Hoe zagen de eerste dieren eruit? Chromosomale aanwijzingen onthullen het begin van de evolutie van dieren

Jacobsschelpgelei (Kydippida)

Een nieuwe studie gebruikte een unieke benadering op basis van de chromosoomstructuur om te bepalen dat kamgelei, ook bekend als ctenophores, de eerste afstamming was die afweek van de dierlijke levensboom, met sponzen die volgden als de volgende tak. Eerder was het onduidelijk of sponzen of geleikammen de eerste tak waren vanwege onduidelijke onderzoeken naar de gensequentie. Dit onderzoek draagt ​​bij aan ons begrip van de vroege evolutie van dieren en biedt inzicht in de oorsprong van belangrijke kenmerken van de dierbiologie, zoals het zenuwstelsel, de spieren en het spijsverteringskanaal.

Chromosoomanalyse beslecht het debat over de zustergroep van alle dieren. Het zijn gelkammen, geen sponzen.

Onderzoekers gebruikten een nieuwe op chromosomen gebaseerde benadering om te onthullen dat sint-jakobsschelpgelei de eerste lijn was die afweek van de dierlijke levensboom, vóór sponzen. Dit onderzoek, dat nieuwe inzichten geeft in de vroege evolutie van dieren, verbetert ons begrip van hoe belangrijke biologische eigenschappen evolueerden.

Al meer dan een eeuw vragen biologen zich af hoe de eerste dieren waren toen ze een half miljard jaar geleden voor het eerst in oude oceanen verschenen.

Zoekend onder de meest primitieve dieren van vandaag naar de oudste tak van de dierenboom des levens, hebben wetenschappers geleidelijk de mogelijkheden teruggebracht tot twee groepen: sponzen, die hun hele volwassen leven op één plek doorbrengen en voedsel uit zeewater filteren; en gelei kammen, vraatzuchtige roofdieren die door de oceanen van de wereld peddelen op zoek naar voedsel.

In een nieuwe studie die deze week in het tijdschrift is gepubliceerd Natuur, onderzoekers gebruiken een nieuwe benadering op basis van de chromosoomstructuur om tot een definitief antwoord te komen: het kammen van gelatines, of teen’-a-fores, was de eerste lijn die zich vertakte van de dierenboom. Sponzen volgden, gevolgd door de diversificatie van alle andere dieren, inclusief de lijn die naar de mens leidde.

Hoewel de onderzoekers vaststelden dat de ctenophore-lijn vertakte vóór sponzen, bleven beide groepen dieren evolueren van hun gemeenschappelijke voorouder. Evolutiebiologen geloven echter dat deze groepen nog steeds kenmerken delen met de eerste dieren, en dat het bestuderen van deze vroege takken van de dierenboom des levens licht kan werpen op hoe dieren ontstonden en zich ontwikkelden tot diversiteit.[{” attribute=””>species we see around us today.

Ctenophore (Hormiphora californensis)

Hormiphora californensis, called the California sea gooseberry, is a comb jelly, or ctenophore, common in California coastal waters. Ctenophores have eight sets of cilia running down their side, which they use to propel themselves through the oceans in search of food. This specimen was observed on 2016 by MBARI’s remotely operated vehicle (ROV) Doc Ricketts in the Monterey Canyon at a depth of approximately 280 meters. Credit: Monterey Bay Aquarium Research Institute

“The most recent common ancestor of all animals probably lived 600 or 700 million years ago. It’s hard to know what they were like because they were soft-bodied animals and didn’t leave a direct fossil record. But we can use comparisons across living animals to learn about our common ancestors,” said Daniel Rokhsar,

“We developed a new way to take one of the deepest glimpses possible into the origins of animal life,” said Schultz, the lead author and a former UC Santa Cruz graduate student and researcher at the Monterey Bay Aquarium Research Institute (MBARI) who is now a postdoctoral researcher at the University of Vienna. “This finding will lay the foundation for the scientific community to begin to develop a better understanding of how animals have evolved.”

Bioluminescent Deep-Sea Sponge

A newly discovered and still undescribed bioluminescent deep-sea sponge observed in 2019 by MBARI’s ROV Doc Ricketts offshore of Central California at a depth of approximately 3,970 meters. Credit: Monterey Bay Aquarium Research Institute

What’s an animal?

