A Profound Role for the Expansion of Trypsin-Like Serine Protease Family in the Evolution of Hematophagy in Mosquito

The trypsin-like serine protease (Tryp_SPc) family is ubiquitous in animals and plays diverse roles, especially in the digestive system, in different phyla. In the mosquito, some Tryp_SPc proteases make important contributions to the digestion of the blood meal. Here, we have defined the complete Tryp_SPc gene repertoire in the genome of the malaria mosquito, a repertoire that has expanded remarkably compared with that of Drosophila. Phylogenetic analysis also indicates that the large-scale lineage-specific expansion occurred leading to mosquitoes. Expression of Tryp_SPc genes elevates after a blood meal, and the expression level of genes that belong to subfamilies that specifically expanded on the mosquito lineage increased significantly more than genes that belong to subfamilies that did not expand in number, suggesting a profound role for the Tryp_SPc genes, especially the expanded subfamilies, in the hematophagous trait of the mosquito. The mosquito Tryp_SPc genes are mostly distributed in a tandem manner on chromosomes, suggesting a role for tandem duplication in the expansion of the subfamilies. Furthermore, evidence for positive selection was found for some genes. Structural modeling indicates that the positively selected sites locate to the surface that is conjugated by protein inhibitors. Our results suggest that the expansion and diversification of the Tryp_SPc domain family in mosquito was driven by positive selection and helps to explain the adaptive hematophagy of the mosquito.