The Geography of Phytochemical Races

7.1 Ambrosia chamissonis (Asteraceae) 287
of three specimens collected near San Francisco exhibited a chemotype closely
resembling the southern populations. This appears to be a case of “founder effect”
after long-distance dispersal from a northern population. Spread of the species
southward from the Monterey area by local dispersal was ruled out owing to the
absence of suitable habitats; south of Monterey, sandy beaches are replaced by
steep cliffs for a distance of several hundred kilometers (Big Sur).
With the possible origin of the southern Californian populations accounted for,
we can turn our attention to the matter of the origin of Chilean A. chamissonis. An
examination of several populations from the Chilean coast (Nakatani et al., 1973)
revealed comparatively uniform morphology and sesquiterpene lactone chemistry,
a situation parallel to that just discussed for the southern California members of
the species. The lactones isolated from the Chilean populations were identifi ed as
chamissonin [574], costunolide [575], and chamisselin [576]. Long-distance dispersal
was again indicated, with morphology and chemistry both pointing to a source
population north of the San Francisco Bay area.
Ambrosia chamissonis also exhibits a rich array of acetylenic compounds.
Although not restricted to Asteraceae, polyacetylenic compounds—or polyynes—
do constitute one of the characteristic secondary metabolites of the family.
A comprehensive review of the structures and patterns of occurrence of these
compounds, including sources other than Asteraceae, was published by Bohlmann
et al. (1973). More recently, a series of review articles appeared discussing
polyacetylene occurrences in Anthemideae (Christensen, 1992), Cynareae
(Christensen and Lam, 1990), Heliantheae (Christensen and Lam, 1991a), and
Astereae (Christensen and Lam, 1991b). Many of the acetylenic compounds
identifi ed from these taxa possess, in addition to the requisite triple bonds, other
functional groups such as one or more double bonds, a hydroxyl group, an epoxide
group, a halogen atom, or some combination of these, for example, chlorohydrin
derivatives (Balza and Towers, 1990). Also of interest are acetylenic
compounds that bear one or more sulfur atoms. One of the simplest of these is
terthienyl [581] (see Fig. 7.2 for structures 581–589), a component of common
garden marigolds and other species. Compounds bearing two sulfur atoms are
also found in a number of species, as seen in the study of the components of
Ambrosia chamissonis. A study of the roots of this species collected from two
widely separated sites along the North American coast showed that compounds
[582–584] made up 85% of the acetylenic fraction with compound [585] making
up 5%. The remaining 10% of the acetylenic fraction from roots collected
near San Francisco, California (Marin Co.) was made up of compounds [586 and
587]. However, material collected in extreme southwestern British Columbia
(near Tsawwassen) lacked the latter two compounds but exhibited compounds
[588 and 589] instead.
Although these data suggest relationships, clear-cut phylogenetic connections
among the various populations must await the application of macromolecular
techniques. In addition to providing insight into the evolutionary relationships,
such data would also likely provide an idea of when colonization might have
occurred.