Mosquitoes have evolved sophisticated cocktails of salivary proteins that affect the hemostasis, immunity and inflammation of host animals, so aiding the insect’s blood meal. Since saliva is so important in the transmission of arboviruses (arthropod borne viruses), studies unravelling its composition are of interest to biologists and medics alike.
In recent years, rapid progress in the analysis of salivary RNA – termed the sialotranscriptome – has seen the development of a large database of secreted salivary proteins from several different mosquito genera. José Ribeiro from the NIH National Institute of Allergy and Infectious Diseases, USA, and colleagues, are the first to study salivary proteins of the Psorophora genus, as published in a recent study in BMC Genomics. Members of this genus are known in the USA for their super-sized mosquitoes with vicious bites, but more importantly Psorophora is linked to the transmission of several viruses in the Western hemisphere, including equine encephalitis and West Nile fever.
Using Next-Generation Sequencing (NGS) techniques and proteome analysis, Ribeiro and colleagues analyzed the salivary glands of 50 field-collected Psorophora albipes from Brazil. Over 3,000 coding sequences were recovered, which were classified into 83 salivary protein families. Protein sequences were also compared to other members of the Culicinae subfamily that encompasses over 100 different mosquito genera, including Psorophora, Aedes and Culex. BLAST score comparisons showed that protein sequences of Psorophora were most similar to Aedes (69 percent), followed by Culex (6.5 percent) and lastly Anopheles (4.7 percent), the latter of which belongs to the subfamily Anophelinae. However P. albipes did present some mosquito protein characteristics found previously only in Culex.
A ‘missing link’ was also discovered between Culex and Aedes aegypti sequences in the form of a Psorophora protein with similarities to a Culex protein family, suggesting that this gene family is ancestral in all Culicines. Two orphan protein families i.e. groups of proteins whose genes are not known to share homologues in the genomes of other organisms, were ‘deorphanized’ – one from Aedes and one from Ochlerotatus. Several new protein families were also identified. Four of these were described as unique to Psorophora, having no match to any other known mosquito salivary sequence, a finding that may prompt future sequencing of other Psorophora species.
The highly divergent sequences of secreted salivary proteins between species, and the accumulated nucleotide polymorphisms within species, suggest a rapid evolution of salivary proteins in mosquitoes. This may be in response to continuously evolving vertebrate host immunity. This study not only brings us one step closer to understanding mosquito evolution but is also relevant for the development of exposure markers to mosquito bites and vector-borne diseases transmitted by mosquitoes.