RNA-Seq analysis of the Drosophila transcriptome in response to infection by entomopathogenic nematodes and their mutualistic bacteria. Julio C. Castillo, Ioannis Eleftherianos. The George Washington University, Department of Biological Sciences, Washington, DC 20052.

   The Drosophila immune system is capable of activating a variety of responses against microbial infections. However, how the fly responds to parasitic infections is currently unknown. The nematode Heterorhabditis bacteriophora is an insect parasite that forms a mutualistic relationship with the gram-negative bacterium Photorhabdus luminescens. Following infection, the nematodes release Photorhabdus bacteria that quickly multiply within the insect and produce several toxins that eventually kill the host. Although we currently know that the insect immune system interacts with Photorhabdus, information on similar interaction with the nematode vector is lacking. The objective of the current study is to identify the number and nature of Drosophila genes that are regulated upon infection with the nematode and its bacteria. We have used next generation RNA-sequencing to analyze the transcriptional profile of adult flies infected by axenic Heterorhabditis nematodes (lacking Photorhabdus bacteria), symbiotic Heterorhabditis nematodes (carrying Photorhabdus bacteria), and Photorhabdus alone. In total, we have obtained around 54 million reads from the different infection types. Preliminary analysis shows that Photorhabdus infection induces several genes encoding recognition and antibacterial effector molecules in Drosophila. Interestingly, Heterorhabditis infection leads to regulation of genes that are involved in lipid homeostasis and metabolism. Our data provide valuable information on what Drosophila genes are activated or repressed following infection with the two pathogens, either separately or together. This study provides exciting clues on the molecular immune events that may take place in Drosophila upon infection with a potent entomopathogenic nematode-bacteria complex. Such large-scale transcriptomics studies set the stage for future functional studies aimed at identifying the exact role of key factors in the Drosophila anti-nematode immune response.