Most familiar animals, including worms, flies, mollusks, sea stars, and

The evolutionary relationships among these diverse creatures — specifically, the order in which each of the lineages branched off from the main trunk of the animal tree of life — has been controversial.

With the rise of

Just looking at them, sponges seem quite primitive. After their free-swimming larval stage, they settle down and generally remain in one place, gently sweeping water through their pores to capture small food particles dissolved in sea water. They have no nerves or muscles, though their hard parts make nice scrubbers in the bath.

“Traditionally, sponges have been widely considered to be the earliest surviving branch of the animal tree, because sponges don’t have a nervous system, they don’t have muscles, and they look a little bit like colonial versions of some unicellular protozoans,” Rokhsar said. “And so, it was a nice story: First came the unicellular protozoans, and then sponge-like multicellular consortia of such cells evolved and became the ancestor of all of today’s animal diversity. In this scenario, the sponge lineage preserves many features of the animal ancestor on the branch leading to all other animals, including us. Specializations evolved that led to neurons, nerves and muscles and guts and all those things that we know and love as the defining features of the rest of animal life. Sponges appear to be primitive, since they lack those features.”

The other candidate for earliest animal lineage is the group of comb jellies, popular animals in many aquariums. While they look superficially like jellyfish — they often have a bell-like shape, although with two lobes, unlike jellyfish, and usually tentacles — they are only distantly related. And while jellyfish squirt their way through the water, ctenophores propel themselves with eight rows of beating cilia arranged down their sides like combs. Along the California coast, a common ctenophore is the 1-inch-diameter sea gooseberry.

Chromosomes to the rescue

To learn whether sponges or ctenophores were the earliest branch of animals, the new study relied on an unlikely feature: the organization of genes into chromosomes. Each species has a characteristic chromosome number — humans have 23 pairs — and a characteristic distribution of genes along chromosomes.

Rokhsar, Simakov, and collaborators had previously shown that the chromosomes of sponges, jellyfish and many other

A smoking gun

Remarkably, when the team compared the chromosomes of these diverse animals and non-animals, they found that ctenophores and non-animals shared particular gene-chromosome combinations, while the chromosomes of sponges and other animals were rearranged in a distinctly different manner.

“That was the smoking gun — we found a handful of rearrangements shared by sponges and non-ctenophore animals. In contrast, ctenophores resembled the non-animals. The simplest explanation is that ctenophores branched off before the rearrangements occurred,” he said.

“The fingerprints of this ancient evolutionary event are still present in the genomes of animals hundreds of millions of years later,” Schultz said. “This research … gives us context for understanding what makes animals animals. This work will help us understand the basic functions we all share, like how they sense their surroundings, how they eat and how they move.”

Rokhsar emphasized that the team’s conclusions are robustly based on five sets of gene-chromosome combinations.

“We found a relic of a very ancient chromosomal signal,” he said. “It took some statistical detective work to convince ourselves that this really is a clear signal and not just random noise, because we’re dealing with relatively small groups of genes and perhaps a billion years of divergence between the animals and non-animals. But the signal is there and strongly supports the ‘ctenophore-branched-first’ scenario. The only way the alternative sponge-first hypothesis could be true would be if multiple convergent rearrangements happened in both sponges and non-ctenophore animals, which is very unlikely.”

For more on this research, see Genetic Linkages Illuminate Earliest Animal Evolution.

Reference: “Ancient gene linkages support ctenophores as sister to other animals” by Darrin T. Schultz, Steven H. D. Haddock, Jessen V. Bredeson, Richard E. Green, Oleg Simakov and Daniel S. Rokhsar, 17 May 2023, Nature.
DOI: 10.1038/s41586-023-05936-6

Jessen Bredeson of UC Berkeley also contributed to this work.

Funding for this research was provided by the David and Lucile Packard Foundation, MBARI, the National Science Foundation (GRFP DGE 1339067 and DEB-1542679), the European Research Council’s Horizon 2020: European Union Research and Innovation Programme (grant No. 945026), internal funds of the Okinawa Institute of Science and Technology Molecular Genetics Unit, the Chan Zuckerberg Biohub Network and the Marthella Foskett Brown Chair in Biological Sciences.

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