Looking Back in Evolutionary Time

Researchers in Japan are studying how one monkey manages to see in the dark. Their work might change our entire perception of how the primate family tree evolved.

Most primate researchers agree that the common ancestor of today’s monkeys was nocturnal. Strangely, most modern primates are active during the day and have poor night vision. Azara’s owl monkey, which is active at night and has good night vision, is the notable exception.

This exception has led to a controversy  in the field. Primate researchers cannot seem to agree if all monkeys lost the ability to see at night before the ancestors of the owl monkey gained it back. On the other hand, it is possible that the ancestors of the owl monkey were just lucky; maybe they never lost the ability to see at night.

Enter Akihiko Koga and his team at Kyoto University in Japan. The Koga group is a team of geneticists who think that the owl monkey is in the process of gaining back the night vision that was lost by the early ancestors of today’s monkeys.

But peering back into time is hard to do. So Koga and his team needed a way to study if the owl monkeys are evolving better night vision or if they are already optimized for night sight.

They relied on a finding from another group that showed that the low light-sensing cells (rods) of the eyes of nocturnal animals package their DNA in a different way than most cells. Most cells keep gene-rich DNA in the center of the nucleus. This sets up a little hotspot for cellular machinery to get in and turn on genes. But the rods of nocturnal animals pack their gene-rich DNA around the edges of the nucleus. It seems like this DNA packaging pattern makes it easier for light to pass through the rod cell in nocturnal animals.

When Koga and his group looked at the rods of the owl monkey eye, they found that the DNA is packed in a manner that is halfway between that of nocturnal and daytime animals. This suggests that owl monkeys have decent night vision, but they probably are not the best at seeing in the dark.

The team still wanted to know if the owl monkey was on the evolutionary path to regaining night vision or if it had been stuck that way since the early nocturnal ancestor of primates. They found that a piece of repetitive DNA that is usually packed into the center of rod nuclei in nocturnal animals has been expanding like a virus into new locations throughout the owl monkey genome. This could explain how the DNA packing has been changing over evolutionary time in the owl monkey nuclei. Most importantly, it could mean that the monkeys are regaining the night vision that was lost by their ancestors. Maybe they are still evolving toward better night vision.

We are all just monkeys with bad night vision. It is interesting to think that we have some DNA lurking in us–because we share most of our DNA with our primate cousins–that has been co-opted in the owl monkey to bring back night vision. More importantly, this is a new way that biologists have been able to go against the grain of time and peer backward in evolution.

Image: Aotus Azarae by Rich Hoyer, Flickr


Did Viruses Teach Us Sex?

Sex is a weird thing. At its core, the process involves a cell from one organism meeting up with another cell from another organism. These two cells have to become one when they collide, and they do this by fusing. New evidence suggests that the molecules responsible for this fusion might have come from viruses.

A new paper in the journal Current Biology noticed that proteins called fusogens in a single-celled organism are remarkably similar to another group of proteins produced by several types of viruses. Both types of fusogens are responsible for cell fusion in their respective organism/virus. In the single-celled organism, called Tetrahymena, fusogens dot the outside of the cell and allow the cells to undergo fusion and a primitive version of sex. Viruses, on the other hand, use fusogens to invade their cellular hosts.

Sex in Tetrahymena — Image: Jmf368w (CC BY-SA 4.0 via Wikimedia Commons)

The researchers involved in the present study were surprised when they saw just how similar Tetrahymena and viral fusogens look. Since proteins are just long strings of amino acids that are folded into complex shapes, we can represent a protein as a string of letters similar to what is done with DNA sequences. Then we can use a computer program to align the protein strings together by similarity. If two protein sequences align with a great deal of similarity–such that there is relatively little difference between the amino acid sequences of the two–it is often inferred that the proteins share a recent ancestor in evolutionary time. This is because evolutionary change is due to change at the DNA level, and the DNA change ultimately determines the protein change. When the viral and Tetrahymena fusogens were aligned, they appeared to be closely related based on similarity.

Not only did viral and Tetrahymena fusogens look strikingly similar, but the researchers were also able to show that they behaved quite similarly in a test tube. Specifically, they were similar in how they interacted with the chemicals that make up the exteriors of cells. The researchers concluded from this that both the structure and function of fusogens are conserved between viruses and Tetrahymena. When we see conservation of structure and function in biology, it is usually suggests that structures share an evolutionary origin.

So could viruses have passed sex on to us by leaving behind fusogens in our ancestor’s cells? Maybe, but the team that wrote this paper is not sure, and even admit that it might have happened the other way around. The bottom line is that we will need more evidence to know for sure, but this is certainly good circumstantial evidence that sexually reproducing organisms might owe a debt of gratitude to our infectious viral frenemies.