Plant-pollinator interactions have important ecological implications as pollination dynamics influence the identity and amount of pollen arriving to stigmas. The composition of deposited pollen loads ultimately affects plant population genetics and fitness. Evidence suggests that pollen load size is an important mediator of offspring fitness, with larger pollen loads (number of grains) leading to greater pollen competition, selecting for higher quality offspring. However, it is unclear how other aspects of pollen load, like the number of pollen donors, affects pollen competition and offspring quality. Therefore, we tested the pollen competition hypothesis, which states that pollen competition will select for higher quality offspring. We specifically evaluated how greater pollen competition from greater donor diversity influences offspring fitness. We performed hand-pollinations on Allium stellatum, depositing pollen mixtures from either one, two, or three donors while controlling for pollen load size. We collected and germinated seeds from hand-pollinations and evaluated how donor diversity influenced seed and seedling characteristics. We found a weak correlation between the number of pollen donors and seed set, which was associated with an overall trade-off between seed number and seed size; hand-pollinations with a greater number of donors produced seeds that were smaller and consequently grew slower. We also found that the flowers that received pollen from a greater number of donors also had a greater number of seeds that germinated. Our results support the pollen competition hypothesis, with pollen competition between different pollen donors benefitting female reproductive success, but with a possible trade-off in progeny vigor. The degree to which pollen competition benefits progeny can potentially be influenced by life history strategies that determine resource allocation to offspring. Our results clearly show donor diversity of pollen loads is an important component of reproductive and offspring fitness. Pollinators, which deposit pollen loads of varying size and diversity, are therefore important selective factors for plant population dynamics. Ultimately, knowledge on how pollinators influence plant population dynamics can guide restoration and conservation actions.
