The Geography of Phytochemical Races

190 4 Intercontinental Disjunctions
resulted from condensation of seco-loganin with tryptamine and/or DOPA fragments
as illustrated in deoxytubulosin [326].
Group 3 consists of two South American genera: Poraqueiba and Emmotum.
The presence of seco-loganoside and its methyl ester have been reported from
members of the former, but it is with these genera that the chemical evolutionary
process appears to have turned its attention to the production of sesquiterpenes
based on the eudesmane system [327], [328], and [329]. Further elaboration of terpenoid
biosynthesis is seen in the compounds of Group 4, which include a complex
array of diterpenoid derivatives, including the alkaloids icaceine [330] and
de-N-methylicaceine [330 less one of the N-methyl groups]. One of the underlying
ideas in Gottlieb’s analyses is the increasing level of oxygenation as taxa evolve
(become more “advanced”). Perhaps the reader can appreciate this idea better by
examining icacinone, compound [331], with reference to the number of oxygen
atoms involved. A more subtle point concerning that molecule is that it consists
of only 19 carbon atoms rather than the 20 that form the diterpene system. Loss of
the carbon is attributed to yet another oxidation resulting in the elimination of the
angular methyl group at C-13 (Kaplan et al., 1991).
Despite the chemical sophistication of the Icacinaceae study, and others like it,
it remains very diffi cult to assess the signifi cance of the conclusions without some
clear idea of phylogeny. Molecular genetic data that indicate evolutionary relationships
among the genera involved would allow interpretation of the secondary
chemical data from a much safer vantage point. As pointed out a few paragraphs
earlier, this criticism, more a suggestion really, could be made about any set of relationships
based upon secondary chemical data.
4.3 North Pacifi c
Few disjunctions have attracted more attention than those involving taxa whose
ranges are disjunct between eastern Asia and eastern North America or between
eastern Asia, western North America, and eastern North America. A substantial literature
has accumulated on this subject, including the historically important work of
Asa Gray (1846, 1859, 1878), a large number of descriptive works (e.g., Hu, 1935;
Li, 1952, 1972; Wolfe and Leopold, 1967; Graham, 1972a, b; Hara, 1972; Wolfe,
1975; Hsü, 1983; Hong, 1993; N. S. Lee et al., 1996), a thoughtful history of the
subject beginning at the time of Linnaeus by Boufford and Spongberg (1983),
and a recent review of relevant molecular phylogenies by Wen (1999). Additional
insights and critical comments on several issues relating to the similarities between
the eastern Asian and eastern North American fl oras can be found in Tiffney (1985).
As the reader can easily see from the partial list of publications above, the subject
is a massive one, indeed. It is not possible to discuss all applications of secondary
metabolite data to these disjunctions in the space available here. Therefore, the
following examples have been selected to represent a reasonable cross-section of
the information available.