An article published in the journal “Science Advances” reports a study on Australian lungfish (Neoceratodus forsteri) that offers new information on the evolution of fish fins into tetrapod limbs. A team of researchers led by Dr. Joost Woltering of the German University of Konstanz studied, in particular, the anatomical and genetic characteristics of Neoceratodus forsteri’s embryos. The result was the confirmation of the importance of some genes with new information on the genes involved and the discovery of a primitive hand in this fish’s fins, suggesting an evolution of fingered limbs through embryonic changes.
Experiments using genetic manipulation techniques showed the importance of some genes in the formation of the limbs of tetrapods such as hoxa13 and hoxd13. Now Joost Woltering’s team studied embryos of Australian lungfish (photo ©Mitch Ames), one of the few still existing in the group of Dipnoi, lungfish that appeared over 400 million years ago, in the Devonian period. They’re considered living fossils because they remained similar to their distant ancestors, and this species is today’s fish most closely related to tetrapods.
Dr. Joost Woltering explained the exam of the Australian lungfish’s embryonic development conducted to understand fin development, especially the importance of various genes. Some “architect” genes shape shape a group of precursor cells causing the complete growth of fins and they’re the same ones that in tetrapods determine limb growth.
The hoxa13 gene is activated in the formation of hands and a similar skeletal region in the Australian lungfish’s fins. This is a type of development that has never been observed in other fish more distantly related to tetrapods. This suggests that the primitive hand found in this fish’s embryos was already present in the ancestors of land animals.
There are still a lot of differences between the Australian lungfish’s fins, with the primitive hand developed at the embryonic level, and tetrapod limbs. For example, in tetrapods, the hoxd13 gene is activated in the development of various structures that determine the growth of fingers. Instead, other genes, such as alx4 and pax9, are switched off in the development of fingers.
The explanation of the evolution from fins to limbs is not yet complete. For this reason, the researchers intend to continue their examinations at the embryonic level, also in other fish species, to understand the differences in fin development. The use of genetic techniques can also help to understand which genes are crucial in the development of the various anatomical structures and how they change by activating and deactivating the various genes.
