Overview of the generic status of Acacia (Leguminosae: Mimosoideae)

Abstract

The systematic treatment and circumscription of the group of plants presently recognised as the genus Acacia has a complex history. The genus was first described by Philip Miller in 1754 and until 1842, when George Bentham clearly defined it's generic limits (by restricting the name Acacia to mimosoid plants having numerous free stamens), a number of species which are now referable to genera within tribes Ingeae and Mimoseae had been referred to it. As presently defined, Acacia is a cosmopolitan genus containing in excess of 1350 species and together with the monotypic genus Faidherbia Chev. (which occurs in Africa and the Middle East), comprises tribe Acacieae within subfamily Mimosoideae. The current classification of Acacia views the genus as comprising three large subgenera, namely subg. Acacia (c. 161 species, pantropical), subg. Aculeiferum Vassal (235 species; pantropical) and subg. Phyllodineae (DC.) Seringe (syn. subg. Heterophyllum Vassal) (960 species, largely confined to Australia). In 1986, Pedley proposed that these three subgenera be attributed generic rank, namely Acacia, Senegalia Rafinesque and Racosperma C.Martius, respectively, but this proposal was not widely adopted. Subsequently, the results of monographic and floristic works have greatly expanded knowledge, not only of Acacia, but also of its presumed relatives in tribes Ingeae and Mimoseae. Cladistic analyses of chloroplast genes have been especially informative in developing a better understanding of phylogenetic relationships of the group. The new data clearly show that the genus as presently defined (i.e. Acacia sens. lat.) is not monophyletic. Furthermore, five separate monophyletic groups can be recognised within Acacia sens. lat. and it is recommended that these each be recognised as a distinct genus. The five genera correspond to those recognised by Pedley, except that Senegalia sens. lat. is now regarded as comprising three genera, namely Senegalia sens. str., Acaciella Britton & Rose [based on Acacia subg. Aculeiferum sect. Filicinae (Benth.) Pedley] and an undescribed genus based on a group of species related to Acacia coulteri Benth. Acacia subg. Acacia appears to be located in tribe Mimoseae. The relationships of subg. Phyllodineae, subg. Aculeiferum sens. str., sect. Filicinae, the 'Acacia coulteri' group and Faidherbia are not fully resolved, although in all studies these groups are shown to be monophyletic. Although it is appropriate that each be recognised as a distinct genus, the application of the names Acacia and Racosperma is currently under consideration and it is therefore not appropriate to use these names until this matter is resolved.