Mutations, anomalies … Why do we have a coccyx and not a tail (like many other mammals)?

– Mutations, anomalies … why do

This particularity may seem all the more surprising since it is. Nevertheless, possible to see a tail on the human embryo. Furthermore, All human embryos temporarily develop a tail between the fourth. Therefore, the eighth week of gestation, which disappears well before birth. However, Genetics work reveal the molecular mechanisms that cause tail loss.

Viral traces in human DNA

DNA retains in its sequences the memory of large transitions. Similarly, upheavals that have shaped life over time, where each DNA fragment tells a stage in our biological mutations, anomalies … why do history.

From 8 to 10 % of the human genome comes from ancient viruses that infected our ancestors millions of years ago. For example. Nevertheless, endogenous retroviruses are the vestiges of ancestral virus that have integrated into DNA and have been transmitted from generation to generation. Therefore, Some of these viral sequences have long been considered “trash DNA”. Meanwhile, This trash DNA designates all the sequences of the genome which do not code for proteins. Nevertheless, whose biological function was initially deemed nonexistent or useless. Moreover, In reality, some of these viruses have played key roles in our biology, especially during embryonic development. For example, they have enabled placenta to the expression of proteins necessary for the development and functioning of this organ.

Other viral elements. In addition, called jumpers or transposable elements which are mobile DNA sequences capable of moving or copying in the genome, influence the expression of mutations, anomalies … why do neighboring genes. Therefore, These elements regulate for example key genes during the embryonic development of reproductive organs in mice. In addition, by activating in a specific way depending on the sex and the stage of development.

Viral insertion causing the loss of the tail

25 million years ago. a transposable element inserted in the TBXT gene of the hominoid ancestors. The TBXT (or Brachyury) gene plays a central role in the formation of the Chorde. an embryonic structure essential to the development of the spine and the body axis. In vertebrates, this gene regulates the differentiation of cells that will give birth to muscles, bones and the circulatory system. TBXT mutations have been identified in short or absent animals, such as the Manx cat and sheep developing vertebral anomalies. In humans, Mutations of TBXT are linked to malformations such as Spina Bifida. These malformations affect the development of the spine. mutations, anomalies … why do spinal cord: the vertebrae do not close completely in their dorsal part around the marrow, sometimes leaving a part of the exposed nervous tissue.

Articulated with the sacrum. the coccyx is a bone room located at the lower end of the spine, which therefore constitutes a vestige of the tail of the mammals. TBXT mutation would alter the conformation of the protein. disturbing its interactions with molecular signaling pathways which regulate for example cell proliferation and the formation of structures at the origin of the vertebrae. The introduction in mice of a mutation of the TBXT gene identical to those in the wild has made it possible to observe short -tailed animals. whose embryonic development is disturbed (6 % of embryos develop anomalies similar to Spina Bifida). The study shows that the TBXT mutation changes the activity of several genes of the WNT route. essential to the normal formation of mutations, anomalies … why do the spine. Experiences on mice show that the simultaneous expression of the complete shape. the truncated shape of the gene product induces a total absence of tail or a shortened tail, depending on their ratio.

These works explain why humans and large apes have a coccyx instead of a functional tail. The insertion of this mobile DNA sequence. or a transposable element, acted as a genetic switch: it partially deactivates TBXT, stopping the development of the tail while allowing the formation of the coccyx.

Mutations, anomalies … why do

An expensive evolutionary compromise?

The loss of the tail marked a major evolutionary turning point for hominoids. By modifying the center of gravity. it would have facilitated the emergence of bipedia, allowing our ancestors to release their hands to handle tools or carry food. But this adaptation was accompanied by an increased risk of congenital malformations. such as the Spina Bifida, which mutations, anomalies … why do affects approximately 1 birthday.

If mutations in the TBXT gene are involved. other risk factors have also been identified, such as nutritional deficiencies (a lack of folic acid (vitamin B9) in mother), taking anti-epileptic drugs (valproate), diabetes, obesity, lifestyles related to tobacco or alcohol. More recently. a study has shown that high exposure to PM particles₁₀ (particles less than 10 microns) during pregnancy increases the risk by 50 % to 100 % the development of a spina bifida.

These results illustrate a form of evolutionary compromise: the disappearance of the tail. advantageous for bipy, was favored while an increased risk of vertebral malformations has remained “tolerable”. Today. the Coccyx embodies this paradox of an advantage preserved at the cost of vulnerability: useful for fixing essential muscles for posture and continence (support of the pelvic floor), it remains a “fragile” vestige. Falls can fracture it.

In conclusion. the coccyx of hominoids mutations, anomalies … why do illustrates an evolutionary paradox: an ancient viral mutation has carved their anatomies, but also created vulnerabilities. DNA fragments. from ancient viruses, have become over the evolution of the essential cogs of embryonic development: they accelerate growth, coordinate the specialization of cells and regulate the expression of genes at the right time.

Jean-François Bodart. university professor, in mutations, anomalies … why do cellular biology and development biology, University of Lille

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