Were sponges or comb jellies the first to split from the animal family tree? A new approach at settling this question, which is critical to understanding the evolution of animals, points strongly to comb jellies – but not all researchers are convinced.
All animals alive today are thought to be descended from a common ancestor that lived more than 600 million years ago. Until fairly recently, researchers thought that sponges were the first group to split from this common ancestor and begin evolving separately. The next group to diverge from the animal family tree was then comb jellies.
But this idea was challenged by a 2008 study based on newly sequenced genomes that found comb jellies appeared to have split off before sponges. Since then, papers using similar methods to argue both sides have flown back and forth “like a ping-pong match”, says Darrin Schultz at the University of Vienna in Austria. “People feel like they’ve been banging their heads against the wall.”
Now, Schultz and his colleagues have pursued a new line of evidence. Where previous studies compared small-scale changes in the DNA sequences of comb jellies, sponges and other animals, his team looked at larger-scale patterns in the order of genes on their chromosomes.
The idea is that these patterns — known as synteny — are more stable over longer periods of evolutionary change, says Schultz. While individual genes can be reshuffled by evolution, the reordering of linked groups of genes caused by mixing and fusing chromosomes is both a rare and irreversible event.
Schultz’s team compared shared patterns of synteny between two species of comb jellies, two species of sponges and two species from other animal groups. In order to determine patterns of synteny prior to any divergence, the researchers looked specifically at 31 groups of genes shared between comb jellies and at least one of three single-celled ancestors of all animals.
In seven of these groups of genes, the comb jellies had patterns of synteny present in at least one single-celled ancestor, but that were missing in sponges and the other animal groups. This suggests that the comb jellies split from the other animals prior to the reordering events that gave the other animals distinct shared patterns of synteny, says Schultz. The possibility that the pattern occurred by random chance is extremely unlikely, he says.
“I’d say this is the strongest evidence to date in favour of the jellies-first hypothesis,” says Aoife McLysaght at Trinity College Dublin in Ireland, whose own work has come down in favour of sponges. But she would like to see more work to understand how to reconcile the finding with the small-scale DNA sequence-based approaches that have found sponges split first.
Davide Pisani at the University of Bristol, UK, says the finding is important, but cautions that there are other ways to define a synteny pattern, and that Schultz’s team analysed weak patterns that may be down to chance rather than evolutionarily significant. “Is it real, or is it just some kind of random signal?” he says.
If the synteny results hold up, this would have wide-ranging implications for understanding the evolution of neurons, muscles and other organ systems in animals, says Kenneth Halanych at the University of North Carolina Wilmington. For instance, sponges don’t have neurons, but comb jellies do. If comb jellies split first, it could mean neurons independently evolved in comb jellies and other animals groups.
But no single study can completely resolve the sponge versus comb jellies debate, says Halanych. “For 150 to 200 years, people have always assumed sponges are near the base of the tree,” he says. “You need multiple sources of the strongest data to really convince people.”