The Geography of Phytochemical Races

256 6 Oceanic Islands
been cytogenetic (Carr and Kyhos, 1981, 1986), enzyme electrophoretic studies
(Witter and Carr, 1988), crossing studies (Carr, 1995), and DNA sequence-based
studies (Carr et al., 1996) of the alliance.
The fi rst suggestion that the silverswords had a connection with North
American Heliantheae tribe Madiinae, came from Gray (1852), who was familiar
with Argyroxiphium. This idea was rejected by Keck (1936), a noted expert on
North American Madiinae, who failed to see the morphological similarities and,
not surprisingly, was dissuaded by the distances involved that a colonizer would
have to travel from the North American mainland. The issue attracted the attention
of Carlquist (1959a, b, 1966, 1967, 1983), who presented detailed anatomical
evidence that clearly pointed to a close relationship of the Hawaiian taxa to the
tarweeds.
A North American origin for the silverswords clearly emerged from the application
of DNA-sequence techniques to members of the alliance and representative
Californian species (Baldwin et al., 1990, 1991). The conclusions from these studies,
based on chloroplast restriction site analysis were subsequently corroborated by
ITS nuclear ribosomal DNA studies. A comprehensive review of these studies can
be found in Carlquist et al. (2003).
Extensive studies of fl avonoids of mainland and Hawaiian Madiinae have
revealed a high degree of similarity in terms of both the components of the sticky
glandular material (the “tar” of the tarweeds) and vacuolar constituents (Bohm and
Fong, 1990; Crins and Bohm, 1990; Bohm et al., 1992; Bohm, 1999). The exudate
profi les consist of, in various combinations, fl avanones, fl avones, and fl avonols with
varying degrees of extra oxygenation (substitution at C-6 or C-6 and C-8) plus a
comparatively high level of O-methylation. Profi les of vacuolar fl avonoids—those
characterized by glycosidic linkages—are also similar in both sets of species.
Qualitative and quantitative studies of vacuolar and glandular fl avonoids of
Wilkesia gymnoxiphium A. Gray from several populations showed only minor levels
of variation (Bohm and Fong, 1990; Yang and Bohm, unpublished data). Five
specimens of each Dubautia scabra (DC.) Keck subsp. scabra and D. ciliolata
(DC.) Keck subsp. glutinosa G. Carr also showed species homogeneity (Crins et
al., 1988a).
Perhaps the most useful contribution made by fl avonoids in this group of plants,
however, was the assistance they provided in studies of natural hybridization
between Dubautia scabra and D. ciliolata (Crins et al., 1988a). Dubautia scabra
is a pioneer plant that colonizes new lava moderately; D. ciliolata occurs on somewhat
older lava. Where newer fl ows overlap older ones, one can often fi nd plants
with features intermediate between the two species. This phenomenon is readily
observed in the vicinity of Kilauea Volcano on the island of Hawaii, where one
fi nds D. scabra subsp. scabra on newer substrate and D. ciliolata subsp. glutinosa
on the older substrate. Since the two taxa exhibit different fl avonoid profi les, and we
know that fl avonoid profi les are inherited in an additive fashion, hybrids would be
expected to exhibit fl avonoid markers inherited from each parent. A major advantage
in this study was the availability of known F 1 hybrids that had been produced
from controlled crosses at the University of Hawaii (by G. D. Carr). Analysis of the