Stamen and pollen heteromorphism linked to the division of labour in Melastomataceae species

Abstract

The occurrence of stamen heteromorphism within pollen flowers (or heteranthery) has been mostly explained by a division of labour hypothesis, which suggests that some anthers produce pollen for pollination (pollinating anthers) while others are specialized in attracting and rewarding floral visitors (feeding anthers). Despite more than 20.000 species presenting stamen heteromorphism, the morphological and functional traits associated with it have been poorly investigated, especially at the pollen level. Here we evaluated the structural and possible functional roles of stamen and pollen heteromorphism in 16 Melastomataceae species belonging to four tribes. We investigated how a number of traits associated with floral heteromorphisms - anther size, pollen quantity and quality, pollen volume and exine ornamentation - might interact with pollination efficacy and pollinator feeding behaviour. Our results showed morphological differences between anthers and pollen grains that might affect pollinating and feeding functions in the studied species. Pollinating anthers were larger and produced more numerous and more viable pollen grains than feeding anthers. Pollen grain volume between the anther sets was mostly similar in the analysed species, with the exception of three species. Three species of tribe Microlicieae and one from the Marcetieae presented differences in pollen exine ornamentation between pollinating and feeding anthers. Overall, our findings support the division of labour hypothesis for some, but not all, species included in our study. We showed that stamen heteromorphism on the 16 studied species may lead to subtle and specific differences in anthers and pollen morphology which may contribute to their functional role.