tas8831
Well-Known Member
... indicates that it might not have been a huge deal.
Attended a cool scientific meeting this weekend - one of the more intriguing talks for me was a presentation by a Harvard researcher regarding his group's work on zebrafish - the production of supernumerary (extra) long bones in the pectoral fin skeleton:
PD = proximal-distal.
The micrographs shown were amazing - actual joint capsules formed, complete with their own muscular investment.
Just a little gene tweaking, and voila! Extra bones, with joints and all.
The full paper is available in a couple of formats so the interested can read for themselves, but some highlights from the experimental procedures and such (emphases are mine):
"To identify genes affecting the form and pattern of the adult skeleton in zebrafish, we conducted a forward mutagenesis screen focused on dominant mutations (13). In our screen, we isolated a mutant that breaks from the teleost ground plan by forming supernumerary long bones along the PD axis of the pectoral fin endoskeleton 20 (Fig. 1C). We call these new bones ‘intermediate radials.’ They are found between the posterior proximal radials and distal radials, and they do not articulate directly with the shoulder. The overall pattern and size of the fin rays are not dramatically altered. We named this mutant rephaim (reph)."
"Using whole-exome sequencing, we mapped the reph mutation to chromosome 4 (Figure 29 S3) (15). Analysis of sequence data from the linked interval identified a missense mutation in the gene wiskott-aldrich syndrome-like b (waslb), changing a serine to a proline at amino acid position 265...
To verify that the waslb mutation causes the reph phenotype, as well as to determine the genetic nature of the dominant reph allele, we used CRISPR-Cas9 to generate targeted frameshift mutations in wild-type waslb as well as in cis to the candidate reph S265P mutation, and assessed their effect on fin skeleton patterning (Fig. 1D, Figures S4, S5). Loss-of-function of wild-type waslb does not have any obvious effect on fin patterning. However, frameshift mutations generated upstream in cis to the missense mutation lead to reversion of the mutant phenotype, restoring the wild-type radial pattern. Small, in-frame deletions in cis to the mutation failed to rescue the wild-type phenotype. Together, these results demonstrate that the identified S265P mutation in waslb causes the reph phenotype, likely through a gain-of-function effect. The altered residue in reph is conserved across vertebrates..."
So - long story short - they generate mutant zebrafish and then screen them for resulting phenotypes (this lab has a wall of tanks with mutant zebrafish in them). One mutant strain exhibited these extra bones in the pectoral fin 'shoulder' region. The potential cause of this mutant phenotype was located and characterized, and various tests were done to ensure that this mutation is in fact the likely cause. And interestingly, what is a mutant in zebrafish is CONSERVED across vertebrates.
Work is ongoing, but this seems pretty big to me - a possible 'root cause' mutation (at least a big contributor) of the transition from fins to limbs.
And it is but a SINGLE amino acid-altering mutation (changing one amino acid to another in a protein utilized during development).
I have long thought that the notion that some huge number of specific and coordinated mutations is required for phenotypic change is wrong. The earlier geneticists/evolutionary biologists, like JM Smith, thought our genomes and numbers of genes would be larger, it seems to me via my reading of their positions, because we are so special and that the great transitions would require many mutations - a bit of human chauvenism. The more recent such claims, that to me seem to come primarily from creationists, is due to much the same reasons. The difference being, as we continue to learn about 'evo-devo', those old human-centric scientists have either died off or abandoned their earlier largely un-supported positions.
On the other hand, if anything, I predict heckling and nit-picking and rock-throwing from the sidelines form creationists.
