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DENDRON » No/Nr: 43 » November 2011
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review
review
Morphology
Vachellia
1. Capitate inflorescences (bright golden yellow with
some pale-yellow to white).
2. Stipules spinescent.
3. Seeds with large areoles that conform to the
outline of the seed shape.
4. Development of fascicular secondary leaves.
5. Inflorescences have an involucel on the peduncle.
6. Ovaries sessile or sub-sessile with no disc
(nectary).
7. Floral bracts persist to seed set.
8. Pod pericarp consists of only longitudinal fibres,
or is absent.
9. Differences in pod venation.
Palynology
10. Pollen grain with furrows on the surface.
anatomy
11. Differences between petioles.
12. Differences in ontogeny of stipule tissues.
13. Anatomy of leaflet tissue types differ.
14. Differences between seed coats.
Genetics
15. Diploid chromosome number of 2n=52.
16. Differences between nucleolar diameters and
chromatin lengths.
Biochemistry
CLASSIFICATION AND NOMENCLATURE OF THE GENUS ACACIA
17. A very distinctive amino acid distribution found in
the seeds.
18. Seeds contain unusual amino acids that are not
present in the other group.
19. Gum has a higher molecular weight-viscosity than
the other group.
20. Heartwood has flavonoids of the resorcinol A-ring
group.
Morphology
Senegalia
1. Spicate inflorescences (pale-yellow to white
flowers).
2. Stipules non-spinescent.
3. Seeds with small horseshoe-shaped central
areoles.
4. No development of fascicular secondary leaves.
5. Inflorescences do not have an involucel on the
peduncle.
6. Pedicellate ovaries with a cup shaped disc
(nectary).
7. Floral bracts shed before flower opens.
8. Pod pericarp consists of longitudinal and
latitudinal fibres.
9. Differences in pod venation.
Palynology
10. Pollen grains without furrows on the surface.
anatomy
11. Differences between petioles.
12. Differences in ontogeny of stipule tissues.
13. Anatomy of leaflet tissue types differ.
14. Differences between seed coats
Genetics
15. Diploid chromosome number of 2n=26.
16. Differences between nucleolar diameters and
chromatin lengths.
Biochemistry
17. Not a distinctive amino acid distribution in the
seeds.
18. Seeds contain unusual amino acids that are not
present in the other group.
19. Gum has a lower molecular weight-viscosity than
the other group.
20. Heartwood has flavonoids of the pyrogallol A-ring
group.
TABLE 3.– Differences between the two mainly African groupings of Acacia s.l. now known as Vachellia
and Senegalia, as summarized by Ross (1979). (Exceptions to these trends exist, but they are few and
many can be explained as hybridization events or recent convergence.)
4.2 Recent evidence
The evidence in Table 3 was not enough, however,
to convince people to take up classifications splitting
Acacia s.l. into different genera (such as that of Pedley
(1986)). It is only recently that the world of systematics
has become more inclined to embark upon the
huge taxonomic endeavour of splitting this large
genus—fuelled primarily by a large quantity of sound
phylogenetic evidence using molecular markers
(Kergoat et al. 2006).
Probably the most comprehensive phylogenetic study
done on the acacias and its relatives in the tribes
Acacieae, Ingeae and Mimoseae, is that of Bouchenak-
Khelladi et al. (2010). They found that Mimosoideae is
monophyletic, but that the distinction between the taxa
within this subfamily is not well defined and should be
investigated further. In addition, they found that the
tribe Mimoseae is paraphyletic because Vachellia, a
member of the tribe Acacieae, nests within it. This led
them to believe that, despite being characterized by
synapomorphic characters, the tribe does not share
a common ancestor. Vachellia, Senegalia, Mariosousa
and Acaciella are all supported as monophyletic
groups, but Vachellia is considered to be basal while
Senegalia, Mariosousa and Acaciella are more derived
and representative of early-diverging lineages from
the Ingeae and Acacia s.s. clade. Although their
phylogenetic relationships are not yet resolved, the
tribe Ingeae and Acacia s.s. form a monophyletic
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group. All these relationships are illustrated in Figure
2, a simplified version of the phylogenetic tree by
Bouchenak-Khelladi et al. (2010).
Kergoat et al. (2006) attempted to use alternative
sources of information to provide additional insights
into Acacia s.l. systematics. They argued that
phytophagous insect species that are obligate feeders
on a restricted number of host plant taxa should display
a similar evolutionary pattern to that of the host plant.
This pattern is termed ‘taxonomic conservationism in
host plant use’. Kergoatet al. (2006) was particularly
interested in the specialized seed-beetles or Bruchid
beetles (Coleoptera, Chrysomelidae, Bruchinae)
because they generally exhibit a strong level of
taxonomic conservationism in host-plant use and are
known to develop in Acacia s.l. seeds.
It was found that species of the bruchid genera
Acanthoscelides, Mimosestes and Bruchidius clade
II are preferentially associated with Vachellia and
members of the tribes Cassieae and Ceasalpinieae.
Species of Merobruchus feed almost exclusively on
Acacia s.s., Acaciella, Mariosousa and Senegalia as
CLASSIFICATION AND NOMENCLATURE OF THE GENUS ACACIA
review
review
well as members of the tribe Ingeae. Lastly, Stator
and Bruchidius clade I species are able to feed on a
much larger set of host plants including both Vachellia
and other Acacia s.l. species (Kergoat et al. 2006).
The phylogeny of Mimosoideae seed predators thus
support the phylogeny depicted in Figure 2 and the
idea that Acacia s.l. is paraphyletic. Some groups of
beetles are associated only with the basal Vachellia,
Cassieae and Ceasalpinieae, while others are
associated with the more derived Acacia s.s., Acaciella,
Mariosousa, Senegalia and Ingeae.
Further noteworthy observations from the Kergoat
et al. (2006) study are that the patterns of host-plant
associations seem to be strongly mutually exclusive.
In other words, predators that feed on Vachellia
and Ceasalpinieae seed will not feed on Acacia
s.s., Acaciella, Mariosousa and Senegalia seed and
vice versa. Also, species of seed predators that are
associated only with Vachellia are closely related,
suggesting a conservative host shift to the genus.
From all the above mentioned evidence it is thus
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DENDRON » No/Nr: 43 » November 2011