Keratin: another exemplarily evolving gene

Keratin is most familiar as the structural protein of hair – seems innocuous enough, and certainly not something that would be vital to the evolution and diversification of a group as major as the tetrapods (four-legged land animals).  However, a recent paper in Molecular Biology and Evolution reveals just that: not only did the keratin genes diversify and radiate as the different tetrapod taxa evolved and diverged; they were also the basis of the acquisition of traits integral to a life on land.

Keratins are split into two groups: beta keratins which are found only in the sauropsids (reptiles, birds and their fossil ancestors); and alpha keratins which are found in all tetrapods.  These alpha keratins are themselves split into two groups, types I and II: both groups are important in the structure of the cytoplasmic cell network that gives appendages like hairs and beaks their toughness.  The authors took known keratin gene sequences, and used these to search for keratin genes in the sequence data from the genomes of representatives of all tetrapods (frogs, birds, echidnas, marsupial mammals and placental mammals).  Interestingly, all genes were located on one of two gene clusters (which to me points to tandem duplication (when one gene is doubled by mistake during cell replication, with the new copy placed next to the original) being responsible for their duplication).

They then aligned these sequences by similarity and produced a phylogenetic tree to show their evolutionary relationships.  To minimise the number of duplications produced by this process they used the Minimal Early Duplication (MED) model, hence avoiding accusations of their duplication numbers being an artefact of the alignment and treeing process.

They produced 4 significant results.  Firstly, they found that all type I keratins are monophyletic (all members of the group came from one ancestral gene, and the group encompasses all descendants of that ancestor), as are all type II.  This exemplifies beautifully the process of molecular evolution: mistakes during the replication of one ancestral gene produce copies that are subtley different from the original, therefore changing the phenotype they produce in a way beneficial to the organism.

Secondly, the highest rate of keratin diversification was seen between 400 and 200 million years ago, during which time the stem amphibians, reptiles, birds and mammals all evolved.  Combined with functional data suggesting that the keratin genes with the most dramatic radiations function in appendage formation (including one human homologue of a sauropsid keratin gene which reinforces palms and soles), this provides persuasive evidence for the driver of tetrapod keratin gene diversification being their new terrestrial habit.

Thirdly, the biggest diversification of keratins in the mammals was seen in the hair keratins, and orthologues (sequences with the same ancestral sequence, but separated by a speciation event) of the mammalian hair keratins were found in amphibians.  Keratins involved in hair production therefore likely evolved early in tetrapod evolution (in the stem tetrapods, the taxon that existed before the divergence of the tetrapods and is the ancestor of the amphibians as well), rather than later (in the amniotes, a taxon that formed after the split of the amphibians and that is the ancestor of anything with an egg adapted to land) as was previously thought.  Instead of hair keratin genes evolving anew by mutations in the amniotes, they suggest that they evolved from genes already present in the stem tetrapods.  Based on this, the authors suggest an alternative hypothesis regarding the evolution of hair itself: in the words of the authors, “hair may have originated from glandular alpha-keratinized bumps in stem tetrapods”, rather than evolving from sensory appendages (as is currently thought).

And lastly, they found the same genes next to the keratin genes in fish and tetrapods – hence it is likely that the keratin gene cluster was organised before the divergence of fish and tetrapods (with subsequent duplication and gene evolution occurring within the confines of this genomic region).

References

Vandebergh, W. and Bossuyt, F. (2011) Radiation and diversification of alpha keratins during early vertebrate evolution.  Molecular Biology and Evolution, first published online 31.10.11

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