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Carboniferous World

Genes Shed Light on Fish Fingers

by Rebecca Morelle, 23 May 2007. Updated 14 December 2017

A genetic study published in 2007 shed light on the mystery of how fish made the move from water to land millions of years ago.

Previous research had suggested that fish had made an abrupt genetic jump to acquire land-friendly limbs. But a US team was now able to show that this event was not an evolutionary novelty and that the transition was far more gradual than had previously been assumed.

The study, published in the journal Nature, followed the recent discovery in Arctic Canada of a fossil which was described as showing features that made it the 'missing link' between fins and limbs (see the feature, A Key Stage in Evolution, via the link in the sidebar).

In 2004, the fossilised remains of the Tiktaalik roseae revealed an animal with fins that were equipped for a life in the water but also for support on land. The crocodile-like creature, which lived about 380 million years ago, was said to 'blur the distinctions' between land and water-dwelling animals.

Overnight transformation

Marcus Davis, lead researcher of the paper and a scientist at the University of Chicago, was of the opinion that the Tiktaalik and other then-recent fossil finds suggested that the structures that really make land animals unique - hands and feet and fingers and toes - did not simply appear out of nowhere.

However, he added that this was in contrast to evidence seen in previous genetic studies, which suggested an abrupt transition from fins to limbs. These studies focussed on the Hox genes, which play a vital role in limb development. Scientists had looked at the expression of the genes in the developing limbs of land animals (tetrapods) and the developing fins of zebrafish, which are often used in embryological studies.

In tetrapods, these studies showed that there were two separate phases of Hox genes that turn on within the developing appendage. Early in the development there is the first phase, and then there is a second very characteristic phase which plays a role in where fingers and toes form.

But if you look at a zebrafish during development, it has the first phase, but it doesn't have this second hallmark phase. Based on this, the hypothesis was that the second phase of Hox expression must be a developmental and evolutionary novelty that correlated with the origin of hands and feet.

'Weirdos'

However, Dr Davis and his colleagues decided to repeat the studies - but this time using paddlefish, which have a fin pattern similar to primitive fish. They found a very clear second phase in their fins - and this shows that the second key phase of Hox-expression is in fact a much more ancient pattern of development.

It seems that some fish have always had this genetic toolkit to modify their fins. Tetrapods on the other hand seem to have modified it in this unique and elaborate way to produce land-dwelling animals.

The reason why some of these primitive fish went on to become land-living animals while others remained in the water was most probably influenced by their environment, according to Dr Davis. A change to the ecosystem from deep water to shallow streams may have driven some fish to make use of their genetic limb-building capability.

The study was also interesting because it revealed that zebrafish were the 'weirdos of the bunch'. They managed to do something very unique - they appear to have lost the second phase of Hox expression altogether. After this find, the expectation was that a number of similar patterns would emerge in other fish in the future.

 

 

     
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