Genome Sequence of the Pea Aphid Acyrthosiphon pisum
Figure 12
Orthologs of circadian clock genes, some significantly diverged, are found in the pea aphid genome.
Shown is a schematic representation of pea aphid orthologs of the circadian clock genes arranged in a two-loop model, as proposed for Drosophila [92],[130]. Genes constituting the core of the clockwork in Drosophila are in filled shapes; other genes relevant to the clock mechanism in Drosophila are in empty ovals. In Drosophila, the per/tim feedback loop is centered on the transcription factors PER and TIM encoded by the genes period (per) and timeless (tim). Kinase 2 (CK2) and Shaggy (SGG), the Protein phosphotase 2a (PP2A), and the degradation signaling proteins Supernumerary limbs (SLMB) and jetlag (JET) participate in this loop either by stabilizing or destabilizing PER and TIM. Light entrainment is mediated through the participation of Cryptochrome 1 (CRY1) and JET, which promote the degradation of TIM. Absence of JET in A. pisum is indicated by a dashed cross. The positive feedback loop in Drosophila is centered on the gene Clock (Clk), whose expression is regulated by the products of the genes vrille (VR1) and Pdp1 (PDP1). In addition to all these genes, the pea aphid genome contains two copies of a mammalian-type cryptochrome, CRY2, which is present in all other insects examined except Drosophila. CRY2 has been proposed to be part of the core mechanism [93], acting as a repressor of CLK/CYC (indicated by a question mark). Some pea aphid orthologs have diverged significantly compared with orthologs in other insects (dashed outlines). This is most dramatic for PER and TIM proteins (double dashed outlines), whose sequences differ significantly from those of other insects. Wavy lines indicate rhythmic transcription in Drosophila. Thick arrows and lines ending in bars indicate positive and negative regulation, respectively.