Agriculture Reference
In-Depth Information
One of the most extensive applications of indirect marker systems to
investigate the evolution of virulence has been in
P. striiformis
f.sp.
tritici
. Justesen
et al.
(2002) developed a set of amplified fragment length polymorphisms (AFLP)
for this fungus and showed that groups of related AFLP genotypes were often
associated with characteristic pathotypes, as determined by virulences to a differen-
tial set of wheat varieties. Hovmøller
et al.
(2002), using information from the
distribution of AFLP variants, showed that long-distance dispersal of
P. striiformis
f.sp.
tritici
isolates from the UK to Denmark occurred on at least five occasions,
each of which was associated with an epidemic of yellow rust on wheat varieties
possessing certain resistance genes. The role of long-distance dispersal in the evolution
of plant pathogens in general was reviewed by Brown and Hovmøller (2002).
AFLP markers were also used by Enjalbert
et al.
(2005) to investigate the
evolution of
P. striiformis
f.sp.
tritici
in France. As in the UK and Denmark, the
pathogen has a population structure consistent with clonal reproduction but
populations in the north and south of France were quite distinct in their
distribution of AFLP types. It was proposed that this has resulted partly from
selection by local wheat varieties but also perhaps from adaptation to local
climatic conditions.
Another significant conclusion from the work of Justesen
et al.
(2002) and
Hovmøller
et al.
(2002) is that the rate of evolution of virulence and avirulence,
under pressure of selection by resistance genes in crop varieties, is an order of
magnitude faster than that of the DNA markers used. This implies that molecular
markers may be appropriate for delimiting the likely range of pathotypes of an
isolate but cannot be treated as a substitute for pathology tests of virulence.
3.5 CHARACTERISING PATHOGEN POPULATIONS
For practical use, data on a set of isolates must be reduced to a summary form so
that the composition of the sample, and by inference that of the population, as well
as changes in response to natural selection and other factors, can be described
concisely. Here, the analysis of virulence and fungicide response data is discussed.
3.5.1 Virulences
The frequency of virulence to a resistance gene or a differential variety is
straightforward to estimate from a random sample of a pathogen population
(assuming that a reasonably random sample can be obtained). Two other important
aspects of the characterisation of virulence frequencies which have concerned
pathologists involve associations between virulences and frequencies of pathotypes
defined by several virulences.
(a) Associations between virulences
Associations between alleles of pairs of genes may be summarised by the linkage
disequilibrium,
D
(also known as the gametic disequilibrium or the haploid phase
