Many crustaceans, including lobsters, crabs and barnacles, have a cape-like shell protruding from the head which can perform various roles, such as a small cave to store the eggs or a protective shield to keep the gills moist.
This shell (carapace), it was proposed, did not evolved from a similar composition in the crustacean ancestor, but arose de novo (or out of the blue) by somewhat random co-optation of genes that also specify insect wings.
However, in a new study from the Maritime Organic Laboratory (MBL), research associate Heather Bruce and director Nipam Patel provide evidence from another perspective: the carapace, as well as other Plate-like buildings in arthropods (crustaceans, insects, arachnids, and myriapods) all evolved from a lateral leg lobe in a common ancestor.
This evidence buttresses nt their proposal of a new idea of how new buildings evolve – an idea that suggests that they are not so new, after all. The study, on the shell of the crustacean Daphnia, appears online in Existing Biology.
“How new buildings arise is a central problem in evolution,” says Bruce. “The prevailing idea, called gene co-option, is that genes that function in one context, for example to make insect wings, end up in an unrelated context, where they make, for example, a shell,” explains Bruce. “But here we show that Daphnia’s carapace did not come out of nowhere.”
They propose rather than the plate-like ancestral leg lobe which has evolved into both a wing and a carapace was probably present in the ancestor of all living arthropods. But because the wing and carapace are so different from this ancestral plate and other plates from neighboring arthropod lineages, no one realized they were all the same thing.
“We are beginning to realize that buildings that look nothing alike – wings, shells, tergal plates – are in fact peers,” explains Bruce. “This suggests they have a special origin that is much older than anyone would have thought, dating back to the Cambrian period years ago.”
It was there all along (cryptic persistence)
Bruce calls his model for how new buildings emerge “the cryptic persistence of serial counterparts”.
“Serial counterparts are things like hands and feet, or the vertebrae of our spine, or the many legs that repeat on the body of a centipede,” she says. “The may look really different, but you can see similarities, and they’re all built using the same initial genetic pathways. In some cases, the full frame doesn’t develop – you may have a stubby centipede leg, or it’s really subtle and tiny. Although the cells have been programmed to grow on the leg, they don’t actually grow on the leg.”
In Bruce’s opinion, these dormant rudiments – legs, plates, and so on. – can persist for tens of millions of years, as long as another repeat of the structure is still present elsewhere in the animal. And when the time is right, the construct can regrow and take on different shapes in different species – a wing in an insect, for example, or a shell in a crustacean.
“If an ancestral framework is no longer needed, mother nature is probably truncating or shrinking this fabric rather than removing it completely. But the fabric is still there and can be elaborated again in later lines, and seems to us to be new,” says Bruce. evolution and genetic networks are so interdependent,” explains Bruce. “if one genetic pathway or tissue were to be deleted, another pathway or tissue would be affected.”
“I think cryptic persistence may be an explanation for many ‘novel’ structures,” says Bruce.
The authors have drawn their conclusions by analyzing gene expression patterns in several arthropod species and eliminating other hypotheses about the possible evolution of the carapace.
“The ancient origin and commonality of all these plate-like structures suggests that the gene networks that model these structures are highly evolutionary and plastic. They are able to generate an impressive amount of diversity,” says Bruce.