Acacia subg. Acacia, 1999

Maslin, B. R., Miller, J. T. & Seigler, D. S., 2003, Overview of the generic status of Acacia (Leguminosae: Mimosoideae), Australian Systematic Botany 16, pp. 1-18 : 9-11

Acacia subg. Acacia

( Fig. 4 )

A cosmopolitan group containing about 161 species ( Table 1) that are distributed as follows:

New World —c. 60 species [species numbers derived from information presented in Ebinger et al. (2000) and Clarke et al. (2000)].

Africa (including Madagascar)—73 species [species numbers derived from information presented in Ross (1979), Lock (1989), Boulos (1995), Thulin and Hassan (1996), Thulin (1998), and Du Puy and Villiers (2002)].

Asia (including about 15 species that occur also in Africa)—36 species [species numbers derived from information presented in Ali (1973), Nielsen (1992), Thothathri (1992), Lock and Simpson (1991), Lock and Heald (1994), S. Kumar and P. V. Sane (unpubl. data) and D. S. Seigler (unpubl. data)].

Australia —seven species [species numbers derived from Kodela and Tindale (2001)].

The subgenus may be characterised in the following ways (see Table 4 for further details): Trees or shrubs. Branchlets usually lenticellate. Prickles absent. Stipules spinose, in pairs, often being larger and more prominent on young growth, sometimes enlarged and inhabited by ants, rarely absent. Leaves bipinnate, with usually numerous small pinnae (1–15 pairs); leaflets 1–50 or more pairs, from less than 5 to 35 mm long. Petiolar glands mostly small and non-specialised, but large (up to 10 mm in length) and modified in some species. Inflorescence systems simple or racemose; flowers yellow-orange or whitish-yellow, arranged in globose to subglobose heads or cylindrical spikes (8–20 mm in diameter), pentamerous or tetramerous. Floral bracts linear, spatulate to peltate. Ovary sessile, nectariferous tissue not evident. Pods rectangular in cross section or terete, dehiscent or indehiscent, pericarpic strip present or not. Funicle exarillate. Pleurogram large to small.

Many studies have shown subg. Acacia to be monophyletic and clearly distinct from the other two subgenera of Acacia , for example the cladistic results of Chappill and Maslin (1995) based on morphological data and molecular results of Robinson and Harris (2000), Clarke et al. (2000), Miller and Bayer (2000, 2001, 2003) and Luckow et al. (in press). In one of these studies (Luckow et al., in press), Acacia subg. Acacia is shown to be nested deeply within tribe Mimoseae , a relationship also suggested by Guinet (1990) on the basis of pollen data. There is strong evidence supporting recognition of subg. Acacia as a distinct genus (i.e. Acacia ). However, as discussed below, a formal proposal will be made to retypify Acacia and, if this is successful, the generic name Vachellia then would apply to this group.

Currently, there is no universally accepted classification that meaningfully subdivides the species of subg. Acacia into subordinate groups. Bentham’s (1875) scheme, which was based on the position of the involucre on the peduncle, is no longer considered suitable for discriminating infra-generic groups ( Ross 1979). Vassal’s (1972) classification, which recognised two subordinate groups (namely subsect. Pluriseriae and subsect. Uniseriae) on the basis of the arrangement of seeds within the pod, has not been widely adopted. Although Ross (1979) was unable to subdivide the African species meaningfully, a number of those in the Americas were recently accommodated in reasonably distinct, albeit small, informal species groups, namely the A. rigidula group ( Lee et al. 1989), A. constricta group ( Clarke et al. 1990), A. farnesiana group ( Clarke et al. 1989; Guinet 1990), A. daemon group ( Clarke and Seigler 1991) (also called the A. acuifera group), the Acacia macracantha group and the ant acacia group (Janzen 1974; Seigler and Ebinger 1988; D. S. Seigler, unpubl. data). These species

groups were supported by chloroplast data, except for the Acacia farnesiana group ( Miller and Bayer 2003) and the Acacia macracantha group ( Clarke et al. 2000). Miller and Bayer (2003) found a strong separation between New World and African–Asian taxa of subg. Acacia . The African members of subg. Acacia that shared some similarities with Acacia farnesiana formed a clade separate from American members of the Acacia farnesiana group. These chloroplast data ( Clarke et al. 2000; Miller and Bayer 2003) also support multiple origins of ant associations in the New World and Africa.

Figures

Fig. 4. Indicative distribution of Acacia subg. Acacia (excluding the distribution of A. farnesiana in Australia).