Genome Sequence of the Pea Aphid Acyrthosiphon pisum
The cell division cycle typically consists of four phases: two growth phases (G1 and G2), a DNA synthesis or replication phase (S), and mitosis (M). Distinct and overlapping sets of regulatory genes are required for orderly progression through these phases. (A) Genes important for G1 and S phase progression are similar in number to other insects (orange box). G1/S Cyclin/Cyclin-dependent kinase (Cdk) protein complexes, along with E2F transcription factors, are critical for entry into G1 and progression into DNA replication and are opposed by cell cycle inhibitors such as p21/p27 family members and pRb/p107 family (Rbf) members, respectively. (B) Genes important for G2 and M phases have expanded in pea aphids (blue box). Polo kinases, Aurora kinases, Cdc25 phosphatases, and G2/M Cyclin/Cdk protein complexes are all critical for promoting entry into and progression through mitosis and meiosis. Negative regulators of Cdk1 and entry into mitosis include the Wee1/Myt1 kinase family. However, while Cdk1 has undergone aphid-specific duplication, no expansion of its activation subunits, Cyclins A and B, has been observed. Expanded gene families are in bold italics. Copy number was compared to that in Drosophila melanogaster, Tribolium castaneum, Pediculus humanas, Nasonia vitripennis, Culex quinquefasciatus, Anopheles gambiae, Aedes aegyptii, Bombyx mori, and Apis mellifera. aNo Myt1 orthologs were identified in the A. pisum genome. bAmong sequenced insects other than the pea aphid, Cdc25 is duplicated only in Drosophilids. cThree Aurora kinase orthologs are also present in Nasonia and Aedes while other insects possess two orthologs.