Mutations in a Novel Cadherin Gene Associated with Bt Resistance in Helicoverpa zea

Abstract

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.

Keywords: Cadherin; Cry1Ac; genome scanning; resistance; selection.

Figure 1

(A) Z-transformed F_ST values were used to estimate genetic divergence between GA and GA-R in 40-kb sliding windows with a 20-kb step size along the H. zea genome. (B) Z-transformed pooled heterozygosity in the GA-R line for the same 40-kb sliding windows with a 20-kb step size. Each of the 2,975 scaffolds that comprise the H. zea reference genome are indicated by the alternating light and dark gray points. Genomic windows identified as potentially under selection are in red.

Figure 2

Frequency distributions of (A) Z-transformed F_ST values for the comparison of genetic divergence between GA and GA-R, and (B) Z-transformed pooled heterozygosity in the GA-R line.

Figure 3

(a) F_ST between lines and (b) pooled heterozygosity in the GA-R line along 40-kb sliding windows surrounding the putative selective sweep on scaffold 20. cad-86C is in gray.

Figure 4

Unrooted neighbor-joining tree indicating the phylogenetic relationships between CAD-86C, CAD2, and BtR. Numbers in red are bootstrap support values (N = 1000) for the tree nodes. A scale bar for genetic distance is in the lower left corner.