1st Sept (Sat) 9:40 - 10:20

Plenary Talk 3

Chairperson: Colin Hughes

Phylogenomic complexity and legume evolution in deep time

Erik Koenen* (Institute of Systematic and Evolutionary Botany, University of Zurich)

Although the study of legume evolution and phylogeny has progressed tremendously in recent decades, there is still uncertainty concerning resolution of the deepest divergences and the age of the family. Furthermore, there is phylogenetic uncertainty within several subfamilies, perhaps nowhere more so than within mimosoids, where relationships among the c. 40 genera of the Ingeae/Acacieae p.p. clade are almost entirely unresolved in phylogenies based on one or a few markers. Improved estimates of phylogeny and divergence times are needed to investigate how key legume traits evolved and how the enormous contemporary species diversity arose. Analyzing genome-scale molecular sequence data sets, as pursued here, is a promising means of solving difficult phylogenetic problems. For the deepest divergences within the family, results indicate strong disagreement among gene trees about the relationships between subfamilies, suggesting a complex origin of the legumes involving incomplete lineage sorting and/or hybridization linked to ancient whole genome duplication. Hybrid capture data for mimosoids show that the Ingeae/Acacieae p.p. clade is resolved into several subclades but persistent lack of phylogenetic signal among the large majority of genes leaves relationships among these subclades still unresolved. This suggests that initial diversification of this clade occurred nearly instantaneously, such that the backbone of the clade should perhaps be seen as a hard polytomy. Time calibration analyses indicate that legumes likely originated in the Maastrichtian (Late Cretaceous) or possibly the early Paleocene and suggest that early legume evolution is closely associated with polyploidy and the Cretaceous-Paleogene (K-Pg) boundary. During the Cenozoic, legumes appear to have experienced multiple pulses of fast diversification, in line with evidence from the fossil record. Simulations using time-varying birth-death models under different extinction scenarios suggest significant episodic species turnover through time.