Contents - Faperta

Physico-Chemical and Molecular Markers for Resistance to Insect Pests 185
against the target insect pest(s). In addition, crops frequently suffer from a number of
primary herbivores. This suggests that single and multiple transgenes will need to be
combined in the same variety with other sources, mechanisms, and targets of insect pest
resistance in order to generate highly effective and sustainable seed-based technologies.
From an evolutionary point of view, the development of plants with several genes for
insect resistance will be expected to decrease the probability of insect pests to overcome
newly deployed seed-based resistance technologies and thereby increase the effectiveness
and prolong the life of such new varieties (Hadi, McMullen, and Finer, 1996; Karim,
Riazuddin, and Dean, 1999), for example, the activity of Bt genes in transgenic plants is
enhanced by the serine protease inhibitors (MacIntosh et al., 1990; Zhao et al., 1997) and
tannic acid (Gibson et al., 1995). However, this may have some metabolic cost to the plant
in some cases, and different resistance gene products may also have deleterious or
nullifying interactions.
Combining transgene- and QTL-mediated resistance can be used as a viable strategy for
insect control. A QTL conditioning maize earworm resistance in soybean PI 229358 and
the cry1Ac transgene from the recurrent parent Jack-Bt has been pyramided into BC 2F 3
plants by MAS (Walker et al., 2002). Segregating individuals were genotyped through SSR
markers linked to an antibiosis/antixenosis QTL on LG M, and tested for the presence of
cry1Ac. MAS was used during and after the two backcrosses to develop a series of BC 2F 3
plants with or without the cry1Ac transgene and the QTL conditioning insect resistance.
The BC 2F 3 plants homozygous for parental alleles at markers on LG M, and which either
had or lacked Cry1Ac, were assigned to one of four possible genotype classes. These plants
were used in no-choice detached leaf feeding bioassays with corn earworm, H. zea and
soybean looper, P. includens. Fewer larvae of either species survived on leaves expressing
the Cry1Ac protein. Though not as great as the effect of Cry1Ac, the PI 229358-derived LG
M QTL also had a detrimental effect on larval weights of both species, and on defoliation
by maize earworm, but did not reduce defoliation by the soybean looper. Weights of soybean
looper larvae fed on foliage from transgenic plants with the PI-derived QTL were
lower than those of larvae fed transgenic tissue with the corresponding Jack chromosomal
segment (Walker et al., 2002). Therefore, combining transgene- and QTL-mediated resistance
to lepidopteran insects may be a viable strategy for insect control.
Marker-Assisted versus Phenotypic Selection
Expression of physicochemical markers is infl uenced by the environment, and this makes
them less reliable than molecular markers. A number of methods have been used for mapping
QTLs associated with the traits of interest (Knot and Hailey, 1992; Karp et al., 1997).
Among the molecular markers, RFLPs are the most reliable (Caetano-Anolles, Bassam, and
Gresshoff, 1991; J.J. Lin et al., 1996), and can be converted into SCARs by sequencing the
two ends of the DNA clones and designing oligonucleotide primers based on end sequences
(Williams et al., 1991). Molecular markers and QTLs associated with insect resistance can
be used to transfer the resistance genes from sources of resistance into high-yielding cultivars.
It is important that the marker cosegregates with the gene, and is closely linked
(1 cM or less) with the trait of interest. There are several advantages of using molecular
markers to breed for insect resistance. Some types of molecular markers behave in a
codominant manner to detect heterozygotes in segregating populations. Morphological