Contents - Faperta

156 Biotechnological Approaches for Pest Management and Ecological Sustainability
may be valuable tools for introgression of genes of interest from the wild relatives into the
cultigen. The F 6-8 progenies of crosses involving a resistance source from the wild relatives
and the cultivated types can also be used as RILs for mapping insect resistance (provided
the population has been advanced through the generations in a correct manner).
Linkage between a resistance gene and a linked marker may vary greatly. The two may
be completely linked, where no crossing over occurs between the resistance gene and the
marker during meiosis, and the gene and marker are always transferred together from one
generation to another. The resistance gene and a molecular marker at times are incompletely
linked, and crossing over may occur between the gene and the marker during
meiosis, thus breaking the linkage between the marker allele and the resistance allele of
the parents. Estimates of the recombination between the resistance gene and the linked
marker are measured as the recombination frequency (RF), which is measured among
backcross progenies of segregating F 2s, F 2:3 families, or RILs by matching the phenotype
and genotype of each progeny (Lander et al., 1987). Mapping simultaneously estimates all
recombination frequencies for markers segregating as dominant, recessive, and codominant
traits in the mapping population. The linkage between QTL and marker loci is based
on the distribution patterns for the resistance characters linked to insect resistance genes
and the molecular marker at each locus (Lincoln, Daly, and Lander, 1993).
To estimate recombination frequency between a resistance gene and a molecular marker,
researchers often analyze between 100 and 500 F 2:3 progenies or RILs derived from crosses
of parents with known resistance or susceptibility. DNA is collected from the resistant and
susceptible parents, as well as from each F 2 plant or several plants in F 6 RILs. Different DNA
markers from a variety of chromosome locations are screened to identify those detecting
polymorphisms between the parents. If the parent polymorphisms are apparent in the
bulked segregant DNA samples, the marker is referred to as a putatively linked marker,
and DNA of all F 2 plants or RILs families is also evaluated. At this point, QTL mapping
software packages such as Mapmaker, Joinmap, QTL cartographer, etc., are used to correlate
the phenotype and genotype of the plants in a mapping population to develop the
genetic linkage map. Linkage mapping software packages contain mapping functions such
as the Haldane’s (1919) and Kosambi’s (1944) functions to correct any under-estimation.
Markers associated with insect resistance can be physico-chemical factors of the plants
(plant hairs or trichomes, fl ower and seed color, leaf size and shape, thickness of the cell
wall, protein, amino acid, sugar, and fatty acid profi les, secondary metabolites such as
alkaloids, terpenoids, fl avonoids, isozymes, and protease inhibitors) or DNA based [RAPDs
(random-amplifi ed polymorphic DNA), RFLPs (restriction fragment length polymorphisms),
AFLPs (amplifi ed fragment length polymorphic DNA), SCARs (sequence characterized
amplifi ed regions), STS (sequence tagged sites), SSRs (simple sequence repeats),
ALPs (amplicon length polymorphisms), and DArTs (diversity array technology)] (Crouch
et al., 2005).
Physico-chemical Markers Associated with Resistance to Insects
Morphological Markers
Plants have acquired several physicochemical characteristics that contribute to insect resistance
in different crops (Panda and Khush, 1995; Smith, 2005) (Table 6.1). The role of various
physico-chemical components in host plant resistance to insects is discussed below.