Unique genetic variants in dihydrolipoamide dehydrogenase (dld) gene confer strong resistance to phosphine in the rusty grain beetle, Cryptolestes ferrugineus (Stephens)

Abstract

The rusty grain beetle, Cryptolestes ferrugineus, a major pest of stored commodities, has developed very high levels (>1000×) of resistance to the fumigant phosphine. Resistance in this species is remarkably stronger than reported in any other stored product pests demanding the need to understand the molecular basis of this trait. Previous genetic studies in other grain insect pests identified specific variants in two major genes, rph1 and rph2 in conferring the strong resistance trait. However, in C. ferrugineus, although the gene, rph1 was identified as cytochrome-b5-fatty acid desaturase, the rph2 gene has not been reported so far. We tested the candidate gene for rph2, dihydrolipoamide dehydrogenase (dld) using the recently published transcriptome of C. ferrugineus and identified three variants, L73N and A355G + D360H, a haplotype, conferring resistance in this species. Our sequence analysis in resistant strain and phosphine selected resistant survivors indicates that these variants occur either alone as a homozygote or a mixture of heterozygotes (i.e complex heterozygotes) both conferring strong resistance. We also found that one of the three variants, possibly L73N expressing “dominant” trait at low frequency in resistant insects. Comparison of dld sequences between Australian and Chinese resistant strain of this species confirmed that the identified variants are highly conserved. Our fitness analysis indicated that resistant insects may not incur significant biological costs in the absence of phosphine selection for 19 generations. Thus, we propose that the observed high levels of resistance in C. ferrugineus could be primarily due to the characteristics of three unique variants, L73N and A355G + D360H within dld.