Functional interpretations of the insect tracheal system – a branched series of tubes supplying oxygen from the environment to every metabolizing tissue in the insect's body – are plagued by two related myths. The first is that gases move through the tracheal system entirely by diffusion. This idée fixe persists despite many studies showing that, across vast taxonomic distances, insects actively ventilate the tracheal system in astonishing and subtle ways. The second myth is that the tracheal system's direct connection between environment and tissue renders unnecessary other respiratory complications, like circulatory systems and respiratory proteins. Of course, oxygen-carrying proteins (called hemocyanins) are known from insects. But their taxonomic distribution, in basal insects, reinforces the idea that advanced insects, equipped with highly evolved tracheal systems, don't need additional help. Another class of respiratory proteins (hemoglobins) is found in a few groups of insects adapted to parasitic or aquatic habitats that expose them to hypoxia. But these were usually explained away as strange solutions to alternative lifestyles.
What a shock therefore when reports started surfacing a few years ago of a Drosophila hemoglobin. An initial report, by Thorsten Burmester and Thomas Hankeln, identified a gene called dmeglob1 (for Drosophila melanogaster globin). A follow up report from the same group also showed that the protein product was not exported into the hemolymph. It remained intracellular, had oxygen-binding kinetics much like other known hemoglobins, and occurred at high levels in the tracheal system and fat body.
Next the team turned to an evolutionary analysis of hemoglobin gene sequences from eight additional Drosophila species separated by up to 65 million years.
Publishing their findings in FEBS Journal, Burmester and his colleagues describe how that found a remarkable degree of conservation across the hemoglobin genes, despite the passage of time.
Comparative data such as these can provide enormous insight into functional questions by identifying whether genes, or regions within them, are under strong selective constraint. The comparative data did not disappoint: the glob1 gene appears to be highly conserved across the fly species. Moreover, glob1's coding regions contained many more synonymous base substitutions (which do not change the encoded amino acid) than non-synonymous substitutions (which change the encoded amino acid). High ratios of synonymous to non-synonymous substitutions provide good molecular evidence that the gene has been under strong selection in the past. As a bonus, the team identified two additional globin genes in Drosophila, glob2 and glob3. Although their sequences differed substantially from the sequence of glob1, the amino acid sites required for heme- and oxygen-binding were conserved. The new genes' functions are unknown. However, expression levels of glob2 protein were much lower than glob1, leading them to exclude a respiratory role for it. They suggest tentatively that it may be related to nitric oxide metabolism.
What is so fascinating about the Drosophila story is the possibility it raises. If Drosophila can and does make physiological use of hemoglobins, then perhaps insects in general can and do so. As usual, the simple story – in this case, of oxygen supply by simple diffusion through branched tubes – looks increasingly naïve.
- © The Company of Biologists Limited 2006