The shell of crustaceans evolved from the leg of an ancestor

Aug. 2 () –

New research provides evidence that the shell of crustaceans evolved from a lateral lobe of the leg in an arthropod ancestor more than 500 million years ago.

Many crustaceans, such as lobsters, crabs, and barnacles, have a cape-like shell protruding from the head that can serve several functions, as a small cave to store eggs or a protective shield to keep the gills moist.

It has been proposed that this shell did not evolve from any similar structure in the crustacean ancestor, but instead appeared de novo (or out of the blue) through a somewhat random co-option of genes that also specify insect wings.

Nevertheless, in a new study by the MBL (Marine Biological Laboratory), 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 lobe of the shell. leg in a common ancestor.

This evidence supports his proposal for a new concept of how novel structures evolve, one that suggests that they are not so novel 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,” says Bruce. “The prevailing idea, called gene co-option, is that genes that work in one context, say, to make insect wings, end up in an unrelated context, where they make, say, a shell,” says Bruce in a release. “But here we show that Daphnia’s shell did not appear out of nowhere“.

Rather, they propose that the ancestral plate-like leg lobe that developed into both the wing and the shell was likely present in the ancestor of all living arthropods. But because the wing and shell look so different from this ancestral plate and from other plates in neighboring arthropod lineages, Nobody realized that they were all the same.

“We’re starting to realize that structures that don’t look anything alike — wings, shells, thergal plates — are actually homologous,” says Bruce. “That suggests they have a single origin that is much older than anyone would have thought, way back in the Cambrian period, [500 millones] of years”.

Bruce calls his model of how new structures arise “Cryptic persistence of serial homologues”. “Serial homologues are things like hands and feet, or the vertebrae in our backbone, or the many repeating legs on a centipede’s body,” he says. it’s a statement. “The [repeticiones] they may 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; you might have a truncated centipede leg, or it’s really subtle and tiny. Although the cells have been programmed to form the leg, they are not actually growing out of the leg.”

In Bruce’s opinion, these dormant rudiments (legs, plates, etc.) can persist for millions of years, as long as another repetition of the structure is still present in some other part of the animal. And when the time is right, the structure can grow back and take different forms in different species: a wing in an insect, for example, or a shell in a crustacean.

“If an ancestral structure is no longer needed, nature is likely to simply truncate or reduce that tissue rather than eliminate it entirely. But the tissue is still there and can be made again in later lineages, and that strikes us as novel,” says Bruce.

“This type of truncation is probably common in evolution because genetic networks are so interdependent.“, Bruce explains. “If one genetic pathway or tissue were deleted, some other pathway or tissue would be affected. I think cryptic persistence can be an explanation for many ‘novel’ structures,” says Bruce.

The authors drew their conclusions by analyzing gene expression patterns in various arthropod species and ruling out other hypotheses about how the shell might have evolved.

“The ancient common origin of all these plate-like structures [en los artrópodos] suggests that the gene networks that model these structures are highly evolutionary and plastic. They are capable of generating an astonishing amount of diversity,” Bruce says.