Molecular characterisation of the recovery process in the entomopathogenic nematode Heterorhabditis bacteriophora

In Heterorhabditis bacteriophora, an insect-parasitic nematode, the third juvenile is the infective, developmentally arrested form. When it infects a suitable host, the infective juvenile recovers from developmental arrest and resumes growth and development. This process is called recovery and it is the first outcome of the host-parasite interaction. Recovery is also very important from a commercial point of view. To characterise the recovery in H. bacteriophora, we sought to identify genes involved in this process. A large-scale bioassay for recovery was established and subtraction libraries of recovering infective juvenile from arrested infective juvenile transcripts were constructed at different time points. Most of the genes identified as differentially expressed between recovering and developmentally arrested infective juveniles belonged to metabolic pathways. Elevated expression levels of 23 selected genes during recovery were confirmed by quantitative PCR. For eight of these genes, transcription silencing in H. bacteriophora resulted in a significant decline in infective juvenile recovery rates, suggesting that these genes are critical to the recovery process. Two of the genes were associated with the insulin-like growth factor-1 (insulin/IGF-1) pathway, known to regulate dauer formation in the free-living nematode Caenorhabditis elegans, whereas the other six genes were associated with pathways not previously associated with recovery in nematodes. These results suggest that although little is known about parasitism-unique genes, the pathways regulating recovery in H. bacteriophora include those activated in C. elegans and those that might be unique to parasitic nematodes; the latter may be activated in response to host signals and enable the parasite to recognise its host.