Evolutionary implications of dipluran hexamerins.

Hexamerin, as a member of the highly conserved arthropod hemocyanin superfamily, has been shown to be a good marker for the phylogenetic study of insects. However, few studies have been conducted on hexamerins in basal hexapods. The first Diplura hexamerin CspHex1 was reported only recently (Pick and Burmester, 2009). Remarkably, CspHex1 was suggested to have evolved from hexapod hemocyanin subunit type 2, which is very different from all insect hexamerins originated from hexapod hemocyanin subunit type 1. Does this finding suggest double or even multiple origins of hexamerins in Hexapoda? To find more evidence on the evolution of dipluran hexamerins, eight putative hexamerin gene sequences were obtained from three dipluran species, as were three hemocyanin genes from two collembolan species. Unexpectedly, after adding the new sequences into the phylogenetic analyses, all dipluran hexamerins including CspHex1 grouped together and as sister to the insect hexamerins, with high likelihood and Bayesian support. Our analysis supports a single origin of the hexamerins in Hexapoda, and suggests the close relationship between Diplura and Insecta. In addition, our study indicates that a relatively comprehensive taxa sampling is essential to solve some problems in phylogenetic reconstruction.