Croton is a mainly tropical genus with about 1,300 species of herbs, shrubs, and trees. Many species are ecologically prominent, particularly in dry areas, and the genus is rich in secondary compounds (some medicinal). The most recent overview by Grady Webster used morphological criteria to separate the genus into 40 sections. We began reevaluating the genus using molecular techniques, at the same time working to improve floristic knowledge worldwide. Our approach involves four steps: a) establish a Croton research network, b) develop a web page with a taxonomic database, image gallery, and other data on the genus, c) begin worldwide molecular sampling based on Webster’s sectional system, and d) student training. A webpage is now operational (www.wisc.edu/herbarium/croton), and a core group of about 20 researchers and students is actively collaborating. Preliminary data are presented for the first molecular studies of the genus using 82 species (77 ingroup and 5 outgroup taxa) and sequences of the nuclear ITS and the chloroplast trnL-F regions. The data so far indicate that the genus is monophyletic, and the earliest branching lineages in Croton are New World taxa, with section Astraea (C. lobatus and allies) basalmost. There is good support for a number of New World sections, but little or no support for Old World sectional delimitations. Finally, all Old World species sampled so far group together as a single clade. Cytological data is phylogenetically informative, with n=10 (22 species/10 sections), n=14 or 28 (4 species/one section), n=8 (3 species/2 sections), n=32 or 64 (3 species/one section), n=16 or 32 (one species), and n=9 (one species). We now require broader sampling in Croton (particularly Madagascar, Cuba, and Brazil), additional outgroup candidates (including Cuban Moacroton), and inclusion of more slowly evolving genes such as ndhF to improve phylogenetic resolution at the base of genus tree.

Key words: Croton, Euphorbiaceae, floristics, molecular phylogeny