Any fitness advantage of dioecy should be conditioned to low-pollination-risk situations. Then we would expect a nonrandom distribution of dioecy and autonomously selfing hermaphrodites of sexually polymorphic species in landscapes with contrasting conspecific densities and frequencies of other pollinator-competing, coflowering species. Nevertheless, sexually polymorphic species’ dwelling habitats with contrasting pollination risks are uncommon. Methodology. Using categorical data modeling, we studied associations between unisexuality and pollen risks in the rare sexually polymorphic Opuntia robusta, with male, female, and hermaphrodite individuals distributed in nine populations and with densities varying from 0.2 to 206.3 individuals per hectare. The frequencies of the nearest neighboring coflowering conspecific plants ranged from ~1% to ~90%. Pivotal results. Dioecy prevails in landscapes with abundant conspecific pollen donors or recipients, where, on the basis of previous results, sex allocation can be optimized. Hermaphroditism prevails in landscapes where conspecific pollen is expected to be low or there is abundant heterospecific pollen; in such situations selfing might assure seed set and reduce reproductive interference. Conclusions. Pollen landscapes help contextualize the reproductive environment that conditions the relative success of dioecy and hermaphroditism in a single species. Pollen landscapes can be estimated by the abundance of conspecific sexual partners and the frequency of heterospecific, coflowering, pollinator-competing nearest neighbors. This species-specific landscape property likely determines the risks of outcrossing pollen failure. Hermaphroditism is conceivably both a self-assurance mechanism and a barrier against heterospecific pollen transfer. Dioecy might outperform hermaphroditism, even if inbreeding depression is undetectable, but it is presumably conditioned in low-pollination-risk landscapes.