Tackling a Taxonomic Giant:  The Genus Croton (Euphorbiaceae)

Project Summary

Croton (Euphorbiaceae) is one of the largest genera of flowering plants, with nearly 1,300 species of herbs, shrubs, and trees that are ecologically prominent and often important elements of secondary vegetation in the tropics and subtropics worldwide. The genus is most diverse in semiarid regions, but also occurs in habitats ranging from beaches to tropical rain forests. Crotons produce clear to reddish or yellow latex, which is rich in secondary metabolites like alkaloids and terpenoids. Several species are used medicinally, such as Croton oil and "dragon's blood." Croton species are characterized by diverse arrangements of the unisexual flowers, varying degrees of development of the floral parts, different kinds of specialized glands, and variations on the genus' unusual stellate or scalelike hairs. The rich array of extrafloral nectaries in Croton plays an important role in ant interactions, and Croton's slender clusters of flowers exhibit shifts between insect and wind pollination and from monoecy to dioecy. Areas of extreme diversity in the genus occur in the West Indies, southern Brazil, and Madagascar.
Croton is a prime example of the "giant genus" problem -- large, complex taxonomic groups that have been passed over for study in favor of smaller, more clearly delimited ones. This proposal addresses whether the genus is a cohesive phylogenetic unit and how previous morphologically-based classifications compare to newly generated molecular phylogenies. It also focuses on the need to provide tools that will facilitate and stimulate floristic and taxonomic studies of Croton worldwide.
The proposed strategy for tackling the taxonomy of Croton is one that can be applied to other large groups of organisms and involves four steps:

1) Create a Croton Research Network, linking Croton researchers worldwide and promoting close collaboration among them.

2) Create a comprehensive taxonomic/floristic information resource for Croton that will be openly available from the onset of the project via a designated web site and centralized database.

3) Carry out a broad molecular sampling aimed at producing robust phylogenetic hypotheses within Croton and its outgroups.

4) Incorporate a strong training component for graduate and undergraduate students at several institutions.

Preliminary data are presented for the first molecular studies of the genus using 82 species and sequences of the nuclear ITS and the chloroplast trnL-F regions. The data show that the genus so far appears to be monophyletic; the earliest branching lineages in Croton are New World taxa; there is good support for a number of New World sections but little or no support for Old World sectional delimitations; and finally, all Old World species sampled so far group together as a single clade. With the inclusion of the more slowly evolving ndhF chloroplast gene, there should be ample phylogenetic resolution at all levels of the genus tree.
The value of this project resides in its boldness to tackle such a large and formerly intractable group that represents 0.5% of all species of flowering plants on earth; an effective molecular sampling strategy that will cover about 25% of all species in the genus as well as the geographical and morphological diversity of Croton; the recruitment of a network of researchers in Croton with decades of experience and technical expertise as well as a cadre of graduate students and undergraduates; and the combination of state-of-the-art molecular techniques with more traditional field work, herbarium studies, and cytological analyses.