The Geography of Phytochemical Races

270 6 Oceanic Islands
appropriate position on the O-bound sugar residue (the inner sugar). All four taxa
in question have the capacity to make quercetin 3-O-diglucoside, which allows us
to remove it from further consideration. The putative ancestral taxon, G. tinctoria,
has the capacity to convert its 3-O-galactoside into 3-O-digalactoside, but G. peltata
apparently does not. Gunnera peltata, interestingly, has the capacity to transfer a
xylose unit to its quercetin 3-O-glucoside, resulting in the formation of quercetin
3-O-xylosylglucoside. This represents gain of a feature by which these two taxa can
be distinguished. Thus, G. peltata has lost the capacity to make the digalactoside,
but has gained the capacity to make the xylosylglucoside, a second feature by which
they can be distinguished. This argument is based on the assumption that there are
two enzymes involved in attaching the outer sugar, one specifi c for glucose, which
both taxa have, and one in G. peltata specifi c for xylose. If this were the case, then
alterations in the glycosyltransferase systems have occurred during the migration to
Masatierra and subsequent differentiation. The situation becomes even more intriguing
when the diglycoside chemistry of G. masafuerae is considered. This Masafuera
endemic, thought to have arisen from G. peltata, has the most complex diglycoside
chemistry of the group. The fi rst thing that can be noted is the capacity of this species
to form the xylosylglucoside, which, as we have just seen, is otherwise seen
only in its progenitor, G. peltata. This co-occurrence clearly points to a close link
between the two. Somewhat perplexing, however, is the reappearance of quercetin
3-O-digalactoside, which, as the reader will recall, was seen in the mainland ancestor
but not in G. peltata. It would be of interest to determine if this is simply a matter
of relative amounts caused by some drop in the level of activity of the appropriate
enzyme(s) in the intermediate species (G. peltata) with a return to the earlier level
in G. masafuerae. Obviously, quantitative studies of enzyme activity are necessary
to answer such questions. However, probably the most signifi cant change in diglycoside
biochemistry within this group of taxa is the capacity of G. masafuerae to
make the 3,7-di-O-diglucoside. Whereas the above assumption concerning outer
sugar specifi city is speculative, the involvement of a new position of glycosylation,
and thus an enzyme of different specifi city, is much less so. Position specifi city in
fl avonoid substitution reactions is a well-known phenomenon. This is also likely to
be the case in a shift from transfer of a sugar to the outer position in a diglycoside
to transfer to a new phenolic position, in this case, the 7-OH group. One of the diffi -
culties in addressing problems of this sort is our comparative ignorance of the sugar
transferases, particularly what happens to control these steps as evolutionary divergence
occurs. It is generally thought, for example, that control of glycosylation can
be upset in hybridization, even between closely related species. This may explain
some of the observations in an instance of interspecifi c hybridization, involving the
two Gunnera species on Masatierra.
Additional evidence indicating a close relationship between G. bracteata and
G. peltata came from the observation of apparent natural hybridization between
them in Quebrada Villagra on Masatierra. A comparison of morphological features
of both parents and putative hybrid individuals taken along two transects showed
clear-cut intermediacy in the latter (Pacheco et al., 1991b). In addition, introgressive
hybridization was indicated by the presence of individuals with intermediate values