Recircumscription of the Neotropical Croton section Eutropia using multiple sources of evidence

The current classification of the New World Croton recognizes 31 monophyletic sections. Ten of these sections consist of isolated lineages containing a single species. Croton section Eutropia is one of these species-poor lineages, including only the eastern Brazilian endemic Croton polyandrus. Recent anatomical and palynological studies suggest that another Brazilian species, C. imbricatus (currently classified in C. section Lamprocroton) exhibits morphological characters in common with C. polyandrus and instead lacks traits typical of C. section Lamprocroton. Prompted by these findings, we obtained additional data, including DNA sequences and complementary anatomical evidence, to reassess the delimitation of C. section Eutropia. Phylogenetic analyses of the ITS and trnL-F genetic regions recovered C. imbricatus outside C. section Lamprocroton and as the closest relative of C. polyandrus. This close relationship was supported by a set of anatomical and morphological characters, including the presence of acropetiolar/basilaminar nectaries in both species (absent in C. section Lamprocroton). In light of our results, here we present an updated taxonomic treatment of C. section Eutropia by providing emended taxonomic descriptions, detailed anatomical characterization of both species, and updated geographic ranges. We also propose the designation of a neotype for C. polyandrus. This work expands both the morphological and ecological scope of this Brazilian clade which now comprises two closely related but distinct species with highly divergent habitats and disjunct geographic distributions. A preliminary conservation assessment indicates that C. polyandrus should be categorized as Vulnerable and C. imbricatus as Endangered.