Xenoturbella species of marine worm. Image credit: Joacim Näslund via Wikimedia Commons (CC BY 4.0)
Xenoturbella bocki is a small marine worm predominantly found on the seafloor of fjords along the west coast of Sweden. This simple organism’s unusual evolutionary history has long intrigued zoologists as it is not clear how it is related to other animal groups. The worm may belong to one of the earliest branches of the animal kingdom, which would explain its simple body. On the other hand, it could be related to a more complex group, the deuterostomes, which includes a wide range of animals, from mammals and birds to sea urchins and starfish.
Understanding X. bocki’s evolution could provide valuable insights into how bilaterians evolved as a whole. Unlike its close relatives, the acoelomorphs, X. bocki evolves more slowly, which makes it simpler to study its genome. As a result, it serves as a starting point for investigating the evolutionary processes and genetics underpinning the broader group of bilaterians.
To better understand the evolution of X. bocki’s simple body, Schiffer et al. asked whether its genome is simpler or differs in other ways from that of more complex bilaterian organisms. Sequencing the entire X. bocki genome revealed that it has a similar number of genes to that of other animals and includes the genes required for complex biochemical pathways. Reconstructing the worm’s chromosomes – the structures that house genetic information – showed that the X. bocki genes are also distributed in a manner similar to those in other animals.
The findings suggest that, despite its simple body plan, X. bocki has a complex genome that is typical of bilaterians. This challenges the idea that X. bocki belongs to a more primitive, simplified sister group to Bilateria and provides a starting point for further studies of how this simple worm evolved.
