The Geography of Phytochemical Races

Chapter 4
Intercontinental Disjunctions
4.1 Across the Atlantic Ocean
4.1.1 Datiscaceae
Until comparatively recently, Datiscaceae were thought by some taxonomists
(e.g., Cronquist, 1981), to consist of four species in three genera, the monospecifi
c Octomeles (O. sumatrana Miq.) and Tetrameles (T. nudifl ora R. Br. ex Benn.)
and Datisca. Octomeles and Tetrameles are large trees native to southeastern Asia;
Datisca consists of D. cannabina L., which occurs from southwestern Asia to
Crete, and D. glomerata (Presl) Baill., which occurs in California and northern Baja
California, Mexico.
Earlier work on the fl avonoid components of D. cannabina by Grisebach
and Grambow (1968) and Pangarova and Zapesochnaya (1974) had revealed
the existence of an unusual array of fl avonoids that included glycosides of the
rare 3,5,7-trihydroxyfl avone galangin [299] and its 7-methyl ether [300], and
3,5,7,2′-tetrahydroxyfl avone, datiscetin [301], and its 7-methyl ether [302] (see
Fig. 4.1), along with the common fl avonols: kaempferol and quercetin. Since galangin
and datiscetin, and their methyl ethers, represented unusual substitution patterns
among the fl avonoids, it seemed reasonable to look at the other members of the
putative family for their possible presence. A detailed study of D. glomerata showed
that the unusual B-ring-substituted fl avonoids were indeed present as major components,
whereas the Asian taxa exhibited only common glycosides of kaempferol and
quercetin (Bohm, 1988). The fl avonoid profi le of D. glomerata was homogeneous.
This is not the case with isozymes or chloroplast DNA.
Despite the morphological similarity of D. cannabina and D. glomerata, and
their distinctive fl avonoid profi les, an electrophoretic examination revealed an
entirely different picture (Liston et al., 1989). Calculation of the mean genetic identity
among populations of both species, based upon variation of enzymes coded for
at 21 loci, yielded a value of I = 0.142. Intraspecifi c genetic identities were I = 0.649
for D. cannabina and 0.847 for D.
The authors suggested the following sequence of events as a possible explanation
for the present-day distribution. It has been thought that the DG-S plastome,
B.A. Bohm, The Geography of Phytochemical Races,
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