title: Molecular adaptation and resilience of the insect's nuclear receptor USP.
abstract: BACKGROUND
  The maintenance of biological systems requires plasticity and robustness. The function of the ecdysone receptor, a heterodimer composed of the nuclear receptors ECR (NR1H1) and USP (NR2B4), was maintained in insects despite a dramatic divergence that occurred during the emergence of Mecopterida. This receptor is therefore a good model to study the evolution of plasticity. We tested the hypothesis that selection has shaped the Ligand-Binding Domain (LBD) of USP during evolution of Mecopterida.
  
  
  RESULTS
  We isolated usp and cox1 in several species of Drosophilidae, Tenebrionidae and Blattaria and estimated non-synonymous/synonymous rate ratios using maximum-likelihood methods and codon-based substitution models. Although the usp sequences were mainly under negative selection, we detected relaxation at residues located on the surface of the LBD within Mecopterida families. Using branch-site models, we also detected changes in selective constraints along three successive branches of the Mecopterida evolution. Residues located at the bottom of the ligand-binding pocket (LBP) underwent strong positive selection during the emergence of Mecopterida. This change is correlated with the acquisition of a large LBP filled by phospholipids that probably allowed the stabilisation of the new Mecopterida structure. Later, when the two subgroups of Mecopterida (Amphiesmenoptera: Lepidoptera, Trichoptera; Antliophora: Diptera, Mecoptera, Siphonaptera) diverged, the same positions became under purifying selection. Similarly, several positions of the heterodimerisation interface experienced positive selection during the emergence of Mecopterida, rapidly followed by a phase of constrained evolution. An enlargement of the heterodimerisation surface is specific for Mecopterida and was associated with a reinforcement of the obligatory partnership between ECR and USP, at the expense of homodimerisation.
  
  
  CONCLUSIONS
  In order to explain the episodic mode of evolution of USP, we propose a model in which the molecular adaptation of this protein is seen as a process of resilience for the maintenance of the ecdysone receptor functionality.
authors.affiliation: Irstea, UR MALY, Lyon F-69336, France.
authors.forename: Arnaud
authors.initials: A
authors.lastname: Chaumot
authors.forename: Jean-Luc
authors.initials: JL
authors.lastname: Da Lage
authors.forename: Oscar
authors.initials: O
authors.lastname: Maestro
authors.forename: David
authors.initials: D
authors.lastname: Martin
authors.forename: Thomas
authors.initials: T
authors.lastname: Iwema
authors.forename: Frederic
authors.initials: F
authors.lastname: Brunet
authors.forename: Xavier
authors.initials: X
authors.lastname: Belles
authors.forename: Vincent
authors.initials: V
authors.lastname: Laudet
authors.forename: François
authors.initials: F
authors.lastname: Bonneton
mesh_terms: D000818:Animals; D003058:Cockroaches / Q000235:genetics*; D001517:Coleoptera / Q000235:genetics*; D018428:Drosophilidae / Q000235:genetics*; D019143:Evolution, Molecular; D008958:Models, Molecular; D010802:Phylogeny; D017434:Protein Structure, Tertiary; D018160:Receptors, Cytoplasmic and Nuclear / Q000235:genetics; D011987:Receptors, Steroid / Q000737:chemistry / Q000235:genetics*
chemical_list: D018160:Receptors, Cytoplasmic and Nuclear; D011987:Receptors, Steroid; C055469:ecdysone receptor