title: Immune genes undergo more adaptive evolution than non-immune system genes in Daphnia pulex.
abstract: BACKGROUND
  Understanding which parts of the genome have been most influenced by adaptive evolution remains an unsolved puzzle. Some evidence suggests that selection has the greatest impact on regions of the genome that interact with other evolving genomes, including loci that are involved in host-parasite co-evolutionary processes. In this study, we used a population genetic approach to test this hypothesis by comparing DNA sequences of 30 putative immune system genes in the crustacean Daphnia pulex with 24 non-immune system genes.
  
  
  RESULTS
  In support of the hypothesis, results from a multilocus extension of the McDonald-Kreitman (MK) test indicate that immune system genes as a class have experienced more adaptive evolution than non-immune system genes. However, not all immune system genes show evidence of adaptive evolution. Additionally, we apply single locus MK tests and calculate population genetic parameters at all loci in order to characterize the mode of selection (directional versus balancing) in the genes that show the greatest deviation from neutral evolution.
  
  
  CONCLUSIONS
  Our data are consistent with the hypothesis that immune system genes undergo more adaptive evolution than non-immune system genes, possibly as a result of host-parasite arms races. The results of these analyses highlight several candidate loci undergoing adaptive evolution that could be targeted in future studies.
authors.affiliation: Institute of Evolutionary Biology, School of Biological Sciences, Ashworth Laboratories University of Edinburgh, Edinburgh EH9 3JT, UK. smctagga@staffmail.ed.ac.uk
authors.forename: Seanna J
authors.initials: SJ
authors.lastname: McTaggart
authors.forename: Darren J
authors.initials: DJ
authors.lastname: Obbard
authors.forename: Claire
authors.initials: C
authors.lastname: Conlon
authors.forename: Tom J
authors.initials: TJ
authors.lastname: Little
mesh_terms: D000818:Animals; D003621:Daphnia / Q000235:genetics* / Q000276:immunology* / Q000469:parasitology; D019143:Evolution, Molecular; D055261:Immunogenetic Phenomena; D012641:Selection, Genetic