What’s new under the sun? Researchers offer an alternate view on how ‘novel’ structures evolve
Many crustaceans, including lobsters, crabs and barnacles, have a cape-like shell that protrudes from the head that can serve several functions, such as a small cave for holding eggs, or a protective shield to keep the gills moist.
This shell, it has been proposed, did not evolve from a similar structure in the crustacean ancestor, but appeared de novo (or out of nowhere) through a somewhat random co-option of the genes that also specify insect wings.
However, in a new study from the Marine Biological Laboratory (MBL), research associate Heather Bruce and director Nipam Patel provide evidence for an alternative view: The carapace, along with other plate-like structures in arthropods (crustaceans, insects, arachnids and myriapods) all evolved from a lateral leg lobe in a common ancestor.
This evidence supports their proposal for a new concept of how new structures evolve — one that suggests they aren’t so new after all. The study, on the shell of the crustacean Daphnia, appears online in Current Biology.
“How new structures arise is a central question in evolution,” Bruce says. “The prevailing idea, called gene co-option, is that genes that function in one context, say to make insect wings, end up in an unrelated context, where they make a shield, for example,” Bruce says. “But here we show that the Daphnia shield didn’t just appear out of nowhere.”
Instead, they propose that the ancestral, plate-like leg lobe that evolved in both the wing and carapace was likely present in the ancestor of all living arthropods. But because the wing and shield look so different from this ancestral plate and from other plates in neighboring arthropod genera, no one realized they were all the same.
“We’re starting to realize that structures that don’t resemble each other — wings, shields, tergal plates — are actually homologous,” Bruce says. “That suggests they have a single origin much older than anyone would have thought, way back in the Cambrian period, [500 million] years ago.”
It was always there (cryptic persistence)
Bruce calls her model for the emergence of new structures “cryptic persistence of serial homologs.”
“Serial homologs are things like hands and feet, or the vertebrae of our spine, or the many legs that repeat in a centipede’s body,” she says. “The [repeats] can look very different, but you can see similarities, and they are all built using the same initial genetic pathways. In some cases, the entire structure does not grow out – you can get a truncated centipede bone, or it is very subtle and small. Although the cells are programmed to form the bone, they don’t actually grow out of the bone.”
According to Bruce, these dormant principles—legs, plates, etc.—can persist for millions of years, as long as another repetition of the structure is still present elsewhere in the animal. And when the time is right, the structure can re-grow and take different shapes in different species — for example, a wing in an insect or a shield in a crustacean.
“If an ancestral structure is no longer needed, nature probably just cuts or shrinks that tissue instead of removing it completely. But the tissue is still there and can be reworked in later generations, and it seems new to us.” says Bruce. .
“This kind of truncation is probably common in evolution because genetic networks are so interdependent,” explains Bruce. “If one genetic pathway or tissue were removed, another pathway or tissue would be affected.”
“I think cryptic persistence could account for a lot of ‘new’ structures,” Bruce says.
The authors drew their conclusions by analyzing gene expression patterns in different arthropod species and by eliminating other hypotheses about how the carapace may have evolved.
“The ancient, common origin of all these plate-like structures [in arthropods] suggests that the gene networks that make up these structures are highly evolvable and plastic. They are capable of generating an amazing amount of diversity,” says Bruce.
How the insect got its wings: scientists tell the story (finally!)
Heather S. Bruce et al, The Daphnia shield and other new structures evolved via the cryptic persistence of serial homologs, Current Biology (2022). DOI: 10.116/j.cub.2022.06.073
Quote: What’s new under the sun? Researchers offer an alternative look at how ‘new’ structures evolve (2022, August 1), retrieved August 1, 2022 from https://phys.org/news/2022-08-sun-alternate-view-evolve.html
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