title: Transcriptional profiling of reproductive development, lipid storage and molting throughout the last juvenile stage of the marine copepod Calanus finmarchicus.
abstract: BACKGROUND
  Calanus finmarchicus, a highly abundant copepod that is an important primary consumer in North Atlantic ecosystems, has a flexible life history in which copepods in the last juvenile developmental stage (fifth copepodid, C5) may either delay maturation and enter diapause or molt directly into adults. The factors that regulate this developmental plasticity are poorly understood, and few tools have been developed to assess the physiological condition of individual copepods.
  
  
  RESULTS
  We sampled a cultured population of C. finmarchicus copepods daily throughout the C5 stage and assessed molt stage progression, gonad development and lipid storage. We used high-throughput sequencing to identify genes that were differentially expressed during progression through the molt stage and then used qPCR to profile daily expression of individual genes. Based on expression profiles of twelve genes, samples were statistically clustered into three groups: (1) an early period occurring prior to separation of the cuticle from the epidermis (apolysis) when expression of genes associated with lipid synthesis and transport (FABP and ELOV) and two nuclear receptors (ERR and HR78) was highest, (2) a middle period of rapid change in both gene expression and physiological condition, including local minima and maxima in several nuclear receptors (FTZ-F1, HR38b, and EcR), and (3) a late period when gonads were differentiated and expression of genes associated with molting (Torso-like, HR38a) peaked. The ratio of Torso-like to HR38b strongly differentiated the early and late groups.
  
  
  CONCLUSIONS
  This study provides the first dynamic profiles of gene expression anchored with morphological markers of lipid accumulation, development and gonad maturation throughout a copepod molt cycle. Transcriptomic profiling revealed significant changes over the molt cycle in genes with presumed roles in lipid synthesis, molt regulation and gonad development, suggestive of a coupling of these processes in Calanus finmarchicus. Finally, we identified gene expression profiles that strongly differentiate between early and late development within the C5 copepodid stage. We anticipate that these findings and continued development of robust gene expression biomarkers that distinguish between diapause preparation and continuous development will ultimately enable novel studies of the intrinsic and extrinsic factors that govern diapause initiation in Calanus finmarchicus.
authors.affiliation: Biology Department, Woods Hole Oceanographic Institution, 45 Water Street, Woods Hole, MA 02543 USA.
authors.forename: Ann M
authors.initials: AM
authors.lastname: Tarrant
authors.affiliation: Biology Department, Woods Hole Oceanographic Institution, 45 Water Street, Woods Hole, MA 02543 USA.
authors.forename: Mark F
authors.initials: MF
authors.lastname: Baumgartner
authors.affiliation: SINTEF Materials and Chemistry, Environmental Technology, N-7465 Trondheim, Norway.
authors.forename: Bjørn Henrik
authors.initials: BH
authors.lastname: Hansen
authors.affiliation: BioTrix, N-7022 Trondheim, Norway.
authors.forename: Dag
authors.initials: D
authors.lastname: Altin
authors.affiliation: SINTEF Materials and Chemistry, Environmental Technology, N-7465 Trondheim, Norway.
authors.forename: Trond
authors.initials: T
authors.lastname: Nordtug
authors.affiliation: Department of Biology, Norwegian University of Science and Technology, N-7491 Trondheim, Norway.
authors.forename: Anders J
authors.initials: AJ
authors.lastname: Olsen
keywords: Arthropod; Crustacean; Gene expression; Molt cycle; Transcriptomics