Contents - Faperta

378 Biotechnological Approaches for Pest Management and Ecological Sustainability
early phases of Bt resistance evolution in the fi eld has been viewed as crucial, but extremely
diffi cult, especially when resistance is recessive. Mechanisms associated with development
of resistance in insect populations to Bt are discussed below.
Reduced Protoxin-Toxin Conversion
Reduced protoxin-toxin conversion is one of the mechanisms of resistance to Bt. A
resistant colony of P. interpunctella displayed reduced capacity to activate Cry1Ac (Oppert
et al., 1996), linked to a genetic linkage between absence of major gut proteases and susceptibility
of Cry1Ac (Oppert et al., 1997). Altered proteolytic activity is also one of the mechanisms
of resistance in this insect to Bt toxins (Johnson et al., 1990). However, altered
proteolytic activity is not associated with development of resistance to Bt in H. virescens
(Marrone and MacIntosh, 1993), and P. xylostella (Liu and Tabashnik, 1997; Tabashnik,
Finson, and Johnson, 1992). Slower activation and faster degradation of Cry1Ab toxin in
midgut extract in a resistant strain of H. virescens is associated with resistance to this toxin
(Forcada et al., 1996). Reduced protoxin-toxin conversion is also one of the mechanisms of
resistance to Cry1Ca in P. xylostella (MacIntosh et al., 1991).
Reduced Binding to Receptor Proteins
Reduced binding is one of the mechanisms of resistance in P. interpunctella to Cry1Ab (Van
Rie et al., 1990). However, there were no differences between the resistant and susceptible
strains in binding levels for Cry1Ca, to which the insect does not display any resistance.
Nearly 60-fold reduction in binding has been observed in another colony to Cry1Ab
(Herrero, Oppert, and Ferre, 2001). However, there were no differences in binding to
Cry1Ac. Resistance to Cry1Ac in H. virescens is not linked to binding to Cry1Ac or Cry1Ab,
but binding to Cry1Ca was reduced drastically, to which the insect showed moderate levels
of cross-resistance (Van Rie et al., 1990; Lee et al., 1995). Competition binding studies
indicated that Cry1Aa binds to receptor A, Cry1Ab to receptors A and B, and Cry1Ac to
receptors A, B, and C. Altered Cry1Aa binding site may contribute to resistance to all three
Cry1A proteins (Lee et al., 1995). Cry1Fa and Cry1Ja also share the binding site A with
Cry1A toxins in H. virescens (Jurat-Fuentes and Adang, 2001). In another colony of H. virescens,
small or no compensatory changes were observed in binding to Cry1Ab or Cry1Ac
(MacIntosh et al., 1991; Gould et al., 1992). Reduced binding is one of the mechanisms of
resistance to Cry1Ac in H. armigera (Akhurst et al., 2003).
Reduced binding is one of the mechanisms of resistance in P. xylostella to Cry1Ab
(Tabashnik et al., 1997b) and Cry1Ac (Tabashnik et al., 1994b, 1997b). In another colony,
binding to Cry1Ab was reduced dramatically, but not to Cry1Aa and Cry1Ac (Tabashnik
et al., 1997b; Ballester et al., 1999). Competition experiments have indicated the presence
of a site recognized only by Cry1Aa, another site by Cry1Aa, Cry1Ab, Cry1Ac, Cry1Fa,
and Cry1Ja, and two additional sites by Cry1Ba and Cry1Ca (Ferre et al., 1991; Ballester
et al., 1994, 1999; Granero, Ballester, and Ferre, 1996). In the resistant strain, there was
no binding to Cry1Ab and Cry1Ac, while Cry1Aa binds equally well to brush border
membrane vesicles (BBMV) of the resistant and susceptible strains (Tabashnik et al.,
1997b). In general, patterns of cross resistance correspond to binding patterns of different
toxins to receptors in the BBMV. Receptor specifi city corresponds to levels of amino acid
homology in domain II (Tabashnik et al., 1996). Lack of binding to Cry1Ab has also been
demonstrated through histopathological studies (Bravo, Jansens, and Peferoen, 1992).
Gahan, Gould, and Heckel (2001) suggested that disruption of a cadherin-superfamily