The function of the ecdysone receptor (EcR) during development and molting has been thoroughly investigated in some arthropods such as insects but rarely in crustacean copepods such as the salmon louse Lepeophtheirus salmonis (L. salmonis) (Copepoda, Caligidae). The salmon louse is an ectoparasite on Atlantic salmon that has major economical impact in aquaculture due to the cost of medical treatment methods to remove lice from the fish. Handling of salmon louse infestations is further complicated by development of resistance towards available medicines. Understanding of basic molecular biological processes in the salmon louse is essential to enable development of new tools to control the parasite. In this study, we found L. salmonis EcR (LsEcR) transcript to be present in the neuronal somata of the brain, nuclei of muscle fibres and the immature intestine of the salmon louse. Furthermore, we explored the function of LsEcR during development using RNA interference mediated knock-down and through infection trials. Our results show that knock-down of LsEcR in the salmon louse is associated with hypotrophy of several tissues, delayed development and mortality. In addition, combined knock-down of LsEcR/LsRXR resulted in molting arrest during early larval stages.
The salmon louse Lepeophtheirus salmonis (Copepoda, Caligidae) is an important parasite in the salmon farming industry in the Northern Hemisphere causing annual losses of hundreds of millions of dollars (US) worldwide. To facilitate development of a vaccine or other novel measures to gain control of the parasite, knowledge about molecular biological functions of L. salmonis is vital. In arthropods, a nuclear receptor complex consisting of the ecdysone receptor and the retinoid X receptor, ultraspiracle, are well known to be involved in a variety of both developmental and reproductive processes. To investigate the role of the ecdysone receptor in the salmon louse, we isolated and characterised cDNA with the 5'untranslated region of the predicted L. salmonis EcR (LsEcR). The LsEcR cDNA was 1608 bp encoding a 536 amino acid sequence that demonstrated high sequence similarities to other arthropod ecdysone receptors including Tribolium castaneum and Locusta migratoria. Moreover, in situ analysis of adult female lice revealed that the LsEcR transcript is localised in a wide variety of tissues such as ovaries, sub-cuticula and oocytes. Knock-down studies of LsEcR using RNA interference terminated egg production, indicating that the LsEcR plays important roles in reproduction and oocyte maturation. We believe this is the first report on the ecdysone receptor in the economically important parasite L. salmonis.
Totiviridae is a virus family well known to infect uni-cellular organisms like fungi and protozoa. In more recent years, viruses characterized as toti-like viruses, have been found in primarily arthropods, but also a couple in planarians and piscine species. These toti-like viruses share phylogenetic similarities to totiviruses; however, their genomes also includes additional coding sequences in either 5′ or 3′ ends expected to relate to more advanced infection mechanisms in more advanced hosts. Here, we applied next generation sequencing (NGS) technologies and discovered three new toti-like viruses, one in wild common carp and one in bluegill from the USA and one in farmed lumpsucker from Norway. These are named common carp toti-like virus 1 (CCTLV-1), bluegill toti-like virus 1 (BGTLV-1), and Cyclopterus lumpus toti-like virus (CLuTLV), respectively. The genomes of these viruses have been characterized and compared to the three previously known piscine toti-like viruses, piscine myocarditis virus (PMCV) found in Atlantic salmon and the two from golden shiner, now named golden shiner toti-like virus 1 and 2 (GSTLV-1 and -2), and also to totiviruses and other toti-like viruses. We found that four piscine toti-like viruses had additional gene(s) in the 3′ end of the genome, and also clustered phylogenetically based on both capsid and RdRp-genes. This cluster constituted a distant branch in the Totiviridae, and we suggest this should be defined as a separate genus named Pistolvirus, to reflect this major cluster of piscine toti-like viruses. The remaining two piscine toti-like viruses differentiated from these by lacking any additional 3′ end genes and also by phylogenetical relation, but were both clustering with arthropod viruses in two different clusters.
The salmon louse is a marine ectoparasitic copepod on salmonid fishes. Its lifecycle consists of eight developmental stages, each separated by a molt. In crustaceans and insects, molting and reproduction is controlled by circulating steroid hormones such as 20-hydroxyecdysone. Steroid hormones are synthesized from cholesterol through catalytic reactions involving a 7,8-dehydrogenase Neverland and several cytochrome P450 genes collectively called the Halloween genes. In this study, we have isolated and identified orthologs of neverland, disembodied and shade in the salmon louse (Lepeophtheirus salmonis) genome. Tissue-specific expression analysis show that the genes are expressed in intestine and reproductive tissue. In addition, levels of the steroid hormones ecdysone, 20-hydroxyecdysone and ponasterone A were measured during the reproductive stage of adult females and in early life stages.
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