The Geography of Phytochemical Races

6.2 The Hawaiian Islands 251
have, until comparatively recently, been mostly speculative. The measurement of
genetic divergence in terms of changes in gene sequences, coupled with the knowledge
of how fast a genome is thought to undergo mutation, provides a much more
reliable means of estimating ages of island species. Fleischer et al. (1998) discuss
molecular-rate calibration using three examples from the animal literature.
The results of studies of secondary metabolites of Hawaiian endemics are primarily
useful in assessing levels of variation within taxa, but some generalizations
relating to relationships with likely ancestors can be made. We start our survey with
a genus well known to North Americans, Bidens, commonly called beggars’ ticks.
6.2.1 Bidens (Asteraceae)
As we have seen often in this review, the study of secondary metabolites does not
usually allow defi nitive statements to be made about the origin of an island endemic;
such information can nonetheless be useful in suggesting possible relationships, as
well as helping to establish the uniformity, or otherwise, of the taxa in question. Much
the same situation exists for the examples in this section. Two studies of secondary
metabolites in the Hawaiian species of Bidens have been described, one involving
polyacetylenes, and the other involving fl avonoids. The investigation of the polyacetylene
(syn.: polyyne) components involved an examination of 19 species and six subspecies
(the entire set sensu Ganders and Nagata, 1990) resulting in the identifi cation
of 18 compounds (Marchant et al., 1984). All taxa accumulated acetylenes in their
roots, but only 13 contained these compounds in their leaves as well. In general, each
taxon exhibited a unique array of compounds and, with a single exception, showed no
interpopulational variation. The exception was B. torta Sherff, where each population
had a unique array. (Bidens torta exhibited a unique array of fl avonoids as well; see
below.) The overall profi le of acetylenes exhibited by the Hawaiian species was quite
uniform, which the authors took as additional support for all species having been
derived from a single ancestral immigrant (Ganders et al., 2000). Most compounds
are known elsewhere in the genus, but one, 2-(2-phenylethyne-1-yl)-5-acetoxymethyl
thiophene [525] (see Fig. 6.3 for structures 525–535), observed in all tested specimens,
is otherwise unknown in the genus.
An examination of the fl avonoids of 33 populations of Bidens, representing 26
taxa (all species and all but one subspecies), was described by Ganders et al. (1990).
The overall pigment chemistry is in general agreement with reports of fl avonoids
from other species of Bidens. Aurones based on sulfuretin [526] (see Fig. 6.3 for
structures) and maritimetin [527]; chalcones based on butein [528] and okanin
[529]; the fl avanone eriodictyol [530]; the fl avones, apigenin [531] and luteolin
[532]; and the fl avonols kaempferol [533] and quercetin [534] were identifi ed and
shown to occur as a variety of O-glycosides in most species. O-Methylated derivatives
of some of the aglycones were also seen, but their distribution was more
sporadic. It was noted above that B. torta was the only taxon tested for acetylenes