The amphipod Parhyale hawaiensis is a model organism of growing importance in the fields of evolutionary development and regeneration. A small, hardy marine crustacean that breeds year-round with a short generation time, it has simple lab culture requirements and an extensive molecular toolkit including the ability to generate targeted genetic mutant lines. Here we identify canonical core and regulatory clock genes using genomic and transcriptomic resources as a first step in establishing this species as a model in the field of chronobiology. The molecular clock of P. hawaiensis lacks orthologs of the canonical circadian genes cryptochrome 1 and timeless, in common with the mammalian system but in contrast to many arthropods including Drosophila melanogaster. Furthermore the predicted CLOCK peptide is atypical and CRY2 shows an extended 5' region of unknown function. These results appear to be shared by two other amphipod species.
Introduction 1The subphylum Crustacea has a rich history of chronobiological research. One of the earliest recorded 2 examples of a persistent circadian phenomenon in animals was the colour changes seen in the prawn Hip-3 polyte varians (Gamble and Keeble, 1900), while investigations on the circadian and circatidal rhythms 4 of the fiddler crab Uca pugnax helped lay the basis for modern chronobiology (Palmer, 1991), with pub-5 lished works on the persistence of chromatophore and locomotor activity rhythms in constant conditions; 6 temperature compensation; persistance of rhythms in isolated tissue; and the phase-shifting effect of light 7 pulses, this latter observation coming a decade before the phase response curve (PRC) was incorporated 8 into circadian concepts (De Coursey, 1960). A variety of endogenously controlled rhythmic phenomena 9 have been documented in wide range of crustaceans, not only circadian rhythms but also tidal, lunar 10 and semi-lunar (Strauss and Dircksen, 2010), under the control of zeitgebers such as light, hydrostatic 11 pressure, immersion, wave motion and salinity. Where the clade has historically proved lacking, how-12 ever, is in providing an organism with a molecular toolkit comparable to that of the fruit fly Drosophila 13 melanogaster, in which transgenics, reporter genes and binary expression systems have been used to 14 characterise, locate and dissect the endogenous biological clock at a neurological level 15 2005).
16At the centre of the Drosophila clock are the bHLH-PAS proteins CLOCK (Allada et al., 1998; 17 Gekakis et al., 1998, CLK) and CYCLE (Rutila et al., 1998, CYC), which form a heterodimer that drives 18 transcription of clock-controlled genes including period (Konopka and Benzer, 1971, per) and timeless 19 (Sehgal et al., 1994, tim), whose protein products dimerise and interact with CLK:CYC to inhibit its 20 transcription activity and thus their own expression. TIM is degraded by the light-activated CRYP-21 TOCHROME (Emery et al., 1998, CRY), exposing PER to degradation in turn and ending CLK:CYC 22 repression. The transcriptio...