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In Bruce’s view, these dormant rudiments - legs, plates, etc - can persist over 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 may grow out again and take different forms in different species – a wing in an insect, say, or a carapace in a crustacean.

“If an ancestral structure is no longer needed, nature probably just truncates or reduces that tissue rather than deleting it completely. But the tissue is still there and can be elaborated again in later lineages, and appear to us to be novel,” Bruce says.

“This kind of truncation is probably common in evolution because genetic networks are so interdependent, “Bruce explains. “if a genetic pathway or tissue were to be deleted, some other pathway or tissue would be affected.”����

“I think cryptic persistence can be an explanation for a lot of ‘novel’ structures,” Bruce says.

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

“The ancient, common origin of all these plate-like structures [in arthropods] suggests the gene networks that pattern these structures are very evolvable and plastic. They are capable of generating an awesome amount of diversity,” Bruce says.

Citation:

Heather S. Bruce and Nipam H. Patel (2022) The Daphnia carapace and other novel structures evolved via the cryptic persistence of serial homologs. Current Biology, DOI:

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