Evolution of the occurrence of phytomelanin in aerial stems of American Vernonieae subtribes (Asteraceae)

Phytomelanin is a dark pigment frequently reported in fruits and/or seeds, but recently described in aerial stems of species of the tribe Vernonieae, in subtribe Lychnophorinae. We hereby report the presence of this pigment in aerial stems of additional species belonging to this tribe and investigate the evolution of its occurrence in these lineages. Samples from 42 species belonging to the main lineages of American Vernonieae subtribes, and two species belonging to an outgroup, were analyzed with traditional plant anatomy techniques. The ancestral states of phytomelanin occurrence were reconstructed with maximum likelihood methods, using a recent phylogenetic hypothesis of American Vernonieae based on molecular data. Of the species listed for the present study, the secretion of phytomelanin, when present, was observed in the intercellular spaces of the sclereids. The pigment is more frequently found in the pith, secondary phloem, and cortex. The ancestral state reconstruction suggests that the most recent common ancestor of tribe Vernonieae had phytomelanin in those three regions. The phylogenetic signal for presence of phytomelanin in these regions may indicate this character was preserved throughout the evolutionary history of the tribe. Thus, the ability to produce phytomelanin would have been conserved in the more nested lineages of the American Vernonieae subtribes, although it could also reflect distinct ecological adaptations across lineages.