The salmon louse (Lepeophtheirus salmonis) is an ectoparasite responsible for huge economic losses to Atlantic salmon aquaculture. Parasitic stages feed upon the host’s skin epithelium, mucus and blood, causing skin damage, physiological stress and increased susceptibility to secondary diseases. The free-swimming copepodid finds and attaches to the host, this involving chemosensory detection of salmonid kairomones. Any transcriptomic responses that occur following detection of kairomones by the copepodid, likely reflect processes associated with the transition to a parasitic state. We investigated the transcriptomic profiles of pre-parasitic copepodids exposed to a) 3 concentrations of the semiochemical 6-methyl-5-hepten-2one in seawater (HP), b) Atlantic salmon conditioned seawater (SCW) c) a control comprising untreated seawater (SW), to investigate responsive gene regulation and function.
Age matched groups of copepodids were exposed to SW, SCW and HP for 18 h at 8oC. Samples consisted of pools of 50 copepodids per treatment. RNA was extracted and subjected to strand specific (50m paired end 150bps reads) RNAseq analysis using NovaSeq X plus. qRT-PCR analysis was conducted for multiple genes of interest identified from highly differentially expressed genes (DEGs) as well as from single-nuclei RNAseq (sn-RNAseq) (Chromium, 10X Genomics).
Transcriptomic responses indicated that exposure to SCW induces significant regulation of genes associated with cuticle processing, immunomodulation, antimicrobial and secretory protease activity. qRT-PCR analysis corroborated the expression profiles of selected genes from the RNA-seq and snRNA seq datasets with 5-30 fold expression changes observed for genes encoding trypsin, histidine rich glycoprotein and prisilkin-like proteins.
Determining the transcriptional shifts of host-cue stimulated copepodids provides greater understanding of the first stages of host-pathogen interaction, as copepodids are exposed to host kairomones during initial infection. Further investigation of the relationship of key differentially expressed transcripts to the process of host infection can pave the way to the development of new tools to disrupt this process, providing novel solutions to the problem of salmon louse infection.