The unique morphology of resin-producing multilocellate anthers and their evolution in Clusia (Clusiaceae)

Clusia shows high infrageneric and sometimes infraspecific diversity in growth form, floral morphology and pollination biology. Species offering floral resin as a reward to pollinating bees often show unique androecial structures. We investigate the stamen structure of two groups of Clusia with resin flowers and anthers with numerous locelli: all the species of section Polythecandra and Clusia nigrolineata (Clusia section Cordylandra). In section Polythecandra the flowers have stamens with a short, thick filaments and anthers with numerous locelli (microsporangia) without thecal organization. When bees try to remove resin, in most species the whole anther detaches from the rest of the stamen and then dehisces in the detachment zone in areas where the cells have thinner cell walls, a unique mechanism. Stamens of C. nigrolineata also have numerous locelli and a broad and short filament, but pairs of locelli are disposed in two parallel series and the anthers show a clear thecal organization. We discuss the stamen characters in relation to the floral biology of the species and suggest that the athecal multilocellate anthers of section Polythecandra evolved to minimize contact of pollen with the sticky floral resin. The locellate anthers of C. nigrolineata, on the other hand, offer a resin-pollen mixture to flower visitors in the same way as other species of section Cordylandra with normal anthers. In this case, locelli might have evolved to cope up with the enlargement of the anthers compared to other species of the section. The difference in the organization of the anthers of C. nigrolineata and the species of section Polythecandra indicate that multilocellate anthers have evolved independently in these groups.