The Geography of Phytochemical Races

140 3 After the Ice
3.1.7 Ambrosia psilostachya (Asteraceae)
Ambrosia psilostachya DC. is a ragweed that displays a high level of morphological
variation throughout its range, which extends from Canada to central Mexico.
It was noted by the University of Texas group, however, that populations on the
Texas Gulf Coast islands were noticeably more homogeneous than the species
at large, suggesting to them that the island populations had arisen from a single
source. The possible usefulness of sesquiterpene lactone data in shedding some
light on relationships among populations of this species emerged from observations
by Miller et al. (1968) that plants from an island population exhibited the
same set of sesquiterpene dilactones based on psilostachyin, for example, psilostachyin
[276] and psilostachyin-B [277] (see Fig. 3.5) that they observed in plants
from the mainland dunes suggesting that the latter population may have been the
source of propagules that gave rise to the island populations. It was also noted
that plants from the vicinity of Vera Cruz, Mexico, exhibited the same array of
dilactones.
The sesquiterpene lactone chemistry was reinvestigated in greater detail by Porter
and Mabry (1972), who used plant material from 20 populations of A. psilostachya
Fig. 3.6 Map of Texas coast
and adjacent Mexican sites
from Ambrosia psilostachys
(Ap) study. Populations of
Ambrosia cumanensis are
represted by “Ac.”