The analysis of a protein-polymorphism. Evolution of monomeric and homodimeric haemoglobins (erythrocruorins) of Chironomus thummi thummi (Insecta, Diptera).
The evolutionary history of 12 Chironomus thummi thummi (CTT) haemoglobins of known primary structures was reconstructed by the maximum parsimony method. This reconstruction demonstrates that the 12 CTT haemoglobin lineages originated monophyletically from a common ancestor within early Insecta and have the lineage to monomeric blood worm haemoglobin as their closest sister group. It can be further deduced that the earliest ancestral CTT haemoglobins were monomers and that a branch to all extant dimeric CTT haemoglobins emerged later in phylogeny near the base of Chironomidae, but perhaps still 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, CTT haemoglobins appear as relatively slow-evolving proteins, showing a preponderance of guanine in equilibrium adenine transitions at the first nucleotide position of the codons but not at the second. The most conservatively-evolving amino acid positions are haem contacts; the next most conservative are in interhelical contacts and interior positions involved in stabilization of tertiary structure. Further elucidation of the phylogenetic origins and adaptive evolution of the multiple haemoglobins found in Chironomus will be possible by the maximum parsimony method once haemoglobins or, in their absence, haemoglobin pseudogenes are sequenced in species throughout Chironomidae and related taxa.