title: Contrasted patterns of selective pressure in three recent paralogous gene pairs in the Medicago genus (L.).
abstract: BACKGROUND
  Gene duplications are a molecular mechanism potentially mediating generation of functional novelty. However, the probabilities of maintenance and functional divergence of duplicated genes are shaped by selective pressures acting on gene copies immediately after the duplication event. The ratio of non-synonymous to synonymous substitution rates in protein-coding sequences provides a means to investigate selective pressures based on genic sequences. Three molecular signatures can reveal early stages of functional divergence between gene copies: change in the level of purifying selection between paralogous genes, occurrence of positive selection, and transient relaxed purifying selection following gene duplication. We studied three pairs of genes that are known to be involved in an interaction with symbiotic bacteria and were recently duplicated in the history of the Medicago genus (Fabaceae). We sequenced two pairs of polygalacturonase genes (Pg11-Pg3 and Pg11a-Pg11c) and one pair of auxine transporter-like genes (Lax2-Lax4) in 17 species belonging to the Medicago genus, and sought for molecular signatures of differentiation between copies.
  
  
  RESULTS
  Selective histories revealed by these three signatures of molecular differentiation were found to be markedly different between each pair of paralogs. We found sites under positive selection in the Pg11 paralogs while Pg3 has mainly evolved under purifying selection. The most recent paralogs examined Pg11a and Pg11c, are both undergoing positive selection and might be acquiring new functions. Lax2 and Lax4 paralogs are both under strong purifying selection, but still underwent a temporary relaxation of purifying selection immediately after duplication.
  
  
  CONCLUSIONS
  This study illustrates the variety of selective pressures undergone by duplicated genes and the effect of age of the duplication. We found that relaxation of selective constraints immediately after duplication might promote adaptive divergence.
authors.affiliation: INRA - Institut National de la Recherche Agronomique, UMR AGAP, Montpellier, 34060, France.
authors.forename: Joan
authors.initials: J
authors.lastname: Ho-Huu
authors.forename: Joëlle
authors.initials: J
authors.lastname: Ronfort
authors.forename: Stéphane
authors.initials: S
authors.lastname: De Mita
authors.forename: Thomas
authors.initials: T
authors.lastname: Bataillon
authors.forename: Isabelle
authors.initials: I
authors.lastname: Hochu
authors.forename: Audrey
authors.initials: A
authors.lastname: Weber
authors.forename: Nathalie
authors.initials: N
authors.lastname: Chantret
mesh_terms: D020440:Gene Duplication; D029902:Medicago / Q000145:classification* / Q000235:genetics*; D026901:Membrane Transport Proteins / Q000235:genetics; D008969:Molecular Sequence Data; D010940:Plant Proteins / Q000235:genetics; D011096:Polygalacturonase / Q000235:genetics; D012641:Selection, Genetic
chemical_list: D026901:Membrane Transport Proteins; D010940:Plant Proteins; D011096:Polygalacturonase