Genomics of in vitro dauer juvenile recovery of the entomopathogenic nematode Heterorhabditis bacteriophora in monoxenic liquid culture with its symbiotic bacterium Photorhabdus laumondii

The entomopathogenic nematode (EPN) Heterorhabditis bacteriophora is a biological control agent against insect pests. The dauer juvenile (DJ) carries cells of Photorhabdus bacteria, invades the host, and delivers bacterial cells into the insect haemolymph. The events chain in which the DJ perceives haemolymph signals and exits the arrested stage to reach sexual maturity is called DJ recovery. In monoxenic liquid cultures, DJs depend on unknown bacterial food signals to trigger the recovery. A rapid, synchronized, and high DJ recovery is a key factor for commercial production of EPN, and its further understanding is crucial to improve the mass production of EPN.

By using Photorhabdus supernatant as predictor bioassay, we evaluated the DJ recovery phenotypic variability in H. bacteriophora wild type and EMS mutant lines. More than 150 single nucleotide polymorphisms (SNPs) were characterized within more than 160 mutant lines via high throughput genotyping, and four SNPs resulted robustly associated with the DJ recovery. Thereafter, we carried out a detailed geno- and phenotypic characterization of 14 Photorhabdus isolates and evaluated their influence on the DJ recovery in a set of H. bacteriophora materials. It was evidenced that the bacterial component plays a subordinate role, whereas the nematode genetic pre-disposition is a main factor in the regulation of the DJ recovery in this species. Furthermore, we conducted RNA-seq along early DJ recovery stages (0.5 – 6 h) in two mutant lines with contrasting phenotype. We determined that H. bacteriophora DJs discriminate at early stages the source of the recovery signal (bacteria or haemolymph). More than 14,000 gene models were analysed in connection with functional databases and homologies with Caenorhabditis elegans. As outcome, nine gene models are postulated as potential targets for future approaches.

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