Does pollen form follow function? Effects of stigma type, pollination mode and habitat on pollen morphological traits

Abstract

Pollen grains display striking morphological diversity, yet the ecological drivers underlying this variation remain poorly understood. We tested whether pollen morphology reflects adaptation to pollination mode, habitat moisture and stigma type, three ecological factors that impose selective pressures on pollen dispersal, desiccation tolerance and hydration dynamics. We analysed 13 morphological traits for 1151 Central European angiosperm species using phylogenetic regression models. Traits included pollen size and dispersal units, number and type of apertures, coatings and both external and internal features of the exine. Insect-pollinated species produced larger, coated grains with thicker exine layers and more reticulate ornamentation, while wind-pollinated species had smaller, smoother grains with thinner walls and more frequent Ubisch bodies. Species from wetter habitats were more likely to disperse pollen in clumps and showed distinct ornamentation patterns, but several traits hypothesized to reflect harmomegathic responses (e.g. aperture number, furrows, opercula) showed no moisture dependence. Stigma type also influenced pollen form: dry-stigma species had more coatings, thicker exine and more frequent opercula, while wet-stigma species had larger, thinner-walled grains with non-perforate ornamentation. Synthesis. Our results support the idea that pollen morphology is functionally adaptive and shaped by distinct phases of pollen performance: dispersal, environmental exposure and stigma interaction. These findings highlight the potential of trait-based approaches for understanding reproductive strategies in angiosperms.