Skipping the dry diagonal: Spatio-temporal evolution of Croton section Cleodora (Euphorbiaceae) in the Neotropics

Abstract

Croton is one of the largest angiosperm genera, with > 1200 species in the tropics worldwide. The arborescent Croton section Cleodora stands out for its disjunct geographical distribution with two main centres of diversity in the Amazonian and Atlantic Forest regions, separated by the Caatinga, Cerrado and Chaco biomes (the 'Dry Diagonal'). This disjunction is found in other Neotropical lineages and attributed to Neogene geological and climatic events. We inferred a nearly complete phylogenetic reconstruction of Croton section Cleodora based on DNA sequences of nuclear ITS and five plastid regions (rps16, trnH-psbA, trnL-F, trnT-L and ycf1). We further estimated divergence times and reconstructed ancestral ranges using Bayesian methods. Our results show that Croton section Cleodora is monophyletic with two main clades; we also confirm the phylogenetic adscription of eight Croton spp. recently described or assigned to section Cleodora based on morphology. Divergence of section Cleodora from its sister clade occurred c. 25 Mya, and diversification within this group started 20 Mya. Biogeographic analyses suggest the section originated in the Amazonian region, from where it dispersed to other forested Neotropical regions, including the Atlantic Forest. Divergence between Amazonian and Atlantic Forest taxa appears to have been triggered by the formation of the South American Dry Diagonal.