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.
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.