What profound insight into ancestral spine structure was revealed by the 'spondo' mutant line?
It revealed that the genetic recipe for the ancestral, more ancient spine structure is still accessible via regulatory changes
The discovery involving the *spondo* mutant, caused by a single-letter change in a teleost-specific gene regulating notochord patterning, provided a dramatic demonstration of evolutionary developmental biology. This mutation resulted in the fish developing a severely malformed spine where vertebrae appeared cleaved in half, closely mirroring fossilized specimens of ancestral fish. This observation strongly suggests that the genetic 'recipe' required to build that ancient, shared vertebrate spine structure was not erased during evolution but remains latent within the zebrafish genome. The single genetic alteration in *spondo* effectively bypassed the more modern regulatory system, reverting the developmental program to an ancient state, highlighting how developmental pathways are layered rather than entirely overwritten.
