Contents - Faperta

186 Biotechnological Approaches for Pest Management and Ecological Sustainability
markers typically behave in a dominant or recessive manner and do not detect heterozygotes
(Staub, Serquen, and Gupta, 1996). Molecular markers are:
• Unaffected by the environment;
• Phenotype neutral; and
• Detectable at all stages of plant growth.
Marker-assisted selection can be used to accelerate the pace and accuracy of transferring
resistance genes into improved cultivars. The MAS takes three to six years, thus speeding
up the pace of transferring the traits of interest into the improved varieties, and it does not
require large-scale planting of the segregating progenies up to crop harvest, as only the
plants with marker allele indicating the presence of the trait or QTL need to be maintained
up to maturity. Narvel et al. (2001) used microsatellite markers to identify soybean QTLs
for resistance to foliar feeding lepidopteran insects, to determine the degree to which different
QTLs have been transferred into soybean cultivars over a 30-year period. Very few
resistant genotypes possessed multiple QTLs from different soybean linkage groups, and
MAS was suggested as a means of introgressing minor genes linked to soybean resistance
to insects into elite soybean cultivars. MAS in barley for resistance to cereal cyst nematode,
Heterodera avenae Woll. could be accomplished approximately 30 times faster and for 75%
lower cost compared to phenotypic selection (Kretschmer et al., 1997). Similar cost and
labor savings have also been documented for microsatellite markers linked to cyst nematode,
Heterodera glycines Ichinohe resistance in soybean (Mudge et al., 1997).
In contrast to the markers linked to resistance genes inherited as simple dominant traits,
the improvement of polygenic traits (QTLs) through MAS is diffi cult due to involvement of
a number of genes, and their interactions (epistatic effects). Resolving these problems often
involves multiple fi eld tests across several environments. However, such experiments often
display signifi cant QTL-environment interactions. Several studies on QTLs linked to stem
borer resistance in maize underscore the problems involved in using QTLs in MAS. The
relative effi ciency of phenotypic and MAS has been found to be similar (Groh et al., 1998a,
1998b; Willcox et al., 2002). However, phenotypic selection was more favorable due to lower
costs. MAS and phenotypic selection for leaf feeding resistance to D. grandiosella and
D. saccharalis improved the effi ciency of selection by 4%, indicating that MAS is less
effi cient than phenotypic evaluation (Bohn et al., 2001). Maximum progress has been made
in breeding for insect resistance in common bean by using a combination of phenotypic
performance and QTL-based index, followed by QTL based index, and conventional selection
(Tar’an et al., 2003). Although the cost of MAS is approximately 90% less than the cost
of conventional selection, accurate identifi cation of QTL position and the cost to generate
initial data for use in MAS make conventional selection more cost effective. However,
marker-assisted selection of large DNA segments can still be highly effective. Stromberg,
Dudley, and Rufener, (1994) did not get a better response to MAS than to conventional
selection for resistance to southwestern corn borer, D. grandiosella. Three putative QTLs
accounted for 28% of the phenotypic variance, and no signifi cant differences were observed
for leaf damage ratings or larval weights between lines selected by the two methods.
The use of DNA-based markers for indirect selection offers the greatest potential gains
for quantitative traits with low heritability, as these are the most diffi cult characters to
work with through conventional phenotypic selection. However, it is also diffi cult to
develop effective markers for such traits. The expression of such traits is infl uenced by
genotype-environment interaction and epistasis, which in addition to diffi culties involved