Structure and evolution of polysporangiate anthers in Melastomataceae

Abstract

Polysporangiate anthers, i.e., anthers bearing more than four sporangia, are an unusual condition that has been reported in several angiosperm families, including Melastomataceae. The structure of anthers in Melastomataceae and taxa from its sister lineage was investigated in 302 species, which were categorized as polysporangiate or non-polysporangiate based on original and literature data. Additionally, the evolutionary history of the anther type in the family was assessed by stochastic character mapping. The polysporangiate anthers of Melastomataceae exhibit two corrugated thecae with transverse septa of parenchymatous tissue partitioning the four pollen sacs into several small locules, this being the most common type of sporangia multiplication among angiosperms. Despite the fact that non-polysporangiate anthers are the most common character-state observed and have been reconstructed in most ancestral nodes of the phylogeny, a relatively large number of transitions to polysporangiate anthers were estimated in our analyses. One event was detected in Meriania, four events were detected in Miconia, and two (with a reversal) or three independent transitions in Microlicia. Most species with polysporangiate anthers occur in Microlicia, representing about 41 % of the species in the genus. Even though tetrasporangiate anthers are the most common condition among angiosperms, dithecal polysporangiate anthers occur in at least 74 taxa of Melastomataceae, in a unique association with poricidal dehiscence.