Contents - Faperta

134 Biotechnological Approaches for Pest Management and Ecological Sustainability
wheat to Hessian fl y, M. destructor (Smith, 2005), and rice resistance to brown planthopper,
N. lugens (Khush and Brar, 1991). The vertical type of resistance for tolerance or moderate
levels of resistance provides stable resistance to insect pests (Panda and Khush, 1995). Since
availability of new genes for resistance may be a limiting factor, it is important to manage
known resistant genes to maximize their effect and durability through: (1) recycling and
sequential release of resistance genes, (2) gene pyramiding, (3) regional biotype-specifi c
deployment of resistance genes, and (4) development of synthetics involving multilines
with different genes for resistance.
Polygenic Resistance
Polygenic resistance is controlled by several genes. It is also called horizontal resistance.
Inheritance of such genes is usually complex, and it takes a long time to transfer polygenic
resistance into improved cultivars. Pedigree breeding involving multiple crosses or population
improvement approaches can be used to breed for horizontal resistance. Because
of the additive effect of several genes, the insects take a longer time to overcome such
resistance and thus the chances of evolution of new insect biotypes are low or minimal.
Resistance to several insect species in crops such as cotton, legumes, maize, rice, and wheat
is under polygenic control (Panda and Khush, 1995; Smith, 2005). Progress in breeding for
insect resistance involving a horizontal or additive type of gene action has been slow, and
suffi cient levels of resistance have not been achieved in cultivars with desirable agronomic
backgrounds. The horizontal type of resistance does not exert a selection pressure on pest
populations, and is quite durable. However, strong environmental infl uence and genetic
variability in insect populations complicate the identifi cation and transfer of resistance
genes into high-yielding cultivars.
Selection for the additive type of gene action should be deferred until later generations,
when a desired level of homozygosity is achieved. In the early generations, selection may be
exercised against lines highly susceptible to the target insect pests. Indirect selection using
physico-chemical traits associated with insect resistance can also be practiced in the early
generations, for example, glossy leaf trait for resistance to shoot fl y, A. soccata and short and
tight glumes for resistance to midge, S. sorghicola in sorghum (H.C. Sharma and Nwanze,
1997), trichomes for H. zea resistance in tomato (Farrar and Kennedy, 1987), leaf hairs for
resistance to leafhopper, Amrasca biguttula biguttula Ishida in cotton (H.C. Sharma and Agarwal,
1983), and nonglandular trichomes in wild relatives of pigeonpea, Cajanus scarabaeoides (L.)
F. Muell. for resistance to H. armigera (Romeis et al., 1999; Rupakala et al., 2005). Molecular
markers linked with insect resistance genes or the factors associated with insect resistance
can be used to improve the effi ciency of screening and breeding for resistance to insects,
including the additive type of gene action (H.C. Sharma, Abraham, and Stenhouse, 2002).
Cytoplasmic Effects
Cytoplasmic resistance is due to the factors in the cytoplasm of the host plant. Cytoplasmic
inheritance is maternal, and can be detected by making reciprocal crosses. The fi rst cytoplasmic
male-sterile line in rice was developed by substituting nuclear genes of the indica
variety, Taichung Native 1 (Athwal and Virmani, 1972), and since then, a large number of
cytoplasmic male-sterile (CMS) lines have been developed by exploiting both intra- and
interspecifi c CMS systems (Virmani and Wan, 1988). The male sterile cytoplasm affects rice
plant reactions to pathogens, as the WA male sterile cytoplasm in rice is less susceptible to
Pyricularia oryzae Cav. and Xanthomonas oryzae pv. oryzae (Swings et al.) than the fertile
cytoplasm (R.C. Yang et al., 1989). In maize, the use of CMS-T cytoplasm for hybrid production