The Analysis of a Protein-Polymorphism. Evolution of Monomeric and Homodimeric Haemoglobins (Erythrocruorins) ofChironomus thummi thummi(Insecta, Diptera)

Abstract

The evolutionary history of 12 C. t. thummi Hb of known primary structures was reconstructed by the maximum parsimony method. This reconstruction demonstrates that the 12 C. t. thummi Hb lineages originated monophyletically from a common ancestor within early Insecta and have the lineage to monomeric blood worm Hb as their closest sister group. The earliest ancestral C. t. thummi Hb were monomers and a branch to all extant dimeric C. t. thummi hemoglobins emerged later in phylogeny near the base of Chironomidae, but perhaps before Chironomus itself evolved. This ancient, pre-Chironomus history suggests that among insect taxa, now lacking expressed globins, remnants of globin genes might exist as unexpressed pseudogenes. By the parameter of base replacement frequencies, C. t. thummi Hb appear as relatively slow-evolving proteins, showing a preponderance of guanine .dblarw. adenine transitions at the 1st nucleotide position of the codons but not at the 2nd. The most conservatively-evolving amino acid positions are heme contacts; the next most conservative are interhelical contacts and interior positions involved in stabilization of tertiary structure. Further elucidation of the phylogenetic origins and adaptive evolution of the multiple Hb found in Chironomus will be possible by the maximum parsimony method once Hb or, in their absence, Hb pseudogenes are sequenced in species throughout Chironomidae and related taxa.