Skip to main content

The evolution of simplicity: comparative morphology, embryology and genomics of Xenacoelomorpha, the fourth phylum of deuterostome

Final Report Summary - XENOTURBELLA (The evolution of simplicity: comparative morphology, embryology and genomics of Xenacoelomorpha, the fourth phylum of deuterostome.)

We have been studying a recently identified major group (phylum) of animals called the xenacoelomorphs. All members of the Xenacoelomorpha phylum are marine worms and most are tiny - less than a millimetre in length. They have been known about for many years but were initially misclassified and wrongly associated with the flatworms (phylum Platyhelminthes).

Their true position within the evolutionary tree of animals has been controversial. The two possibilities being first that these simple worms represent an early diverging branch within the animals and that their simplicity relative to other animals is due to their having evolved early. The second possibility is that these simple worms are members of the deuterostomes - a more complex group of animals that include chordates (such as vertebrates) and echinoderms (e.g. starfish).

One of the major results from the project that depended on the new data from the DNA sequence we have created has been to show that the latter scenario is correct. Our finding implies that these simple worms derive from a complex ancestor and must therefore have lost complexity over evolutionary time.

Having sequenced the genome using a new technique that allows us to get a very complete assembly, we have been able to investigate the apparent simplicity of these animals (in particular the worm Xenoturbella bocki) more closely. What genes do they possess? Have they lost genes in becoming more simple and if so can we discern meaningful patterns in the losses? We find that Xenoturbella has fewer genes than many other animals but that the number of missing genes is fewer than for other simplified animals such as parasites.

Finally we wanted to understand just how simple these worms really are. To understand the cellular make up of these animals we have used a very new technology called single cell sequencing. This allows us to ask how many different types of cells there are. To see where each of these cell types is found in the body we have developed a new method to examine where in the body different genes are switched on. One important result has been a better characterisation of the cells in the nervous system. While the nervous system of Xenoturbella is typically described as a simple nerve net, we find hidden complexity in the different types of nerve cells we can find.

More generally we have carefully characterised the morphology of these animals using techniques such as scanning and transmission electron microscopy, sectioning the animals to identify the distribution of different tissues and have used a synchrotron to do a version of a CAT scan on the adult Xenoturbella to see them in 3D. These techniques have allowed us to understand these worms better in terms of their genetics and their morphology in order to compare them to other better known animal models.