On the evolution of salivary gland genes in Anophelines.

BACKGROUND: Saliva of adult female mosquitoes assists blood feeding by impairing host hemostasis and affecting inflammation and immunity. The salivary potion of mosquitoes is complex and in Anopheles gambiae it is estimated to contain the product of at least 75 genes, most being expressed solely in the adult female salivary glands. The sequencing of 16 anopheline genomes, in addition to those of An. gambiae and An. darlingi allows an unprecedented opportunity to obtain insights into the evolution of salivary genes associated with blood feeding in mosquitoes. METHODS: Using Artemis we manually re-annotated and analyzed the salivary gland genes of the 18 species, and proceeded with their phylogenetical analysis. SnIPRE and the HYPHY suite FEL and MEME programs were used to investigate the evolutionary signatures. RESULTS: A scenario of high plasticity of these genes emerges following annotation of the existing 18 anopheline genomes, revealing a relatively large number of gene gains/losses involving both individual salivary genes and multigene families. SnIPRE analysis indicated that salivary genes show the shortest average divergence times, the smallest negative constraint effect, and highest proportion of non-synonymous mutations that are non-lethal of all gene classes. Both FEL and MEME analysis showed that salivary gland genes are among those having the highest rate of positively selected codons among seven gene classes. CONCLUSIONS: Results support the estimates that salivary gland genes associated to blood feeding are at an accelerated pace of evolution and are remarkably driven by positive selection.