Contents - Faperta

Applications of Biotechnology in Agriculture: The Prospects 25
Marker-Assisted Selection
It takes fi ve to six generations to transfer a trait within a species into high-yielding, locally
adapted cultivars through conventional breeding, and one has to plant a large number of
progenies to be able to select the plants with appropriate combination of traits (Figure 2.1).
The improved lines developed then have to go through a set of multilocation tests, before
a variety can be identifi ed for cultivation by farmers. This process takes a minimum of
seven to ten years. Recombinant DNA technologies, besides generating information on
gene sequences and function, allows the identifi cation of specifi c chromosomal regions
carrying genes contributing to traits of economic interest (Karp et al., 1997). The identifi -
cation of DNA markers for traits of interest usually depends on making crosses between
two genotypes with substantial and heritable differences in the trait(s) of interest.
Depending on the crop and traits involved, mapping populations are then derived from
the progeny of this cross by selfi ng once, many times, backcrossing to one of the parental
genotypes (BC), or subjecting plants to tissue culture to generate doubled haploids (DH).
The major advantage of most mapping populations is that each line is homozygous and
therefore can be multiplied indefi nitely through self-pollination. This then allows the
population to be evaluated under many environments and seasons, facilitating a much
more accurate estimate of phenotypic variation, on which to base the mapping exercise.
Mapping populations also allow scientists from many diverse disciplines to study different
aspects of the same trait in the same population. This approach can only be used
when parental genotypes can be identifi ed with opposing phenotypes for the trait of
interest. Interspecifi c crosses can also be used to good effect, but linkage maps derived
from such crosses may have limited relevance in crop improvement programs (Fulton
et al., 1997).
Wide
species
Wide
crossing
Biotechnological applications for crop improvement
X Elite line X
Line with trait
of interest X Elite line
Hormones/
embryo rescue/
tissue culture
F1 F1 X
Recurrent
parent
F1 BCF 1
*BCF n
7–10 years
Phenotypic
selection
Molecular
markers
F 6-8
3–6 years
BC 1
BC 3-5
Field
evaluation
Multilocation
testing
Conventional
breeding
F 5
Improved lines
Varieties with
specific traits
7–10 years
FIGURE 2.1 Application of tools of biotechnology in crop improvement.
Genetic
transformation
Tissue culture
Transformation with
novel genes
Plant
regeneration
Bioassay
Characterization
T 1 - T 3-5
Stable transgenic
plants
Field evaluation
5–6 years