The Geography of Phytochemical Races

182 4 Intercontinental Disjunctions
Turner speculated that the two families arose from a common ancestor that likely
occupied xeric or halophytic habitats in Gondwana in an area that encompassed what
were to become southwestern Africa and southeastern South America. The ancestral
types do not still exist, but both families share the unusual, betanidin-based fl oral
chemistry, and the equally unique sieve-tube plastids seen in Caryophyllales but are
absent elsewhere in the plant kingdom (Behnke and Turner, 1971).
Cactaceae are certainly among the most readily recognized groups of fl owering
plants known. Their distinctiveness is based upon a wide variety of growth forms,
including, but by no means limited to, the striking barrel and columnar cacti of the
American deserts. The very extensive diversity exhibited by this group has led a
number of workers over the years to conclude that the family must be an old one in
order to accommodate the level of diversity now evident. Speculation put the age of
the family before or at the beginning of the Tertiary, perhaps 65–90 mya (Axelrod,
1979; Gibson and Nobel, 1986; Mauseth, 1990). That amount of time could both
account for the diversity of structure and coincide with the separation of South
America and Africa and thus explain the absence of Cactaceae in Africa.
Recent studies employing macromolecular techniques have brought that scenario
into question. A study by Hershkovitz and Zimmer (1997) employing ITS
sequence divergence of nuclear ribosomal DNA revealed that the cacti nested
phylogenetically within arid-adapted members of Portulacaceae. The degree of
sequence divergence suggested an origin of the family in about mid-Tertiary, perhaps
30 mya with later diversifi cation coincident with the formation of American
deserts. Most recently, Nyffeler (2002) described studies aimed at determining phylogenetic
relationships within the family. Although not specifi cally concerned with
the age of the family, the recorded information on sequence divergence of the trnK
intron, the matK gene, and trnL-trnF sequences, support the time frame argued by
Hershkovitz and Zimmer (1997) for the appearance and subsequent diversifi cation
of the family.
4.2 Across the Indian Ocean (Primarily)
4.2.1 Hymenaea verrucosa (Fabaceae)
Hymenaea verrucosa (Gaertn.) Oliv. occurs along the coast of eastern Africa
(Kenya, Tanzania, and Mozambique) and on the Islands of Madagascar, Mauritius,
the Seychelles, and Zanzibar (Fig. 4.6). S. S. Martin et al. (1973) examined populations
from Kenya and Madagascar for their leaf-pocket resins and identifi ed the
following sesquiterpenes: α-cubebene, α-copaene, copacamphene, caryophyllene,
β-humulene, γ-muurolene, α- and β-selinenes, and δ-cadinene. Profi les from the
two areas were qualitatively identical, but differed signifi cantly (1% level) in the concentration
of two components, copaene [314] (11.9 and 7.6%, Kenya vs. Madagascar,
respectively) and caryophyllene [315] (31.2 and 41.3%, Kenya vs. Madagascar,