Foraging preferences of bees and birds – assessing the adaptive value of heteranthery in flowers of Merianieae (Melastomataceae)

Heteranthery describes flowers with two or more types of stamens that differ in colour, size and/or shape and it is particularly common in bee-pollinated plants. The evolution of heteranthery has been explained by a ‘division-of-labour’ hypothesis driven by the dual function of pollen as pollinator reward and as male gametes. Hence, the more conspicuous stamen type may primarily function in rewarding pollinators and the other, more cryptic stamen type functions in efficient pollen transfer. Recently, gradual pollen presentation as a pollen-dosing strategy has been proposed as an alternative explanation for heteranthery.
In Merianieae (Melastomataceae), bee pollination is ancestral and most widespread, but repeated independent shifts to vertebrate pollination (passerine birds, hummingbirds, bats, rodents) have occurred. With these pollinator shifts, flowers have evolved alternative rewards (food bodies, nectar) and pollen has lost its rewarding function. If ‘division-of-labour’ is the source of heteranthery in Merianieae, pollinator shifts may lead to a reduction of heteranthery. Alternatively, if the need of gradual pollen presentation drives the evolution of heteranthery, it may also be found in species with vertebrate pollinators. Finally, the functionality of heteranthery may greatly depend on pollinator behaviour during the flower visit.
Using comparative phylogenetic methods and field experiments, we show that heteranthery has evolved independently multiple times in Merianieae and persists in some vertebrate pollinated species. Indeed, foraging passerine birds discriminate between stamen whorls and prefer showier stamens. In nectar-rewarding species, however, pollinators do not interact with the stamens directly and heteranthery was lost as it is no longer functional.