Evolution: fin to limb genes in zebrafish (Evolution)

by David Turell , Thursday, February 04, 2021, 21:53 (1169 days ago) @ David Turell

Expression does cause limb=like changes:

https://phys.org/news/2021-02-fin-limb-mutations-zebrafish-fins.html

"Unlike tetrapod limbs, which have complex skeletons with many bones that articulate at many joints, zebrafish pectoral fins have a simple endoskeleton that lacks joints. To their surprise, Hawkins and colleagues found mutants that modified their fins into a more limb-like pattern by adding new bones, complete with muscles and joints. These results reveal that the ability to form limb-like structures was present in the common ancestor of tetrapods and teleost fishes and has been retained in a latent state which can be activated by genetic changes. (my bold)

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"We look at some of the developmental aberrations and ask, can they inform us of the processes that were happening underlying some of these large shifts in evolution," said senior author Matthew P. Harris, Associate Professor of Genetics at Harvard Medical School and Orthopedic Research at Boston Children's Hospital. "And when you see something that shouldn't be there, ever, I mean 400 million years type of never, it's a major finding." (my bold)

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"Zebrafish belong to the teleost lineage of ray-finned fishes. Teleosts are a diverse lineage of about 30,000 species that includes goldfish, salmon, eels, flounder, clownfish, pufferfish, catfish and zebrafish. There are more teleost species than all birds, mammals, reptiles, and amphibian species combined. Yet, despite the large number of species and wide ranges of shapes, sizes, and habitats, the pectoral fin of teleosts is surprisingly simple and unchanged.

"The fin-to-limb transition in the tetrapod lineage modified and elaborated the ancestral fin to include many bones that articulate end on end. From the same ancestral starting state, teleost fins were reduced and simplified, such that there is no end-on-end articulation, only the side-by-side arrangement of the proximal radials. This structure allowed researchers to determine which aspects of development are uniquely limb and which features are common across teleost and tetrapod fins and limbs.

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"...they discovered that mutations in the waslb and vav2 genes cause the fin phenotype. This was a surprising finding as these genes have not previously been known to play roles in patterning the body.

"'It was a big question as to how waslb and vav2 were changing fin patterning," said Hawkins. "These genes were not known to interact with any of the very well characterized pathways that guide limb development. However, we found that these mutations cause an increase in the expression of the gene hoxa11b. This gene is very exciting because the Hox genes are in part responsible for patterning the vertebral column as well as the regions of the limb. And the Hox11 genes in particular are required to make the forearm."

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"Histological analysis revealed that the new bones had muscle attachment, which occurs extensively in limb bones, but not in the fin. In teleost fishes there are no muscles attached to the bones. Instead, the bones provide an intermediate support in the fin and the muscles extend directly from the shoulder out to the bony fin rays, bypassing the bones all together. The new bones are fully integrated into the fin, complete with joints for articulation and attachment to the fin muscles.

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"'Prior to this there aren't any examples where we have genes or mutations that actually elaborate the structure and make it even more complicated," said Hawkins. "Even in the case of limbs we only know how to make a limb smaller or less complex, but we didn't have any information on how add elements to a fin or a limb."

"'That was very surprising as well," confirmed Harris. "We had no hard experimental examples where you take a gene, turn it up, make it work more and get a more complex mature structure at the end. In our findings we actually found some of the dials that can turn up the genetic pathways and get a more complicated structure in the end."

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"This finding also fits well with another recent discovery that Hox13 genes are required to form the distal regions of both fins and limbs. Altogether these discoveries reveal that the appendage Hox code was likely present in the common ancestor of tetrapods and teleost fishes and is not specific to the tetrapod lineage."

Comment: This is a beautiful example of my theory that God had preplanning in His coding of early DNA