The Geography of Phytochemical Races

102 2 Examples Within Continents
study may, in fact, reveal that taxonomic dissection may not be justifi ed at all. The
problem may be purely academic, however, as both forms are considered at risk.
A more recent report from these workers (Dodd and Helenurm, 2002) described
an electrophoretic study of seven populations of the mainland subspecies and
all known populations (24 in all) of the two island subspecies. Information was
obtained representing 19 loci. Although the mainland populations have higher levels
of polymorphic loci (33.6% vs. 24.5%) and greater number of alleles per locus
(2.61 vs. 2.15) than the island populations, observed heterozygosities are lower for
the mainland populations (0.041 vs. 0.071), which was attributed to lower levels
of outcrossing for the mainland populations compared to island populations. These
values fall within the limits usually associated with comparisons of insular and
mainland subspecies (Hamrick and Godt, 1989). The island subspecies are nearly
identical genetically (mean I = 0.997). The island populations were very closely
related with no taxonomic distinction possible. Values for mainland populations
ranged from 0.752 to 1.0 (mean 0.929)
2.7.12 Isomeris arborea (Capparaceae)
Glucosinolates, the naturally occurring glucosides from which the so-called mustard
oils arise, have been studied in considerable detail with regard to their putative role
as defensive chemicals in a variety of brassicaceous taxa, for example, Cardamine
cordifolia A. Gray (Louda and Rodman, 1983a, b). Blua et al. (1988) have also
investigated ecological factors that may infl uence the formation of the glucosinolate
produced by Isomeris arborea Nutt. (Capparaceae) in Southern California. This species
offers the advantage of possessing a single glucosinolate, methyl glucosinolate,
also known as glucocapparin [213]. The presence of only one compound offers the
combined advantages of ease of quantitative analysis and absence of interactional
effects of a more complex array of compounds. Plants were collected from four sites
in southern California, allowing a comparison between desert and nondesert environments
to be made. The desert sites sampled were the low desert in Anza-Borrego
State Park (San Diego County) and a high desert site in the Mojave Desert (Kern
County). A valley grassland site near Bakersfi eld (Kern County) and a coastal sage
scrub site in Torrey Pines State Preserve (San Diego County) provided material from
nondesert environments. Glucocapparin was quantifi ed from fi ve tissues: immature
leaves, mature leaves, fl ower buds, capsule walls, and seeds. Seeds were eliminated
from further comparison owing to the lack of any statistically signifi cant difference
among their glucocapparin concentrations. Signifi cant differences were observed
Fig. 2.64 Breakdown of the glucosinolate glucocapparin, compound 212