88 L. Pathak et al.
of binding site, absence of specific receptors,
hindrance in pore formation, and removal of a
highly conserved glycosylation pathway (Heckel
1994 ). Candas et al. ( 2002 ) studied the changes
in the levels of proteins identified in the midgut
epithelium of Indian-meal moth larvae resistant
to Bt and found that the levels of aminopeptidase
N, vacuolar ATPase subunit B, phosphopyruvate
hydratase, cytochrome oxidase subunit I, NADH
dehydrogenase subunit V, 3-Dehydroecdysone
reductase, F 1 F 0 -ATPase/resistant variant, and
GSH transferase showed significant increase.
Ferre and van Rie ( 2002 ) proposed a model
for binding site modification of Cry proteins to
brush border membrane of midgut cells of P. xy-
lostella larvae and found conformational changes
in binding site can alter the degree of specificity
of toxin binding. Aroian et al. ( 2003 ) proposed
an oligosaccharide receptor-based model for
bre genes in intestinal cells of Caenorhabditis
elegans and reported that bre genes synthesize
an oligosaccharide which serves as receptor for
crystal toxins. Loss of any of the bre enzymes
leads to high level of resistance to Cry5B sug-
gesting that oligosaccharide is the major receptor
for Cry5B. In the case of Cry14A, the reduced
level of resistance conferred by mutants suggests
that other receptors can partly compensate in the
absence of bre oligosaccharide.
Genetic reasons for resistance development
can be intraspecific variation in baseline sus-
ceptibility, frequency of resistance gene allele,
mode of inheritance of resistance, and stability
of resistance. Sayyed et al. ( 2004 ) studied the
toxicity of commercial formulations and purified
Cry toxins to susceptible laboratory and resistant
field populations of P. xylostella and reported
that among the various toxins tested, susceptible
populations were less sensitive to Cry1Ab, Cry-
1Aa, and Cry1Fa, respectively. While among the
resistant field populations tested, they developed
severe resistance especially to Cry1Ac and MVP
(Table 6 ) along with resistance development to
other toxins tested. This suggests that there is an
intraspecific variation in the baseline susceptibil-
ity among susceptible populations.
Kranthi ( 2008 ) has been assigned job to
monitoring the shifts in baseline susceptibility
(development of tolerance/resistance) in the H.
armigera against Cry1Ac toxin in various cotton
growing regions of the country. Yu Cheng Zhu
et al. ( 2009 ) studied frequencies of resistance al-
leles to Bt cotton in field population of H. ar-
migera. In 1999, the allele frequency estimated
for population (0.0058) significantly fluctuated
during 2003–2005. F1 and F2 screens conducted
in 2006 and 2007 revealed > 3-fold increase of
resistant gene frequency compared to the levels
of 2003–2005, > 18-fold increases over 1999 in
same population.Table 6 Toxicity of commercial formulations and purified Cry toxins to susceptible laboratory and resistant field
populations of P. xylostella. (Sayyed et al. ( 2004 ))
Population Toxin used LC50 (μg/ml) Resistance ratio
LAB-UK (susceptible) Dipel 0.0039 –
MVP 0.027 –
Cry1Ac 0.007 –
Cry1Ab 0.47 –
Cry1Aa 0.039 –
Cry1Fa 0.20 –
Karak (resistant field) Dipel 2.97 770
MVP 9800 363,000
Cry1Ac > 40 > 5710
Cry1Ab 77.0 164
Cry1Aa 32.9 845
Cry1Fa 82.7 414