The effects of sex allocation coevolution on the coexistence of two closely related plant species interacting via interspecific pollen transfer

Interspecific pollen transfer (IPT), the pollen movement between plant
species via shared pollinators, reduces the reproductive success of
pollen-recipient plants due to hybridization with heterospecific pollen
grains. As a result, IPT hinders coexistence of sympatric, co-flowering
species by reducing their reproductive success. IPT likely exerts
selective pressure on plant reproductive systems, particularly on sex
allocation (resource investment to pollen versus ovules). However, the
influence of IPT on the evolutionary dynamics of sex allocation and
associated ecological processes remains poorly understood. Here, we
explore how the female costs incurred by IPT affect the co-evolution and
coexistence dynamics of two plant species. To address this, we construct a
mathematical model capturing interactions between two plant species
through pollen transfer and resource competition, and reduced
fertilization caused by IPT. Our analysis focuses on a scenario where an
invasive species with female-biased sex allocation enters a habitat
occupied by a resident species with evolutionarily stable, equal sex
allocation (Fisherian sex allocation). Using adaptive dynamics theory, we
demonstrated that, irrespective of IPT strength, natural selection drives
both species toward equal sex allocation, consistent with the Fisherian
sex allocation theory. We present two key predictions of eco-evolutionary
outcomes. First, when the impacts of IPT are comparable for both species,
the resulting eco-evolutionary dynamics lead to their stable coexistence.
In contrast, if IPT from the invasive species is disproportionately
stronger than IPT from the resident species, the invasive species’
evolutionary shift from female-biased to equal sex allocation drives the
resident species to extinction. Since local mate competition in small
populations can favour female-biased sex allocation, our results suggest
that invasive species experiencing reduced local mate competition may
drive resident species to extinction. Our findings underscore the
intricate role of IPT-driven sex allocation evolution in shaping
coexistence of closely related plant species.