Ecomorphology and history of a recurrent evolutionary adaptation to long-distance bird dispersal in Asteraceae

Background and aims: Genera of Asteraceae, belonging to different tribes, present a repeated pattern of morphological adaptations to epiornithochory and have unique distributions (widely disjunct and/or inhabit remote islands). This pattern requires the interplay between plant morphology and environmental aspects (ecomorphology), and between morphology and the evolutionary history (homology, homoplasy, and phylogeny). Adenostemma and Adenocaulon are investigated to: 1) establish some aspects of morphological and histochemical similarity in their diaspores, and associate that similarity to long-distance dispersal by birds, 2) superimpose these similarities in the phylogenetic tree of Asteraceae to determine whether they are homologous or homoplastic. M&M: Previous work on Adenostemma and Adenocaulon is reviewed for analyzing their diaspores morphology and histochemistry, geographic distribution, and mode of dispersion. One of the current molecular-based phylogenetic trees of Asteraceae is employed as a template to perform evolutionary inferences. The criteria for distinguishing homology from homoplasy, and parallelism from convergence are applied. Results: Adenostemma and Adenocaulon are located in the recently radiated crown (tribe Eupatorieae) and in one of the earliest diverging branches (tribe Mutisieae) of the Asteraceae tree, respectively. The set of features shared by both genera in relation to epiornithochory is homoplastic: two parallelisms, one reversion and one functional convergence. Conclusions: A set of diaspore characters optimal for successful external dispersion (compatible with the ‘Dispersal Syndrome hypothesis’) evolved as homoplasies in several genera of the same family. Similar environmental pressures are expected to elicit similar adaptive morphologies, suggesting that morphological and histochemical homoplasy is often a consequence of natural selection.