Fast evolutionary rate of mosquito salivary proteins: an effect of the host immune system?

Inter-specific comparisons of mosquito protein repertoires indicated that salivary proteins (S) show much higher diversity than housekeeping ones (H). For example, in the anophelines An. gambiae–An. stephensi on average the S proteins show a ~62% identity whereas H proteins share ~93% aa residues. We hypothesized this may result from the evolutionary pressure of the host immune system on proteins essential for hematophagy and, therefore, strongly affecting mosquito fitness. In such a scenario the host antibody response may favor the selection of variants with conserved functions but with different antigenic properties. Accordingly, one may expect multiple alleles to coexist in natural populations, perhaps in a condition of balanced polymorphism. This is an interesting subject with practical implications since salivary proteins are currently under development as markers of human exposure to vectors and as potential vaccine against arthropod-borne diseases. We analyzed the degree of poly