Evolution of the outer ovule integument and its systematic significance in Melastomataceae

Abstract

Ovule characters have been shown to be informative at higher taxonomic levels and are therefore of interest in angiosperm systematics. In this study, we aimed to describe the evolution of the outer ovule integument thickness in Melastomataceae and to evaluate its systematic and biological significance. This character was investigated in 137 species, and data from the literature were compiled for 22 additional species, totalling 159 taxa. Integument origin was studied in selected species to clarify whether different conditions were associated with different ontogenetic origins. The most recent common ancestor of Melastomataceae probably had an ovule with a two-cell-layered outer integument. A multilayered outer integument evolved independently at least six times in the family. Moreover, reversals to the two-cell-layered plesiomorphic condition have also occurred more than once. The two-cell-layered and multilayered outer integuments have different ontogenetic origins, enhancing the taxonomic usefulness of this character. The thickness of the outer ovule integument is of systematic significance in Olisbeoideae, Rupestrea, Melastomateae and Miconieae. The multilayered outer integument may have some adaptive significance for Miconieae and Mouriri, groups with endozoochoric fleshy fruits. Evidence of a multiplicative testa was observed in a few species with both types of integuments and it may be associated with a massive seed coat found in large seeds.