Introduction | Previous work on Euphorbia relevant to this project | Objectives | Methods | Web Based Monograph and Identification tools | Phylogenetic Component | References

Introduction

According to the latest taxonomic estimates, 57 angiosperm genera include more than 500 species each, 19 of them with over 1000 species and two with over 2000 species. The total of 53,000 species in these genera accounts for 15% of all species of flowering plants. The spurge family (Euphorbiaceae s.l.) includes four of these giant genera, with a combined total of over 5000 species. Molecular studies have confirmed the monophyly of each of the large euphorb genera (some expanded to include former segregates), and together they offer a unique opportunity to study a significant portion of the world's angiosperm diversity and address issues inherent to large genera such as the role of adaptive or key innovations, accelerated diversification rates, and ways of identifying and monitoring the conservation status of a large part of the world flora.

This study will focus on the spurge genus itself, Euphorbia. With over 2000 accepted species, it is second in size only to the legume genus Astragalus among the flowering plants. We have assembled an international team of Euphorbia systematists who will be capable of providing a comprehensive monograph of the genus through appropriate field exploration, study of museum and living collections, and molecular and developmental analyses.

Euphorbia corresponds to the former subtribe Euphorbiinae, which comprises over 2000 species (Webster, 1994; Steinmann and Porter, 2002). The genus is morphologically best defined by the shared presence of a cyathium, which is a complex pseudanthial inflorescence. Nuclear and chloroplast DNA sequence data from 223 species in the clade strongly support the monophyly of Euphorbia s.l., with all former segregate genera such as Chamaesyce, Monadenium, Pedilanthus, and Poinsettia (as E. pulcherrima) nested well within it (Steinmann and Porter, 2002). However, the data show a marked incongruence with most of the traditionally recognized subgeneric groupings within Euphorbia, clearly indicating that previous morphologically based delimitations of subgenera or sections within the genus should not be taken at face value. The genus is in fact rife with striking examples of morphological convergence in cyathial and vegetative features, which justifies a global approach to studying the genus to obtain a proper phylogenetic understanding of the whole group.

The spurges have a cosmopolitan terrestrial distribution and vary in habit from prostrate annuals to well-developed trees. They achieve their greatest diversity in arid areas of Africa and Madagascar, where most of them are cactus-like succulents; indeed they occupy the same ecological niche in the Old World as cacti do in the New World. Because of their high degree of local endemism and sensitivity to habitat disturbance by humans, a high percentage of spurge species are rare and several species are considered endangered. Euphorbia is the only genus of plants known to include all three major photosynthetic systems (CAM, C3, and C4), and a proper phylogenetic framework will allow a better understanding how these different systems evolved within a single lineage. Euphorbia species all have white latex that is often rich in secondary metabolites; some species are considered toxic, and others are treated as noxious weeds. The genus is much prized for its ornamental species, which include the poinsettia, crown-of-thorns, milk bush, some showy temperate spurges, and the numerous tropical to subtropical succulents.

Previous work on Euphorbia relevant to this project

We are fortunate to have access to a recently revised and comprehensive nomenclator for the entire genus/subtribe, in the World Checklist and Bibliography of Euphorbiaceae by Govaerts et al. (2000). The consolidation of this nomenclator is one of the first priorities in the first year of the project.

The last complete monograph of Euphorbia was the treatment by Boissier (1862) in de Candolle's Prodromus, in which 740 species were recognized. Steinmann and Porter (2002) provide a thorough taxonomic history of the genus since then. The bottom line is that a number of clades have been placed inside or outside of Euphorbia at different times (e.g. Chamaescyce, Cubanthus, Endadenium, Monadenium, Pedilanthus, Poinsettia, and Synadenium), and between 9 and 11 subgenera have been recognized at one time or another. Especially in light of the molecular findings of Steinmann and Porter (2002), few of the subgeneric circumscriptions hold up under DNA sequence analysis.

