The Geography of Phytochemical Races

6.4 Juan Fernandez Islands 267
upon glycosides of quercetin, quercetagetin (syn. 6-hydroxyquercetin), apigenin,
luteolin, and 6-C-glucosylacacetin in a variety of combinations. There was insuffi
cient variation within species to render any defi nitive statements unwise, although
the overall patterns were in general agreement with relationships suggested by the
morphological data. An unusual fl avonoid occurrence will be discussed in detail
below.
The combination of morphological data with the fact that fi ve of the six species
occur on the younger island (Masafuera) was taken by those workers to suggest that
colonization of the younger island occurred fi rst, followed by differentiation and ultimate
migration of E. fernandezianus (or an ancestor) to Masatierra. The fl avonoid data
do not support this scenario, suggesting instead that evolution of E. fernandezianus
involved colonization of Masatierra from the mainland, with subsequent colonization
of Masafuera. The fl avonoid profi le of E. fernandezianus from Masatierra has,
in addition to compounds present in other individuals of the species, such as luteolin
7-O-diglucoside [554] and 6-C-glucosylacacetin 7-O-diglucoside [555] (see Fig.6.9
for structures). These two compounds are not present in E. fernandezianus from
Masafuera, but are present in four of the fi ve species of Erigeron from the mainland.
The most parsimonious explanation would involve colonization of Masatierra from
the mainland with subsequent expansion of its range to Masafuera accompanied by
loss of the capacity to make the two compounds in question. Morphological data
suggest, however, that E. fernandezianus originated on Masafuera, along with the
other fi ve species, and subsequently migrated to Masatierra. This would require
that the individuals that colonized Masatierra regain the capacity to make the two
compounds. In order for these plants to regain the capacity to make both these compounds,
three reactions are necessary: (1) placement of the “outer” glucose moiety
to form luteolin 7-O-diglucoside; (2) establishment of the 6-C-glucosyl linkage;
and (3) activation of the enzyme necessary to O-methylate apigenin to form acacetin.
The total loss of the genes responsible for controlling these reactions, in the
colonization of Masafuera from the mainland, and their seemingly miraculous reappearance
in the Masatierra plants seems a rather unlikely process. It is much more
likely that, given the route of evolution supported by morphology, that the genes
involved were not lost, but simply silenced. Upon establishment of the new populations
on Masatierra, the suite of genes was released from whatever repression was
responsible, with the result that the full suite of compounds was again produced.
Fig. 6.9 Compounds 554 and 555, the odd fl avonoids from Erigeron