LIFE SCIENCE

Ctenophores are probably the oldest group of animals


A team of researchers from the University of Vienna in Austria and the University of California, Berkeley, found that among ancient animals, ctenophores, not sponges, were the ancient “relatives” of all other animals. That is, ctenophores were close relatives of the first animals, from which humans also emerged.

A species of ctenophore (Ctenophore). Photo credit: Shutterstock/SaskiaAcht

All living animals on Earth today are believed to be descendants of their common ancestor more than 600 million years ago. Previously, researchers thought that sponges were the first group to break off from a common ancestor and begin to evolve independently, and the next group to diverge from the animal family tree was ctenophores.

But that view was challenged by a 2008 genome-based study based on a new sequencing of the genome that found that ctenophores appeared to split earlier than sponges. Darrin Schultz of the University of Vienna in Austria says papers that argue both sides in a similar way have flown around “like ping-pong balls” since then.

Schultz’s team is looking for new evidence. While previous studies have compared small-scale changes in the DNA sequences of ctenophores, sponges and other animals, Schultz’s team has studied large-scale patterns in gene sequence on their chromosomes.

The idea of Schultz’s team is that these patterns are more stable over long periods of evolutionary change. He said that while individual genes can be reshuffled through evolution, the reordering of linked gene populations caused by chromosomal mixing and fusion is a rare and irreversible event.

The researchers compared the collinear patterns of two species of ctenophore, two species of sponge, and two other animal groups. To determine patterns prior to any differentiation, the researchers specifically looked at 31 sets of genes shared between Ctenophore and at least one of three single-celled ancestors.

Of the 7 sets of genes, ctenophores had collinear patterns in at least one single-celled ancestor, but not in sponges and other animal groups. Schultz said this suggests that ctenophores were isolated from other animals before the reordering events that gave other animals unique patterns. He said it is extremely unlikely that this pattern will occur randomly.

“This is the strongest evidence to date to support the jellyfish-first hypothesis,” said research by Aoife McLysaght of Trinity College Dublin, Ireland, which supports sponges splitting first, but she would like to see more work to understand how this finding can be reconciled with a small-scale DNA sequence-based approach that found that sponges split first.

Davide Pisani of the University of Bristol in the UK said the findings were important. But he cautions that there are other ways to define collinear patterns, and that the weak patterns analyzed by Schultz’s team may be accidental or random rather than evolutionarily significant.

Kenneth Halanych of the University of North Carolina at Wilmington says that if the results of the synthesis hold, it will have broad implications for understanding the evolution of neurons, muscles and other organ systems in animals. For example, sponges don’t have neurons, but ctenophores do, and if ctenophores split first, this could mean that neurons from ctenophores and other animal groups evolved independently.

But Halanych believes that no study can completely resolve the debate between sponge jellyfish and ctenophore, and that it would take the most robust data from multiple sources to really convince people. (Source: China Science News Xin Yu)

Related paper information:https://doi.org/10.1038/s41586-023-05936-6



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