Attended a cool scientific meeting this weekend - one of the more intriguing talks for me was a presentation by a Harvard researcher regarding his group's work on zebrafish - the production of supernumerary (extra) long bones in the pectoral fin skeleton:
"Here, we identify zebrafish mutants that form supernumerary long bones along the PD axis of the pectoral fin. These new bones are integrated into musculature, form joints, and articulate with neighboring bones. This phenotype is caused by activating mutations in previously unrecognized regulators of appendage patterning, vav2 and waslb, that function in a common pathway. We find this pathway is required for normal appendage development across vertebrates, and loss of Wasl in developing mouse limbs results in patterning defects similar to those seen in Hoxa11 knockout mice. Concordantly, we show that formation of supernumerary fin long bones in the zebrafish mutants requires the function of hoxa11 paralogs, indicating developmental homology with the forearm. These data suggest the existence of latent but functional Hox code patterning the fin endoskeleton. Our findings reveal an inherent limb-like patterning ability in fins that can be activated by simple genetic perturbation, resulting in the elaboration of the endoskeleton."
PD = proximal-distal.
The micrographs shown were amazing - actual joint capsules formed, complete with their own muscular investment.
Just a little gene tweaking, and voila! Extra bones, with joints and all.
The full paper is available in a couple of formats so the interested can read for themselves, but some highlights from the experimental procedures and such (emphases are mine):
"To identify genes affecting the form and pattern of the adult skeleton in zebrafish, we conducted a forward mutagenesis screen focused on dominant mutations (13). In our screen, we isolated a mutant that breaks from the teleost ground plan by forming supernumerary long bones along the PD axis of the pectoral fin endoskeleton 20 (Fig. 1C). We call these new bones ‘intermediate radials.’ They are found between the posterior proximal radials and distal radials, and they do not articulate directly with the shoulder. The overall pattern and size of the fin rays are not dramatically altered. We named this mutant rephaim (reph)."
"Using whole-exome sequencing, we mapped the reph mutation to chromosome 4 (Figure 29 S3) (15). Analysis of sequence data from the linked interval identified a missense mutation in the gene wiskott-aldrich syndrome-like b (waslb), changing a serine to a proline at amino acid position 265...
To verify that the waslb mutation causes the reph phenotype, as well as to determine the genetic nature of the dominant reph allele, we used CRISPR-Cas9 to generate targeted frameshift mutations in wild-type waslb as well as in cis to the candidate reph S265P mutation, and assessed their effect on fin skeleton patterning (Fig. 1D, Figures S4, S5). Loss-of-function of wild-type waslb does not have any obvious effect on fin patterning. However, frameshift mutations generated upstream in cis to the missense mutation lead to reversion of the mutant phenotype, restoring the wild-type radial pattern. Small, in-frame deletions in cis to the mutation failed to rescue the wild-type phenotype. Together, these results demonstrate that the identified S265P mutation in waslb causes the reph phenotype, likely through a gain-of-function effect. The altered residue in reph is conserved across vertebrates..."
So - long story short - they generate mutant zebrafish and then screen them for resulting phenotypes (this lab has a wall of tanks with mutant zebrafish in them). One mutant strain exhibited these extra bones in the pectoral fin 'shoulder' region. The potential cause of this mutant phenotype was located and characterized, and various tests were done to ensure that this mutation is in fact the likely cause. And interestingly, what is a mutant in zebrafish is CONSERVED across vertebrates.
Work is ongoing, but this seems pretty big to me - a possible 'root cause' mutation (at least a big contributor) of the transition from fins to limbs.
And it is but a SINGLE amino acid-altering mutation (changing one amino acid to another in a protein utilized during development).
I have long thought that the notion that some huge number of specific and coordinated mutations is required for phenotypic change is wrong. The earlier geneticists/evolutionary biologists, like JM Smith, thought our genomes and numbers of genes would be larger, it seems to me via my reading of their positions, because we are so special and that the great transitions would require many mutations - a bit of human chauvenism. The more recent such claims, that to me seem to come primarily from creationists, is due to much the same reasons. The difference being, as we continue to learn about 'evo-devo', those old human-centric scientists have either died off or abandoned their earlier largely un-supported positions.
On the other hand, if anything, I predict heckling and nit-picking and rock-throwing from the sidelines form creationists.
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