The Geography of Phytochemical Races

292 7 Polar Disjunctions
(1978), and seven species from South America by Simpson et al. (1979). In all cases, the
lipid fractions consisted solely of 3-acetoxy fatty acids based on C 16 , C 18 , and C 20 chains,
for example, [601] (see Fig. 7.3 for structure). The authors concluded, quite reasonably,
that the possession of identical profi les of unusual fatty-acid derivatives argues for a close
relationship among the species of the two disjunct areas and further, that these compounds
are likely to have been present in the ancestral taxon.
7.7 Larrea (Zygophyllaceae)
The genus Larrea, the creosote bush, has been the subject of extensive examination
from many points of view, not the least of which is the origin of the North
American–South American disjunction (see Hunziker et al., 1977 and Mabry et al.,
1977 for detailed reviews). Although our primary interest involves the two disjunct
taxa (or one with disjunct populations, depending upon one’s taxonomic perspective),
information on the entire genus is necessary to address the question of direction
of evolution within the group. In the broader context, the genus consists of fi ve
species grouped into two sections on the basis of leaf and fl ower size. Four species
occur in southern South America, and the fi fth occurs in northern Mexico and
the southwestern United States (jointly referred to as the “North American” taxon
for convenience). Section Larrea consists of two species characterized by multifoliolate
leaves and small fl owers: L. nitida Cav. (2n = 26), which occurs in western
Argentina and in Chile, and L. amieghinoi Speg. (2n = 26), which occurs in southern
Argentina from Santa Cruz Province north to Neuquén. Section Bifolium, characterized
by bifoliolate leaves and larger fl owers, comprises the remaining three species:
L. divaricata Cav. (2n = 26) from the semidesert of central Argentina and disjunctly
west of the Andes in isolated pockets in Chile and Peru; L. cuneifolia Cav. (2n = 52)
from the central desert of Argentina (the Monte); and L. tridentata (DC.) Colville,
the North American species.
Larrea tridentata is cytologically the most complex member of the genus with
diploids (2n = 26) in the Chihuahuan Desert, tetraploids (2n = 52) in the Sonoran
Desert, and hexaploids (2n = 78) in the Mojave Desert. Some authorities prefer to
consider the North American L. tridentata and the South American L. divaricata
as conspecifi c, a suggestion based upon close similarities in morphology, and supported
by their capacity for forming fertile hybrids. Electrophoretic data, reviewed
briefl y by Hunziker et al. (1977), clearly distinguished among the South American
species and, because of the high degree of similarity of their protein profi les, provided
additional support for the hypothesis that L. divaricata and L. tridentata are
closely related, if not conspecifi c. Moreover, the electrophoretic data suggested that
L. divaricata from northern Patagonia is most closely related to the diploid form of
L. tridentata from New Mexico. Comparisons of chromatographic profi les of all
fi ve taxa also revealed signifi cant differences among the South American taxa and
a high degree of similarity between L. divaricata and L. tridentata. Details of the
chemical composition of Larrea can be found in Mabry et al. (1977).