Copying may seem like the slacker’s way; but for natural selection, it only increases the work it has to do. When a gene in an organism is duplicated – as can happen for a single gene, a string of genes, a whole chromosome or even the whole genome – the organism suddenly has new roads of evolutionary possibility open to it. Before duplication a gene has a hard time evolving, because deviation from its original function is usually harmful; however, after duplication the original copy can continue to carry out the original function, and natural selection can act on the spare copy, leading to the evolution of a new function.
A recent paper in Molecular Biology and Evolution tracked the evolution of the globin genes through vertebrate history, and showed that whole-genome duplication (AKA WGD, where every gene in the organism is replicated) has been integral to the evolution of their diverse functions.
Their main findings were:
1). Two rounds of WGD occurred during the evolution of the early vertebrates
2). The first duplication produced two copies of the globin gene (as well as every other gene in the genome): one copy was the ancestor of the myoglobins and cytoglobins; the other was the ancestor of the haemoglobins
3). The second round of WGD produced two copies of the haemoglobin gene (and all other genes): these two copies specialized into the alpha and beta globins
These findings are important because:
1). While these two rounds of WGD in early vertebrate evolution have been known about for a while, this provides another concrete example of their products, highlighting further the importance of gene duplication in evolution
2). We can pinpoint more specifically the timing of the division of labour between the myoglobins (which evolved to store oxygen in muscles) and the haemoglobins (which evolved to transport oxygen to respiring tissues), which was a crucial evolutionary development
3). We now know that the alpha and beta globins were produced by a second round of WGD, which allowed these copies to specialize into genes that are key to haemoglobin’s affinity for oxygen
The paper is also noteworthy for its sophisticated use of gene sequence comparison to reveal the evidence for two rounds of WGD. When a genome is replicated twice, four copies of each gene are expected; however, puzzlingly the authors initially found only three copies of the globin gene. Luckily for them, WGD also preserves the order of genes on the replicated chromosomes; therefore if one gene is missing, but the rest of a chromosome is present, we can infer that the chromosome (and the whole genome in this case) was in fact duplicated, and this gene must have been removed. The authors therefore looked for regions of chromosomes which were syntenic (similar in gene order and arrangement) with the chromosomal regions containing the three globin genes, and traced the missing fourth globin gene copy to human chromosome 19. They therefore had more evidence of two rounds of WGD, and showed that one copy of the globin gene had been lost, something that may have been missed had they just scanned the genome for similar sequences.
Overall, this paper is a great example of the importance of gene duplication to the evolution of the globin gene family, as well as to genome organisation and physiology generally. It also highlights the subtle and powerful analyses made possible by genome sequencing.
Hoffman, F.G., Opazo, J.C. and Storz, J.F. (2011) Whole-genome duplications spurred the functional diversification of the globin gene superfamily in vertebrates. Molecular Biology and Evolution, Advance Access