Marker-Assisted Introgression of Major QTLs for Grain Yield Under Drought in Rice - Translational Genomics for Crop Breeding: Abiotic Stress, Yield and Quality - page 50

Fig. 4.1. Development strategies for different types of mapping populations in self-pollinated species (a) Recombinant
inbred lines (RILs), (b) Backcross inbred lines (BILs), (c) Doubled haploids (DH).

Moreover, the relative ease of maintaining the
purity of the lines, resulting from a high percent-
age of self-pollination in the subsequent genera-
tions after crossing, also makes population devel-
opment easier. The complexity of genetic control
of grain yield under drought in rice requires the
development of large populations for the map-
ping of QTLs. A population size of more than
300 lines can be considered suitable for the map-
ping of these QTLs. A variety of populations can
be used for mapping QTLs for high grain yield
under drought. Some of them are described in
the sections below.
Recombinant inbred lines (RILs) are devel-
oped by crossing two parents that contrast for the
trait of interest, followed by subsequent selfing
and advancement through the single seed descent
(SSD) method to achieve nearly homozygous
lines (Figure 4.1a). Backcross inbred lines
(BILs) are another form of mapping populations,
which are widely used for QTL mapping. F 1 s
developed by crossing two parents contrasting
for the target trait are backcrossed ( n number of

times) to one of the parents to develop a BC n F 1
population (Figure 4.1b). Most often, the parent
used for backcrossing is the susceptible parent
into which the QTL has to be finally introgressed.
These BCnF 1 plants are then selfed through the
SSD method to a BC n F 3 generation and then
bulked to develop BC n F 3:4 lines that can be used
for drought screening and the identification of
QTLs. These populations are especially suitable
for introgression of exotic germplasm from wild
species to domestic varieties (Peterson 2002).
Although most of the donors of drought QTLs
are traditional varieties or landraces possessing
valuable genes for drought tolerance, their yield
potential is usually low under irrigated condi-
tions. BIL populations are highly suitable for
such cases. Apart from this, the relatively higher
percentage of recipient genome in the back-
ground leads to a more convenient transfer of
QTLs to the recipient. Advanced backcross pop-
ulations are also suitable for fine-mapping pur-
poses when large near-isogenic line (NIL) popu-
lations are needed with different segments of the