The Geography of Phytochemical Races

6.1 The Galapagos Islands 247
offensive to the senses of herbivores or toxic. In response to the idea that these
chemicals would be lost in a shift to an island lifestyle, the occurrence of secondary
plant chemicals in island plants was reviewed leading to the conclusion that losses
were minimal or not apparent in the vast majority of cases (Bohm, 1998b).
One of the few examples where the loss-in-new-local hypothesis has been tested
involves a detailed examination of cyanogenic plants on the Galapagos Islands.
Adersen et al. (1988) examined 475 specimens using either fresh material tested
in the fi eld, or herbarium specimens available in Copenhagen using the picric acid
color test (picric acid-soaked fi lter paper turns brown in the presence of HCN).
Of 97 freshly collected species, 24 were considered to be strongly cyanogenic
(10 mg HCN kg −1 plant material), while 27 additional species were scored as weakly
to moderately cyanogenic (2.5–10 mg HCN kg −1 plant material). Taking the analysis
a step further, these workers observed that only 45% of species endemic to the
islands were HCN-positive, as compared to 62% of the species that occur on the
archipelago and on the mainland. When herbarium specimens were tested, only
17.5% of endemic species gave a positive color test compared to 18.6% for the nonendemics.
This dramatic drop in reactivity with herbarium specimens is likely the
result of the breakdown of cyanogenic glycosides during long standing. The reduction
of cyanogenesis in the Galapágos fl ora, relative to the mainland, can be taken as
an indication that selection pressures that keep the level of toxic compounds on the
mainland high may not be functioning to the same level of intensity on the islands.
A more recent study provides additional evidence for reduction of defenses in an
island setting. In a study of herbivory on Santa Cruz Island, one of the California
Channel Islands, Bowen and Van Vuren (1997) found that endemic plants had lower
levels of defensive chemicals, as well as reduced mechanical defenses, that is, spines
and thickened cuticles, compared to related taxa on the mainland (Santa Ynez Mountains).
Feeding trials demonstrated that sheep, which are not natural inhabitants on
the Channel Islands, grazed the endemics to a greater extent than they did mainland
plants. The putatively defensive chemicals studied were expressed as total phenols
and total tannins. Although cyanogenic compounds were not included in the study,
three of the genera involved, Cercocarpus, Heteromeles, and Prunus, are members of
Rosaceae, a family with many HCN-yielding species.
Returning to cyanogenic compounds for the moment, it is interesting to look at the
glycoside profi le in Passifl ora foetida L., a species with a comparatively wide distribution,
including the Galapagos Islands and Réunion Island in the Indian Ocean. The
Galapagos material has been accorded varietal status, P. foetida var. galapagensis
Killip, on the basis of differences in morphology of stipules, glands, and hairs, and
appears as such in the Flora of the Galapagos Islands (Wiggins, 1971). This taxonomic
recognition has not been universally accepted (Lawesson, 1988), however.
Andersen et al. (1998) studied the cyanogenic glycosides of P. foetida from both locations.
Plants from the Galapagos Islands, grown from seed, yielded tetraphyllin-A
[519] (see Fig. 6.2 for structures 519–524), tetraphyllin-B [520], tetraphyllin-B sulfate
[521], and volkenin [522], and related cyclopentene-based compounds. Material
from Réunion Island afforded tetraphyllin-B, its sulfate, and volkenin, but was
distinguished from the Galapagos material by the accumulation of linamarin [523],