title: Molecular evolution of Drosophila Sex-lethal and related sex determining genes.
abstract: BACKGROUND
  Sex determining mechanisms are evolutionarily labile and related species often use different primary signals and gene regulatory networks. This is well illustrated by the sex determining cascade of Drosophila fruitflies, which have recruited Sex-lethal as the master switch and cellular memory of sexual identity, a role performed in other insects by the gene transformer. Here we investigate the evolutionary change in the coding sequences of sex determining genes associated with the recruitment of Sex-lethal. We analyze sequences of Sex-lethal itself, its Drosophila paralogue sister-or-Sex-lethal and downstream targets transformer and doublesex.
  
  
  RESULTS
  We find that the recruitment of sister-or-Sex-lethal was associated with a number of adaptive amino acid substitutions, followed by a tightening of purifying selection within the Drosophila clade. Sequences of the paralogue sister-or-Sex-lethal, in contrast, show a signature of rampant positive selection and relaxation of purifying selection. The recruitment of Sex-lethal as top regulator and memory gene is associated with a significant release from purifying selection in transformer throughout the Drosophila clade. In addition, doublesex shows a signature of positive selection and relaxation of purifying selection in the Drosophila clade. A similar pattern is seen in sequences from the sister Tephritidae clade.
  
  
  CONCLUSIONS
  The pattern of molecular evolution we observe for Sex-lethal and its paralogue sister-or-Sex-lethal is not characteristic of a duplication followed by neo-functionalization. Rather, evidence suggests a sub-functionalization scenario achieved through the evolution of sophisticated splicing. As expected, we find that transformer evolves under relaxed purifying selection after the recruitment of Sex-lethal in Drosophila. Finally, the observation of doublesex adaptation in both Drosophila and Tephritidae suggests that these changes are due to ongoing adaptation of downstream sex-specific regulation, rather than being associated the recruitment of Sex-lethal and the resulting change in the topology of the sex determining cascade.
authors.affiliation: Department of Genetics, Environment and Evolution, University College London, Darwin Building, Gower Street, London WC1E 6BT, UK.
authors.forename: Charles
authors.initials: C
authors.lastname: Mullon
authors.forename: Andrew
authors.initials: A
authors.lastname: Pomiankowski
authors.forename: Max
authors.initials: M
authors.lastname: Reuter
mesh_terms: D000818:Animals; D029721:Drosophila Proteins / Q000235:genetics*; D004331:Drosophila melanogaster / Q000235:genetics* / Q000378:metabolism; D019143:Evolution, Molecular; D005260:Female; D019476:Insect Proteins / Q000235:genetics; D008297:Male; D009117:Muscidae / Q000145:classification / Q000235:genetics; D016601:RNA-Binding Proteins / Q000235:genetics*; D019849:Sex Determination Processes; D033621:Tephritidae / Q000145:classification / Q000235:genetics
chemical_list: D029721:Drosophila Proteins; D019476:Insect Proteins; D016601:RNA-Binding Proteins; C067777:Sxl protein, Drosophila