Evolution of extrafloral nectaries: adaptive process and selective regime changes from forest to savanna.

Much effort has been devoted to understanding the function of extrafloral nectaries (EFNs) for ant-plant-herbivore interactions. However, the pattern of evolution of such structures throughout the history of plant lineages remains unexplored. In this study, we used the hypothesis of plant defense mediated by ants as a theoretical framework to test more specific hypotheses about the adaptive role of EFNs during plant evolution. Emphasis was given to different processes (neutral or adaptive) and factors (habitat change and tradeoffs with new trichomes) that may have affected the evolution of ant-plant associations. We measured 7 EFN quantitative traits in all 105 species included in a well-supported phylogeny of the tribe Bignonieae (Bignoniaceae), and collected field data on ant-EFN interactions in 32 species. We identified a positive association between ant visitation (a surrogate of ant guarding) and the abundance of EFNs in vegetative plant parts, and rejected the hypothesis of phylog...