The Geography of Phytochemical Races

204 4 Intercontinental Disjunctions
Fig. 4.13 Compounds 357 and 358 from Castanospermum and Alexa. Compounds 359–364,
diterpenes from Scoparia dulcis
Castanospermum, an Australian genus that consists of the single species C. australe
A. Cunn. & C. Fraser ex Hook., and Alexa, a genus of eight or so species from
South America (Kaplan, 1995); both belong to the tribe Sophoreae of Papilionoideae.
Other, related genera will be commented upon below.
Castanospermine was obtained from Castanospermum australe (Hohenschutz
et al., 1981), while australine was isolated from the same species by Molyneux et al.
(1988). Nash et al. (1988) identifi ed these compounds as components of Alexa.
Related compounds that differ in the number of hydroxyl groups, and respective
stereochemistries, have been isolated from these taxa as well. It is of interest that
swainsonine [359], closely related to castanospermine, has been identifi ed (Colegate
et al., 1979) as a component of Swainsona canescens (Benth.) F. Muell. Swainsona
is a genus of some 50 species (Mabberley, 1997, p. 692), found only in Australia
and on the south island of New Zealand (one species). Swainsonine has also been
isolated from North American species of Astragalus and Oxytropis (Colgate et al.,
1979; Molyneux and James, 1982). Neither of these genera is represented in the
Australian fl ora. Kaplan (1995) suggested that these chemical data point to a “ . . .
common ancestor thriving on the old South Pangaean continent . . .” from which
Castanospermum and Alexa arose. Kaplan goes on to suggest that ancestors of
Astragalus and Oxytropis may have occupied the “continent” as well. The authors
were not precise in their estimates of the time scale involved in these events. Given
that the age of the legumes has been reasonably estimated at about 50 million years,
based on fossil evidence, it ought to have been possible to narrow the possibilities
for the time of separation of the ancestors of the taxa involved. In any event, it
seems unlikely that compounds of the sort seen in these taxa developed independently
in several unrelated ancestral (or modern) taxa, although one can never rule