Geoscience Reference
In-Depth Information
constraints have imposed severe limitations on efficient utilisa-
tion of QTL mapping information in plant breeding through
MAS. Salient among these constraints are
1. Identification of a limited number of major 'QTLs with
more phenotypic variance controlling specific traits
2. The notion that QTL identification is required whenever
additional germplasm is used
3. Inadequacies/experimental deficiencies in QTL analysis
resulting due to either overestimation or underestimation
of the number and effects of QTLs
4. Lack of universally valid QTL marker associations appli-
cable over different sets of breeding materials
5. Strong QTL-environment interaction; and difficulty in
precisely evaluating epistatic effects
Increasing the potency of MAS for quantitative traits need
improved field experimentations/designs, robust mathematical
models and comprehensive statistical methods. As an exam-
ple, with composite interval mapping (CIM), field data from
different environments are often integrated into a joint analy-
sis to evaluate the Q × E interactions; thus, enabling identi-
fication of stable QTLs across environments. Besides, with
a detailed linkage map, CIM permits an explicit identifica-
tion of the QTL in the genome and better identification of
linked QTL (in coupling phase) from the identical parental
line (Babu et al. 2004).
Favourable QTL from even a phenotypically inferior paren-
tal line (in repulsion phase) also can be effectively identified and
utilised by DNA-based markers. Tanksley and Nelson (1996)
proposed an advanced backcross-QTL (AB-QTL) approach for
enhancing the QTL mapping in tandem with MAS. This analysis
involves crossing between elite germplasm and unadapted gen-
otype/wild relatives with favourable genes/QTLs, followed by
two generations of backcrossing for developing several hundred
sibling lines. These lines, each containing different genomic
segments of the wild relative/unadapted genotype, are then
genotyped using DNA markers. In effect, these lines become
a set of NILs that individually dissect the effects of potential
QTL in the background of the elite parent. At the same time,
the BC lines additionally give comparatively mounted mate-
rial for an essential step of replicated phenotypic evaluations.
The core of this approach—revealing and accessing the desir-
able alleles from wild relatives or unadapted genotype—indi-
cates that QTL mapping can go hand-in-hand with MAS rather
Search WWH ::




Custom Search