The Geography of Phytochemical Races

6.4 Juan Fernandez Islands 269
Although all tested species (and populations thereof) exhibited quercetin glycosides,
the array of compounds in the individual species proved of particular
interest. Six quercetin derivatives were identifi ed: (1) 3-O-arabinoside, (2) 3-Oglucosylgalactoside,
(3) 3-O-galactosylgalactoside, (4) 3-O-glucosylglucoside,
(5) 3-O- xylosylglucoside, and (6) 3-O-glucoside-7-O-glucoside. The occurrence
data are summarized in Table 6.3.
Although each taxon exhibits a unique fl avonoid profi le, the overall pattern
similarities between the island endemics and G. tinctoria are striking, both in the
compound types they have in common, as well as the shared absence of kaempferol
glycosides. If, as the accumulated morphological and cytological evidence indicates,
G. tinctoria is the likely ancestor of G. peltata, some comments are in order
concerning the changes in the fl avonoid profi les that appear to have occurred during
the evolutionary process. Pacheco et al. (1993) discuss the changes in terms of loss
or gain of single compounds, in this case, the various quercetin diglycosides. It is
perhaps more instructive to examine the situation in terms of changes in biochemical
steps. The fi rst step in the biosynthesis of fl avonoid diglycosides is the formation
of the monoglycoside at the position in question, in this case the 3-OH group. Three
quercetin 3-O-monoglycosides are needed to account for the array of diglycosides
observed, that is, the arabinoside, the glucoside, and the galactoside. Since there
are no diglycosides with arabinose, we can focus on the fates of the glucoside and
the galactoside. Diglycosides are formed by transfer of a second sugar unit to the
Table 6.3 Flavonol glycoside profi les of Juan Fernandez Islands species of Gunnera compared to
species from South America and Mexico (from Pacheco et al., 1993)
Species Quercetin glycosides K-Glysa 1 2 3 4 5 6
Juan Fernandez Is.
G. bracteata
1 site + + − − − − −
6 sites
G. masafuerae
+ + − + − − −
2 sites + + + + − −
3 sites
G. peltata
+ + + + + + −
1 site + + − + − − −
4 sites
Mainland
G. tinctoria
+ + − + + − −
2 sites + + + − − − −
2 sites + + − + − − −
5 sites + + + + − − −
G. boliviari + + − + − − +
G. boliviana + + − − − − +
G. margaretae + + − − − − +
G. peruviana + + − + − − +
G. mexicana + + − + − − +
a Presence or absence of any kaempferol glycoside.