EMBL researchers study Platynereis dumerilii, a worm that retains many features of ancient species, aiming to discover mechanisms of evolution
After more than 500 million years of existence as a species, the worm Platynereis dumerilii still retains some ancient features that most other animals have moved on from over evolutionary time. Yet, this species has been able to survive and adapt to a variety of environments.
Scientists in Detlev Arendt's group at EMBL Heidelberg have tracked this marine worm over time and space, in a quest to learn the many lessons it offers about evolution. They recently took advantage of EMBL's Traversing European Coastlines (TREC) project to broaden their research perspective, resulting in both interesting science and some incredible stories.
"We don't know which parts are new and which are old in us humans," said Phil Oel, Field Expedition scientist in the Arendt Group. "Since we can't time travel, Platynereis offers a good path backwards to compare and contrast. We can identify traits still there and infer basic traits - what the original ancestors looked like, so to speak."
In other words, the scientists hope to uncover the species's missing biological history. For example, many brain parts and functions in these worms have retained relatively ancient forms, in contrast to other animals that have more drastically updated their brains to suit complex needs. This is what makes Platynereis dumerilii special.
"Since these worms haven't had much pressure to change, their evolution has been slow," Tobias Gerber, a postdoc in the Arendt Group, said, "However, we still don't know how they've adapted to different environments - one of the questions we're researching."
Platynereis worms live around the world and even though they look the same, they live differently. Normally, species specialise to live in a particular habitat and rarely tolerate changes in their environment. However, Platynereis worms are able to deal with a diverse range of water salinities and temperatures. They can survive in polluted waters as well as in pristine seawater, which intrigues the scientists.
"We want to know what changed in Platynereis so they could adapt to so many different environments," Victoria Witte, PhD student in the Arendt Group, said.
Broadening the research scope
When the EMBL-led TREC expedition started its field sampling phase in early 2023, it offered the researchers an opportunity to gather Platynereis from a variety of places along Europe's coasts. By collecting and comparing Platynereis from different locations, the scientists could investigate how the worm populations evolved in different habitats and conditions.
The main challenge, however, came from the worm's unique mating ritual: a beautiful dance following the lunar cycle. On specific nights, female and male worms sense the presence of each other and begin swimming in circles, during which the eggs and sperm are released into the water and eggs get fertilised. Capturing these events can be quite tricky, resulting in several unsuccessful sampling missions.
Finally, after several night shifts, the researchers were able to catch Platynereis worms right before mating. Afterwards, the worms were placed in a little ocean on the boat - a simple glass of water - in which the worms could mate. At this point, the clock stops for the worms since their lifespan comes to an end after releasing the sperm and eggs. In contrast, the clock starts running for scientists who are interested in studying the progeny at different ages.
"TREC allowed us to study the microevolution of Platynereis and understand how the conditions close to the North and Baltic sea challenged the population compared to the Mediterranean sea, for example," Gerber said.
Using tiny worms to study nervous system evolution
The size of this species also determines the type of analyses that scientists can do. Gerber's postdoctoral work has him comparing genes expressed in individual cells from across the whole organism, a technique called single-cell RNA sequencing. "I study the worms at exactly six days of age since they are already tiny worms by then, equipped with the main organs and body parts that will be maintained until their adult age," he said. At six days of age, the worms are only a few micrometres long (one micrometre being about one-hundredth the thickness of human hair). This allows Gerber to profile all cell types across the whole worm body simultaneously.
Witte, on the other hand, is interested in how the Platynereis nervous system evolves. Her PhD project looks at cell types present in the worm's brain to get a better fundamental understanding of them and also to potentially compare them to species that are evolutionarily related.
"During my first week at EMBL, I travelled to Villefranche-sur-Mer, close to Nice in France, to join TREC scientists and start collecting Platynereis that were already around one year of age," Witte said. "At this time, the worms have specific behaviours and a capacity to learn, so we wanted to know how their brains adapted to different environments encountered at the Mediterranean versus the North sea."
Both Gerber's and Witte's research projects will help build the first atlas at cellular resolution for a whole Platynereis animal, especially for the nervous system, including early development and differentiation stages. This work will allow, in the future, not only to select a cell anywhere in the worm body and know what genes that cell expresses but also the other way around: discover which cell expresses a specific group of genes. These discoveries, when merged with evolutionary biology studies, will make it possible to determine, for example, which cells are evolutionarily old or new in the worms' brains.
Before TREC, the scientists mostly did lab-controlled temporal evolutionary studies where they grew worms in the lab and studied different stages over time. "TREC made spatial evolution studies possible," Oel said. "After collecting samples in different locations, we can now merge both datasets and characterise Platynereis across time and space."
Oel's own research looks at photoreceptors. "They are little cells that can detect light and then send this information to the brain," he explained. "Platynereis need these cells. For example, they help indicate when the worms are too close to the sea surface, where they might be eaten by larger animals, and should therefore swim into deeper waters."
During evolution, cells can change from one type to another. By analysing how photoreceptors in Platynereis have evolved across time and space, Oel is trying to understand how organisms make new kinds of cells.
TREC's sampling phase ended this summer, and now the three scientists are ready to use the collected samples to construct a complete story of Platynereis dumerilli evolution.
"With the work of Victoria, Phil, and Tobias, we have now built new fantastic resources that enable us to explore the nervous system of Platynereis at the cellular level," said Detlev Arendt, EMBL Group Leader. "From this, we expect many more exciting insights into the evolution of the nervous system. This will be regarding both the origin of nervous systems and also their capacity to adapt to changing conditions, such as the challenges resulting from climate change."
