The evolution of chromosome number during the diversification of the tribe Vernonieae (Asteraceae)

Abstract

Changes in chromosome number have played an important role in the diversification and evolution of angiosperms. In Asteraceae, tribe Vernonieae are one of the most variable groups with regard to chromosome number. Previously, chromosome numbers n = 9 and 10 were thought to characterize the Old World members of the tribe, and n = 14, 16, 17 and 18 the New World members. This scenario was revised as a result of reports of new chromosome numbers, but the events leading to this wide variation remain unknown. Here we carried out a phylogenetic analysis of Vernonieae in a temporal framework, assessing patterns of diversification and establishing possible relationships with chromosome events. Chromosomal evolution was analysed with ChromEvol, from a phylogenetic tree dated in BEAST. Shifts in diversification rates using Bayesian analysis of macroevolutionary mixtures were inferred. Vernonieae originated ~46 Mya and the diversification rate increased sharply ~11 Mya after the Mid-Miocene Climatic Optimum. The ancestral chromosome number for the tribe was n = 10, which remained stable for Old World taxa, whereas n = 9 was the ancestral number for New World species. The tribe has undergone 32 chromosome rearrangements throughout its evolutionary history, with dysploidy and polyploidy possibly explaining the observed diversification pattern.