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Background: Microsporidia are highly specialized, parasitic fungi that infect a wide range of eukaryotic hosts from all major taxa. Infections cause a variety of damaging effects on host physiology from increased stress to death. The microsporidian Facilispora margolisi infects the Pacific salmon louse ( Lepeophtheirus salmonis oncorhynchi), an economically and ecologically important ectoparasitic copepod that can impact wild and cultured salmonids. Results: Vertical transmission of F. margolisi was demonstrated by using PCR and in situ hybridization to identify and localize microsporidia in female L. salmonis and their offspring. Spores and developmental structures of F. margolisi were identified in 77% of F generation copepods derived from infected females while offspring from uninfected females all tested negative for the microsporidia. The transcriptomic response of the salmon louse to F. margolisi was profiled at both the copepodid larval stage and the pre-adult stage using microarray technology . Infected copepodids differentially expressed 577 transcripts related to stress, ATP generation and structural components of muscle and cuticle. The infection also impacted the response of the copepodid to the parasiticide emamectin benzoate (EMB) at a low dose of 1.0 ppb for 24 h. A set of 48 transcripts putatively involved in feeding and host immunomodulation were up to 8-fold underexpressed in the F. margolisi infected copepodids treated with EMB compared with controls or either stressor alone. Additionally, these infected lice treated with EMB also overexpressed 101 transcripts involved in stress resistance and signalling compared to the other groups. In contrast, infected pre-adult lice did not display a stress response, suggesting a decrease in microsporidian virulence associated with lice maturity. Furthermore, copepodid infectivity and moulting was not affected by the microsporidian infection. Conclusions: This study demonstrated that F. margolisi is transmitted vertically between salmon louse generations and that biological impacts of infection differ depending on the stage of the copepod host. The infection caused significant perturbations of larval transcriptomes and therefore must be considered in future studies in which impacts to host development and environmental factors are assessed. Fitness impacts are probably minor, although the interaction between pesticide exposure and microsporidian infection merits further study. [ABSTRACT FROM AUTHOR]

Background: Outcomes of infections with the salmon louse Lepeophtheirus salmonis vary considerably among its natural hosts (Salmo, Oncorhynchus spp.). Host-parasite interactions range from weak to strong host responses accompanied by high to low parasite abundances, respectively. Parasite behavioral studies indicate that the louse prefers the host Atlantic Salmon (Salmo salar), which is characterized by a weak immune response, and that this results in enhanced parasite reproduction and growth rates. Furthermore, parasite-derived immunosuppressive molecules (e.g., proteases) have been detected at higher amounts in response to the mucus of Atlantic Salmon relative to Coho Salmon (Oncorhynchus kisutch). However, the host-specific responses of the salmon louse have not been well characterized in either of the genetically distinct sub-species that occur in the Atlantic and Pacific Oceans. Results: We assessed and compared the transcriptomic feeding response of the Pacific salmon louse (L. salmonis oncorhynchi,) while parasitizing the highly susceptible Atlantic Salmon and Sockeye Salmon (Oncorhynchus nerka) or themore resistant Coho Salmon (Oncorhynchus kisutch) using a 38 K oligonucleotide microarray. The response of the louse was enhanced both in the number of overexpressed genes and in the magnitude of expression while feeding on the non-native Atlantic Salmon, compared to either Coho or Sockeye Salmon. For example, putative virulence factors (e.g., cathepsin L, trypsin, carboxypeptidase B), metabolic enzymes (e.g., cytochrome B, cytochrome C), protein synthesis enzymes (e.g., ribosomal protein P2, 60S ribosomal protein L7), and reproduction-related genes (e.g., estrogen sulfotransferase) were overexpressed in Atlantic-fed lice, indicating heightened parasite fitness with this host species. In contrast, responses in Coho- or Sockeye-fed lice were more similar to those of parasites deprived of a host. To test for host acclimation by the parasite, we performed a reciprocal host transfer experiment and determined that the exaggerated response to Atlantic Salmon was independent of the initial host species, confirming our conclusion that the Pacific salmon louse exhibits an enhanced response to Atlantic Salmon. Conclusions: This study characterized global transcriptomic responses of Pacific salmon lice during infection of susceptible and resistant hosts. Similar parasite responses during infection of Coho or Sockeye Salmon, despite differences in natural immunity to infection between these host species, indicate that host susceptibility status alone does not drive the parasite response. We identified an enhanced louse response after feeding on Atlantic Salmon, characterized by up-regulation of virulence factors, energy metabolism and reproductive-associated transcripts. In contrast, the responses of lice infecting Coho or Sockeye Salmon were weaker, with reduced expression of virulence factors. These observations indicate that the response of the louse is independent of host susceptibility and suggest that co-evolutionary host-parasite relationships may influence contemporary host-parasite interactions. This research improves our understanding of the susceptibility of Atlantic Salmon and may assist in the development of novel control measures against the salmon louse. [ABSTRACT FROM AUTHOR]

