Agriculture Reference
In-Depth Information
It is now possible, with the use of molecular markers derived from a variety of
technologies, to effi ciently select breeding lines based on a plant's genotype rather than
its phenotype. Use of this technology is expanding rapidly as the genetic knowledge
underpinning it is developed. Molecular markers can only be used where the resistance
genes or modifi ers required for resistance expression have been linked on the chromo-
some to markers and where there are a suffi cient number of markers polymorphic across
the breeder's germplasm available to enable the rapid and accurate identifi cation of
specifi c resistant genotypes. These conditions have only been met for a limited number
of resistance genes used in breeding programs and so much development work remains
to be done before the technology is mature. Breeding programs are also keen on having
'perfect' markers rather than linked markers. Perfect markers show complete associa-
tion with the resistance gene and provide unambiguous data on the presence of specifi c
resistance alleles. In many cases, such markers are based on single-nucleotide polymor-
phisms (SNPs) that underlie variation in the resistance genes. In other cases, insertions or
deletions (indels) in the DNA are involved. Few such markers are currently available for
most crop species, although progress in their development is being made rapidly.
Once developed, molecular markers allow more precise and rapid selection than pheno-
typic selection. They also allow a plant breeder to reliably pyramid two or more resistance
genes into a single variety more readily, thereby increasing the likelihood that the variety
will have durable resistance to multiple strains.
6.4.2
Pedigree breeding
This is standard procedure for many inbreeding crop programs. It involves the selection
of individual plants in the F
2
and subsequent generations, so that a precise pedigree of
each line can be traced through the program. Generally it is based on simple crosses, but
frequently involves topcrosses to a third parent or, less often, more complex crosses.
Pedigree breeding allows for plant selection where maximum diversity is expressed
amongst the segregating progeny. The method is most likely to be used where the resis-
tant donor parent is adapted to the region of cultivation or where both adapted parents
have some useful resistance and where the disease is reliably expressed in a single plant.
Early generation selection for disease resistance ensures that resistance is retained during
the selection processes for other traits of economic signifi cance. This is only likely to be
achieved, however, where high throughput and low cost screening systems are in place.
This is often the case for foliar pathogens that are highly visible and can be inoculated
into disease nurseries. It is less likely to be the case for diseases that are more diffi cult to
observe, inoculate uniformly or which are costly to screen.
For pedigree breeding, MAS is most likely to be used in later generations where the
numbers of lines are smaller and when a greater proportion of genes are homozygous.
This may change as the cost of marker screening is reduced.
6.4.3
Backcrossing
Backcrossing aims to retain a large proportion of the genome of an adapted variety, while
introgressing one or a small number of specifi c genes. It involves the screening of single
