Larval spongy moth transcriptomic response to ingestion of broad-versus narrow-spectrum insecticidal Chromobacterium species

The PRAA4-1^T strain of Chromobacterium subtsugae was the first insecticidal bacterium to be registered by the U.S. Environmental Protection Agency for use in crop protection applications since approval for Bacillus thuringiensis was granted in 1961. C. subtsugae, a Gram-negative betaproteobacterium, exhibits oral toxicity against a broad range of important insects, including dipteran, coleopteran, lepidopteran, and at least some hemipteran and tetranychidan pests. Chromobacterium sphagni is a closely related bacterium exhibiting a distinctly narrower activity spectrum than that of C. subtsugae: it is toxic to lepidopteran, but not dipteran or coleopteran pest insects. The molecular mode of activity for either species is not well characterized at present, and it remains unclear whether these bacterial species affect insects similarly, notwithstanding their close evolutionary relatedness. In this study, synchronized third-instar larvae of the destructive lepidopteran forest pest, Lymantria dispar dispar (European spongy moth), were separately fed with cultures of C. subtsugae strain PRAA4-1^T or C. sphagni strain 14B-1^T and sampled after 24 h post infection. Gene expression levels in healthy reference versus treated insects were independently compared at the whole-insect and midgut-only tissue levels to characterize host-specific transcriptional responses to intoxication. Treatment induced up-regulation of such antimicrobial peptides as attacin and cecropin, of two cytochrome P450-encoding genes, and of gelsolin, a molecule involved in actin organization. Some differentially expressed genes were novel or uncharacterized, hence future work with lepidopteran species will be necessary to understand insect physiological responses to Chromobacterium infection.