The genomes of two species of animals known for their astonishing powers of regeneration have been sequenced, and geneticists hope this will open the way to better healing of human injuries.
Axolotls (Ambystoma mexicanum), also know as Mexican walking fish, are amphibians that usually never undergo the sort of metamorphosis tadpoles do to become frogs. Instead they grow to adulthood with gills and inhabit the bottom of lakes (or more recently aquariums, which is just as well for the species, since they are verging on extinction in the wild).
Besides popularity as pets, the species has been taken up by scientists as a model organism because of its remarkable capacity to regenerate from injury. Like some other amphibians, axolotls can quickly regrow limbs lost in fights or to predators, but they go further than most of their relatives, regrowing damaged organs – including even parts of their brains. They will also sometimes grow extra limbs because who hasn't wanted to do a Zaphod Beeblebrox now and then.
A detailed sequencing of the axolotl genome has now been published in Nature, along with a preliminary discussion of the implications. For example, while the Prod1 gene had already been identified as contributing to the axolotl's regeneration capacity, the paper provides evidence other members of the same gene family also play a part. Non-coding sections of the axolotl's DNA also appear to contribute, and the paper identifies genes whose strong expression in regenerating limbs invites further attention.
The work was challenging because the axolotl has an unusually large genome, 10 times the size of humans, full of repeated sequences. It is the largest genome sequenced so far.
It is possible the sheer size of the axolotl genome contributes to its distinctive capacities, which also extend to being able to easily accept transplants of organs, including brain regions, from others of their species and make full use of them. However, the size doesn't seem to be entirely necessary. The same edition of Nature has an accompanying paper on the sequencing of the genome of the flatworm Schmidtea mediterranea, which has only 800 million bases, about a fortieth of the axolotl. (Does that make it an axolittle?). Nevertheless, this creature is a regenerating superstar, capable of being sliced and diced into many pieces and regrowing from there.
S. mediterranea has been sequenced before, but as a rough draft, now improved upon. The study found 124 genes used by vertebrates to repair DNA are missing from S.meditterannea's genome, including many we would expect to be essential for an animal's survival. It is hoped an exploration of how they do this, and comparison between the two organisms, will help us understand what life needs to survive, as well as to recover.