Fish Fingers: The genes we share with our scaly cousins

Researchers have taken the fin-forming genes of a primitive fish and used them to generate the wrists and digits of mice, adding genetic evidence to the already rich fossil record of the fin to hand transition.

From before bony animals emerged onto land, the ocean was filled with a diversity of fishes. And from these primordial ancestors, many fish with many limb-types evolved.

Over the eons some groups evolved complex fins built with ‘rays’, the familiar fish fins you might imagine. Other groups moved down evolutionary estuaries towards limbs even more familiar.

Four hundred million years ago or so, some fish had fins that could bend in ways similar to our own elbows and knees. Such jointed fins are known from the fossil record. More recently in the fossil record are fossils with similar joints but that also bear fleshy fins with wrists and digits, and more recently still, limbs with hand bones comparable to four-legged land animals.

Though this transition is captured beautifully with fossils, no modern-day fish has wrist bones, or even bony remnants of a wrist – these transitional states exist only in the past, in the fossil record.

A modern fin equivalent to wrists and digits would firmly complement the fossil record, and that’s what researchers of the current study were seeking. Since the limb bones of modern fish do not resemble hands, researchers took to the fish genome to identify any wrist-digit equivalents.

For this genetic enquiry, the researchers analysed the genomes of the best understood fishes: the teleosts, the group of bony fish in which most species are a part.

The researchers were able to identify the genetic trigger for teleost fin development to test if there was such a genetic relationship between fins and hands. Yet, upon introducing these fin-triggering genes into developing mice, nothing happened.

Why did the fishes’ fin genes – which were supposedly shared with cousins such as the mouse – not trigger the development of mouse wrists and digits? It seems the team was comparing cousin species with too wide a fork between them, tines with too much change.

The teleosts had a more complex than expected back catalogue of genes, muddying the chemical cascade in mice. In fact, the team discovered an enormous genetic event in the history of teleost fishes, one that completely changed the way they grow their fins: a ‘whole-genome duplication’.

This is a genetic phenomenon where a creature’s entire genome gets duplicated. If this global mutation spreads throughout a population, this opens up a world of possibility for further change.

Genes that were once responsible for life-saving functions are free to mutate in any direction. In the intervening millions of years since one group of fish had evolved wrists and digits, the other group had undergone comparably radical mutations to their own ray-finned limbs.

The whole-genome duplication event for these ray-finned fish had rejigged their genome beyond recognition to the developing mice.

The researchers calculated the time when this whole-genome event occurred and then sought out a primitive fish that branched off before the genome duplication: the single-genomed ‘spotted gar’.

Not only would the spotted gar have more ancient, ancestral genes for limb development, but its genome had recently been sequenced, too.

Again, the team identified the genes responsible for fin development and inserted these gar genes into developing mice. But this time, the fin genes kick started mouse limbs that were “nearly indistinguishable” from those created by the mouse’s own genes.

This set of gar genes identified in the current study is so ancient that it is integral in the creation of the fins of fish, the hands of mice, and presumably the wings of bats and birds. This discovery is the perfect complement to the fossil record, indeed.

Story source Science Daily/University of Chicago Medical Center

Research paper Andrew R. Gehrke, Igor Schneider, Elisa de la Calle-Mustienes, Juan J. Tena, Carlos Gomez-Marin, Mayuri Chandran, Tetsuya Nakamura, Ingo Braasch, John H. Postlethwait, José Luis Gómez-Skarmeta, and Neil H. Shubin. Deep conservation of wrist and digit enhancers in fish. PNAS, December 22, 2014 DOI: 10.1073/pnas.1420208112