Contents - Faperta

Mechanisms and Inheritance of Resistance to Insect Pests 133
resistance in maize (G.E. Scott, Dicke, and Pesho, 1966; Onukogu et al., 1978). For this
reason, maize hybrid development has been conducted using other breeding methods.
Development of F1 Hybrids Using Cytoplasmic Male Sterility
Male-sterility results from the inability of plants to produce functional pollen due to reproductive
defi ciency in hermaphrodite fl owers. Cytoplasmic male-sterility occurs due to the
mutation of mitochondria or some other cytoplasmic factors outside the nucleus, which
results in the transformation of fertile cytoplasm into sterile cytoplasm. The male-sterile
line is maintained by pollinating it with pollen from the maintainer line, which differs
from the A line only for male-sterility. The F1 hybrids for cultivation are produced by
crossing the male-sterile line with a pollinator that carries the fertility restoration genes.
A pollinator line that results in maximum heterosis for grain yield and other traits of interest
is used for hybrid production. Cytoplasmic male-sterility has been exploited in several
crops to develop hybrids for increasing crop productivity, particularly in cereal crops such
as rice, maize, sorghum, and pearl millet. However, large-scale cultivation of hybrids based
on a single source of male sterility may pose a serious challenge to sustainable crop production
because of decreased genetic diversity and increased susceptibi lity to insect pests.
Therefore, there is a need to develop male-sterile and restorer lines with resistance to
insects to develop insect-resistant hybrids for cultivation by the farmers. To produce insectresistant
hybrids involving a cytoplasmic male-sterility system, it is important to transfer
insect resistance genes into both male-sterile and restorer lines to produce hybrids with
resistance to insects. Sorghum hybrids with resistance to shoot fl y and sorghum midge
have been developed based on cytoplasmic male sterility (H.C. Sharma et al., 1996, 2005c;
Dhillon et al., 2006a; H.C. Sharma, Dhillon, and Reddy, 2006). Hybrids with resistance to
sorghum midge have been widely deployed in Australia for controlling sorghum midge,
S. sorghicola (Henzell et al., 1997).
Genetic Basis of Resistance
Information on mechanisms and inheritance of resistance to insects can be utilized in
selecting parents with diverse mechanisms or with different genes for resistance, selection
of appropriate breeding methodology (pedigree, backcross, or population improvement)
depending on the number of genes involved and nature of gene action, and developing
isolines and mapping populations for resistance to insects. Resistance to insects may be
oligogenic, polygenic, or cytoplasmic.
Oligogenic Resistance
Oligogenic resistance is controlled by one (monogenic or vertical resistance) or a few major
genes, and the gene effects are easy to detect. Such resistance can be transferred into the
elite lines through pedigree or backcross breeding. Resistance to several insects in rice,
sweet potato, corn, and sorghum is monogenic in nature (Painter, 1958; M.D. Pathak, 1970;
Kogan, 1982; H.C. Sharma, 1993). The chances of evolution of insect biotypes capable of
overcoming the monogenic resistance are quite high, and such resistance breaks down
over a period of time. Some of the notable examples of vertical resistance are resistance in