Background: Salmon lice, Lepeophtheirus salmonis (Copepoda: Caligidae), are highly important ectoparasites of farmed and wild salmonids, and cause multi-million dollar losses to the salmon aquaculture industry annually. Salmon lice display extensive sexual dimorphism in ontogeny, morphology, physiology, behavior, and more. Therefore, the identification of transcripts with differential expression between males and females (sex-biased transcripts) may help elucidate the relationship between sexual selection and sexually dimorphic characteristics. Results: Sex-biased transcripts were identified from transcriptome analyses of three L. salmonis populations, including both Atlantic and Pacific subspecies. A total of 35-43 % of all quality-filtered transcripts were sex-biased in L. salmonis, with male-biased transcripts exhibiting higher fold change than female-biased transcripts. For Gene Ontology and functional analyses, a consensus-based approach was used to identify concordantly differentially expressed sex-biased transcripts across the three populations. A total of 127 male-specific transcripts (i.e. those without detectable expression in any female) were identified, and were enriched with reproductive functions (e.g. seminal fluid and male accessory gland proteins). Other sex-biased transcripts involved in morphogenesis, feeding, energy generation, and sensory and immune system development and function were also identified. Interestingly, as observed in model systems, male-biased L. salmonis transcripts were more frequently without annotation compared to female-biased or unbiased transcripts, suggesting higher rates of sequence divergence in male-biased transcripts. Conclusions: Transcriptome differences between male and female L. salmonis described here provide key insights into the molecular mechanisms controlling sexual dimorphism in L. salmonis. This analysis offers targets for parasite control and provides a foundation for further analyses exploring critical topics such as the interaction between sex and drug resistance, sex-specific factors in host-parasite relationships, and reproductive roles within L. salmonis. [ABSTRACT FROM AUTHOR]

Background: Sockeye Salmon are an iconic species widely distributed throughout the North Pacific. A devastating pathogen of Sockeye Salmon is infectious hematopoietic necrosis virus (IHNV, genus Novirhabdovirus, family Rhabdoviridae). It has been postulated that IHNV is maintained in salmon populations by persisting over the life of its host and/or by residing in natural reservoirs other than its susceptible hosts. Herein we demonstrate the presence of IHNV in the brain of Sockeye Salmon that survived an experimentally-induced outbreak, suggesting the presence of viral persistence in this susceptible species. To understand the viral persistent state in Sockeye Salmon we profiled the transcriptome to evaluate the host response in asymptomatic carriers and to determine what effects (if any) IHNV exposure may have on subsequent virus challenges. Results: A laboratory disease model to simulate a natural IHNV outbreak in Sockeye Salmon resulted in over a third of the population incurring acute IHN disease and mortality during the first four months after initial exposure. Nine months post IHNV exposure, despite the absence of disease and mortality, a small percentage (<4 %) of the surviving population contained IHNV in brain. Transcriptome analysis in brain of asymptomatic virus carriers and survivors without virus exhibited distinct transcriptional profiles in comparison to naïve fish. Characteristic for carriers was the up-regulation of genes involved in antibody production and antigen presentation. In both carriers and survivors a down-regulation of genes related to cholesterol biosynthesis, resembling an antiviral mechanism observed in higher vertebrates was revealed along with differences in nervous system development. Moreover, following challenge with poly(I:C), survivors and carriers displayed an elevated antiviral immune response in comparison to naïve fish. Conclusions: IHN virus persistence was identified in Sockeye Salmon where it elicited a unique brain transcriptome profile suggesting an ongoing adaptive immune response. IHNV carriers remained uncompromised in mounting efficient innate antiviral responses when exposed to a viral mimic. The capacity of IHNV to reside in asymptomatic hosts supports a virus carrier hypothesis and if proven infectious, could have significant epidemiological consequences towards maintaining and spreading IHNV among susceptible host populations. [ABSTRACT FROM AUTHOR]

