Sexual dimorphism in specialist pollinated dioecious species with complex flowers

Premise
Pollinators with flower constancy and long nectar-feeding organs should favour less or no sexual dimorphism in the individual flowers of dioecious plants. This hypothesis is deduced because such pollinators can discriminate between intersexual flower size differences, and morphological differences between male and female flowers often diminish pollen transfer.

Methods
We compared floral traits and pollinator behaviour between sexes in the hawkmoth-pollinated species, Trichosanthes cucumeroides. We focused on the roles of elaborate petal fringes in pollinator attraction and pollination success of female and male flowers by conducting field pollinator observations and fringe removal experiments.

Results
Female flowers had a similar front flower size and fringe extension as male flowers, supporting our hypothesis. In contrast, females allocated fewer resources to floral biomass. Additionally, they had smaller and narrower petal lobes, lower fringe density, shorter tubes with inferior nectar rewards, and lower display size than males, which is inconsistent with the hypothesis. Nocturnal hawkmoths prefer flowers with long fringe extensions. Fringe removal significantly decreased hawkmoth visitations to both female and male flowers but reduced success only in females. A literature survey indicated that specialist-pollinated species tend to exhibit similar or larger female attraction to floral organs more frequently than generalist-pollinated species.

Conclusions
Female flowers have evolved similar fringe extensions to male flowers, likely increasing pollinator attraction even slightly, and exhibited lower biomass of other floral parts and nectar production than male flowers. Our findings imply that female-biased resource limitation and flower-size-sensitive pollinators together exert sex-specific selection of floral traits in T. cucumeroides.

Methods
Please see detail in paper "Sexual dimorphism in specialist pollinated dioecious species with complex flowers" in American Journal of Botany.