Contrasting invertebrate immune defense behaviors caused by a single gene, the Caenorhabditis elegans neuropeptide receptor gene npr-1

The nematode Caenorhabditis elegans feeds on microbes in its natural environment. Some of these microbes are pathogenic and thus harmful to C. elegans. To minimize resulting fitness reductions, C. elegans has evolved various defence mechanisms including behavioural responses (e.g. avoidance behaviour) that reduce contact with the infectious microbes. In this study, we characterized the genetic architecture of natural variation in C. elegans avoidance behaviour against the infectious stages of the Gram-positive bacterium Bacillus thuringiensis. We performed an analysis of quantitative trait loci (QTLs) using recombinant inbred lines (RILs) and introgression lines (ILs) generated from a cross of two genetically as well as phenotypically distinct natural isolates N2 and CB4856. The analysis identified several QTLs that underlie variation in the behavioural response to pathogenic and/or non-pathogenic bacteria. One of the candidates is the npr-1 gene. This gene encodes a homolog of the mammalian neuropeptide receptor. Npr-1 was previously indicated to fully contribute to behavioural defence against the Gram-negative bacterium Pseudomonas aeruginosa and food patch-leaving behaviour on Escherichia coli. Interestingly, in our study, npr-1 is not the only gene mediating avoidance behaviour toward Bacillus thuringiensis. Moreover, our functional analyses show that npr-1 alleles appear to influence survival and avoidance behaviour toward Bacillus thuringiensis in exactly the opposite way than toward Pseudomonas aeruginosa. Our findings highlight the role of npr-1 in fine-tuning nematode behaviour in an ecological context depending on the microbe to which C. elegans is exposed. These opposite phenotypes reflect the diversity in innate immunity to pathogens. To understand the mechanism involved in these opposite phenotypes, we carried out a whole-genome transcriptomics study by RNA-Sequencing. This study includes two pathogens: Pseudomonas aeruginosa PA14 and Bacillus thuringiensis B-18247 (BT247), two strains: N2 and npr-1 (ur89), two time points (12 and 24h) and standard lab food E. coli OP50 as control. mRNA profiles of wild type (WT) and npr-1 (ur89) C.elegans exposed to either Bacillus thuringiensis B-18247, Pseudomonas aeruginosa PA14 or standard lab food E. coli OP50 at 12h or 24h were generated by deep sequencing, in double or triplicate, using Illumina HiSeq2000.