Different groups of Euphorbia have been monographed at different times, including Pedilanthus (Dressler, 1957), Poinsettia (Mayfield, 1997), Hawaiian Chamaesyce (Koutnik, 1982), Caribbean Chamaesyce (Burch, 1965), mediterranean subsect. Galarrahei (Simon and Vicens, 1999), and section Tithymalopsis (Huft, 1979; Park, 1995). There have been numerous floristic treatments for different regions, but these are generally done outside of any avowed phylogenetic context. Examples are Carter's 1988 treatment of Euphorbieae for Tropical East Africa, Johnston (1975) for the Chihuahuan desert region, Hargreaves (1987) for Malawi, Airy Shaw (1980) for Borneo, Benedí; et al. (1997) for the Iberian Peninsula, and Vindt (1953) for Morocco. For Madagascar, Haevermans (2003) has produced a checklist of the 170 species known from the island, and he has performed a molecular phylogenetic analysis of subgenus Lacanthis (Haevermans, 2004), which includes the widely cultivated crown-of-thorns, E. milii.

Because of the genus's great horticultural interest, there is a plethora of publications on the cultivated species. Prominent among them are the ten-volume Euphorbia Journal series (LaFon et al., 1983–1997), all lavishly illustrated in color. The International Euphorbia Society (IES) has just begun publishing a new journal called Euphorbia World, and this could become a suitable venue for publishing descriptions of new species and report findings from fieldwork expeditions as part of this project.

Project Objectives

1. Conduct new field research in poorly explored areas or where Euphorbia is particularly diverse or poorly known
2. Carry out in-depth herbarium and literature studies to resolve ongoing taxonomic problems in the genus
3. Publish descriptions of species new to science resulting from these studies
4. Develop a web-based virtual monograph of the roughly 2000 known species of Euphorbia (in the broad sense, including all former segregate genera)
5. Develop a web-based virtual monograph of the roughly 2000 known species of Euphorbia in the broad sense
6. Develop online interactive keys
7. Produce a molecular-based phylogeny of the genus that should lead to a much more informative and predictive classification scheme
8. Gain a better understanding of the evolutionary developmental of the cyathium, as this may have been a 'key innovation' in the evolutionary success of this amazing group of plants.

Methods

Web based Monograph and identification tools

EuphORBia portal and species pages. Access to biodiversity information, particularly in an online environment, is typically through one of four common points of entry: 1) taxonomy (scientific and common names); 2) identification using keys, images, and illustrations; 3) geospatial (e.g., what is in my backyard or national park); and 4) phylogenetic trees. Each point of access represents a use case that has been implemented in a variety of biodiversity informatics projects. There is a unique opportunity through the Euphorbia PBI to provide an integrated approach to users with all of these points of access through a single project interface - the EuphORBia Portal (so named to indicate the comprehensive and planetary scope of the project).

Species pages will represent publicly available views from the EuphORBia project data resulting from any query. Through electronic floristic and monographic treatments, species pages will display authoritative and validated information on Euphorbia taxonomy, nomenclature, phylogenies, descriptions (protologues), specimens cited (including types), images and geographic data. Links to additional internal and external resources will be provided. Geographic information will be displayed through an interactive web mapping application.

Availability of existing collections. The major historical herbarium collections of Euphorbia, especially for species-rich regions of Africa and Madagascar, reside in major European herbaria, such as Kew, Paris, Geneva, Brussels, and Munich. For other areas, there are many regional herbaria that are important for typification issues, such as Pretoria in South Africa and MEXU for Mexico. A real asset in the study of a genus like Euphorbia is the large number of species of ornamental value, which are widely available in living collections. For a genus with so many succulent species, which are hard to convert into herbarium specimens, sources of live plants are particularly valuable for systematic studies. The International Euphorbia Society is now the biggest central clearing house for living collections and Euphorbia enthusiasts, with over 250 members, but there are many other succulent plant societies where Euphorbia is widely grown as well. We have so far received excellent cooperation from Euphorbia growers, and there appears to be an extensive network of amateurs we can tap into for expertise and plant material.