Function and evolution of sterile sex organs in cryptically dioecious Petasites tricholobus (Asteraceae)

Sexual dimorphism in a dioecious species with complex, specialist-pollinated flowers

PREMISE

Pollinators with flower constancy and long nectar-feeding organs should favor 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 behavior between male and female flowers in the hawkmoth-pollinated species, Trichosanthes cucumeroides. In field studies, we removed petal fringes on both sexes and observed pollinators to assess the role of elaborate petal fringes in pollinator attraction and pollination success for each flower sex.

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 female flowers of specialist-pollinated species are similar in size or larger than the males and thus tend to attract more pollinators compared with female flowers of generalist-pollinated species.

CONCLUSIONS

Female flowers have evolved fringe extensions that are similar to those of male flowers, likely increasing pollinator attraction even slightly, and had less biomass in other floral parts and produced less nectar compared with male flowers. Our findings imply that female-biased resource limitation and flower-size sensitivity of pollinators together exert sex-specific selection of floral traits in T. cucumeroides.

Observations on flower and fruit anatomy in dioecious species of Cordia (Cordiaceae, Boraginales) with evolutionary interpretations

Detailed anatomical studies of flowers and fruits may render traits of phylogenetic importance, but are still rare in the Boraginales. An Old World clade of Cordia comprises many dioecious species, but the floral anatomy is largely unexplored and, hence, the structural basis for sex separation. Fresh material of buds, functionally unisexual flowers and fruits were studied using a stack shot camera. Fixed such material of Cordia crenata, C. cf. grandicalyx and C. sinensis was sectioned using a microtome and stained with safranin red and astra blue, and the gynoecial surface was studied using SEM. The stamens of the female flowers are largely intact, but do not produce pollen, whereas the gynoecium of the male flowers is highly reduced though develops ovules lacking an embryo sac. The morphologically rather unreduced stamens of the female flower may have undergone a shift of function from producing pollen towards attracting pollinators by optical signals. The outer parenchyma of the gynoecium is strongly stained and after fertilisation, one through three ovules are maturing towards seeds. The presence of more than a single seed enclosed in the undivided endocarp is rare in Cordiaceae.