Biology Reference
In-Depth Information
the Toluca Valley of central Mexico is a center of diversity for the fungus
(Flier et al. 2003). Fungal strains differing in genetic structure and inter-
breeding potential exist in commercial potato fields, in potatoes grown in
subsistence farm plots, and in association with wild species of Solanum .
Although genetic differentiation for these plant hosts exists, measurable
gene flow does occur among the fungal strains.
Rapid evolution of many fungal species and their hosts frequently
tends to follow a gene-for-gene coevolution involving genes regulating
resistance of the host and genes regulating virulence of the pathogen
(Bergelson et al. 2001). In most cases, the allele of the host gene for resist-
ance is dominant and the allele for avirulence in the pathogen is domi-
nant, so the combinations in table 11.1 indicate situations in which infec-
tion or resistance will be shown. These genes apparently function
according to an elicitor-receptor relationship (DeWit 1992). In the
pathogen, a gene product, the elicitor, is produced, usually a protein or
other organic compound in the pathogen's cell wall. In the plant host,
receptor genes code for protein molecules that recognize the elicitor
components of the cell wall of the pathogen (Bishop et al. 2000). Recog-
nition leads to the induction of specific defense proteins, such as chitinases
that destroy the cell walls of fungi or bacteria. Coevolutionary changes
thus involve the ability of the pathogen to change the structure of its sur-
face compounds and the ability of the plant to detect new configurations.
Table 11.1. Gene-for-gene relationships of a host with alleles for resist-
ance (R) or susceptibility (r) and a pathogen with alleles for avirulence
(V) or virulence (v). Capital letters indicate allele dominance. Boxes in
table body show the combination of host and pathogen genotypes that
can occur and whether or not infection is possible. Infection is impossi-
ble as long as dominant alleles for resistance (R) and avirulence (V) are
combined in host and pathogen.
Host Genotypes
Pathogen
Genotypes
RR
Rr
rr
VV
RR + VV
Rr + VV
rr + VV
Infection prevented
Infection prevented
Infection occurs
Vv
RR + Vv
Rr +Vv
rr + Vv
Infection prevented
Infection prevented
Infection occurs
vv
RR + vv
Rr + vv
rr + vv
Infection occurs
Infection occurs
Infection occurs
Search WWH ::




Custom Search