The Genus Croton (Euphorbiaceae)

Background Information

Generic boundaries and sectional delimitations in Croton: Morphologically, Croton is distinguished from related groups by staminal filaments that are distinctly inflexed in bud. Most species are also characterized by thyrsoid inflorescences, indument of stellate or lepidote hairs, nonarticulated lactifers with clear or reddish to yellow latex, and reduced pistillate petals. On this basis, Webster merged segregate genera like Crotonopsis, Eremocarpus, and Julocroton back into Croton (Webster 1992, 1993). Likewise, taxa originally described in Croton but lacking key synapomorphies like inflexed stamens and stellate hairs, have been removed and placed in other genera (e.g. Esser & van Welzen 2001 for Croton nanus, now Colobocarpos).

Identifying the sister group to Croton is key to understanding whether the diversity of the genus is phylogenetically significant or whether it is a taxonomic artifact (Sanderson & Donoghue 1996, Sanderson & Wojciechowski 1996). Webster (1994) placed Croton in tribe Crotoneae together with African Mildbraedia, Asian Paracroton (= Fahrenheitia), and the Cuban endemic Moacroton. However, the tribal and subtribal delimitations within the inaperturate Crotonoideae are avowedly 'soft' (G. Webster, pers. comm.), and therefore a judicious sampling of potential sister groups needs to be included in a molecular analysis of Croton. For example, Webster (1994) noted that South America Sagotia (placed by him in tribe Codiaeae) is morphologically similar to Croton, and Nowicke (1994) found that pollen of Sagotia racemosa has very large lumina, a character shared only with Croton matourensis among the 69 species of Crotonoideae she sampled. Moacroton species have all the main Croton characteristics except for its few stamens with short, erect filaments in bud. Molecular studies could show that it is simply a distinctive clade nested within Croton, or alternatively, it could prove to be Croton's closest outgroup.

The 19th century classification of Croton by Mueller (1866, 1873) was a highly artificial one, so Webster (1993) set out to revise the division of the genus into sections that would supposedly better reflect their phyletic relationships. He based his system on differences in trichome types; presence of glands on the leaves, stipules, and/or sepals; degree of branching in the styles; aestivation of the sepals of the female flowers; presence or absence of bisexual cymules at the base of the inflorescence; opposite vs. alternate leaves; and the degree of petal reduction in male and female flowers. The result was a realignment of the genus into 40 sections. Some sections are very distinctive and will almost surely prove to be monophyletic, but there are serious problems with many Old World taxa, which do not fit easily into these sections (H. J. Esser, P. Forster, and G. Webster, pers. comm.). The largest sections, Cascarilla and Velamea, are among the least coherent, and Cascarilla has been circumscribed to include taxa from three continents. Webster's system is not always predictive of the placement of newly discovered species (e.g. Croton scutatus, Berry & Gaskin 1998), but it can be very useful as a functional way to subdivide the New World species, which account for most of the species diversity in Croton (as in Murillo's 1999 synopsis of the Colombian species). For now, Webster's sections are the best available framework of hypotheses for future phylogenetic studies.

Regional diversity and floristics of Croton: A number of researchers have made valuable advances in floristic work in Croton in recent years. Areas covered in the Neotropics include Costa Rica (Burger & Huft 1995), Panama (Webster 1988), Cuba (Borhidi & Muñiz 1977), Hispaniola, (Liogier 1986), southern Venezuela (Berry 1999), and Santa Catarina, Brazil (Smith et al. 1988). Radcliffe-Smith (1988) completed a treatment of Croton in tropical East Africa, and he is now updating Leandri's (1939) treatment of Malagasy Croton, which will approach 200 species. H.-J. Esser (pers. comm.) has completed a manuscript for Flora of Thailand, and next he will revise Croton for Flora Malesiana. Paul Forster in Queensland is revising the Australian species of Croton. Other groups are now studying parts or all of Croton in Mexico, Colombia, and Brazil. These are all basically 'bottom-up' approaches that suffer to varying degrees from the lack of a broader regional or phylogenetic context. On the other hand, virtually all of these researchers are interested in collaborating on this project, and they can often facilitate the location of fresh material for molecular and cytological studies.

Although Croton is most typical of semiarid habitats, almost any habitat in the tropics will likely yield one or more Croton species, including salt-water beaches, deserts, cloud forest, lowland rain forest, and even seasonally flooded forests. Certain areas are hotbeds of diversity in Croton and are areas where field work should be focused. In the West Indies, Cuba has 51 species of Croton (33 of them endemic), covering 14 sections, as well as the seven species of the closely related Moacroton (Borhidi & Muñiz 1977) and the monotypic Cubacroton. Hispaniola has 45 species in 12 sections and is nearly as diverse. Even the southern U.S. is surprisingly diverse in Croton, with 42 species in 14 sections. The largest country count is for Brazil, with 356 species in 26 sections; of these, 172 species in 21 sections are found in four states of southeastern Brazil, making it the most species-rich area for the genus in the world. These figures are from our Croton database, which began last year with a download of the data in Govaerts et al. (2000) World Checklist of the Euphorbiaceae. The database is being continually updated with input from our Croton collaborators, and via customized queries we can generate similar data for any country or region in the world.

