Understanding how pollen moves between species is critical to understanding speciation, diversification, and evolution of flowering plants. For co-flowering species that share pollinators, competition through interspecific pollen transfer (IPT) can profoundly impact floral evolution, decreasing female fitness via heterospecific pollen deposition on stigmas and male fitness via pollen misplacement during visits to heterospecific flowers. The pollination literature demonstrates that such reproductive interference frequently selects for reproductive character displacement in floral traits linked to pollinator attraction, pollen placement, and mating systems and has also revealed that IPT between given pairs of species is typically asymmetric. More recent work is starting to elucidate its importance to the speciation process, clarifying the link between IPT and current and historical patterns of hybridization, the evolution of phenotypic novelty through adaptive introgression, and the rise of reproductive isolation. Our review aims to stimulate further research on IPT as a ubiquitous mechanism that plays a central role in angiosperm diversification.

Premise of research. Heterospecific pollen deposition on stigmas can impact plant reproduction by decreasing seed set or inducing fruit abortion. Pollinating bats often carry pollen from many species on their fur, and thus bat-pollinated flowers may exhibit tolerance to heterospecific pollen deposition, but to our knowledge this has never been studied. We investigated the relative impact of increasing heterospecific pollen deposition on female reproduction of two sympatric species of Burmeistera that experience pollen transfer between them by their shared bat pollinators.Methodology. We simulated heterospecific pollen deposition by applying pollen mixtures that differed in the ratio of heterospecific to conspecific flowers used to make them (3∶1; 2∶2, and 1∶3) and measured female reproduction by quantifying fruit abortion, seed number, and seed mass.Pivotal results. For Burmeistera borjensis, applying a higher relative amount of heterospecific pollen led to decreased seed production and lighter seed mass, whereas no significant effect of increasing heterospecific pollen was detected in Burmeistera ceratocarpa. Burmeistera borjensis aborted more fruits than B. ceratocarpa (44.1% vs. 18.8%); however, in both species the probability of fruit abortion was not affected by pollination treatment.Conclusions. We detected asymmetric impacts on female reproduction between this pair of sympatric Burmeistera: while increasing heterospecific pollen deposition decreased seed production and seed mass for B. borjensis, it had no detectable effect on B. ceratocarpa. Prior work shows that in nature B. borjensis receives much lower amounts of heterospecific pollen than B. ceratocarpa, and thus results suggest that B. ceratocarpa may have stronger gametic or postzygotic isolating barriers to be able to tolerate heterospecific pollen deposition. Future work should explore how different isolation mechanisms contribute to the asymmetric effects on female reproduction that we report here and whether tolerance to heterospecific pollen deposition is a common response among bat-pollinated plants and other plant taxa relying on low-fidelity pollinators.

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