Agriculture Reference
In-Depth Information
exactly which resistances it has, the hypothesis should be tested by appropriate
crosses. However, it is not at all practicable to do test-crosses to determine the
resistances of all commercially significant varieties to all important diseases.
Inferences about varietal resistances are most reliable if a well-characterised set
of isolates is used. Such collections exist for many important pathogens and have
usually been built up over several decades, reflecting the successive introduction of
resistance genes to breeding programmes and the development of knowledge about
the pathogen itself. They are therefore themselves important outputs of surveys of
variation in pathogen populations.
It is now possible to identify some commercially important resistance genes by
the DNA sequences of the genes. Inference of resistance genes by molecular data is
simpler than for avirulence or fungicide resistance genes. Since most commercial
crop species have a very small effective population size (Barrett, 1981), the allelic
variation at any one resistance locus is usually limited. In particular, when a
resistance gene has been introgressed into commercial breeding programmes from a
landrace or a wild plant related to the crop species, there may only be one resistance
allele in current use. Continuing with the example of the
Mla
mildew resistance
genes of barley, this is probably the case with most commercially important alleles,
including
Mla1
,
Mla6
,
Mla9
,
Mla12
and
Mla13
(Jørgensen, 1992b, 1994).
Mla7
represents a more complex situation, however, as it appears to have been
introgressed into commercial breeding programmes on four separate occasions
(Brown and Jessop, 1995). In a still more complex case, certain important resistance
genes have not been introduced deliberately into breeding programmes so the
number of functional resistance alleles is not known. For instance,
Stb6
in wheat for
resistance to septoria tritici blotch has entered modern commercial breeding
programmes from at least ten independent landrace sources (Chartrain
et al.
, 2005a)
and that number may be an underestimate, as Chartrain
et al.
studied only one wheat
variety from China and none from North Africa or South Asia, which are important
secondary centres of diversity for wheat. Even when
Stb6
has been isolated, it will
be a challenging task to enumerate every independent functional allele of this gene
used in world wheat breeding.
Molecular sequences do not give complete information, however, as they
describe the genotype at a particular locus, not the resistance phenotype. Some
resistances are suppressed by modifier genes in certain varieties, for example the
Pm8
gene of wheat for resistance to mildew (Ren
et al.
, 1997). In such a case, use of
the DNA sequence alone would incorrectly identify some varieties as resistant
although they had a susceptible phenotype. However, the widespread use of DNA
sequence data, using standard molecular genetic methods, will at least help to
delimit the range of resistances that should be studied by pathology tests, which
require more specialised skills.
(c) Identification of durable resistances
An especially useful output from a long-term survey programme is knowledge about
durable resistance. For example, the
mlo
gene has provided resistance to powdery
