The Geography of Phytochemical Races

154 3 After the Ice
These workers also addressed the problem of abnormal production of 3-carene
by certain trees in the California population of P. longaeva. Controlled crosses with
P. balfouriana individuals having known levels of 3-carene production led to the conclusion
that gene fl ow between P. balfouriana and P. longaeva could be ruled out as
an explanation for the deviant profi les. The alternative explanation would involve the
selective elimination of the capacity to make 3-carene. Again, no immediate selective
advantage of the carene-free state comes to mind. The existence of 3-carene-producing
trees in the fi rst place could have been due to limited gene exchange between P. aristata
(or an aristata-like ancestor) and P. longaeva (likewise) through a path south of the
Grand Canyon of the Colorado River that no longer exists.
The third paper in the bristlecone pine series (Snajberk et al., 1979) addressed
the issue of chemical differentiation within P. balfouriana. This species exhibits
a disjunct distribution with populations known from the mountains of northern
California (Marble Mountains, Salmon Mountains, Trinity Alps, and Yolla Bolly
Mountains), and in the southern Sierra Nevada where the range lies just to the west
of the range of P. longaeva (they are separated by 35 km across the Owens Valley).
Analysis of the monoterpenes of representative specimens revealed that the
northern and southern populations can be distinguished primarily on the basis of
their different foliar α-pinene concentrations. It is interesting to note, recalling the
signifi cance of differences between wood and foliar monoterpenes in the Arizona
populations of P. aristata, that monoterpenes from the wood of P. balfouriana did
not show any differentiation. Other differences between northern and southern populations
of P. balfouriana included higher limonene content in wood from northern
populations and higher 3-carene content in foliar monoterpenes from northern populations.
These chemical differences were taken as support for an earlier suggestion
that the northern and southern forms of this species deserve subspecifi c recognition
( Mastrogiuseppe, 1976).
The fi nal paper in this series Zavarin et al. (1982) described studies of resin
canal structural differences within the subsection, concluding that despite having
P. aristata-like monoterpenes, the trees in the Panamint Mountains of California
rightfully belong in P. longaeva. They also identifi ed a chemical-latitudinal gradient
in trees producing 3-carene. The gradient involves frequency of the I hc allele
(hc = high 3-carene as opposed to lc = low 3-carene), which exhibited the highest
frequency (1.0) in trees in the Panamint Mountains (ca. 36°05′N), and ranged to
a low value of 0.10 in the White Mountains (Esmerelda, Nevada, 37°49′N). Intermediate
frequencies, 0.269 and 0.318, were observed for populations in the Inyo
Mountains.
The level of genetic variation in Pinus longaeva in eastern Nevada and adjacent
western Utah was investigated by Hiebert and Hamrick (1983). Seeds were collected
from fi ve populations, two from the Markagurt Plateau of Utah, and three
from Nevada, one each representing the Snake Range, the Egan Range, and the
White Pine Range. Electrophoretic analysis provided information for seven enzyme
systems representing 14 loci. Variation among populations was low but statistically
signifi cant, whereas variation within populations was high. In general, the overall
level of genetic variation observed in this study of bristlecone pines is in reasonable