Pollination and evolution in neotropical Annonaceae


Email: gerhard.gottsberger@biologie.uni-ulm.de


The most prominent pollination mode in neotropical Annonaceae is pollination by beetles. Flowers are protogynous and have fruity, spicy or unpleasant odors. The floral chambers, formed by the petals closing over the flower center, emit these specific odors which entice the beetles to enter the flowers. There, the beetles are sheltered from daylight and predators and encounter food (tissues and pollen) as well as opportunities for mating. The amount of food offered, the thickness of the petal tissue and the dimension of the flowers increases with size, number and voraciousness of the attracted beetles. Three main groups of beetle-pollinated Annonaceae can be distinguished. Two of them have relatively small floral chambers, exhibit diurnal and/or nocturnal anthesis and are visited and pollinated by relatively small beetles (Nitidulidae, Curculionidae, Chrysomelidae). Large flowers with a large pollination chamber and very thick petals are associated with nocturnally active, large and voracious dynastid scarab beetles, which are attracted by strong odors promoted principally by thermogenetic processes of the flowers during the time of the beetles’ main activity. The dynastid beetle-pollinated species appear to have the most adapted and most specialized flower characteristics in the Annonaceae. Some small-flowered Annonaceae are pollinated by thrips and, for a few species, pollination by flies is suspected. Some genera, such as Guatteria, are uniform with respect to flower biology and seem to have adapted principally to nitidulids as pollinators. In contrast, the genus Annona, basically a group pollinated by dynastid beetles, is diversified with respect to flower morphology and pollination. The neotropical Annonaceae as a whole may have started as a group pollinated by sucking and pollen-eating Thysanoptera and non-destructive beetle groups. Species pollinated by such Thysanoptera and/or rove beetles (Staphylinidae) still show the laminar, plesiomorphic stamen type of Annonaceae with a tongue-shaped connective prolongation. The disc-like sclerified connective shield of the majority of Annonaceae is apparently a secondary and modified structure, especially prominent in the beetle-pollinated species. The densely aggregated stamens with their connective shield appear to be a kind of antipredator structure in Annonaceae adapted to pollination by beetles.