Identification and ligand binding of a chemosensory protein from sea louse Caligus rogercresseyi (Crustacea: Copepoda)

Caligus rogercresseyi is an ectoparasitic copepod that negatively affects the salmon farming industry, causing economic losses. To use phytochemicals as feed additives, or other chemicals that could elicit behavioral responses in C. rogercresseyi, the chemosensory recognition process is crucial. Therefore, to establish how C. rogercresseyi recognizes glucosinolates and their derivates isothiocyanates, a chemosensory protein (CSP) described as specific carrier of these chemicals in sea louse (CrogCSP) was identified in this study. The recombinant CSP and its selectivity against different chemical compounds was tested by fluorescence binding assays. Phylogenetic analysis revealed a close relationship among CrogCSP and other reported CSPs. Our results indicate that phenyl isothiocyanate and isophorone exhibited dissociation constants of 4.17 and 4.28 μM of K_i, respectively, indicating affinity over other chemicals, such as fatty acids and sinigrin. Structural findings suggest a unique binding site capable of accept several types of chemicals, similar to what has been reported for crystallized insect CSPs. Finally, this study lays the foundation for a deeper understanding of CSPs in crustaceans and especially in C. rogercresseyi. Likewise, the identification of chemosensory proteins could serve as the first step towards novel semiochemicals discovery to being applied in the sea louse controlling.

Early development and metabolic rate of the sea louse Caligus rogercresseyi under different scenarios of temperature and pCO2

Anthropogenic CO₂ emissions have led to ocean acidification and a rise in the temperature. The present study evaluates the effects of temperature (10, 15 and 20 °C) and pCO₂ (400 and 1200 μatm) on the early development and oxygen consumption rate (OCR) of the sea louse Caligus rogercresseyi. Only temperature has an effect on the hatching and development times of nauplius I. But both factors affected the development time of nauplius II (<temperature = longer development time). Copepodid survival time was also affected by temperature and pCO₂, at 10 °C and 400 μatm, survival was 30 and 44% longer than at 15 and 20 °C. OCRs were impacted by temperature but not by pCO₂. In all treatments, OCR was lower for nauplius II than for the copepodid. Our results show the need to further evaluate the effects of a combination of environmental drivers on the performance of C. rogercresseyi, in a changing and uncertain future.

Gene silencing reveals multiple functions of Na+/K+-ATPase in the salmon louse (Lepeophtheirus salmonis)

Na⁺/K⁺-ATPase has a key function in a variety of physiological processes including membrane excitability, osmoregulation, regulation of cell volume, and transport of nutrients. While knowledge about Na⁺/K⁺-ATPase function in osmoregulation in crustaceans is extensive, the role of this enzyme in other physiological and developmental processes is scarce. Here, we report characterization, transcriptional distribution and likely functions of the newly identified L. salmonis Na⁺/K⁺-ATPase (LsalNa⁺/K⁺-ATPase) α subunit in various developmental stages. The complete mRNA sequence was identified, with 3003 bp open reading frame encoding a putative protein of 1001 amino acids. Putative protein sequence of LsalNa⁺/K⁺-ATPase revealed all typical features of Na⁺/K⁺-ATPase and demonstrated high sequence identity to other invertebrate and vertebrate species. Quantitative RT-PCR analysis revealed higher LsalNa⁺/K⁺-ATPase transcript level in free-living stages in comparison to parasitic stages. In situ hybridization analysis of copepodids and adult lice revealed LsalNa⁺/K⁺-ATPase transcript localization in a wide variety of tissues such as nervous system, intestine, reproductive system, and subcuticular and glandular tissue. RNAi mediated knock-down of LsalNa⁺/K⁺-ATPase caused locomotion impairment, and affected reproduction and feeding. Morphological analysis of dsRNA treated animals revealed muscle degeneration in larval stages, severe changes in the oocyte formation and maturation in females and abnormalities in tegmental glands. Thus, the study represents an important foundation for further functional investigation and identification of physiological pathways in which Na⁺/K⁺-ATPase is directly or indirectly involved.

Sexual maturation and administration of 17β-estradiol and testosterone induce complex gene expression changes in skin and increase resistance of Atlantic salmon to ectoparasite salmon louse

The crustacean ectoparasitic salmon louse (Lepeophtheirus salmonis) is a major problem of Atlantic salmon aquaculture in the Northern hemisphere. Host-pathogen interactions in this system are highly complex. Resistance to the parasite involves variations in genetic background, nutrition, properties of skin, and status of the endocrine and immune systems. This study addressed the relationship between sex hormones and lice infection. Field observation revealed a sharp reduction of lice prevalence during sexual maturation with no difference between male and female fish. To determine if higher resistance against lice was related to sex hormones, post-smolt salmon were administered control feed and feeds containing 17β-estradiol (20 mg/kg) and testosterone (25 mg/kg) during a 3-week pre-challenge period. After challenge with lice, counts were reduced 2-fold and 1.5-fold in fish that received 17β-estradiol and testosterone, respectively. Gene expression analyses were performed from skin of salmon collected in the field trial and from the controlled lab experiment at three time points (end of feeding-before challenge, 3 days post challenge (dpc) and 16 dpc) using oligonucleotide microarray and qPCR. Differential expression was observed in genes associated with diverse biological processes. Both studies revealed similar changes of several antibacterial acute phase proteins; of note was induction of cathelicidin and down-regulation of a defensin gene. Treatment with hormones revealed their ability to modulate T helper cell (Th)-mediated immunity in skin. Enhanced protection achieved by 17β-estradiol administration might in part be due to the skewing of Th responses away from the prototypic anti-parasitic Th2 immunity and towards the more effective Th1 responses. Multiple genes involved in wound healing, differentiation and remodelling of skin tissue were stimulated during maturation but suppressed with sex hormones. Such opposite regulation suggested that these processes were not associated with resistance to the parasite under the studied conditions. Both studies revealed regulation of a suite of genes encoding putative large mucosal proteins found exclusively in fish. Marked decrease of erythrocyte markers indicated reduced circulation while down-regulation of multiple zymogen granule membrane proteins and transporters of cholesterol and other compounds suggested limited availability of nutrients for the parasites.