The Geography of Phytochemical Races

286 7 Polar Disjunctions
and varieties. Payne (1964), however, preferred to consider A. chamissonis as a
large, heteromorphic species consisting of two varieties, var. cuneifolia (Nutt.)
Payne, ranging from the mouth of the Columbia River north to British Columbia,
and var. chamissonis from the southern part of the species’ range. The latter variety
consists, in turn, of two intergrading forms, forma chamissonis with unlobed leaves,
and forma bipinnatisecta with pinnately decompound leaves (Payne et al., 1973).
Several reports describing different sesquiterpene derivatives from A. chamissonis
collected from different areas suggested that secondary chemistry might be of value
in helping to sort out relationships within the complex (see Geissman et al., 1973
for citations). The major compound reported in those studies was the germacranolide
chamissonin [574] (see Fig. 7.1 for structures 574–580), with other collections
affording costunolide [575]. More recent work, involving a wider sampling of populations
throughout the range of the species, confi rmed the presence of those compounds
and revealed others (Geissman et al., 1973): chamisselin [576], chamissanthin
(8-α-hydroxycostunolide) [577], chamissarin [578], and the two chamissonin epoxides
[579] and [580]. Patterns of occurrence of these compounds have provided important
information on relationships within the species.
Plants collected north of the Monterey area were shown to be variable with
regard to both morphology and sesquiterpene lactone composition. Of particular
signifi cance are the sporadic occurrence of chamissonin and the appearance
of costunolide as a major component in nearly half of the samples analyzed.
However, there was no correlation between chemotypes and morphology. The situation
south of Monterey was shown to be markedly different, where individuals
proved to be morphologically uniform and to have an almost invariant chemotype
characterized by a preponderance of chamissonin and a lack of costunolide. These
data strongly suggested that the propagule(s) that gave rise to the southern populations
came from the central part of the range of A. chamissonis, where one fi nds
the greatest variation within the species. Although the sample size was small, two
Fig. 7.1 Compounds 574–580, terpenes from Ambrosia chamissonis