Tiny worm makes for big evolutionary discovery
UC Riverside scientists have described ‘Uncus,’ the oldest ecdysozoan and the first from the Precambrian period
University of California - Riverside
image:
Scott Evans and Ian Hughes excavating a fossil bed at Nilpena National Park.
view moreCredit: Mary Droser/UCR
Everyone has a past. That includes the millions of species of insects, arachnids, and nematode worms that make up a major animal group called the Ecdysozoa.
Until recently, details about this group’s most distant past have been elusive. But a UC Riverside-led team has now identified the oldest known ecdysozoan in the fossil record and the only one from the Precambrian period. Their discovery of Uncus dzaugisi, a worm-like creature rarely over a few centimeters in length, is described in a paper published today in Current Biology.
“Scientists have hypothesized for decades that this group must be older than the Cambrian, but until now its origins have remained enigmatic. This discovery reconciles a major gap between predictions based on molecular data and the lack of described ecdysozoans prior to the rich Cambrian fossils record and adds to our understanding of the evolution of animal life,” said Mary Droser, a distinguished professor of geology at UCR, who led the study.
The ecdysozoans are the largest and most species-rich animal group on Earth, encompassing more than half of all animals. Characterized by their cuticle — a tough external skeleton that is periodically shed — the group comprises three subgroups: nematodes, which are microscopic worms; arthropods, which include insects, spiders, and crustaceans; and scalidophora, an eclectic group of small, scaly marine creatures.
“Like many modern-day animal groups, ecdysozoans were prevalent in the Cambrian fossil record and we can see evidence of all three subgroups right at the beginning of this period, about 540 million years ago,” said Ian Hughes, a graduate student in marine biology at Harvard University and the paper’s first author. “We know they didn’t just appear out of nowhere, and so the ancestors of all ecdysozoans must have been present during the preceding Ediacaran period.”
DNA-based analyses, used to predict the age of animal groups by comparing them with their closest living relatives, have corroborated this hypothesis. Yet ecdysozoan fossil animals have remained hidden among scores of animal fossils paleontologists have discovered from the Ediacaran Period.
Ediacaran animals, which lived 635-538 million years ago, were ocean dwellers; their remains preserved as cast-like impressions on the seabed that later hardened to rock. Hughes said uncovering them is a labor-intensive, delicate process that involves peeling back rock layers, flipping them over, dusting them off, and piecing them back together to get “a really nice snapshot of the sea floor.”
This excavation process has only been done at Nilpena Ediacara National Park in South Australia, a site Droser and her team have been working at for 25 years that is known for its beautifully preserved Ediacaran fossils.
“Nilpena is perhaps the best fossil site for understanding early animal evolution in the world because the fossils occur during a period of heightened diversity and we are able to excavate extensive layers of rock that preserve these snapshots,” said Scott Evans, an assistant professor of Earth-Life interactions at Florida State University and co-author of the study. “The layer where we found Uncus is particularly exciting because the sediment grains are so small that we really see all the details of the fossils preserved there.”
While the team didn’t set out to find an early ecdysozoan during their 2018 excavation, they were drawn to a mysterious worm-like impression that they dubbed “fishhook.”
“Sometimes we make dramatic discoveries and sometimes we excavate an entire bed and say ‘hmmm, I’ve been looking at that thing, what do you think?’” Hughes said. “That’s what happened here. We had all sort of noticed this fishhook squiggle on the rock. It was pretty prominent because it was really, really deep.”
After seeing more of the worm-like squiggles the team paid closer attention, taking note of fishhook’s characteristics.
“Because it was deep, we knew it wasn’t smooshed easily so it must have had a pretty rigid body,” Hughes said. Other defining characteristics include its distinct curvature and the fact that it could move around — seen by trace fossils in the surrounding area. Paul De Ley, an associate professor of nematology at UCR, confirmed its fit as an early nematode and ruled out other worm types.
“At this point we knew this was a new fossil animal and it belong to the Ecdysozoa,” Hughes said.
The team called the new animal Uncus, which means “hook” in Latin, noting in the paper its similarities to modern-day nematodes. Hughes said the team was excited to find evidence of what scientists had long predicted; that ecdysozoans existed in the Ediacaran Period.
“It’s also really important for our understanding of what these early animal groups would have looked like and their lifestyle, especially as the ecdysozoans would really come to dominate the marine ecosystem in the Cambrian,” he said.
