title: Analysis of C3 suggests three periods of positive selection events and different evolutionary patterns between fish and mammals.
abstract: BACKGROUND
  The third complement component (C3) is a central protein of the complement system conserved from fish to mammals. It also showed distinct characteristics in different animal groups. Striking features of the fish complement system were unveiled, including prominent levels of extrahepatic expression and isotypic diversity of the complement components. The evidences of the involvement of complement system in the enhancement of B and T cell responses found in mammals indicated that the complement system also serves as a bridge between the innate and adaptive responses. For the reasons mentioned above, it is interesting to explore the evolutionary process of C3 genes and to investigate whether the huge differences between aquatic and terrestrial environments affected the C3 evolution between fish and mammals.
  
  
  RESULTS
  Analysis revealed that these two groups of animals had experienced different evolution patterns. The mammalian C3 genes were under purifying selection pressure while the positive selection pressure was detected in fish C3 genes. Three periods of positive selection events of C3 genes were also detected. Two happened on the ancestral lineages to all vertebrates and mammals, respectively, one happened on early period of fish evolutionary history.
  
  
  CONCLUSIONS
  Three periods of positive selection events had happened on C3 genes during history and the fish and mammals C3 genes experience different evolutionary patterns for their distinct living environments.
authors.affiliation: Laboratory for Marine Living Resources and Molecular Engineering, College of Marine Science, Zhejiang Ocean University, Zhoushan, China.
authors.forename: Fanxing
authors.initials: F
authors.lastname: Meng
authors.forename: Yuena
authors.initials: Y
authors.lastname: Sun
authors.forename: Xuezhu
authors.initials: X
authors.lastname: Liu
authors.forename: Jianxin
authors.initials: J
authors.lastname: Wang
authors.forename: Tianjun
authors.initials: T
authors.lastname: Xu
authors.forename: Rixin
authors.initials: R
authors.lastname: Wang
mesh_terms: D000818:Animals; D001483:Base Sequence; D001499:Bayes Theorem; D003176:Complement C3 / Q000235:genetics* / Q000378:metabolism; D019143:Evolution, Molecular; D005399:Fishes / Q000235:genetics* / Q000276:immunology; D008322:Mammals / Q000235:genetics* / Q000276:immunology; D008957:Models, Genetic; D008969:Molecular Sequence Data; D010802:Phylogeny; D012641:Selection, Genetic; D017422:Sequence Analysis, DNA; D013045:Species Specificity
chemical_list: D003176:Complement C3