The genomes of the Mexican axolotl (Ambystoma mexicanum) and the planarian flatworm (Schmidtea mediterranea) are reported in a pair of Nature papers this week, shedding light on the genetic bases of the regenerative abilities of both organisms.
Axolotls are able to regenerate whole limbs, whereas planarian worms can regrow entire bodies even after being minced into hundreds of pieces. Understanding how this is possible has long been a goal of researchers, who seek the underlying genetic mechanisms.
At 32 billion bases long, the axolotl genome, sequenced by Elly Tanaka and colleagues, is ten times the size of the human genome, and is the largest genome assembled so far. The researchers highlight genes and microRNA sequences whose expression is highly enriched in the cells of regenerating limbs as candidates for further study, and show that Pax3, a gene that is essential for development in many animals, is absent in axolotls.
By contrast, the genome of the planarian worm, reported by Jochen Christian Rink and colleagues, is about 800 million bases long, and improves on earlier draft genomes for this species. Some 124 genes that are essential in humans and mice are absent from the worm genome, including genes involved in DNA repair and others that help chromosomes separate correctly during cell division.
Both genomes contain a high level of repetitive DNA sequences, a feature that made their genomes more difficult to analyse. The authors used a new computational method that, together with long-read sequencing, allowed for an improved genome assembly. Certain repetitive sequences, which are known to be involved in embryonic development and stem cell activity, have previously been linked to regeneration in the Spanish newt, so it will be intriguing to learn whether they have a similar role in these two model organisms.