The Geography of Phytochemical Races

248 6 Oceanic Islands
Fig. 6.2 Compounds 519–524, cyanogenic glycosides from Passifl ora species
a valine-derived cyanogenic glucoside. The fi nding of cyclopentene-derived as well
as common α-amino acid-derived cyanogenic glycosides in the same plant species
opens up an interesting area for further investigation. It would be of some signifi cance
to determine if both types of cyanogens are produced by the same set of enzymes.
If that should be the case, it would lead to the question of why valine is used by the
Réunion plants and not by the Galapagos plants. If the two compound types are not
produced by the same enzymes, it would be necessary to determine where in the
pathway the differences lie.
The presence of an unusual cyclopentene-based cyanogenic glycoside in Passifl ora
colinvauxii Wiggins, an endemic species of the Galapagos Islands, further restricted
to Indefatigable Island (Santa Cruz), has been used to argue for a close relationship
between this species and the two species from mainland Ecuador, P. bifl ora Lam.
and P. punctata L. (Adersen et al., 1993). The structure of “passibifl orin” is shown
as [524]. Although the presence of this unusual diglycoside (gulose derivatives are
rare in nature) in the island species certainly is a good indicator of relationship with
the mainland species, it is not possible to state with certainly that the derivation of
the endemic species occurred directly from a mainland species or whether there is
(or was) an intermediate.
It might be mentioned in passing that variation in cyanogenesis has also been
reported for members of Turnera sect. Canaligerae of Turneraceae (Shore and
Obrist, 1992). The cyanogenic glycosides identifi ed from Turneraceae are based
upon the cyclopentene ring system, as are those we have just seen in the related
Passifl oraceae. Geographic patterning was not discussed in that paper, however.
6.1.2 Lycopersicon cheesmanii (Solanaceae)
The existence of a species of tomato on the Galapagos Islands was fi rst noted by
Charles Darwin. Despite a moderate level of variation, Lycopersicon cheesmanii
Riley is considered the only tomato species on the islands. All populations
examined exhibited morphological and physiological characteristics that clearly