Many crustaceans, including lobster, crabs, and barnacles, have cape-like shells on their heads that can be used for a variety of purposes.
The proposed shell didn't evolve from a similar structure in the crustacean, but from random co-option of genes that also specified insect wings.
The carapace, along with other plate-like structures in arthropods, are evidence for an alternate view.
Their proposal for a new idea of how novel structures evolve is supported by this evidence. There is a study on the carapace of the crustacean Daphnia.
The central question in evolution is how novel structures arise. Bruce says that the prevailing idea is that genes that are functioning in one context end up in another. The Daphnia carapace didn't just come out of nowhere.
The ancestors of all living arthropods probably had the plate-like leg lobes that evolved into the wing and carapace. No one realized that the wing and carapace looked different from other plates in the same arthropod family.
Bruce says that structures that don't look alike are actually related. It looks like they have a single origin that is 500 million years old.
It was there for a long time.
Her model for how novel structures emerge is called crypt persistence of serial homologs.
She says that a centipede's body has a number of legs repeating down it's body. The repeats can look different, but they are all built the same way. You can get a truncated centipede leg if the full structure isn't growing out. The cells are not actually growing out the leg.
Bruce believes that the rudiments can persist over millions of years if another repeat of the structure is present in the animal. A wing in an insect or a carapace in a crustacean are examples of different forms of the structure when it grows out again.
If an ancestral structure is no longer required, nature probably just reduces or eliminates the tissue. Bruce says that the tissue is still there and can be elaborated again.
Bruce says that this type of truncation is common in evolution. Some other pathway or tissue would be affected by the deletion of a genetic pathway.
Bruce thinks that cryptic persistence can be an explanation for some structures.
The authors analyzed gene expression patterns in arthropods and eliminated other theories of how the carapace may have evolved.
The ancient, common origin of all these plate-like structures suggests the genes that pattern these structures are plastic. They are capable of generating a lot of diversity.
More information: Heather S. Bruce et al, The Daphnia carapace and other novel structures evolved via the cryptic persistence of serial homologs, Current Biology (2022). DOI: 10.1016/j.cub.2022.06.073 Journal information: Current Biology
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