Tackling a Taxonomic Giant:  The Genus Croton (Euphorbiaceae)

Detailed Project Plan

Euphorbiaceae is a diverse family of plants, with 334 genera recognized in Genera Euphorbiacearum (Radcliffe-Smith 2001) and 8,910 species recognized in the World Checklist and Bibliography of Euphorbiaceae (Govaerts et al. 2000). Despite its large size and ecological prominence, particularly in tropical and seasonally dry areas, the family lags behind other large groups like Asteraceae, Fabaceae, or Solanaceae in depth of systematic studies. Webster (1975) made a major morphologically-based revision of relationships within Euphorbiaceae, recognizing five subfamilies and 52 tribes (later reduced to 49 tribes in Webster 1994). With the advent of the Angiosperm Phylogeny Group classification (APG 1998) and subsequent molecular studies that have used 18S rDNA, rbcL, and atpB sequences (Savolainen et al. 2000, Soltis et al. 2000, Wurdack & Chase 1999 and in prep.), it has been clearly demonstrated that the euphorbs belong to the eurosid I clade, in a broadly circumscribed Malpighiales. These same studies, however, indicate that Euphorbiaceae sensu lato is not monophyletic, and should be separated into at least five lineages within the order. These include Pandaceae; three biovulate groups – the glucosinolate-producing Putranjivaceae, Phyllanthaceae, and Picrodendraceae; and the Euphorbiaceae sensu stricto for the uniovulate subfamilies Acalyphoideae, Crotonoideae, and Euphorbioideae.

Croton belongs to the core Euphorbiaceae in subfamily Crotonoideae, which consists of about 2,400 species in 67 genera and 12 tribes (Webster 1994). Croton accounts for more than half the species in the subfamily, followed in size by Jatropha (± 185 species) and Manihot (± 100 species). A key synapomorphy of the subfamily is an unusual pollen configuration called the ?Croton structure.? This involves a particular architecture of the exine, with triangular supratectal subunits, a network of muri, and short or irregular columellae (Punt 1961, Nowicke 1994). Furthermore, Croton and all but several early branching lineages in subfamily Crotonoideae share inaperturate pollen, an unusual feature among the angiosperms and a strong synapomorphy for most of the subfamily (Nowicke 1994).

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. Giant genera present a larger monophyly dilemma – how do we know if there are previously unsuspected or ?hidden ingroups,? such as Lycopersicon nested within Solanum (Spooner et al. 1993) or Hydnophytum and Myrmecodia within Psychotria (Nepokroeff et al. 1999), or conversely, has the genus has been so broadly circumscribed that it will be shown to be an assemblage of polyphyletic clades, as in Acacia (Miller & Bayer 2001) and Senecio (Nordenstam 1978, Jeffrey 1992)? To assess the monophyly of large genera, we need a solid understanding of the basic taxonomy of the component species and species groups, since this basic knowledge is what guides an effective sampling strategy for molecular phylogenetic studies.

For many botanists and ecologists, Croton has been a taxonomic quagmire. Leon Croizat worked for many years on the genus and described over 100 new taxa (Croizat 1940, 1941, 1943, 1944, 1945, and many others), but he never attained his goal of producing a viable infrageneric classification of Croton. This is partly due to the sheer size of the genus and the proliferation of names and the difficulty of accessing type material scattered among herbaria worldwide (our database currently includes 1,275 accepted names and 1,100 synonyms in Croton). Burger and Huft (1995) bemoaned the ?considerable variation in leaf size, leaf form, pubescence, inflorescences, and floral morphology... [which] can make the determination of individual specimens very difficult.? Some of the features that contribute to Croton?s diversity offer promising research opportunities when coupled with phylogenetic studies. These include a remarkably rich array of extrafloral nectaries that play important roles in ant interactions (Jose & Inamdar 1989), shifts between insect and wind pollination (Domínguez & Bullock 1989, Bullock 1994), separate instances of dioecy evolving from monoecious ancestors, and propensity for weediness in certain lineages. In their revision of Costa Rican crotons, Burger and Huft (1995) noted a general dichotomy of wide-ranging species and local endemics with remarkably narrow ranges, and Gómez-Pompa (1971) felt that the prominence of crotons in secondary vegetation in the tropics was a key factor in the genus? evolutionary diversification.

Croton is also particularly rich in secondary metabolites like alkaloids, terpenoids, and cocarcinogens (Farnsworth et al. 1969). Extracts from ?dragon?s blood? (C. lechleri and related species) are now being marketed by Shaman Botanicals as an antidiarrheal remedy for AIDS patients. Croton oil from Croton tiglium is used as a strong purgative in the Old World, and latex from members of section Cyclostigma like dragon?s blood is widely used by local populations in South America for wound-healing and an array of other ailments (B. Smith, Ph.D. thesis in prep.).

All of these biological features of Croton would benefit from an improved understanding of Croton phylogeny. In the larger picture of cataloging global diversity, which is being actively promoted by organizations like the All Species Foundation and the Global Biodiversity Information Facility, we cannot hope to make significant progress in this venture unless we begin working aggressively on groups like Croton, which alone accounts for 0.5% of all flowering plants.

For now, questions specific to Croton we wish to address in the granting period include:

a) Is a broadly defined Croton monophyletic?

b) Which are Croton?s closest outgroups, and where do they occur?

c) Which groups within Croton – former segregate genera, or sections sensu Webster (1993) –are monophyletic?

d) Do Old World taxa form clades distinct from New World groups?

e) Which morphological characters are concordant or discordant with the molecular phylogeny?

f) Which groups within Croton account for the highest species-level diversity, and how do they differ from smaller clades?

g) How do we provide resources in the short term that will be of assistance to the public needing to identify Crotons?

This sweeping approach to Croton systematics aims to provide the framework into which many individual workers could readily identify meaningful research topics that would contribute more directly to a comprehensive understanding of the genus worldwide.

There are many approaches one could take to tackle taxonomic giants such as Croton, and here we outline a strategy that can produce tangible results over the next three years and could also be applied to a number of the world?s other large, poorly known genera. The strategy is based on the following four objectives:

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

6) 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.

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

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

With this plan, we are confident that the interplay of ongoing molecular analyses of Croton and continually improving species-level treatments will lead to a refinement of the questions that can be addressed by these data both inside and outside the sphere of Croton.