Genes For Tetrapod Limbs In Fishes: An Evidence Of Genetic Unity
Queensland Lungfish (Neoceratodus forsteri)

Scientists have long wondered how transition from sea to land caused the ancestral organisms to adapt to environment on land, of which one of the most important adaptation is the evolution of limbs from fishes’ fins.

During embryonic development, clusters of genes hoxa and hoxd, found in both fishes and their descendants on land cause the limbs and fins to develop. From the comparison of these gene clusters in zebrafish and mice embryo, researcher Denis Duboule and his colleagues at University of Geneva and Federal Institute of Technology in Lausanne, Switzerland and the EPFL, Switzerland, found the similarity between 3-dimensional DNA organization of the gene clusters, proving that the mechanism of the formation of tetrapod limbs was already present in fishes, before the transition of their descendants on land.

But, an important point scientists have discovered is that the DNA of fish ,although has gene for limb formation, it lacks the genes for the formation of the digits, this was proved when scientists inserted the fish Hoxd gene in transgenic mice embryos, during the development of which the inserted gene was active only in the arm region but not in the digits. This evidence proves that the formation of limbs in land animals is the result of modification in genetic material of ancestral water animals.

In mammals, during embryonic development, hox genes or the ‘architect’ genes play the main role in the organizing body structures including the limbs. Hoxd gene are activated by ‘bimodal 3-dimensional DNA’ structure that directs the formation of subdivisions as arms and digits,when these genes and their expression mechanisms were compared in zebrafish and mice embryos,the results were surprising as the same biomodal 3-dimensional DNA was found in zebrafish as well. Next, the geneticists inserted the hox genes in transgenic mice embryo but again the result was unexpected, as during development the hox genes were active only in the arms and not in digits.

This study concludes that the genes for the formation of limbs were already present in fishes before the first animal walked on land about 350 million years ago, but these genes were further modified in land organisms resulting in division of arm and digits from the ancestral fins.

Our closest fish relative, the lungfishes, crawls on its four legged pointy fins. So,the transition from water to land resulted in modified genetic material thus formation of tetrapod limbs and digits which are found in nearly all land animals. Further research is needed to show what mechanisms resulted in change from fins to limbs.

[Image Credit: Wikipedia]

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