Contents - Faperta

Physico-Chemical and Molecular Markers for Resistance to Insect Pests 155
Physical maps can be used for gene isolation through microdissection and microcloning.
Gene structure and function can also be elucidated by comparing the mapped
positions of genes with their physical locations, and physical distances between genes can
be used to gain an understanding of the mechanism of chromosome recombination and
rearrangements. High-density genetic linkage maps have been developed for barley,
Hordeum vulgare L.; maize, Zea mays L; potato, Solanum tuberosum L.; rye, Secale cereale L.;
sorghum, Sorghum bicolor (L.) Moench; soybean, Glycine max (L.) Merr.; tomato, Lycopersicon
esculentum Mill; and wheat, Triticum aestivum L. (Paterson, Tanksley, and Sorrells, 1991;
Hernandez et al., 2001; Korzun et al., 2001; Boyko et al., 2002; Sharopova et al., 2002; Song
et al., 2004; Somers, Isaac, and Edwards, 2004). Molecular markers in many of these crops
have also been linked to genes expressing resistance to several insect pests. Developments
in DNA marker technology can be used to accelerate the process of transferring insect
resistance into improved cultivars. Once markers linked closely to the resistance genes
are identifi ed, MAS can be practiced in early generations at early stages of plant development,
and speed up the selection process. The MAS can also be used for pyramiding resistance
genes from diverse sources. Location of the marker away from the gene of interest
may lead to cross over between the gene of interest and the marker, and the marker identifi
ed for a gene in one cross may not be useful in another cross unless the marker is linked
to the resistance gene (Mohan et al., 1997). The MAS takes 3 to 6 years, and thus speeds up
the pace of transferring the traits of interest into improved varieties, and it does not require
large-scale planting of the segregating material up to crop harvest, as only the plants
showing the presence of markers associated with QTLs linked to resistance alleles need to
be maintained up to maturity.
Mapping Populations
It takes fi ve to six generations to transfer insect resistance traits into the high-yielding
cultivars through conventional breeding, while gene transfer from wild relatives may take
a considerably longer time due to the complexity of achieving interspecifi c hybrids on a
suffi ciently large scale to identify stable progeny with an acceptable combination of traits.
In either case, MAS can dramatically speed up the process by reducing the number of generations
and the size of the populations required to identify individuals with appropriate
combination of genes, while having the minimal amount of linkage drag from the wild
relatives. The improved lines with insect resistance thus developed will need to be tested
across seasons and locations, before a variety can be identifi ed for cultivation by farmers.
This process takes 7 to 10 years. In MAS programs, the elite breeding line or cultivars can
be crossed with the source of resistance, and the F 1 hybrid recrossed with the recurrent
parent (invariably the elite parent) (BC 1), and the gene transfer can be monitored through
MAS up to BC 3-5 (until a line with the QTL or the gene of interest in the genomic background
of the elite line with a minimum of donor parent genome is identifi ed).
Near isogenic lines (NILs), F 2 and backcross populations, doubled haploids, and recombinant
inbred lines (RILs) can be used for gene mapping in many crops (Mohan et al., 1994,
1997). Mapping populations from interspecifi c crosses are often used for genetic linkage
studies due to high levels of detectable polymorphism, but linkage maps derived from
such crosses may have limited relevance in crop breeding programs due to different
recombination patterns (Fulton et al., 1997). However, markers developed from such maps