Differential gender selection on flower size in two Neotropical savanna congeneric species

Abstract

Larger flowers greatly increase among-individual pollen exchange within populations. However, water costs associated to transpirational cooling also increase with increasing flower size. Overall, the interplay between pollen and resource limitation determines the intensity of selection on flower size and this process is mostly dependent on gender and ecological context. To examine how pollinators and water use affect flower size, we determined corolla transpiration, pollen limitation, and selection through male and female fitness in two Kielmeyera species from the Brazilian cerrado flowering at different seasons. Hand-pollination experiments suggested pollen limitation through female fitness in both species, but K. coriacea showed lower limitation levels than K. regalis. For male fitness, the percentage of pollen removal was 1.5-times higher in K. coriacea. Higher air temperature and water deficit during flowering season of K. coriacea resulted in 4-fold higher corolla transpiration rates compared to K. regalis. Selection on flower size through male function was positive and significantly higher than selection through female components in both species. We also detected stabilizing selection in K. coriacea and positive selection in K. regalis on flower size through seed number. Our results suggest that selection on flower size in K. coriacea was mainly limited by water, whereas in K. regalis it was more limited by pollen. We demonstrate that differences in pollen and abiotic resource limitation determine gender-specific selection on flower size.