Expression of Heterorhabditis bacteriophora C-type lectins, Hb-clec-1 and Hb-clec-78, in context of symbiosis with Photorhabdus bacteria

Abstract

Insect-parasitic nematodes of the genus Heterorhabditis live in a symbiotic relationship with a gram negative Gamma-proteobacteria of the genus Photorhabdus. This nematode-bacteria pair is a simple and genetically tractable model to study animal-microbe symbiosis. Here we investigated the role of Heterorhabditis nematode C-type lectin (clec) genes in context of nematode-bacteria symbiosis. The in silico analysis identified seven clec genes in H. bacteriophora and three clec genes in H. indica. Two of the clec genes, H. bacteriophora clec-1 (Hb-clec-1) and H. bacteriophora clec-78 (Hb-clec-78) were further characterized. Both of these genes were present in a single copy in the H. bacteriophora genome. The phylogenetic analysis revealed that H. bacteriophora CLEC proteins were close to CLEC-1 and CLEC-78 proteins of free living Caenorhabditis but not to the CLEC proteins of insect-parasitic Steinernema nematodes which share a similar symbiotic relationship with Xenorhabdus bacteria. In situ hybridization showed that expression of Hb-clec-1 and Hb-clec-78 was localized to the alimentary canal of infective juveniles (IJs) in the region of terminal bulb, oesophago-intestinal valve and anterior part of intestine. Hb-clec-78 gene expression displayed significant positive correlation to the presence of bacteria during various stages of symbiosis: it was up-regulated during all the nematode developmental stages when Photorhabdus was symbiotically associated, but down-regulated at the post-IJ recovery stage when the developing nematodes were free of bacteria. Hb-clec-1 gene expression did not show any correlation with presence or absence of symbiont bacteria. Subject to genetic validation, our study suggests that Hb-clec-78 might be actively involved in modulation of symbiosis with Photorhabdus symbionts.