Background: Salmon species vary in susceptibility to infections with the salmon louse (Lepeophtheirus salmonis). Comparing mechanisms underlying responses in susceptible and resistant species is important for estimating impacts of infections on wild salmon, selective breeding of farmed salmon, and expanding our knowledge of fish immune responses to ectoparasites. Herein we report three L. salmonis experimental infection trials of co-habited Atlantic Salmo salar, chum Oncorhynchus keta and pink salmon O. gorbuscha, profiling hematocrit, blood cortisol concentrations, and transcriptomic responses of the anterior kidney and skin to the infection. Results: In all trials, infection densities (lice per host weight (g)) were consistently highest on chum salmon, followed by Atlantic salmon, and lowest in pink salmon. At 43 days post-exposure, all lice had developed to motile stages, and infection density was uniformly low among species. Hematocrit was reduced in infected Atlantic and chum salmon, and cortisol was elevated in infected chum salmon. Systemic transcriptomic responses were profiled in all species and large differences in response functions were identified between Atlantic and Pacific (chum and pink) salmon. Pink and chum salmon up-regulated acute phase response genes, including complement and coagulation components, and down-regulated antiviral immune genes. The pink salmon response involved the largest and most diverse iron sequestration and homeostasis mechanisms. Pattern recognition receptors were up-regulated in all species but the active components were often species-specific. C-type lectin domain family 4 member M and acidic mammalian chitinase were specifically up-regulated in the resistant pink salmon. Conclusions: Experimental exposures consistently indicated increased susceptibility in chum and Atlantic salmon, and resistance in pink salmon, with differences in infection density occurring within the first three days of infection. Transcriptomic analysis suggested candidate resistance functions including local inflammation with cytokines, specific innate pattern recognition receptors, and iron homeostasis. Suppressed antiviral immunity in both susceptible and resistant species indicates the importance of future work investigating co-infections of viral pathogens and lice. [ABSTRACT FROM AUTHOR]

Background: There are a growing number of genomes sequenced with tentative functions assigned to a large proportion of the individual genes. Model organisms in laboratory settings form the basis for the assignment of gene function, and the ecological context of gene function is lacking. This work addresses this shortcoming by investigating expressed genes of sockeye salmon (Oncorhynchus nerka) muscle tissue. We compared morphology and gene expression in natural juvenile sockeye populations related to river and lake habitats. Based on previously documented divergent morphology, feeding strategy, and predation in association with these distinct environments, we expect that burst swimming is favored in riverine population and continuous swimming is favored in lake-type population. In turn we predict that morphology and expressed genes promote burst swimming in riverine sockeye and continuous swimming in lake-type sockeye., Results: We found the riverine sockeye population had deep, robust bodies and lake-type had shallow, streamlined bodies. Gene expression patterns were measured using a 16 k microarray, discovering 141 genes with significant differential expression. Overall, the identity and function of these genes was consistent with our hypothesis. In addition, Gene Ontology (GO) enrichment analyses with a larger set of differentially expressed genes found the "biosynthesis" category enriched for the riverine population and the "metabolism" category enriched for the lake-type population., Conclusions: This study provides a framework for understanding sockeye life history from a transcriptomic perspective and a starting point for more extensive, targeted studies determining the ecological context of genes.