Disentangling the evolutionary history of Senna: Current research and perspectives.

BRIGITTE MARAZZI; FATEN FILINBAD; TOBY PENNINGTON; ELENA CONTI; BARBARA MCKINDER; SABINA KNEES

Johannisbourg

Congreso; VI International Leguminosae Conference 2013 (ILC6); 2013

Senna is with 300-350 species one of the most species-rich, widely distributed, and morphologically diverse caesalpinioid legume genera. Yet, its evolutionary history remains little understood. Recent molecular phylogenetic work in the genus provided the first framework to test its current infrageneric taxonomic classification, hypotheses on floral morphological specialization, and analyze patterns in its large-scale diversification. Currently, research in Senna focuses especially on (1) biogeography and (2) morphological evolution. (1) Compared to American taxa (ca. 75% of the genus), non-American taxa especially from Africa and the Near East are underrepresented in current molecular phylogenies and poorly documented in general. Biogeographical studies aiming at disentangling the taxonomy and history of Senna species of the Arabian Peninsula, demonstrate that there are eight native species and one widely naturalized species in the region. Phylogenetic analyses of chloroplast matK and rpS16 sequence data do not recover the native Arabian species as monophyletic but, suggest that Senna arrived in the Arabian Peninsula several times independently. (2) Senna species are morphologically highly diverse. Over 80% of the species bear extrafloral nectaries (EFNs) to attract and reward aggressive ants that protect developing leaves and flowers from consumers. Reconstruction of the remarkable morphological diversification of Senna EFNs suggests that two distinct EFN morphologies evolved independently in unrelated clades within the genus and on different bearing organs. The EFNs attained different degrees of morphological differentiation or ‘individualization’ from their bearing organ. ‘Individualized’ EFNs (conspicuous, gland-like) have experienced more complex evolutionary history than ‘non-individualized’ EFNs (cryptic, embedded within their bearing organ), perhaps reflecting variations in their ecology and evolutionary role. Finally, the increasing taxonomic scope of Senna phylogenies and growing knowledge about its biogeography and morphology will allow us to progress towards disentangling its evolutionary history, proposing a new classification, and making Senna a study system in and beyond legumes.