The Geography of Phytochemical Races

10 2 Examples Within Continents
of Aloë plicatilis (L.) Miller leaf exudate. Those workers speculated that it might
be restricted to this anomalous species, the sole member of section Kumura.
A more recent study involving a much larger sampling of species revealed a very
interesting geographic distribution pattern. Viljoen et al. (1999) investigated the leaf
exudate chemistry of 380 taxa of Aloë using high-performance column chromatography
coupled with a diode array detector. Twenty species were found to have
plicataloside, often as the only detectable compound. It was accompanied in a few
species by trace amounts of compounds normally encountered in Aloë, namely,
chromones [16] and anthrones [17], or by unidentifi ed substances. Most of the
plicataloside- positive species occur in Kenya, Tanzania, and Uganda with outliers
in extreme southwestern Angola (A. palmiformis Baker) and extreme southwestern
South Africa (A. plicatilis, the original source). Other positive taxa originated in
Mozambique, Zimbabwe, and northeastern South Africa. Aloe schweinfurthii Baker
was also found to contain plicataloside. This taxon occurs in a narrow band running
from southern Sudan through Central African Republic, southern Chad, northern
Cameroon, and through central Nigeria, Benin, Togo, and east-central Ghana. Species
endemic to Madagascar available for study lacked plicataloside.
The authors were very conservative about any phylogenetic speculations based
upon the single chemical apomorphy. The genus is known to have relatively few
morphological apomorphies at the infrageneric level, and no morphological feature
was held in common by the plicataloside-positive taxa. The workers’ (Viljoen et al.,
1999) concluding remarks are best appreciated directly: “We do not wish to suggest
that chemical characters should enjoy preference over the morphological characters
as all problems (e.g., convergence) encountered with morphological characters are
prevalent for chemical characters. However, we do believe that the presence of this
unique compound should not be completely dismissed as chemotaxonomic coincidence,
and we should at least explore further the possibility of taxonomic affi nity
between [sic] these plicataloside-containing taxa.”
It is obvious in situations like this that a clear phylogenetic picture is necessary
in order to gain some understanding of the distributional history of the taxa in question,
in this case the plicataloside-positive ones. This would appear to be a situation
where gene sequence information might well provide the needed framework.
It would also be of interest to learn where the biosynthesis of plicataloside diverges
from the pathway that leads to the anthrones normally seen in Aloë.
Viljoen et al. (2002) have described the occurrence of a new anthrone derivative,
which they named “homonataloside B,” from 14 species of Aloë (of the 380 tested).
Species containing this compound, a hitherto unknown diglycoside, were obtained
from scattered sites in East Africa, the Horn of Africa, and southern parts of the Arabian
Peninsula. The compound was not observed in any of the species native to Madagascar.
2.1.6 Aspalathus linearis—Rooibos tea (Fabaceae)
Aspalathus is an endemic South African legume genus, particularly well represented in
the Cape District. According to Mabberley (1997, p. 61), 278 species are recognized.