Insect Control: Biological and Synthetic Agents - Index of

this toxin (Ferré and Van Rie, 2002). Several receptor
molecules for Cry1 toxins in H. virescens and
P. xylostella have been hypothesized based on crossresistance
and susceptibility of different Cry1
resistant populations (Ferré and Van Rie, 2002).
For H. virescens, three different receptors have been
proposed: receptor A which binds Cry1Aa, Cry1Ab,
and Cry1Ac toxins; receptor B which binds
Cry1Ab and Cry1Ac toxins; and receptor C which
recognizes Cry1Ac toxin. Based on the susceptibility
of resistant insect populations, it was concluded that
receptors B and C are unlikely to be involved in
toxicity (Ferré and Van Rie, 2002). In the case of P.
xylostella, four binding sites have been proposed
(Ferré and Van Rie, 2002): receptor 1 is only recognized
by Cry1Aa toxin; receptor 2 binds Cry1Aa,
Cry1Ab, Cry1Ac, Cry1F, and Cry1J; receptor 3
binds Cry1B; and receptor 4 binds Cry1C (Ferré
and Van Rie, 2002). Cross-resistance of Cry1A toxins
with Cry1F and Cry1J correlates with sequence
similarity of loop regions in domain II, in particular
loop 2, of these toxins (Tabashnik et al., 1994;
Jurat-Fuentes and Adang, 2001) (see Section
7.3.3). In the case of four P. xylostella strains that
developed resistance in the field, reduced Cry1A
binding correlated with resistance (Schnepf et al.,
1998; Ferré and Van Rie, 2002).
7: Bacillus thuringiensis: Mechanisms and Use 267
In the majority of the selected resistant insect lines
described above there is no information regarding
the characterization at the molecular level of mutations
that leads to Cry resistance. In the case of the
laboratory selected H. virescens Cry1Ac resistant
line, YHD2, it was shown that a single mutation
was responsible for 40–80% of Cry1Ac resistance
levels. The mutation was mapped and shown to be
linked to a retrotransposon insertion in the cadherin-like
gene (Gahan et al., 2001). As mentioned
previously (see Section 7.4.5), this result shows that
binding of Cry1A toxins to cadherin-like receptors
is an important event in the mode of action of these
toxins. Based on these results, it has been suggested
that if mutations of cadherin genes are the primary
basis of resistance in the field, the characterization
of cadherin alleles could be helpful in monitoring
field resistance (Morin et al., 2003). The characterization
of cadherin alleles in field derived and laboratory
selected strains of the cotton pest pink
bollworm (Pectinophora gossypiella) revealed
three mutated cadherin alleles that were associated
with resistance in this lepidopteran insect (Morin
et al., 2003). Figure 9 shows the structural features
of the four mutated alleles of cadherin genes
associated with resistance in H. virescens and
P. gossypiella.
Figure 9 Gene structure of resistant cadherin alleles of Heliothis virescens (Gahan et al., 2001) and Pectinophora gossypiella (Morin
et al., 2003). Sig corresponds to signal sequence for protein export; CR, cadherin repeat or repeated ectodomains; MPR, membrane
proximal region; TM, transmembrane region; CYT, cytosolic domain; , stop codon; red triangle, retrotransposon insertion; solid
lines, deletions. Red sections on CR7 and CR11 are Cry1A binding epitopes mapped on M. sexta Bt-R 1 cadherin receptor.