Biology Reference
In-Depth Information
Tanoak, one of the most seriously affected species, is a common
understory tree in redwood (
Sequoia sempervirens
) and Douglas-fir
(
Pseudotsuga menziesii
) stands in California and may pass the sudden oak
death fungus to these species. In 2002, sudden oak death fungus infections
were found in young Douglas-firs and redwoods in the wild, and labora-
tory tests showed that healthy young trees of both species could be
infected. These observations raised the fear that the fungus was adapting
to two of the most valuable forest trees in North America. In addition, the
fungus appears capable of infecting northern red oak (
Quercus rubra
) and
pin oak (
Q. palustris
) from eastern North America (Rizzo et al. 2002).
Thus, an alien, lethal fungus is spreading and adapting to new hosts at
alarming rates. Already devastating to oaks and tanoaks, this evolutionary
experiment might change oak-dominated woodlands of the Pacific coast
and possibly elsewhere (Gilbert 2002). No one can predict the outcome.
The Evolution of Disease Agents and Their Hosts
All higher plants and animals are host to a variety of microorganisms that
may be beneficial, harmless, or pathogenic. Some are organisms that have
evolved a mutualistic relationship with their hosts. Others are facultative
forms that feed on dead tissues or, in animals, intestinal contents, but can
sometimes become pathogenic, as in the case of the human colon bac-
terium,
Escherichia coli
. Still others are pathogens that are in active coevo-
lution with the host (Levin and Udovic 1977).
The relationship between a pathogen and a particular host is the result
of an evolutionary interaction between the defenses of the host and the
virulence of the pathogen (see, e.g., Clay and Kover 1996). Natural selec-
tion favors an increase in host resistance, on the one hand, and the level
of virulence that maximizes the fitness of the pathogen, on the other
(Gilbert 2002). In the case of an Australian flax,
Linum marginale
, and its
rust pathogen,
Melampsora lini
, for example, the balance between host
resistance and rust virulence varies in different populations of the flax
(Thrall and Burdon 2003). In the rust, virulence and transmission proba-
bility show a strong trade-off. In flax populations most exposed to rust
attack, strong rust resistance was favored in the flax plants and high viru-
lence was favored in the rust. In more isolated flax populations, plant
resistance was not as highly favored, while high spore production and
lower virulence were favored in the rust.
Resulting relationships between a host and an infective agent may vary
greatly in their nature.The relationship may be commensal, with neither