Morphological characters in Croton: Among the difficulties in arriving at a satisfactory systematic arrangement within Croton, Webster (1993) cited the 'pervasiveness of parallelism and convergence' in many of the classical morphological characters. Examples include bifid vs. multifid styles, stellate vs. lepidote trichomes, and leaves with or without basal glands. It is indeed a challenge to try to determine which of the potential morphological characters can be used in a phylogenetic study of the genus, but part of the solution is to reevaluate and atomize the characters in a way that may avoid what can be called 'superficial homoplasy.' One example of this are the stellate to lepidote trichomes that are so characteristic of Croton. In a detailed SEM study of foliar trichomes in the genus, Webster et al. (1996) demonstrated that there are not two, but five basic trichome types in Croton leaves - stellate, fasciculate, multiradiate/rosulate, dendritic, and lepidote. Additional kinds of trichomes can be found on different parts of the flowers and inflorescences. In a somewhat extreme case, Berry (1999) documented how Croton matourensis from the Guiana Shield has different trichome types on different parts of organs (leaf midvein vs. surface), at different developmental stages (juvenile vs. mature), or on different parts of the plant (sun vs. shade leaves). Ricarda Riina, a student in Berry's lab, demonstrated in a plant morphology class project that an undescribed species from Venezuela has five different trichome morphologies on different surfaces (upper leaf, lower leaf, sepals, petals, and capsules). Most species are actually quite constant in their vestiture, but the point is that one must break down a character like trichomes into the proper components and do so for different parts of the plants and at different developmental stages.

Foliar glands have been used in sectional classifications, usually as present or absent, or on stipules vs. lamina. Jose & Inamdar (1989) found five kinds of extrafloral nectaries on one species, Croton bonplandianus. Distinguishing between petiolar, pedicellate, stipular, bracteolar, and laminar nectaries and whether they are discoid, stipitate, or globular will better atomize this character for other species and sections. Stigma branching is also more complicated than simply bifid vs. multifid. There are several elaborate stigmatic variations in Croton, such as the unusual fan-shaped divisions of the stigmas in sect. Astraea, which may be a good synapomorphy for that section.

With fairly reduced flowers to begin with, it is not clear what significance there is for the kinds of floral variations that have been used in Croton classification. Webster (1993) thought that species with well-developed female petals were primitive in the genus, but in the related genus Moacroton, six species are apetalous and the seventh has petals as large as the sepals (Borhidi 1991). Several sections of Croton are characterized by inflated calyces with connate sepals, or else by 'reduplicate-valvate' sepals (folded back and outwards), and we would like to know if the different groups share a common ancestry. Seed and fruit characters have barely been explored in Croton, but Webster (1993) believes they could supply informative characters within the genus.

With a more effective discrimination of the vegetative and floral characters that have been used in the past and inclusion of little used characters such as seeds, we can make a better attempt at evaluating the phylogenetic usefulness of morphological characters in Croton. Also, we should take the approach of Evans et al. (2000), who explored incongruence between morphological and molecular datasets in Commelinaceae by identifying which particular morphological characters contributed most to the conflicts between datasets.

Creation of the Croton Research Network and web page. Early in 2001, Rafael Govaerts from Kew provided us with an electronic output of the Croton section of their World Checklist of the Euphorbiaceae publication (Govaerts et al. 2000). We have modified this into an Access database and added to it a series of geographical queries and determined sectional assignments for hundreds of previously unassigned species (up from 500 out of 1278 taxa to 700). We have also incorporated images (live plants, type specimens, photomicrographs, and SEMs) contributed by Brian Smith from his Ecuadorian sect. Cyclostigma revision (in prep.). Based on specimen and literature analysis, as well as consultations with local specialists, we regularly update the taxonomy and geographical distribution data in the database. In July 2001, Berry visited Kew Botanic Gardens to confer with the resident Euphorbiaceae specialists, Petra Hoffman and Alan Radcliffe-Smith. There was not enough time to examine herbarium material on this visit, but Berry then spent three days in the Paris herbarium reviewing New World Croton collections and making digital photographs of several hundred type specimens, including the important Humboldt & Bonpland collection. Most of these are now available for viewing on this web site, and more will be added as they are processed. For now, a static version of the Kew checklist is available on the website, as well as a synopsis of Webster's 1993 sectional revision of the genus.

Berry has continued to develop the Croton Research Network through personal visits and by communicating updates on this project. Two senior researchers are key to this collaboration, because they have devoted so much of their lives to Euphorbiaceae research, and a considerable part of it on Croton. They are Grady Webster (emeritus, UC-Davis) and Alan Radcliffe-Smith (emeritus, Kew Botanic Gardens), and both have begun to share their expertise and materials with us. A junior, but equally key collaborator -- especially for the molecular component of our project, is Kenneth Wurdack, at the Cullman Program of the New York Botanical Gardens and finishing his Ph.D. at UNC. Ken has extensively sampled the traditional euphorb genera using rbcL and trnL-F, and he has a keen interest in Croton. He plans to postdoc this year at the Smithsonian Institution and will continue to work with us on the molecular phylogeny of Croton. Two of Berry's graduate students at UW-Madison, Ricarda Riina and Benjamin van Ee, have contributed significantly to the preliminary results and are eager to develop their doctoral theses as part of this project.

Other persons who are contributing to the Croton Research Network include: Hans-Joachim Esser (Harvard University, southeast Asian Croton), Petra Hoffmann (Kew, Euphorbiaceae and Madagascar), Brian Smith (UC-Davis, systematics of section Cyclostigma), José Pirani and his doctoral student Letícia Ribes de Lima (Universidade de São Paulo, Brazilian Croton), Inés Cordeiro (Instituto Botânico, São Paulo, section Julocroton), Ricardo Secco (Museu Goeldi, Belem, Brazil, Amazonian Croton), Paul I. Forster (Brisbane, Australian Croton), José Murillo (Colombian Croton), Marta Martínez-Gordillo (Mexican section Barhamia), Mark Olson (UNAM, Mexican Euphorbiaceae), and William Burger (Field Museum of Natural History, Central American Croton). There are other specialists we plan to contact (e.g. Steven Ginzbarg, Victor Steinmann, Emily Lott, Willem Punt, P. van Welzen, among others), but we wish to secure further funding before committing too many people to this endeavor.