Biology Reference
In-Depth Information
pines that survived the blister rust outbreak at the Champion Mine site
in Oregon showed a several hundredfold increase in frequency of a gene
for blister rust resistance (Kinloch et al. 2003).The selective effect of this
rust outbreak was thus enormous.
The incidence of resistance genes to white pine blister rust is unex-
pectedly high in many populations of several of the pines in western
North America, being one, two, or more orders of magnitude greater than
expected by mutation alone (Kinloch and Dupper 2002). In sugar pine,
the frequency of the resistant gene varies greatly over the range of the
species. In much of Oregon, at the northern edge of the range, and in Baja
California, Mexico, at the southern edge, incidence is zero. In the south-
ern Sierra Nevada and the Transverse ranges of California, however, the
incidence reaches 6.6-8.2%. In most locations, the frequencies of geno-
types are those expected under the Hardy-Weinberg equilibrium. In
some stands heavily affected by blister rust, however, selection appeared to
be acting to increase incidence of the resistance allele above Hardy-Wein-
berg expectation (Kinloch 1992). In western white pine, a higher general
frequency of the resistance gene also exists in the Sierra Nevada of Cali-
fornia (Kinloch et al. 2003).
The substantial frequency of resistance among the several pines noted
above is difficult to explain.All of these pines, however, occur in environ-
ments where singleleaf pinyon (
Pinus monophylla
) and Colorado pinyon
(
P. edulis
) occur nearby on drier sites. These pines are often infected by
pinyon pine blister rust (
Cronartium occidentale
), a close relative of white
pine blister rust. Although this rust does not now attack five-needled
western pines, infection can be induced artificially (Kinloch and Dupper
2002). Thus, it appears that the incidence of resistance in western pines
may reflect past interaction between these pines and
C. occidentale
or a
closely related form of rust, perhaps during the late Pleistocene when
sugar pines and singleleaf pinyons grew in closer association.
In the case of the American chestnut, breeding programs have both
sought to find resistance within the American chestnut (
Castanea dentata
)
and to introduce genetic resistance from Asian relatives that are highly
resistant to the disease. Hybridization with the Chinese chestnut (
C. mol-
lisima
) and backcrossing with the American chestnut has resulted in a
blight-resistant tree with characteristics close to the American chestnut
(Griffin 2000). Recent studies (Rieske et al. 2003) suggested, however,
that these chestnut trees may be more susceptible to gypsy moth her-
bivory than are pure American chestnuts. Efforts have also been made to
introduce a strain of the disease fungus with low virulence to surviving