Blog post by Colin Hughes, University of Zurich
The three most important taxonomic ranks used to classify organisms are family, genus and species, especially the latter two, which make up the scientific binomials used to communicate about biodiversity, and indeed about all aspects of biology. While the description of a new plant family is now a very rare event, the same is not true for genera. Indeed, delimitation of genera within many plant families remains in a state of considerable flux, because many traditionally recognized genera do not correspond to evolutionary groups. This causes unwelcome instability in scientific names of species and is why work to delimit genera lies at the heart of much current research in systematic botany.
This is very much the case for subfamily Caesalpinioideae, the second largest subfamily of the legume family, which is the focus of this new special issue of the open-access, peer-reviewed journal PhytoKeys. With around 4,600 species of mostly trees, shrubs and lianas, distributed right across the tropics in rainforests, dry forests and savannas, Caesalpinioideae represent a spectacularly diverse lineage of tropical woody plants.
New analyses of DNA sequences of 420 species of Caesalpinioideae presented here reveal that 22 of the 152 currently recognized genera do not coincide with natural evolutionary groups, i.e., in phylogenetic terms, they are non-monophyletic. The aim of this special issue is to re-define as many of these problematic genera as possible in order to bring them into line with natural evolutionary lineages. To achieve this, nine new genera of Caesalpinioideae are described, five previously recognized genera are resurrected, and three genera shown to be nested within other genera are consigned to synonymy.
Many of the species in these new genera are important, conspicuous, ecologically abundant, and, in some cases, geographically widespread trees in tropical forests. For example, the three species of the new genus Osodendron are important large canopy trees in tropical rain forests and riverine gallery forests across a broad swathe of west and central Africa. In recent decades these species have been successively placed in different genera including Cathormion, Samanea and Albizia, among others. The neglected generic placement of these African trees has finally been resolved via analyses of DNA sequences, and a new generic home for them has been established.
In contrast, two of the genera newly described in this special issue, Mezcala and Boliviadendron, each with just a single species, are much more elusive plants occupying very narrowly restricted geographical ranges. Mezcala occurs across just a few square km of the central Balsas Depression in south-central Mexico and Boliviadendron is known from just two interior valleys of the Bolivian Andes. Establishing these two lineages as distinct genera highlights the importance of conserving these globally rare evolutionary lineages.
Choosing names for new taxa is one of the delights and privileges of the practising taxonomist. Derivations of the names of the nine new genera described in this special issue span features of the plants themselves and the locations where they grow, as well as names of fellow legume researchers honoured with genera named in recognition of their contributions. For example, Osodendron is named after ‘Oso’ a food that is prepared in West Africa from seeds of one of the species now placed in the new genus. Mezcala is named for the indigenous Mezcala culture of the Balsas region in Mexico where the genus is found. Boliviadendron is named as such because it is a tree that grows in Bolivia and nowhere else. The new genus name Heliodendron is derived from the Greek helios (sun) and dendron (tree) because it grows in the sunshine state of Queensland in Australia and its flowers are arranged in sun-like globose heads.
Finally, Naiadendron celebrates the Brazilian Amazon where the genus grows, and the famous German botanist Carl Friedrich Philipp von Martius (1794–1868), who named the Brazilian Amazon after the Naiads, Greek mythology’s nymphs of freshwater.
Four of the genera newly described in this Special Issue are named after prominent contemporary legume taxonomists, three women and one man: Gretheria for Rosaura Grether, a Mexican specialist on the genus Mimosa, Ricoa for Lourdes Rico, another Mexican botanist who worked on legumes based at Kew, Marlimorimia, in honour of Marli Pires Morim of the Jardim Botânico do Rio de Janeiro, Brazil in recognition of her contributions to the taxonomy of mimosoid legumes, and Gwilymia named for Gwilym Lewis, in honour of one of the world’s most experienced and productive legume taxonomists who is legume research leader in the Herbarium at the Royal Botanic Gardens, Kew.
One of the central achievements of the work on Caesalpinioideae presented in this Special Issue is that for the first time a truly pantropical analysis of this large group of plants has been accomplished. A global synthesis is essential to work out how many genera there are.
For example, by sampling across Asia, Africa, Madagascar, North and South America, it has become clear that the Old World species of the important pantropical genus Albizia are not closely related to Albizia in the Americas, prompting splitting of the genus and resurrection of the name Pseudalbizzia for the New World species. All elements of the former Albizia – the last so-called ‘dustbin’ genus in the mimosoid legumes – are accounted for in this special issue (here, here and here). Similarly, the genus Prosopis, one of the most important silvopastoral tree genera of the dryland tropics, has traditionally encompassed elements spanning the New and Old Worlds that are here shown to comprise four distinct evolutionary lineages, two in the Old World and two in the Americas, here treated as four separate genera.
Changes to the scientific names of species are not always immediately welcomed by users, but over time, establishment of a classification that is based on robust evidence about evolutionary history will result in greater nomenclatural stability and in named taxa that are aligned with natural groups and hence biologically more informative. This special issue, reshaping the generic system of a species-rich group of legumes, is an important step towards that goal.
Photo credits: Globimar Pereira-Silva, Steen Christensen, William Hawthorne, Colin Hughes, Luciano de Queiroz, Marcelo Simon.