Increasing pollen production at high latitudes across animal-pollinated flowering plants

Abstract

Aim: Plant reliance on animal mutualists is expected to decrease with latitude owing to increasing environmental instability. As a consequence, more erratic animal pollination in the temperate zones than in the tropics could translate into lower efficiency in pollen transfer, and thus increasing pollen wastage. Despite the relevance of this hypothesis for plant reproductive evolution, the implications of a proposed latitudinal gradient in pollinator reliability for pollen and ovule production, traits directly affecting seed siring and seed set, respectively, remain unresolved. Location: Global. Time period: 1971–2020. Major taxa studied: Angiospermae. Methods: Based on a bibliographic survey and our own data, we collated a dataset with information on pollen production (P) and ovule number (O) per flower from 419 studies, including a total of 1,392 animal-pollinated angiosperm species from 141 families distributed worldwide and sampled between the equator and 68.35° latitude. Using phylogenetic general linear mixed models, we investigated latitudinal variation in P and O. We also tested a latitudinal effect on several ancillary plant traits associated with a plant’s mating system and pollinator specialization that might confound any latitudinal effect on gamete production. Results: P but not O was positively associated with latitude, a trend that even became stronger after including the latitudinally varying ancillary traits (i.e., growth form, flower size, flower symmetry and number of pollinator orders). The latitudinal effect on P was more pronounced among species producing large flowers, a trait that could be linked to self-incompatibility and thus outcrossing. A weak phylogenetic patterning of P also indicates high evolvability potential of this trait, which may have an effect on both male and female plant fitness. Main conclusions: Even though a latitudinal trend in pollinator reliability still awaits direct testing, the observed latitudinal increase in P across angiosperms can be interpreted as an evolutionary consequence of an increasingly unpredictable pollination environment.