Evolution of the complement system in protostomes revealed by de novo transcriptome analysis of six species of Arthropoda

• RNA-seq analysis of six arthropod species revealed distribution of complement genes in Arthropoda.
• A simple complement system comprising C3 and FB is present in Chelicerata and centipeda.
• Crustacea, Hexapoda and millipede have no complement component.

To elucidate the evolutionary history of the complement system in Arthropoda, de novo transcriptome analysis was performed with six species among the Chelicerata, Myriapoda, and Crustacea, and complement genes were identified based on their characteristic domain structures. Complement C3 and factor B (FB) were identified from a sea spider, a jumping spider, and a centipede, but not from a sea firefly or two millipede species. No additional complement components identifiable by their characteristic domain structures were found from any of these six species. These results together with genome sequence information for several species of the Hexapoda suggest that the common ancestor of the Arthropoda possessed a simple complement system comprising C3 and FB, and thus resembled the alternative pathway of the mammalian complement system. It was lost at least twice independently during the evolution of Arthropoda in the millipede lineage and in the common ancestor of Crustacea and Hexapoda.