Acacia subg. Phyllodineae

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 : 13-15

Acacia subg. Phyllodineae

( Fig. 8 )

This enormous group of 960 species is largely confined to Australia ( Table 1). Within Australia there are 948 described species [species numbers derived from information presented in Pedley (1999), Kodela and Tame (1999), McDonald and Maslin (2000) and Maslin (2001)]. We estimate that about 100 species are yet to be described. Nineteen species of subg. Phyllodineae occur outside Australia (fide Pedley 1975, in addition to A. sericoflora, which occurs in Papua-New Guinea). Seven of these species also occur in Australia; the extra-Australian endemics are found in the Pacific region (seven species) including Hawaii, Asia (three species) and Madagascar and the Mascarene islands in the Indian Ocean off the east coast of Africa (two species).

The subgenus may be characterised in the following ways (see Table 4 for further details): Trees or shrubs. Prickles absent. Stipules normally present and scarious, sometimes spinose. Leaves bipinnate or more commonly modified to polymorphic phyllodes, rarely reduced to scales or absent. Inflorescence systems simple or racemose, sometimes paniculate; flowers arranged in globular or obloid heads or cylindrical spikes, very rarely with ovary on gynophore. Funicle arillate or exarillate.

All genetic and cladistic studies since 1995 have shown subg. Phyllodineae to be monophyletic ( Chappill and Maslin 1995; Miller and Bayer 2000, 2001; Robinson and Harris 2000) and, as already noted above, we consider it now timely that the group be recognised as a distinct genus. Pedley (1986) proposed use of the generic name Racosperma for subg. Phyllodineae ; however, as discussed below, a proposal is being prepared to retypify Acacia with an Australian type. If this effort is successful, the generic name Acacia would be retained for this group.

Although subg. Phyllodineae is monophyletic, its relationships are seemingly equivocal. Chloroplast and nuclear DNA evidence supports a non-monophyletic tribe Ingeae as sister to subg. Phyllodineae ( Miller and Bayer 2000, 2001; Robinson and Harris 2000; Luckow et al. 2003). This strong evidence contradicts previous notions by Pedley (1986) and Chappill and Maslin (1995) that the sister group of the subg. Phyllodineae is subg. Aculeiferum sens. lat. Further work within the tribe Ingeae is necessary to determine phylogenetic relationships and which lineage is sister to subg. Phyllodineae .

On the basis of cpDNA work, relationships within subg. Phyllodineae appear less resolved than within either subg. Acacia or subg. Aculeiferum ( Miller and Bayer 2000, 2001; J. T. Miller, R. Andrew and R. J. Bayer, unpubl. data). The elucidation of infrageneric categories within subg. Phyllodineae is hampered by the enormous size of the group, which comprises more than 960 species contained in at least 100 informal species groups (Maslin and Stirton 1997). Furthermore, on the basis of currently available sequence data, the small amount of divergence within subg. Phyllodineae will slow development of a meaningful and well-supported infrageneric classification ( Murphy et al. 2000, 2003; J. T. Miller, R. Andrew and R. J. Bayer, unpubl. data). The low resolution in these studies may indicate that at least many of the lineages have undergone a large, recent morphological radiation and that hybridisation among taxa may be a common phenomenon.

Although Pedley’s (1978) classification ( Table 2) provides a useful framework for discussing species of this vast subgenus, this is largely an artificial scheme. Recent cladistic, molecular and morphological studies suggest that, with the possible exception of the relatively small sections Lycopodiifoliae (phyllodes whorled) and Pulchellae (leaves bipinnate), the other sections (which comprise mostly phyllodinous species and constitute about 95% of the subgenus) are paraphyletic or polyphyletic.

Nevertheless, recent examination of subg. Phyllodineae shows that some general trends are beginning to emerge; perhaps the most significant of these are the following:

(1) Bipinnate-leaved species of sect. Botrycephalae and certain members of sect. Phyllodineae with racemose inflorescences (e.g. A. binervata, A. microbotrya and several additional species) form a monophyletic group. This relationship is well supported by the results of a number of independent studies, for example Vassal (1972), Tindale and Roux (1974), Brain and Maslin (1996), Murphy et al. (2000, 2003), Miller and Bayer (2001) and J. T. Miller, R. Andrew and R. J. Bayer (unpubl. data). The results of J. T. Miller, R. Andrew and R. J. Bayer (unpubl. data) and Murphy et al. (2003) indicate that the Botrycephalae is non-monophyletic, suggesting that there have been multiple reversals from phyllodes back to bipinnately compound leaves.

