Ancient retroviruses to thank for evolution of complex brains

Retroviruses may have triggered the evolution of large, complex brains in vertebrates hundreds of millions of years ago according to new research.

Deepening our understanding of brain evolution in vertebrates could help us piece together how human brains evolved to be so large and complex.

A retrovirus genetic element, called a “retrotransposon”, could be the reason mammals, amphibians, reptiles, birds and fish produce myelin – made up of protein and fatty substances. Myelin is an insulating layer that protects the axon of nerve cells, as seen in the diagram below.

Neuron description diagram labelled
Labelled diagram of a neuron. Source: “Anatomy and Physiology” by the US National Cancer Institute’s Surveillance, Epidemiology and End Results Program via Wikimedia commons (CC BY-SA 3.0).

This “sheath” helps speed up the transmission of electrical signals between the neurons. Using myelin meant signals could be sped up without increasing the diameter of the axon. This, in turn, meant that vertebrate brains could pack more neurons tightly together.

The gene sequence, called “RetroMyelin” is described in a paper published in the journal Cell.

“Retroviruses were required for vertebrate evolution to take off,” says senior author and neuroscientist Robin Franklin of Altos Labs-Cambridge Institute of Science. “If we didn’t have retroviruses sticking their sequences into the vertebrate genome, then myelination wouldn’t have happened, and without myelination, the whole diversity of vertebrates as we know it would never have happened.”

Myelin first appeared around the same time that ancient fish evolved jaws.

The first jawed fish, like the 380-million-year-old Bothriolepis canadensis pictured at the top of this page, sparked an evolutionary revolution.

Newly formed jaws coincided with, and may have been intimately linked to, the development of limbs which would eventually see vertebrates leave the ancient oceans, and the evolution of large brains in backboned animals.

“There’s been an evolutionary drive to make impulse conduction of our axons quicker because having quicker impulse conduction means you can catch things or flee from things more rapidly,” says Franklin.

Myelin’s importance in vertebrate evolution has been known about for a long time. But how it first appeared has been a mystery until now.

The researchers delved into this question by studying the gene networks in oligodendrocytes – the cells that produce myelin in the central nervous system.

Green cells on black background
Myelinating oligodendrocytes shown glowing green. Credit: Peggy Assinck Altos Labs-Cambridge Institute of Science.

“Retrotransposons compose about 40% of our genomes, but nothing is known about how they might have helped animals acquire specific characteristics during evolution,” says first author Tanay Ghosh, a computational biologist also at Altos Labs-Cambridge Institute of Science. “Our motivation was to know how these molecules are helping evolutionary processes, specifically in the context of myelination.”

Inhibition of RetroMyelin in rodents showed myelin production stopped. Similar sequences were found in other jawed vertebrates, but not in jawless vertebrates and invertebrate species.

By constructing a phylogenetic tree of 22 jawed vertebrate species, the researchers were able to compare the RetroMyelin sequences. This showed differences between species, suggesting that RetroMyelin was incorporated into the genome multiple times in the evolution of jawed vertebrates.

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