Floral Nectaries in Melastomataceae and Their Systematic and Evolutionary Implications

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Abstract
The vast majority of the 4,500-5,000 species in the primarily tropical family Melastomataceae do not produce floral nectar, instead relying on pollen as the pollinator reward. To determine the anatomical basis for nectar production in the relatively few nectar-producing taxa, we examined 40 species in 17 genera representing known nectar producers and genera and species not reported to secrete nectar. Our anatomical investigations included six species in three genera of Memecylaceae, a family traditionally placed within Melastomataceae. We also conducted field observations to clarify the site of nectar secretion in several genera. No structural nectaries (derived from differentiated parenchyma) were detected in any of the species examined. Rather, most nectar-secreting species appear to produce the nectar from a thickened staminal vascular bundle. Within the order Myrtales, this type of nonstructural androecial nectary is limited to Melastomataceae and is apparently very rare among angiosperms as a whole. Two additional methods of nectar production in Melastomataceae were revealed: secretion from the petal tips in Medinilla, and, although field confirmation is still required, from the stigma in Miconia. Given the ancestral myrtalean nectary type, it seems clear that structural nectaries were lost in the evolutionary lineage ancestral to Melastomataceae and Memecylaceae. In most nectar-producing Melastomataceae the re-evolution of nectaries appears to be related to a shift in pollinator interactions, specifically from vibratile-pollination bees at lower elevations to vertebrates at higher elevation. We consider the independent development of nectaries in several lineages of Melastomataceae to be the most parsimonious explanation for the diversity of nectary types within teh family and for the scattered phylogenetic positions of nectar-producing taxa.