Fin-to-Limb Evolution Explained through Examination of Catsharks

Sure, we know many current land mammals were equipped to live under water millions of years ago. But now we know more about that tricky transition period from flopping fish to walking vertebrate. Researchers looking at catsharks have detailed their findings about that key evolution of fins to limbs--and how fish evolved to live as early tetrapods--in a recent report in the journal eLife.

Researchers from the Tokyo Institute of Technology (Tokyo Tech) and Center for Genomic Regulation (CRG, Barcelona) collaborated on this research regarding the earliest tetrapods--or four-legged, land-living vertebrates. Forelimbs on a tetrapod evolved from pectoral fins on the ancestral fish. In a fish, those fins contain three or more basal bones connected to the pectoral (shoulder) girdle. Most of the basal bones in the anterior side, or the thumb side in a human limb, were lost in early tetrapods. Only the most posterior bone remained as the humerus, or the upper arm of a human today, according to a release.

Because catsharks' pectoral fins contain three basal bones as seen in the ancestral fish, they made a good subject for studying fin development. Their forelimb development was compared with that of mice, which provided the tetrapod data. They found that the sharks had a shift in the balance of anterior and posterior fields in fin buds, compared with the mouse limb buds. For instance, mice have a much stronger emphasis on the "hand" area of the forelimb than a catshark would, the release noted. 

The researchers realized that the regulator protein Gli3 was the reason for this difference. The protein controls the balance of anterior and posterior fields, and regulates where cells are located along the anterior-posterior axis. They also learned that the catshark genome lacked a sequence found in mice and other tetrapods. That sequence is responsible for preventing Gli3 expression in the posterior part of tetrapod limb buds, said the release.

The results concluded that the Gli3 influenced a posterior emphasis in catshark embryos' pectoral fins. The research showed that a major genetic shift occurred as tetrapods evolved, involving the different genetic sequence found in mice.

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