Septoria and Stagonospora Diseases of Cereals - CIMMYT ...

significant evolutionary force at
present. However, we consider it
more likely that some gene flow
continues as a result of the global
commerce in grain. The obvious
mechanism for gene flow on a
regional basis is air-dispersed
ascospores. In the case of P.
nodorum, the most likely
mechanism for intercontinental
dispersal is infected seed (King et
al., 1983). Since it has been shown
that M. graminicola can infect seed
(Brokenshire, 1975) we consider it
likely that this also is the
mechanism for long distance gene
flow in M. graminicola. Whatever
the mechanism, the high degree of
similarity in populations around
the world for both fungi suggests
that they have been transported
around the world by humans.
Evidence for selection
We recently completed an
experiment to measure competition
among 10 genotypes of M.
graminicola in a field setting. The 10
isolates were inoculated onto three
host treatments consisting of a
moderately resistant wheat variety
(Madsen), a susceptible wheat
variety (Stephens) and a 1:1
mixture of these cultivars. Our
most important finding in this
experiment was that intense
competition appeared to occur
among the different genotypes.
Significant changes in the
frequencies of specific pathogen
genotypes occurred over the season
(McDonald et al., 1999). Some
isolates showed evidence for
adaptation to particular hosts. The
results from this experiment
provided our first direct evidence
Population Genetics of Mycosphaerella graminicola and Phaeosphaeria nodorum 81
that selection operates on specific
M. graminicola pathogen genotypes
in a field setting.
We have not yet conducted
similar replicated field experiments
to measure selection in P. nodorum.
But we have indirect evidence that
selection does not result in
widespread clones that are adapted
to specific host genotypes. In an
experiment conducted in
Switzerland in collaboration with
Martin Wolfe’s group, we sampled
50 isolates of P. nodorum from each
of nine wheat fields near Zurich.
Three different wheat varieties were
represented three times each among
the nine wheat fields. Though fields
planted to the same variety used the
same source of seed, no genotypes
were shared among field
populations. Only six pairs of clones
were found among the 432 isolates
that were assayed. Isolates with the
same DNA fingerprints always
came from the same site within a
field (Keller et al., 1997b).
Taken together, all of our
experiments suggest that nuclear
genotypes do not persist through
time for either fungus. Instead, the
genes are the units of selection that
are carried forward across
generations. Selection operates on
the population instead of the
individual. In order to gain a
representative spectrum of the
diversity for virulence in natural
populations, plant breeders should
include the widest possible
diversity of strains when screening
germplasm for resistance to these
fungi. Similarly, chemical
companies should include at least
several hundred strains in their
screens for resistance to fungicides.
Conclusions
Given our present data, we have
drawn the following conclusions
regarding the evolutionary forces
that affect the population genetics
of M. graminicola and P. nodorum:
• For both fungi, the mating system
includes both sexual and asexual
reproduction. Asexual
reproduction may have an
important impact over an area of
a few square meters, but the
sexual reproduction has much
greater consequences for the
evolutionary biology of both
fungi. Genotypes are ephemeral
but genes persist in populations
through time.
• Population sizes are large enough
to make genetic drift negligible
for both fungi. Large population
sizes also ensure that ample
mutations are present in every
population to allow for a rapid
response to selection, e.g.
mutations from avirulence to
virulence for major resistance
genes. The population from
Patzcuaro, Mexico, exhibits a
genetic structure consistent with
a founder effect.
• Gene flow is sufficient to unite
large geographical areas into a
single genetic population. If gene
flow is ongoing, then breeders
should continue to test their
resistant lines over the widest
possible geographical area. If
gene flow is episodic, continued
vigilance is needed to limit the
spread of new virulence genes
and fungicide resistance genes.
Quarantines in areas with low
gene diversity, such as Australia,
should be enforced to limit the
evolutionary potential of these
populations. If CIMMYT
continues to use Patzcuaro as a
field site to screen for resistance