Scientists discover genes that may have caused fish to grow limbs
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An ancient relationship between fins and fingers has been discovered by palaeontologists.
The experts claim to have unravelled the evolutionary changes needed for ancient lobe-finned fish to turn underwater pectoral fins into strong, bony structures.
Until now, scientists were unsure exactly how this happened, but the latest research shows that certain species of fish have the genes needed to turn fins into wrists, fingers, ankles and toes.
By studying the genes of a fish called the spotted gar, scientists have discovered genetic 'switches' (shown in green) in transgenic fish fins, meaning it has the potential to evolve into fingers and wrists. This image shows a view of a transgenic zebrafish pectoral fish fin with cell nuclei in blue and gene activity in green
'Fossils show that the wrist and digits clearly have an aquatic origin,' said Dr Neil Shubin of the University of Chicago.
'But fins and limbs have different purposes.
'They have evolved in different directions since they diverged.
'We wanted to explore, and better understand, their connections by adding genetic and molecular data to what we already know from the fossil record.'
In particular, the scientists wanted to study how the extinct Tiktaalik roseae enabled emerging tetrapods - animals with limbs - to crawl in shallow water or on land.
The Tiktaalik rosae was a lobe-finned fish that lived in the late Devonian period, but had features similar to four-legged animals.
They began by attempting to compare the shape of fins and limb bones, but these tests proved unsuccessful.
A limb is split into three sections - stylopod, zeugopod, and autopod.
A stylopod, for example, contains the humerus and femur, the zeugopod is the ulna and radius, and the autopod contains the wrist and fingers.
Prehistoric fish, such as the Tiktaalik roseae (fossil pictured), whose fins were strong, bony structures, led to the evolution of emerging emerging tetrapods - animals with limbs - to crawl in shallow water or on land, but until now, experts have been unsure how this happened
US experts have found that a freshwater fish called the spotted gar (stock image), has the basic genetics to evolve an autopod, comprising wrists and fingers or ankles and toes
The autopod differs to most fins because the wrist is composed of a series of small nodular bones, followed by longer thin bones.
In contrast, the bones that make up the fins of modern fish look different, with a set of longer bones ending in small circular bones called radials.
The primary genes that shape the bones are known as the HoxD and HoxA clusters, and these also differ between limbs and fins, according to the study in the journal, Proceedings of the National Academy of Sciences (PNAS).
The researchers first tested the ability of genetic switches that control the genes in teleosts - a type of fish - to shape the limbs of developing mice.
They found that the fish control switches did not trigger any activity in the autopod, or mouse in general.
After deciding that teleost fish were not suitable for the study, they inserted genes from the gar fish into developing mice instead.
This time, the researchers discovered that the gar switches caused patterns of activity that were 'nearly indistinguishable from those driven by the mouse genome'.
The autopod-building genetic switches from gar are able to drive gene activity (shown in purple) in the digits of transgenic mice - an activity that was not spotted in any other fish groups studied
In January, the same group of researchers announced that the evolution of hind legs began as enhanced fins. They studied the fossil of the Tiktaalik roseae (illustrated) which is thought was able to 'walk' and support itself on the ground with the aid of extra functional, strong 'fins'
More than 300 million years ago, after fish-like creatures that would become tetrapods split off from other bony fish, a common ancestor of the teleost lineage went through a whole-genome duplication (WGD) - a phenomenon that has occurred multiple times in evolution.
By doubling the entire genetic repertoire of teleost fish, this WGD phenomenon provided the creatures with enormous diversification potential.
Experts think this helped teleosts to adapt, over time, to a variety of environments across the globe.
Graduate student Andrew Gehrke, who led the study, explained: 'In the process, the genetic switches that control autopod-building genes were able to drift and shuffle, allowing them to change some of their function, as well as making them harder to identify in comparisons to other animals, such as mice.
'Not all bony fishes went through the whole genome duplication, however,' he said.
'The spotted gar, a primitive freshwater fish native to North America, split off from teleost fishes before the WGD.'
When the scientists compared Hox gene switches from the spotted gar with tetrapods, they found 'an unprecedented level of 'deep conservation of the vertebrate autopod regulatory apparatus.'
Put simply, they discovered that the tiny radial bones in bony fish fins, and the fingers or toes of tetrapods are remarkably similar.
Mr Gehrke said: 'Overall, our results provide regulatory support for an ancient origin of the "late" phase of Hox expression that is responsible for building the autopod.'
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