Acacia subg. Aculeiferum

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 : 11-12

Acacia subg. Aculeiferum sens. str.

( Fig. 5 )

A cosmopolitan group containing about 203 species ( Table 1) that are distributed as follows:

New World —97 species (plus about another 20 as yet undescribed) (D. S. Seigler, unpubl. data).

Africa (including Madagascar)— 69 species [species numbers derived from information presented in Ross (1979), Thulin and Tardelli (1988), Lock (1989), Thulin (1989), Thulin and Hassan (1990), and Du Puy and Villiers (2002)].

Asia (including about seven species that occur also in Africa)—43 species [species numbers derived from information presented in Ali (1973), Lock and Simpson (1991), Nielsen (1992), Lock and Heald (1994), S. Kumar and P. V. Sane (unpubl. data), and D. S. Seigler (unpubl. data)].

Australia (including A. pennata subsp. kerrii , which extends to Asia) – 2 species (species number from Ross 2001).

If treated as a distinct genus, the name Senegalia would apply to this subgenus.

Acacia subg. Aculeiferum sens. str. may be characterised in the following ways (see Table 4 for further details): Trees or shrubs, often scandent. Prickles usually present, recurved or straight, scattered or 1–3 at nodes. Stipules normally present and scarious, not spinose, often early deciduous. Leaves bipinnate, with 1–50 pairs of pinnae; leaflets 1–80 pairs, 3–80 mm long. Petiole nearly always glandular; glands single or multiple, usually small, not specialised, but occasionally modified. Inflorescence systems racemose, paniculate, fasciculate or simple; flowers arranged in globular or obloid heads or cylindrical spikes, cream-colored, pentamerous. Floral bracts linear to spatulate. Ovary on gynophore (0.4–2 mm long), with a nectariferous disk at the base of the ovary. Pods mostly dehiscent, chartaceous, some coriaceous, a few articulate. Funicle often arillate.

As discussed above, the exclusion of sect. Filicinae and the ‘ Acacia coulteri ’ group from subg. Aculeiferum sens. lat. leaves a core group, subg. Aculeiferum sens. str., which is shown to be monophyletic in all cladistic studies and which is well-supported by bootstrap values.

Subgenus Aculeiferum sens. str. comprises two sections, sect. Aculeiferum (which appears to be confined to Africa and Asia) and sect. Monacanthea (which is pantropical). There are several characters that distinguish these two relatively large sections ( Vassal 1972), including the number and placement of prickles and the nervation of stipules. Species of sect. Aculeiferum have prickles near the nodes, whereas those of sect. Monacanthea have prickles scattered along the stem. Recent evidence, however, suggests that these two sections are non-monophyletic, with a few taxa not grouping according to their sectional classification. These exceptions include A. chariessa, A. riparia, A. gaumeri, A. persiciflora and A. ataxacantha ( Robinson and Harris 2000) , A. ataxacantha ( Chappill and Maslin 1995) and A. eriocarpa ( Miller and Bayer 2003) . Vassal (1972) considered A. ataxacantha to be a member of sect. Monacanthea and A. galpinii to belong to sect. Aculeiferum . Although Vassal did not include A. riparia, A. gaumeri, A. chariessa or A. eriocarpa in his treatment, on the basis of their overall characters, they should fall into sect. Monacanthea, whereas A. persiciflora should be in sect. Aculeiferum . Ross (1979) noted that A. eriocarpa was one of four African species of sect. Monacanthea with spicate inflorescences, which differed from other African species (all of which have capitate inflorescences), in pollen, seed and seedling characters, as well as inflorescence structure. Chappill and Maslin (1995) showed that three African–Asian species of Vassal’s (1972) sect. Aculeiferum (A. caffra, A. mellifera and A. senegal), together with A. ataxacantha (which Vassal placed in sect. Monacanthea), formed a monophyletic group. Biochemical data from both Evans et al. (1977) and Brain (1990) have also shown A. ataxacantha to be atypical within sect. Monacanthea. These independent studies suggest parallel evolution of prickle and inflorescence type in subg. Aculeiferum sens. str. and indicate that further investigation within sections Aculeiferum and Monacanthea on a worldwide scale is needed in order to reassess the boundaries between them.

It is worth repeating here that sampling within subg. Aculeiferum sens. str. has been somewhat limited (see Table 3). It is therefore possible that future study of this group may identify additional monophyletic lineages that warrant generic recognition.

Figures

Fig. 5. Indicative distribution of distribution of Acacia subg. Aculeiferum sens. str. (excludes sect. Filicinae and the ‘Acacia coulteri group’).