A Survey of Reproductive Biology in Neotropical Melastomataceae and Memecylaceae

Abstract

In the Neotropics, the Melastomataceae and Memecylaceae comprise over 3,000 species in 106 genera. Pollination observations have been reported for 126 species in 35 genera of Melastomataceae and for four species in one genus of Memecylaceae. Genetic self-incompatibility has been found in 22 Melastomataceae species, self-compatibility in 25 species. A single Memecylaceae species tested is self-compatible. Agamospermy is known in 19 New World and some Old World species of Melastomataceae. The incidence of polyploidy in Melastomataceae seems high, but only about 8% of the species have been counted. Most Melastomataceae and Memecylaceae are hermaphroditic; dioecism is known in a few species in one genus of Melastomataceae. The principal mode of promoting outcrossing in both families is spatial separation of pollen and stigma (herkogamy), achieved by the pollen being enclosed in poricidal anthers, which have to be manipulated to release pollen. New World Memecylaceae are pollinated in the same way and by some of the same bee species as most Melastomataceae. All bee species known to visit Melastomataceae and Memecylaceae are listed; they comprise a wide spectrum of the neotropical bee fauna. It is not yet clear whether the stamen dimorphism and the conspicuous connective appendages present in many Melastomataceae have a function in the pollination mechanism beyond that of enhancing the visual attractiveness of the flowers and making the stamens easier to grasp. In most Melastomataceae, flowers offer only pollen; however, some 60 species in 8 genera offer nectar as a reward for pollinators (probably, some species in at least on additional genus do, too). The nectariferous species are pollinated by birds, bats, rodents, and bees. The nectar is rich in sucrose and, as a rule, its production is correlated with floral morphological changes relating to the fact that, except for bees, nectar consumers do not vibrate the stamens to collect pollen actively. I suggest that the capacity for developing nectaries is basic in Melastomataceae but suppressed in most modern members. The staminal glands of Memecylaceae produce minute quantities of a secretion containing lipids which, however, is not collected by the pollinators; the glands may in some way enhance stamen attractiveness. In seed dispersal systems, major diversifications occurred, with 40% of the neotropical Melastomataceae having capsular fruits and wind-dispersed seeds and 60% having soft, juicy berries taken mainly by birds, but also by marsupials, monkeys, bats, other mammals, turtles, and other reptiles. All Memecylaceae have berries, and their seeds are dispersed by birds, monkeys, and fish. I conclude that the east and west Gondwanian lineages of Melastomataceae and Memecylaceae, in parallel, coevolved with the earliest bees in the lower mid-Cretaceous. This may have been the time when the ancestors of these families switched from nectar-reward flowers to pollen-reward flowers. In the further extensive radiation of both families, special adaptations to pollinators were of limited importance; diversification in seed dispersal systems, vegetative characters, and edaphic adaptations were more important.