The marine worm Platynereis is considered a living fossil. Here, we prefer the term “durable species.”
Antonella Lauri and Thibaut Brunet, both in Arendt’s lab, identified the genetic signature of the notochord — the combination of genes that have to be turned on for a healthy notochord to form. When they found that the larva of the marine worm Platynereis has a group of cells with that same genetic signature, the scientists teamed up with Philipp Keller’s group at Janelia Farm to use state-of-the-art microscopy to follow those cells as the larva developed. They found that the cells form a muscle that runs along the animal’s midline, precisely where the notochord would be if the worm were a chordate. The researchers named this muscle the axochord, as it runs along the animal’s axis. A combination of experimental work and combing through the scientific literature revealed that most of the animal groups that sit between Platynereis and chordates on the evolutionary tree also have a similar, muscle-based structure in the same position.
The scientists reason that such a structure probably first emerged in an ancient ancestor, before all these different animal groups branched out on their separate evolutionary paths. Such a scenario would also explain why the lancelet amphioxus, a ‘primitive’ chordate, has a notochord with both cartilage and muscle. Rather than having acquired the muscle independently, amphioxus could be a living record of the transition from muscle-based midline to cartilaginous notochord.
The shift from muscle to cartilage could have come about because a stiffened central rod would make swimming more efficient, the scientists postulate.
But did the muscle conduct nerves from the brain throughout the body and back?
See also: Genetic program for a face long predates a recognizable face (Researchers: Our results show that coupling of Hox gene expression to segmentation of the hindbrain is an ancient trait with origin at the base of vertebrates.)
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