Axolotl Genome Sequenced

An international team of researchers has successfully sequenced, assembled and analyzed the genome of the Mexican axolotl (Ambystoma mexicanum), an enigmatic salamander-like creature capable of extraordinary regenerative abilities.

The Mexican axolotl (Ambystoma mexicanum). Image credit: Bouboulski / CC BY-SA 4.0.

The Mexican axolotl (Ambystoma mexicanum). Image credit: Bouboulski / CC BY-SA 4.0.

Axolotls are known only from central Mexico, on the southern edge of Mexico City, in canals and wetlands in the general vicinity of Xochimilco. They are not homogeneously distributed through their range, and congregate in particular places.

They reach lengths on average of 9 inches (20 cm), but can grow up to 18 inches (45 cm) long.

Axolotls are neotenic aquatic salamanders, meaning they retain certain larval characteristics in the adult, reproductive state. They possess a caudal fin extending from behind the head to the vent and never lose their gills, allowing them to remain perpetually underwater.

The animals are also unique in that they can regenerate their limbs, including less vital parts of their brains.

“To fully understand regeneration and to find out why it is so limited in most species, scientists need to have access to genome data to study gene regulation and evolution,” explained Dr. Sergej Nowoshilow from the Research Institute of Molecular Pathology in Vienna, Austria, and co-authors.

“So far, the axolotl genome had evaded a complete assembly, due to its sheer size — at 32 billion base pairs, it is more than 10 times larger than the human genome.”

“The sequence assembly process using existing tools had been confounded by the enormous number of large repetitive sequences in this genome.”

Dr. Nowoshilow and his colleagues are the first to sequence and analyze the axolotl genome, the largest genome ever to be decoded.

“Using the PacBio-platform, a sequencing technology that produces long reads to span large repetitive regions, a total of 72.435.954 reads were sequenced,” the authors explained.

“The analysis of the genome revealed several features that seem to point to the uniqueness of the animal,” they said.

“We found that several genes that only exist in axolotl and other amphibian species are expressed in regenerating limb tissue.”

“Most strikingly, an essential developmental gene named PAX3 is completely missing from the genome, and its functions have been taken over by another gene termed PAX7.”

“Both genes play key roles in muscle and neural development.”

Dr. Nowoshilow added: “we now have the map in our hands to investigate how complicated structures such as legs can be re-grown.”

“This is a turning point for the community of scientists working with axolotl, a real milestone in a research adventure that started more than 150 years ago.”

The research is published in the journal Nature.

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Sergej Nowoshilow et al. The axolotl genome and the evolution of key tissue formation regulators. Nature, published online January 24, 2018; doi: 10.1038/nature25458

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