THE EVOLUTION OF HEXAMERINS AND THE PHYLOGENY OF INSECTS

The evolutionary relationships among arthropod hemocyanins and insect hexamerins were investigated. A multiple sequence alignment of 12 hemo cyanin and 31 hexamerin subunits was constructed and used for studying sequence conservation and protein phylogeny. Although hexamerins and hemocyanins belong to a highly divergent protein superfamily and only Is amino acid positions are identical in all the sequences, the core s tructures of the three protein domains are well con served. Under the assumption of maximum parsimony, a phylogenetic tree was obtained that matches perfectly the assumed phylogeny of the insect orders. An inte resting common clade of the hymenopteran and coleopteran hexamerins wa s observed. In most insect orders, several paralogous hexamerin subcla sses were identified that diversified after the splitting of the major insect orders. The dipteran arylphorin/LSP-1-like hexamerins were sub ject to closer examination, demonstrating hexamerin gene amplification and gene loss in the brachyceran Diptera. The hexamerin receptors, wh ich belong to the hexamerin/hemocyanin superfamily, diverged early in insect evolution, before the radiation of the winged insects. After th e elimination of some rapidly or slowly evolving sequences, a lineariz ed phylogenetic tree of the hexamerins was constructed under the assum ption of a molecular clock. The inferred time scale of hexamerin evolu tion, which dates back to the Carboniferous, agrees with the available paleontological data and reveals some previously unknown divergence t imes among and within the insect orders.