The Geography of Phytochemical Races

266 6 Oceanic Islands
(formerly the monotypic genus Rhetinodendron), and subgen. Robinsonia, which
consists of three sections, sect. Robinsonia with two species, sect. Eleutherolepis
with three species, and sect. Symphyochaeta with one species (Sanders et al., 1987).
Robinsonia masafuerae Skottsb. is the only member of this genus present on the
younger island.
Examination of allozyme diversity of four species of Robinsonia (Crawford et al.,
1992b) allowed an estimation of time of divergence of the four species ranging from
1.7 to 5.5 million years, which is in line with the estimated age of the older island,
Masatierra, on which the four species examined occur. Restriction-site mutations in
chloroplast DNA and the intergenic spacer region (IGS) of nuclear ribosomal DNA
(Crawford et al., 1993) revealed mutations characterizing individual species, but
provided no information on phylogeny. Study of ITS sequences, however, resulted
in a well-resolved cladogram very similar to the tree generated from morphological
information (Sang et al., 1995). The information also indicated that the genus is
monophyletic.
Pacheco et al. (1985) examined 54 populations representing all species of
Robinsonia for fl avonoids; comparatively little variation was observed. The profi les
of R. berteroi and R. macrocephala were identical and the simplest seen in the genus
consisting of quercetin 3-O-mono- and digalactosides. Robinsonia gayana Dcne.
and R. thurifera Dcne., which comprise sect. Robinsonia, exhibited those two compounds
plus quercetin 7-O-glucoside. The greatest differences were observed in
the three species that form sect. Eleutherolepis, each of which exhibited a different
array based on fl avones, fl avonols, fl avanones, and dihydrofl avonols, although not
all of these aglycones were present in all of the three species. Pacheco et al. (1985)
concluded that evolutionary advancement within the genus had been accompanied
by the accumulation of more complex fl avonoid arrays, suggesting that in addition
to making aglycones not seen in the other species, these three also accumulated—
sequestered to use their term—pathway intermediates not seen in the other taxa,
that is, the fl avanone and dihydrofl avonol derivatives. Owing to the comparatively
simple pigment profi les exhibited by these taxa, it is not possible to do more than
say that the compounds observed are in general similar to the fl avonoid chemistry
of Senecio, which is in agreement with the suggestion made on the basis of macromolecular
information that Robinsonia has arisen from a self-incompatible species
of Senecio from the South American mainland.
6.4.3 Erigeron (Asteraceae)
Six endemic species of Erigeron occur on the Juan Fernandez Islands, fi ve of which
occur only on the younger island (Masafuera): E. ingae Skottsb., E. luteoviridis
Skottsb., E. rupicola Philippi, E. stuessyi Valdebenito, and E. turricola Skottsb.
The sixth species, known from both islands, is E. fernandezianus (Colla) Harling.
Flavonoid profi les of the entire group, as well as several likely relatives from the
mainland, were described by Valdebenito et al. (1992b). The profi les were based