The Geography of Phytochemical Races

32 2 Examples Within Continents
divergence from the arrays of fl avonoid compounds sequestered by two species.”
The group at the University of Utrecht, whose work is examined next, has clearly
demonstrated that this is not necessarily true. It has been the combination of fl avonoid
profi le analysis and a thorough understanding of the genetic background of
each profi le, when translated into a measure of divergence that has led to the level
of understanding of the system that we now enjoy. To be certain, the Dutch workers
have studied many aspects of the evolutionary biology of Silene, and in that regard
answer the challenge of Crawford et al. (1992b) that studies of disjunctions—and
other systems to be sure—should include morphological, cytological, ecological,
and geological aspects of the systems under scrutiny. The information to follow
should suggest that study of secondary metabolites, and their genetic control, might
well be added to that list!
The evolutionary history of Silene attracted the attention of members of the
Department of Population and Evolutionary Biology at the University of Utrecht,
whose work dominates the bulk of the following discussion. Pioneering studies
of fl avonoid formation in Silene latifolia and S. dioica had shown a considerable
degree of allelic variation in Europe (Kamsteeg et al., 1978) with at least six loci
involved in the control of glycosylation in the former. Subsequent studies (van
Brederode and van Nigtevecht, 1975; Heimsbroek et al., 1980; van Brederode and
Kamps-Heimsbroek, 1981) provided a detailed view of the reactions involved.
The fundamental fl avonoids present in these plants are isovitexin [67] and its
positional isomer vitexin [68] (see Fig. 2.16 for structures 67–70), 6-C-glucosyl-
and 8-C-glucosylapigenin, respectively. Three of the loci, g, gl, and fg, control
glycosylation of isovitexin. A fourth locus, indicated as V, controls transfer of glucose
(dominant allele is Vg) and xylose (dominant allele is Vx) to the 2″-hydroxyl
group of vitexin. Three alleles have been found at the g locus: g, g G , and g X . The
g G and g X alleles control the transfer of glucose and xylose, respectively, to the
7-OH group of isovitexin. The alleles at the gl locus are designated as gl, gl R and
gl A . The latter two control the transfer of rhamnose and arabinose, respectively,
to the 2″-hydroxyl group of the C-bound sugar of isovitexin. The dominant form
of fg controls the transfer of glucose to the 2″-hydroxyl group of isovitexin. Two
other loci, P and Me, are responsible, respectively, for the 3′-hydroxylation of
isovitexin to isoörientin [69], and 3′-O-methylation of isoörientin to isoscoparin
[70]. The biosynthetic relationships among these compounds are shown in
Fig. 2.17.
Fig. 2.16 Flavone glycosides identifi ed from Silene