Mutations in a novel cadherin gene associated with Bt resistance in Helicoverpa zea

Transgenic corn and cotton produce crystalline (Cry) proteins derived from
the soil bacterium Bacillus thuringiensis (Bt) that are toxic to
lepidopteran larvae.  Helicoverpa zea, a key pest of corn and
cotton in the U.S., has evolved widespread resistance to these proteins
produced in Bt corn and cotton.  While the genomic targets of Cry
selection and the mutations that produce resistant phenotypes are known in
other lepidopteran species, little is known about how selection by Cry
proteins shape the genome of H. zea.  We scanned the genomes of
Cry1Ac-selected and unselected H. zea lines, and identified twelve genes
on five scaffolds that differed between lines, including cadherin-86C
(cad-86C), a gene from a family that is involved in Cry1A resistance in
other lepidopterans.  Although this gene was expressed in the H.
zea larval midgut, the protein it encodes has only 17 to 22% identity with
cadherin proteins from other species previously reported to be involved in
Bt resistance.  An analysis of midgut-expressed cDNAs showed
significant between-line differences in the frequencies of putative
nonsynonymous substitutions (both SNPs and indels).  Our results
indicate that cad-86C is a likely target of Cry1Ac selection in H.
zea.  It remains unclear, however, whether genomic changes at
this locus directly disrupt midgut binding of Cry1Ac and cause Bt
resistance, or indirectly enhance fitness of H. zea in the presence of
Cry1Ac by some other mechanism.  Future work should investigate
phenotypic effects of these nonsynonymous substitutions and their impact
on fitness of H. zea larvae that ingest Cry1Ac.