The paper is titled “An Ediacaran bilateran with an ecdysozoan affinity from South Australia.” Funding for the research came from NASA.
Uncus fossil from Nilpena Ediacara National Park. The numbers correspond to the coordinates of this fossil on the fossil bed surface. Bottom: 3D laser scans enable the researchers to study the fossils’ shape and curvature.
Credit
Droser Lab/UCR
Journal
Current Biology
Article Title
An Ediacaran bilaterian with an ecdysozoan affinity from South Australia
Article Publication Date
18-Nov-2024
How marine worms regenerate lost body parts
The return of cells to a stem cell-like state as the key to regeneration
University of Vienna
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From left to right: Max Perutz Labs group leader Florian Raible, first authors and doctoral students Alexander Stockinger and Leonie Adelmann.
view moreCredit: Max Perutz Labs
Many living organisms are able to regenerate damaged or lost tissue, but why some are particularly good at this and others are not is not fully understood. Molecular biologists Alexander Stockinger, Leonie Adelmann and Florian Raible from the Max Perutz Labs at the University of Vienna have now made an important contribution to clarifying this question in a new study. In it, they explain the molecular mechanism of regeneration in marine worms and thus create a better understanding of the natural reprogramming ability of cells. The study has just been published in the renowned journal Nature Communications.
The ability to regenerate – from individual cell types to entire organs or complex tissues – is of crucial importance for all living species. The human body also regenerates, in short, dead cells are replaced by newly produced ones. In humans, for example, this is the case in the intestinal mucosa or the liver. However, other creatures have much stronger regenerative abilities. For example, annelids such as Platynereis dumerilii can regenerate entire parts of their posterior body after injury. The molecular mechanisms that control this process were hardly known until now. A new study led by molecular biologist Florian Raible from the Max Perutz Labs at the University of Vienna has now provided new insights. The scientists are not only gaining a better understanding of biology in general, but also of the natural reprogramming ability of cells.
The growth of new segments (body parts) in marine worms is controlled by a special growth zone, in which special stem cells are located. New segments then emerge by the division of these cells. But what happens if this special growth zone is lost due to an injury? In their new study, first authors Alexander Stockinger and Leonie Adelmann, together with the team from the Raible laboratory, show which molecular mechanisms can be used to renew a lost growth zone so that the marine worms can form new segments again. What is special about Platynereis dumerilii is that, unlike other species, regeneration in marine worms does not rely on existing stem cells. Instead, differentiated cells undergo what is known as dedifferentiation after the removal of the growth zone. "This means that these cells begin to return to a stem cell-like state within just a few hours in order to build up a new growth zone as quickly as possible," explains Leonie Adelmann, one of the two lead authors of the study.
The researchers also found that the gene expression in these newly formed stem cells actually differs from their precursor cells. "Excitingly, factors related to the transcription factors Myc and Sox2, which are also used in modern medicine to produce stem cells from differentiated human cells, also play a role here," says Alexander Stockinger, the other first author of the study.
"The concept of dedifferentiation was proposed over 60 years ago, but researchers at the time lacked the tools to test this idea. Now we have developed tools to understand dedifferentiation at the molecular level and compare it to this so-called ‘reprogramming’ of cells in modern medicine. This creates a solid basis for future studies," summarises Florian Raible, head of the working group at the University of Vienna.
One of the scientists' special strategies was to investigate cell states using the new method of single-cell RNA sequencing. This technique provided a new type of data set for investigating tissue regeneration. "Single-cell transcriptomics allows us to identify cell types and their states and show how they respond to the loss of body parts at an individual level. In our study, we also combined this technique with data from French colleagues who used fluorescent labelling of cells to help reveal which tissues ultimately arise from certain stem cells," explains Stockinger. "We discovered at least two different stem cell populations - one that regenerates tissues such as epidermis and neurons, and another that forms muscles and connective tissue," says Adelmann.
Shortly after amputation of the posterior end, specialised epithelial cells begin to return to a special stem cell state (red). The missing segments are then recreated with the help of the newly formed growth zone.
Credit
Leonie Adelmann, Universität Wien
Visualisation of specific transcripts (green, magenta) confirmed the identification of different stem cell populations proliferating in regenerating worm tails (cyan).
Credit
Leonie Adelmann
Journal
Nature Communications
Article Title
Molecular profiles, sources and lineage restrictions of stem cells in an annelid regeneration model.
Article Publication Date
18-Nov-2024
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