American comb jelly (Mnemiopsis leidyi)
Shutterstock/Saskiaacht
Were sponges or comb jellies the first to be isolated from the phylogenetic tree of animals? researchers are not convinced.
All animals alive today are thought to be descended from a common ancestor who lived more than 600 million years ago. Until very recently, researchers thought sponges were the first group to split off from this common ancestor and begin to evolve separately. The next group to diverge from the animal tree was the comb jelly.
But this idea was challenged by a 2008 study based on a newly sequenced genome, which found that comb jelly likely divided before sponges. Since then, papers using similar techniques to argue each side have floated back and forth “like a game of ping-pong,” says Darin Schulz of the University of Vienna in Austria. “People feel like they’re banging their heads against the wall.”
Schultz and his colleagues are now pursuing a new line of evidence. While previous studies have compared small-scale changes in his DNA sequences in comb jellies, sponges, and other animals, his team looked at larger-scale patterns in the order of genes on chromosomes. .
These patterns, known as synteny, are the idea that evolutionary changes are more stable over time, Schultz said. Although individual genes can be reshuffled by evolution, rearrangement of linked gene groups caused by chromosome mixing and fusion is a rare and irreversible event.
Schultz’s team compared patterns of synteny shared between two comb jellies, two sponges, and two other animal groups. To determine the pattern of synteny prior to divergence, the researchers specifically examined 31 groups of genes shared between the comb jelly and at least one of the three unicellular ancestors of all animals.
In seven of these gene groups, comb jellies had a pattern of synteny present in at least one unicellular ancestor, but sponges and other animal groups did not. This suggests that comb jelly split from other animals before a sorting event that gave them a distinct common synteny pattern, Schultz said. It’s highly unlikely that this pattern arose by chance, he says.
“I think this is the strongest evidence to date in favor of the jelly-first hypothesis,” says Aoif McRaisagut of Trinity College, Dublin, Ireland. His own research resulted in favor of sponges. But she wants more work to understand how this finding reconciles with a small-scale DNA-sequence-based approach that found that the corpus cavernosum was the first to divide.
Davide Pisani of the University of Bristol, UK, notes that while the finding is important, there are other ways to define synteny patterns, and Schultz’s team is weak, suggesting that it may be due to chance rather than evolutionary significance. It warns that it has analyzed patterns. “Is it real or is it just a random signal?” he says.
Kenneth Haranich of the University of North Carolina, Wilmington, said that if the synteny result is correct, it will have far-reaching implications for our understanding of the evolution of neurons, muscles and other organ systems in animals. there is For example, sponges do not have neurons, but comb jellies do. If the comb jelly split first, it could mean that neurons evolved independently in the comb jelly and other groups of animals.
But no single study can fully resolve the sponge-comb jelly controversy, says Halanich. “For 150 to 200 years, people always thought sponges were near the base of trees,” he says. “You need multiple sources of strong data to really convince people.”
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