(2) Among the phyllode-bearing species, there appears to be an important division between uninerved taxa (sect. Phyllodineae ) and plurinerved taxa (sect. Plurinerves and sect. Juliflorae). The classifications of Vassal (1972) and Pedley (1986) embody these differences. Some support for a uninerves v. plurinerves division is seen in the results of various recent studies, including morphology ( Chappill and Maslin 1995), immunology ( Brain and Maslin 1996) and DNA sequence data ( Murphy et al. 2000, 2003; Miller and Bayer 2001; J. T. Miller, R. Andrew and R. J. Bayer, unpubl. data). The general trend in the morphological and DNA studies was for the uninerved species of sect. Phyllodineae (together with the bipinnate-leaved species of sect. Botrycephalae) to form a terminal monophyletic group, whereas the plurinerved species of sections Plurinerves and Juliflorae formed a series of basal paraphyletic groups.

(3) The two plurinerved groups (sect. Plurinerves and sect. Juliflorae) are traditionally separated on the basis of the shape of their inflorescences, globular in the former and cylindrical in the latter section. However, studies by Chappill and Maslin (1995), Murphy et al. (2000, 2003) and J. T. Miller, R. Andrew and R. J. Bayer (unpubl. data) do not support monophyly of either sect. Plurinerves or sect. Juliflorae. On the basis of immunological evidence, Brain and Maslin (1996) produced a similar result.

(4) Inflorescence structure, racemose v. non-racemose, has traditionally been an important character for recognition of subgroups within sect. Phyllodineae . However, the immunological study by Brain and Maslin (1996) and DNA studies ( Murphy et al. 2000, 2003; J. T. Miller, R. Andrew and R. J. Bayer, unpubl. data) show these two groups not to be as closely related as once thought. In fact, these studies showed that some of the uninerved, non-racemose species of sect. Phyllodineae have their greatest affinities with the plurinerved species of sect. Juliflorae and sect. Plurinerves and practically none with the racemose species of sect. Phyllodineae . These surprising results require further investigation.

(5) The racemose species of sect. Phyllodineae certainly do not form a monophyletic group. As is evident from the results of Brain and Maslin (1996), Miller and Bayer (2001), J. T. Miller, R. Andrew and R. J. Bayer (unpubl. data) and Murphy et al. (2000, 2003), there are at least three main groups of racemosae and these are centred on A. podalyriifolia–A. microbotrya (this group is close to sect. Botrycephalae), A. pyrifolia and A. ligulata–A. myrtifolia (these two species are contained in the expanded Pulchelloidea group as defined by J. T. Miller, R. Andrew and R. J. Bayer, unpubl. data). Testing of more species will undoubtedly reveal further racemose groups, e.g. A. suaveolens and its allies.

(6) The results of molecular studies of Murphy et al. (2000, 2003) and J. T. Miller, R. Andrew and R. J. Bayer (unpubl. data) suggest support for Vassal’s small sect. Pulchelloidea, which brings together bipinnate-leaved species of sect. Pulchellae with a few uninerved and plurinerved phyllodinous species. In addition, J. T. Miller, R. Andrew and R. J. Bayer (unpubl. data) also placed sect. Lycopodifoliae and several species of sect. Alatae into this clade. Vassal’s Pulchelloidea was partially described by seedling ontogenetic characters. These data show that the first two seedling leaves appear simultaneously as a pair of opposite, pinnate leaves in all taxa of this clade (J. T. Miller, unpubl. data). In the majority of taxa outside this clade, the first leaf is pinnate and the second leaf is alternate and bipinnate, a more common pattern of subg. Phyllodineae . However, it is noted that the results of Chappill and Maslin (1995) did not support sect. Pulchelloidea as monophyletic; similarly, Pettigrew and Watson (1975) found no support for this section. Further study of this intriguing group is warranted, particularly as to the inclusion of sect. Lycopodifoliae within its circumscription.

(7) Most studies have shown sect. Alatae to be a polyphyletic assemblage.

Figures

Fig. 8. Indicative distribution of distribution of Acacia subg. Phyllodineae.