Phylogenetic Placement of the Monotypic Genus Schwackaea (Melastomeae: Melastomataceae) and the Evolution of Its Unique Fruit

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Premise of research. The phylogenetic placement of the nearly Central American endemic Schwackaea cupheoides has not been investigated, although this monotypic genus had been hypothesized to belong in the tribe Melastomeae based on morphology. Its distinctive and unusual eight-lobed fruits have been recently observed to break in between the lobes. This study was undertaken to test the phylogenetic placement of S. cupheoides as well as to describe the anatomy of its complex fruits, establish the pattern of dehiscence, and compare the fruits to those of its close relatives.Methodology. Phylogenetic analyses of DNA sequences from the chloroplast spacers accD-PsaI and psbK-psbL, including all species of Melastomataceae available in GenBank, were conducted. Comparative anatomical and morphological studies of fruits of S. cupheoides as well as representatives of closely related genera were carried out.Pivotal results. The phylogenetic placement of Schwackaea in the tribe Melastomeae is corroborated here with analyses of DNA sequences. Strong support was found for a sister relationship between Schwackaea and the Central American endemic genus Pilocosta. This clade was, in turn, sister to another Central American endemic, Centradenia. Anatomical and morphological investigation revealed that all of these genera possess unique fruits with respect to each other but that those of Pilocosta and Schwackaea are most similar in the presence of angles or lobes, and both have continuously distributed druse crystals in their fruit. The most unusual fruit is that of S. cupheoides, which shows a specialization for fruit dehiscence in which druse crystals have been localized in between the hypanthium lobes to aid in dehiscence.Conclusions. Phylogenetic evidence supports a clade of Central American genera that includes Centradenia, Pilocosta, and Schwackaea, all of which have distinctive fruits. It is hypothesized that fruit dehiscence in Schwackaea represents a case of exaptation where druse crystals have been co-opted from an antiherbivory role to a fruit dehiscence function.