Geoscience Reference
In-Depth Information
because of lack of allele-specific markers, practical examples
of this approach in plant breeding are limited. One successful
example is the conversion of traditional maize lines into quality
protein maize (QPM) through marker-assisted transfer of a reces-
sive mutant allele, opaque2, and using allele-specific molecular
markers. In animal breeding, an array of allele-speciic markers
has been available facilitating the applications of this approach on
a commercial scale to eliminate disease and stress-susceptibility
genes. 'Marker-assisted background selection', a term coined by
Hospital and Charcosset in 1997, was initially proposed by Young
and Tanksley (1989), and experimented by numerous research-
ers. This strategy has been extensively used in commercial maize
breeding programmes, particularly for the selection of lines car-
rying transgenes conferring herbicide tolerance or insect resis-
tance. Within the background programme, several parameters
need to be optimised. Flanking markers for the target allele are
essential to get rid of linkage drag. The optimal distance between
the target gene and flanking markers govern the selection inten-
sity that will be exerted. The equations given by Hospital and
Charcosset (1997) and Frisch and Melchinger (2005) are useful
in determining the quantity of BC plants that need to be gener-
ated and typed with a special set of flanking markers. Variety of
gene/marker associations are reported in crop plants that may
probably be used in MAS strategies.
Improving
quantitative
traits using
MAS: Case
study
Most of the traits of agronomic importance are complex and
controlled by many genes. Improvement of such traits through
MAS could be an advanced endeavour, unlike the case of
merely inheritable traits. The genetic quality of quantitative
trait creates difficulty in their manipulation mainly because of
the quantity of genes involved in their expression and interac-
tions among genes (epistasis). Since many genes are involved
in the expression of a quantitative trait, these genes, in gen-
eral, have smaller individual effects on the phenotype, and
the effect of the individual genes are not simply identifiable.
This needs repetitions of field tests to characterise the exact
results of QTLs and to evaluate their stability across envi-
ronments. Assessment of QTL by environment interaction
(Q × E) continues to be a serious limitation on the efficiency
of MAS. Furtrhermore, epistatic interaction among totally
different regions of a genome will induce a skew evaluation
of QTL effects. Also, if the genomic regions concerned in the
interactions are not incorporated in the selection scheme, they
will probably bias the selection process. Despite the explosion
of QTL mapping experiments in recent years, a number of
