Larger pollen sizes correspond to longer styles: correlated evolution between male traits and female preference in plants

In plants, sexual selection is argued to operate through reproduction to shape plant traits. Pollen size and style length are two traits that potentially reflect male-male competition and female choice. Adaptations in one trait may elicit an evolutionary response in the other, leading to correlated evolution at higher taxonomic levels. Many studies have examined their correlation across diverse taxonomic groups, with inconsistent results that limit our understanding of the selective forces and processes affecting these traits. To address this, we conducted a comprehensive meta-analysis on 51 studies from 43 published articles. The results uncovered a significant positive correlation between pollen size and style length across studies. To validate this finding further, we compiled a data set comprising 1041 species from 421 genera in 89 families, with measurements of both pollen size and style length. Phylogenetically controlled analyses confirmed a consistent positive correlation between these traits. Furthermore, our analysis revealed a significant effect of pollination vector on pollen size variation. Specifically, mammal-pollinated species produced substantially larger pollen grains, while wind-pollinated species showed no significant difference in size relative to those pollinated by Diptera, Hymenoptera, or Aves. Pollen size also exhibited a negative correlation with pollen number at macroevolutionary scales. Overall, this study underscores a correlated evolutionary pattern between pollen size and style length across angiosperms, calling for more in-depth investigations on the mechanisms by which sexual selection shapes traits that convey male competitive ability and female preference.