What's new under the sun? Offers an alternative view of how the "new" structure evolved

WOOD HOLE, Mass. – Many crustaceans, including lobsters, crabs, and barnacles, have a cape-like shell protruding from the head that can serve a variety of roles, such as a small cave for storing eggs, or a protective shield to keep the gills moist.

This shell (carapace), it has been proposed, did not evolve from a similar structure in a crustacean ancestor, but emerged de novo (or suddenly) through the rather random co-optation of genes that also determine 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 the lateral toe lobe in a common ancestor.

This evidence underpins their proposal for a new concept of how new structures evolve – which suggests that they are not so new. A study of the carapace of Daphnia crustaceans, appears online in Current Biology.

“How new structures emerge is a central question in evolution,” Bruce said. “The prevailing idea, called gene co-optation, is that genes that function in one context, say to make insect wings, end up in an unrelated context, where they make, say, a carapace,” Bruce said. “But here we show that Daphnia’s carapace doesn’t just appear.”

Instead, they propose an ancestral plate-like foot lobe that evolved into wings and a carapace may have existed in the common ancestor of all living arthropods. But because the wings and carapace looked so different from this ancestral plate, and from other plates in the adjacent arthropod lineage, no one realized that they were all the same thing.

“We’re starting to realize that structures that don’t look the same — wings, carapace, tergal plates — are actually homologs,” Bruce said. “That suggests they had a much more ancient origin than anyone thought, way back in the Cambrian, [500 million] many years ago.”

It’s been there all along (faint persistence)

Bruce calls his model of how new structures emerge “the nebulous persistence of serial homologues.”

“Homolog series are things like hands and feet, or the spine of our spine, or multiple legs repeating the body of a centipede,” he said. “It [repeats] can look very different, but you can see the similarities, and they are all built using the same initial genetic pathway. In some cases, the full structure doesn’t grow – you may get the centipede’s legs clipped off, or it’s very delicate and small. While the cells have been programmed to form the legs, they don’t actually grow the legs.”

In Bruce’s view, these dormant foundations – legs, plates, etc. – can last for millions of years, as long as repeats of other structures are still present elsewhere in the animal. And when the time is right, those structures can grow again and take on different shapes in different species – the wings of an insect, say, or the carapace of a crustacean.

“If the ancestral structure is no longer needed, nature might simply cut or reduce that network rather than completely erasing it. But the network is still there and can be deciphered in later lineages, and it looks like new to us,” Bruce said.

“This kind of truncation may be common in evolution because genetic networks are highly interdependent,” explains Bruce. “If a genetic pathway or network is deleted, some other pathways or tissues will be affected.”

“I think faint persistence could be an explanation for a lot of ‘new’ structures,” Bruce said.

The authors drew their conclusions by analyzing gene expression patterns in several arthropod species, and by eliminating other hypotheses about how the carapace might have evolved.

“The ancient and common origin of all these plate-like structures [in arthropods] demonstrated that the network of genes that make up this structure is highly evolvable and plastic. They are capable of producing incredible diversity,” said Bruce.

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