The Geography of Phytochemical Races

2.7 North and Central America 111
2.7.17 Encelia farinosa (Asteraceae)
Taxonomic troubles with Encelia, a genus of perennial shrubs found primarily in the
Sonoran and Mojave Deserts of southwestern North America (two species occur in
South America and one occurs on the Galapagos Islands), include questions of species
number, relationships with other genera within its tribe, and, in fact, its proper tribal
affi liation. In addition to traditional taxonomic studies, examination of several classes
of secondary metabolites has revealed quite complex patterns of variation. For example,
Wisdom and Rodriguez (1982) analyzed plants from 16 populations of E. farinosa
A. Gray in Torr. for the benzopyran encecalin [225], the benzofuran euparin [226], and
the sesquiterpene lactone farinosin [227] (see Fig. 2.69 for structures 225–227). The
three populations from California, along with three populations from adjacent Baja
California, were characterized by high percentages (>80%) of encecalin with about
equal amounts of the other two compounds making up the balance. One population
from extreme northwestern Mexico and nine populations from Arizona were characterized
by concentrations of encecalin that varied from about 60% to zero. The proportions
of euparin and farinosin varied widely in these populations. Compounding these profi
les was the existence of both seasonal and ontogenetic variation, which, though clearly
evident, did not detract from the existence of a western and an eastern chemotype.
Some years later, Proksch and Clark (1987) examined 18 species (19 taxa) for the
presence of encecalin, 7-demethylencecalin [228] (Fig. 2.69), and euparin. These
compounds allowed a sorting of the taxa into several clear-cut groups. The issue that
concerns us in the present context, however, is those workers’ examination of variation
within E. farinosa, which was done in an effort to assess the degree of stability
of species profi les. In most cases differences between different populations were
quantitative. The species was sampled at ten sites along a transect running from Palm
Springs, California (33°49′N, 116°34′W) to Tempe, Arizona (33°25′N, 111°55′W),
a distance of about 500 km (Fig. 2.70). Highly signifi cant differences were observed
Fig. 2.70 Map of transects from Encelia farinosa study where circle size represents concentration
differences; and transects from Larrea tridentata study. Map of transects from Encelia farinosa
study where eastern (E) and western (W) types are identifi ed