What’s new under the sun? Researchers offer an alternative view of how ‘novel’ structures evolve

Many crustaceans, including lobsters, crabs, and barnacles, have a cape-like shell that protrudes from the head that can serve various functions, such as: B. a small cave for storing eggs or a protective shield to keep the gills moist.

This shell, it has been suggested, did not evolve from a similar structure in the crustacean ancestor, but arose de novo (or out of the blue) through a somewhat fortuitous 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) have evolved all evolved from a lateral leg flap of a common ancestor.

This evidence supports their proposal for a new concept of how novel structures evolve – one that suggests they are not so novel after all. The study of the shell of the crustacean Daphnia appears online in Current Biology.

“How novel structures emerge is a central question of evolution,” says Bruce. “The prevailing idea, called gene co-option, is that genes that function in one context, to make say insect wings, end up in an unrelated context, where they make say armor,” says Bruce. “But here we show that the Daphnia tank didn’t just appear out of nowhere.”

Rather, they suggest that the ancestral plate-like leg lobe, which evolved into both wing and carapace, was probably present in the ancestor of all living arthropods. But because the wings and carapace look so different from this ancestral plate and other plates in neighboring arthropod lineages, no one realized they were all the same.

“We’re starting to see that structures that don’t look alike — wings, carapace, tergal plates — are actually homologous,” says Bruce. “This suggests that they have a single origin, much older than anyone would have thought, way back to the Cambrian period, [500 million] years ago.”

It was there all along (cryptic persistence)

The ancestral arthropods had multiple plates on each leg on each body segment, similar to the living crustacean Parhyale. Later arthropods repress most of these, but each plate can be derepressed on any body segment to form what appear to be novel structures. The Daphnia Shell evolved through de-repression and elaboration of the blue headplate, and the Insect Wing evolved through de-repression and elaboration of the pink breastplate. Photo credit: Bruce and Patel, Current Biology2022

Bruce calls their model for how novel structures arise “cryptic persistence of serial homologues”.

“Serial homologues are things like hands and feet, or the vertebrae of our spine, or the many legs that are repeated on a centipede’s body,” she says. “That [repeats] They can look really different, but you can see similarities, and they’re all built based on the same initial genetic pathways. In some cases, the full structure doesn’t grow out – you can get a cut centipede leg, or it’s really subtle and tiny. Although the cells have been programmed to form the leg, they don’t actually grow out of the leg.”

According to Bruce, these dormant rudiments – legs, plates, etc. – can persist for millions of years as long as another repeat of the structure is still present somewhere else in the animal. And when the time is right, the structure can grow back out and take on different shapes in different species—for example, a wing on an insect or a shell on a crustacean.

“When an ancestral structure is no longer needed, nature is likely merely shortening or reducing that tissue rather than erasing it entirely. But the fabric is still there and can be reworked in later lineages and appears novel to us,” says Bruce.

“This type of truncation is probably common in evolution because genetic networks are so interdependent,” explains Bruce. “If one genetic pathway or tissue were deleted, another pathway or tissue would be affected.”

“I think cryptic persistence can explain a lot of ‘novel’ structures,” says Bruce.

The authors reached their conclusions by analyzing gene expression patterns in several arthropod species and eliminating other hypotheses about how the carapace might 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’re able to generate an incredible amount of diversity,” says Bruce.