Ancient Retroviruses Played a Key Role in the Evolution of Vertebrate Brains
Published:28 Mar.2024    Source:Cell Press

Myelin is a complex, fatty tissue that ensheathes vertebrate nerve axons. It enables rapid impulse conduction without needing to increase axonal diameter, which means nerves can be packed closer together. It also provides metabolic support to nerves, which means nerves can be longer. Myelin first appeared in the tree of life around the same time as jaws, and its importance in vertebrate evolution has long been recognized, but until now, it was unclear what molecular mechanisms triggered its appearance.

The researchers noticed RetroMyelin's role in myelin production when they were examining the gene networks utilized by oligodendrocytes, the cells that produce myelin in the central nervous system. In rodents, the researchers found that the RNA transcript of RetroMyelin regulates the expression of myelin basic protein, one of the key components of myelin. They identified analogous sequences in all other classes of jawed vertebrates (birds, fish, reptiles, and amphibians) but did not find a similar sequence in jawless vertebrates or invertebrates.
Researchers report that ancient viruses may be to thank for myelin -- and, by extension, our large, complex brains. The team found that a retrovirus-derived genetic element or 'retrotransposon' is essential for myelin production in mammals, amphibians, and fish. The gene sequence, which they dubbed 'RetroMyelin,' is likely a result of ancient viral infection, and comparisons of RetroMyelin in mammals, amphibians, and fish suggest that retroviral infection and genome-invasion events occurred separately in each of these groups.