Give us a Hand, Gar: More Insights into the Evolution of Tetrapod Limbs

About 360 million years ago some fish-like creatures crawled out of the water and explored a vast new world. The transition from water to land is well documented in the fossil record with some amazing specimens, such as Tiktaalik. Why these animals decided to leave their safe watery habitat is still largely a mystery (although Gary Larson provides an intriguing hypothesis, see cartoon).

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Broken Bone-genes?

Another blank spot in our knowledge on this transition deals with the so-called autopod (not to be confused with autobots), which refers to the modern structure composed of wrist and fingers or ankle and toes (depending on which side of the organism you are looking at, I prefer the front…).  When you compare this autopod with the fins of extant fish, you don’t to be a scientist (although it might help) to notice they are not the same. Genetic analyses showed that the genes controlling bone-formation, known as the HoxA and HoxD clusters, are also different. Even the “switches” that control these genes differ. When you insert a fish control switch into a mouse, it doesn’t work because of incompatibility issues (and the internet did not provide any solutions either).

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Looking in the Right Spot(-ted Gar)

Why didn’t this work? Well, they were simply looking in the wrong organisms. Until now, experiments focused on teleost fish which experienced a whole-genome duplication some 300 million years ago. This event and the aftermath probably changed the genetic architecture (genes and switches) of their limbs. Following this line of reasoning a group of researchers focused on a species that was not subjected to this whole-genome duplication, the Spotted Gar. When they inserted, genetic switches from this species into mouse embryos, they worked! This result confirms the paleontological evidence and shows that the regulatory system for building limbs has an ancient origin. And with the development of new genetic techniques, we will definitely gain more insights into this fishy history.

The Spotted Gar
The Spotted Gar

 

Reference

Gehrke, A., Schneider, I., de la Calle-Mustienes, E., Tena, J., Gomez-Marin, C., Chandran, M., Nakamura, T., Braasch, I., Postlethwait, J., Gómez-Skarmeta, J., & Shubin, N. (2014). Deep conservation of wrist and digit enhancers in fish Proceedings of the National Academy of Sciences DOI: 10.1073/pnas.1420208112

 

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