Copepods are arthropods in the aquatic environment and the extremely abundant free-living species are an essential part of the first levels of the marine food chain. Although copepods comprise the largest animal biomass on the earth, relatively limited biological information is available at the molecular level and no model species exists.
The salmon louse (L. salmonis
) is an ectoparasitic copepod feeding on skin, mucous and blood from salmonid hosts. Recently it was shown that L. salmonis
infections in farmed fish induce epizootics in wild fish [1
]. The life cycle of L.salmonis
consists of 10 developmental stages separated by ecdysis [3
] and after the final molt, females develop into mature adults that continuously produce eggs for life. The first free-living larvae (naupli I) hatch directly from egg-strings attached to adult females and all three larval stages (naupli I, naupli II and the infectious copepidid stage) can be transported by the ocean currents over large distances depending on hydrographical conditions [5
]. After host settlement the infectious copepodids stage molt into chalimus. The four chalimus stages, all separated by molting, are anchored to the host by a frontal filament [6
], which restricts the feeding area. However, in the succeeding pre-ad I and -II and adult stages the salmon louse can move unrestricted on the host surface resulting in increased virulence [7
Sexual maturation and vitellogenesis are major physiological and behavioral changes in most animal life cycles. Germ cells are typically established early in development but arrested in development until the onset of sexual maturation. The generation of gametes is most conserved in males whereas variation is seen between different species for the development of female gametes. The ovum (i.e. the mature unfertilized egg) is a highly complex cell that is energetically expensive to produce. In order to produce high quality ova the females must undergo physiological adaptations that initiate further gamete development and maturation. Since the reproduction strategy is highly variable between different species and different life strategies (e.g. free living or parasitic) the processes of sexual maturation and the production of eggs also varies. However, there are some common hallmarks that are expected in most animals. After fertilization, the egg must contain sufficient energy to ensure development until external energy sources can be utilized. The ovarium is the site of initial development for female gametes during sexual maturation and reproduction. At ovulation, oogonia are released from the ovarium into the oviduct where growth and maturation take place. This process is typically divided into pre-vitellogenic and vitellogenic development. During vitellogenesis, yolk proteins are incorporated into the oocytes. A molecular hallmark for this process is the transcription of genes encoding egg yolk proteins like vitellogenins (Vgs). Depending on animal group, the transcription of Vgs takes place in different cell types like liver (in vertebrates), fat body (in insects) and hepatopancrease (e.g. in decapods). The Vgs are subsequently transported by the blood or hemolymph to the maturing oocytes, where they are taken up by receptor-mediated endocytosis [8
]. Production of vitellogenin is controlled by steroid hormones, which induce transcription of the target gene through binding of a steroid-receptor complex to the gene promoter. In arthropods, including crustaceans, ecdyson (i.e. E20) has been shown to bind to the heterodimeric ultra-spinacle (UsP) and ecdysteroid receptor (EcR) to an ecdysteroid response element (ERE) in the vitellogenin promoter [10
]. Ecdysteroids are also a key regulatory component in arthropods molting and development [11
The salmon louse reproductive systems have been described at the anatomical and histological levels [13
] but there is no information regarding the timing of the different events during sexual maturation. It has been proposed that males depositing spermatophores triggers the egg-production in other parasitic copepods (Lepeophtheirus pectoralis
], but according to our observations using unfertilized laboratory animals, adult female L. salmonis
produce eggs and external egg string also when males are not present (pers. obs., present study). Unlike crustaceans like shrimp, that produce eggs in seasons and grow/molt their entire life, salmon louse have a final molting, stop growing when egg production has started and then continuously produce eggs for life. It has been shown that L. salmonis
can produce up to 11 sets of egg-strings from a single fertilization [15
]. However, immediately following the last molting, the adult female salmon louse is not fully developed. Prior to egg production the animal matures in a process that includes a large increase of the genital segment and the abdomen, whereas the frontal cephalothorax appears unchanged (present study).
In order to link transcripts to the morphological and anatomical changes that takes place during the transition from pre-adult II to egg producing females we have combined EST sequencing and microarray analysis. The microarray analysis revealed three distinct groups of transcripts that correspond to molting, post molting growth and egg production. The possible function of the regulated transcripts is discussed in relation to the anatomical and physiological changes taking place. Initial analysis of EST sequences of L. salmonis revealed a large proportion of transcripts with no significant hits in public databases. In order to obtain some initial information regarding copepod proteomes we compared at set of L. salmonis proteins to some selected crustaceans, insects and vertebrates and shows that the salmon louse proteins are equally similar to all species.