Biology Reference
In-Depth Information
have been produced. Races of
P. sojae
that can overcome these patterns of
cultivar resistance arise rapidly, however, and many races of the fungus are
known (Forster et al. 1994). More than 200 races of
Phytophthora sojae
,
with different combinations of virulence genes, were detected in Ohio
soybean fields in the late 1990s, for example (Dorrance et al. 2003).About
20% of these showed combined virulence to ten or more resistance genes
in soybean.
In general, for
Phytophthora
fungi, the host-fungus relationship appears
to follow a gene-for-gene pattern of resistance and virulence. For the late
blight fungus (
P. infestans
), 11 sets of resistance and virulence genes have
been discovered in the relationship with plants of the genus
Solanum
(Abu-El Samen et al. 2003a). As we noted in chapter 1, different mating
types of the late blight fungus exist. These have the capacity for meiotic
recombination, so the spread of a second mating type into areas that for-
merly possessed only a single mating type threatens to increase the evo-
lutionary potential of the fungus. Genetic forms of the fungus that might
have been the result of sexual reproduction were detected in the mid-
1990s (Goodwin et al. 1998). In addition, the late blight fungus showed a
substantial ability to alter its virulence characteristics during asexual
reproduction (Abu-El Samen et al. 2003b).
Many other fungal parasites have shown similar ability to overcome
resistance.These include lettuce downy mildew (
Bremia lactucae
), powdery
mildew of wheat and barley (
Erysiphe graminis
), barley leaf blotch fungus
(
Rhynchosporium secale
), sorghum anthracnose (
Colletotrichum sublinolum
),
and rice blast fungus (
Magnaporthe grisea
). Many of these fungi, such as
powdery mildew of barley, exhibit different mating types that can allow
sexual recombination and promote diversity in virulence patterns (Bous-
set and deVallavielle-Pope 2003). Like the situation for the first late blight
fungus, these fungi have a substantial evolutionary potential based on the
introduction of new strains and complementary mating types to agricul-
tural regions where their various crop hosts are grown. Hybridization
among parasitic fungi of the genus
Phytophthora
has given rise to forms
adapted to new hosts, as described in chapter 5.
Alien Animal Disease Agents and Their Evolution
Disease agents of animals, particularly viruses, have an enormous capabil-
ity for rapid evolution. Like plant diseases, these agents are now able to
cross geographical barriers in the vehicles of human travel and commerce.
