Vestigial DNA
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[edit] Introduction
DNA which has lost its function or become reduced in function, can still be retained in the genome, often in truncated or corrupted form, and so can be regarded as a vestigial structure of the genome; as with morphological vestigial structures, these provide us with insight into the evolution of the species in question: most because they are suitable candidates for the application of Molecular Phylogeny, but others are of more specific interest. In this article, we shall look at a few of the more interesting examples.
[edit] Icefish and hemoglobin
The icefishes (family Channichthyidae) are a group of fishes living in the cold waters of Antarctic and sub-Antarctic seas. They are unique among vertebrates in not making hemoglobin, the red pigment in the blood that in other vertebrates is used to transport oxygen in the blood; instead, they simply transport oxygen in their blood plasma. This adaptation helps to prevent the coagulation of the blood that otherwise affects vertebrates at low temperatures[1].
However, despite not producing any hemoglobin, they still have broken remnants of the genes for making alpha-globin and (in some icefish) beta-globin, the polypeptides from which the hemoglobin protein is built[2]. One species of icefish, Neopagetopsis ionah, possesses complete, but non-functional copies of the alpha-globin and beta-globin genes[3].
[edit] Primates and vitamin C
Primates, including humans, have a broken copy of the gene which in other mammals produces L-gulono-gamma-lactone oxidase[4][5], an enzyme crucial for producing L-ascorbic acid, better known as Vitamin C: this is why you can't just make your own vitamin C, but have to get it in your diet.
The evolution of this situation is easily understood: the typical primate diet is rich in vitamin C, so the ability to synthesize their own vitamin C was not required. A mutation rendering the gene non-functional would therefore be neutral with respect to the survival of the organism, so such a mutation could get fixed in the gene pool of a basal primate species by genetic drift. (Obviously, the mechanisms of evolution could not look ahead and see that this mutation would one day cause sailors on long voyages without fresh fruit or vegetables to come down with scurvy!)
[edit] Human chromosome 2
Human chromosome 2 looks just like what you would get if you fused together chimpanzee chromosomes 2a and 2b. Where the chromosomes join, we see remnants of DNA normally characteristic of the ends of chromosomes. Also, human chromosome 2 has (as usual) only one centromere, corresponding in position to the centromere of chimp chromosome 2b; however, where the centromere of chimp chromosome 2a would lie, it has DNA sequences characteristic of centromeres.
This subject is treated at length in our main article on Chromosome 2.
[edit] Axolotls
However, by injecting them with thyroxine hormone, which controls development, or with iodine, which stimulates production of thyroid hormones, scientists can induce axolotls to transform into normal-looking adult salamanders, similar, though not identical, to the adult form of the closely related Mexican Tiger Salamander.
It follows that the axolotl must have all the genetic information necessary to produce the adult form, but without making use of it. The fact that this information have not been corrupted by mutation and genetic drift shows that the neoteny must be a relatively recent evolutionary development, as can be confirmed by molecular phylogeny or more simply by comparing the induced adult form to the adult form of the Mexican Tiger Salamander.
