麻豆淫院

Arrow worms, or chaetognaths, are strange ocean predators that have puzzled scientists for more than a century. They have transparent, torpedo-shaped bodies, sharp grasping spines near their mouths, and sensory structures unlike those of any other animal group. Despite their distinctive look, their place on the evolutionary tree has long been a mystery.

Led by Ferdi Marl茅taz (UCL) and co-authored by Alexandre de Mendoza Soler, Chema Martin and Luke Sarre (formerly) of the School of Biological and Behavioral Sciences, along with other researchers, this new study in Nature, has pieced together their story right down to the level of individual genes and cells, and reveals how their unusual body plan came to be.

A unique place in the animal family tree

Chaetognaths belong to a group called Gnathifera, alongside microscopic animals like rotifers. Unlike their tiny relatives, chaetognaths are larger, active predators that live in open waters feeding on plankton.

Fossils show that their body plan has barely changed since the Cambrian period, over 500 million years ago, making them living snapshots of ancient ocean life. The Gnathifera are the sister group to a large group of animals including earthworms and snails.

Genomes that evolved at high speed

Researchers sequenced the complete genome of a species called Paraspadella gotoi and compared it to other animals. They found that chaetognaths' DNA has been heavily reshuffled over time.

While they possess many genes in common with all animals, they also harbor a large number of gene duplicates. Gene duplication can arise from whole genome duplications, where the genome is doubled and all genes have two copies, such as what happened in our own lineages, the vertebrates. Instead, the chaetognaths have duplicated all genes independently, presenting no indications of whole genome duplication.

Building new cells from new genes

By mapping nearly 30,000 individual cells from arrow worms, the team identified both ancient cell types (like muscles, neurons, and gut cells) and completely new ones (such as cells that produce their grasping spines and sensory papillae). Many of these unique cells rely on brand-new genes that evolved only in chaetognaths, as well as duplicated genes adapted for new roles.

This means arrow worms didn't just repurpose old genes鈥攖hey invented new ones to build their unusual organs.

Surprising gene regulation tricks

Chaetognaths also use unusual methods to control their genes:

• DNA methylation shift : Most animals use DNA methylation to regulate gene activity. In chaetognaths, methylation instead mainly targets "jumping genes" (transposons) to keep them inactive.
• Trans-splicing: Nearly half their genes undergo a process called trans-splicing, which helps produce mature RNAs from multiple gene segments. This is rare among animals and may help them manage their streamlined, rearranged genome.

This study shows that chaetognaths' distinct body plan is the result of massive genetic change, losing many old genes, gaining new ones, and evolving novel ways to regulate them. It suggests that, after a period of simplification in their distant past, arrow worms reinvented their complexity, creating unique organs and behaviors that allowed them to thrive as predators for over half a billion years.

As the researchers put it, chaetognaths are a striking example of how evolution can "rewrite" an animal's genetic toolkit to build something entirely new.