Vernonieae- The Evil Tribe
The Vernonieae (a.k.a. the “evil tribe”) is one of the most successful tribes of the Compositae or Asteraceae, the largest of all flowering plant families (23,000-30,000 species). The Vernonieae, along with the tribes Liabeae, Cichorieae and Arctotideae form the subfamily Cichorioideae and include over one third of all of the Compositae species (Figs. 1, 2). The tribe is further subdivided into 21 subtribes, ~130 genera and 1500-1700 species found in tropical, subtropical and temperate locations around the world, with the exception of Europe, part of Asia north of the Himalayas and Antarctica. There are over 50 monotypic genera (one species each) and ca. 35 genera with fewer than three species each, an extremely unusual situation. This is most likely related to past extinctions due to the age of the tribe (ca. 24 MY) as it radiated out into both the New and Old Worlds (Fig. 3). There is also a broad range of growth forms (Fig. 4) and habitat types (Fig. 5) with species typically found on soils such as serpentine, limestone, bauxite, dolomite, volcanic and others rich in metals. The nickname, the “evil tribe,” derives not only from the large number and varying habits of the species, in combination with its worldwide distribution, but also because of the similarities in appearance of species, even among those found on different continents (Fig. 6) and the very small number of morphological characters that can be used to reliably distinguish them from each other. In addition, similar morphologies occur among the vast majority of the species in the tribe and this has stymied our understanding of relationships at all taxonomic levels for the better part of the past 150 years. For example, until recently there were >1000 species in one genus, the core genus Vernonia. Chloroplast and nuclear DNA sequence data have greatly improved this situation by providing literally thousands of new characters that taken together distinguish species, genera and subtribes, and, in addition, show us their relationships to each other. The first ever phylogeny for the Vernonieae was provided by Keeley et al. in 2007 using DNA sequence data (Fig. 3).
Among the distinctive features of the Vernonieae revealed by molecular studies are two that are particularly noteworthy. First, it is unique in being the only tribe within the family that originated in the Old World (Madagascar/Africa) and whose sister tribe, the Liabeae, originated in the New World (northern Andes, S. America). Second, the Vernonieae is also the only tribe in the family found throughout the New World with large radiations of genera and species from southern Argentina to the eastern United States and the West Indies. These are significant characteristics because the Compositae family originated in northern South America but proliferated in the Old World, primarily in Africa, and only later re-colonized the Americas starting in the southern hemisphere (Fig. 3). (The most derived members of the Compositae, the familiar sunflowers and daisies, are North American, Fig. 1). Additionally, our studies indicate that long-distance over-water dispersal has played an important role in current tribal distribution. The Hawaiian endemic genus, Hesperomannia, for example, is most closely related to the Madagascan genus, Distephanus, and the Central American taxa in the subtribe Leiboldiinae are most closely related to species in southeast Asia. No land connection has ever existed between Hawaii and any other continent or island chain, nor has there been land connecting Asia to Central America within the time frame of the Compositae’s existence (35-60 MY). There have been at least two trans-oceanic dispersals of Vernonieae between the Old and New Worlds, and these have occurred in both directions. However, it is also clear that overland dispersals have also occurred within both hemispheres as there are many closely related species groups (and their descendants) found across vast areas of continental Africa, Asia, and the Americas.
While the patterns of radiation of the Compositae are known in broad outline, the specifics of the history of dispersals and the pathways followed remain unclear. Given that the Vernonieae originated in the Old World and now have many species in the New World, it is likely that there are several inter-connecting lineages. What all these might be and the extent to which different lineages have radiated in each hemisphere remain among the most tantalizing questions in Vernonieae and Compositae evolution.
Economic, Ecological and Medicinal Uses of Vernonieae
Anti-herbivore compounds– Sesquiterpene lactones, bitter tasting secondary products present in leaves and stems deter herbivory by mammals and insects, and in some cases have proven fatal to cattle; in some North American species these compounds prevent larvae of the Southern Army Worm and other pests from completing their development and thus halt reproduction, dropping insect populattions.
Medicinal– Many species are used by indigenous peoples in Africa, India, China, South and Central America and were used by native Americans in the eastern U.S. in the past. These are use to treat skin diseases, for ridding the body of parasites and worms, stomach aches, menstrual and birth pain and as a general analgesic. A variety of pharmacologically active compounds have been isolated and found to be effective against parasites that cause schistosomiasis, malaria (they kill the causative agents, Leishmania major and Plasmodium falciparum, respectively). Brazilian pharmaceutical companies have developed a cancer treatment for non-Hodgkins lymphoma and leukemia, and one African species, Gymnanthemum (formerly Vernonia) amygdalina is used by chimpanzees to self-medicate. New compounds are currently being isolated from Brazilian and African Vernonieae and tested for use against tropical parasites and cancer.
Nutritional-Some species are used as a leaf vegetable in much of Africa. The leaves are rich in Vitamin C and minerals. Farmers may also feed domestic animals with the leaves to supplement the nutrition of pasture grasses.
Ecological-Vernonieae of all types are colonizers of volcanic, serpentine, limestone and ultramafic soils and are found on ancient soils such as found in the cerrado and caatinga of Brazil and the eastern scarp in Africa as well as those more recently derived in forests and lowland areas. Numerous species worldwide are co-evolved with endemic insects. The heads are frequently the site of larval development in North and South American species particularly for Tephridid flies, and there are a number of ant-mutualisms in African species. Many American species are also co-evolved with Puccinia rusts and a number of African species have been shown to enrich soils in nitrogen and have since been used in crop rotations. Overall, few species in the tribe are weedy, but approximately a dozen shrubby species have become nuisances in pastures in North and South America, Africa and India, and one annual, Cyanthillium cinereum, is distributed in tropical localities around the world. However, it causes little if any damage.
Manufacturing– Some Vernonieae seeds provide a source of high quality epoxy oils (and have been grown by the USDA to develop these as crops), in Brazil a number of species have been found to produce large quantities of high fructose syrups which are used in soft drinks, while others provide resins for paint and paper coatings. Strobocalyx arborea is a major timber tree in Malaysia.