An arms race isn’t just for competing countries to build advanced weaponry; it also relates to genes or species competing within an environment and driving evolution forward. This evolutionary pressure could have driven the human genome to its incredible level of complexity, according to new research. Particularly, between “jumping genes” and the genes that regulate that function. The study was led by Sofie Salama of University of California, Santa Cruz, and the paper was published in Nature.
Scientists believe that ancient viruses adapted the ability to insert themselves into animal DNA in order to become replicated. The remainders of those are called retrotransposons, also described as “jumping genes” due to the ability to relocate within the genome. Our genome could be made up of as much as 50% of transposable elements, most of which are retrotransposons. Though only about two percent of the genome codes for protein, if a retrotransposon inserts into one of those genes or the areas that regulate it, it could be problematic.
Genetic mutations aren’t always bad. Many times, the mutation is “neutral” and doesn’t really affect anything at all. Sometimes it can even be advantageous (think X-Men, but slightly more realistic. Like gradually keener eyesight). Of course, insertions into certain areas could cause potentially fatal disease. In order to protect against this, the environment in the genome drove the evolution of genes to suppress the activity of transposable elements. Salama’s team suggested that over time, the genes that regulate retrotransposons also adopted other functions as well.
The team analyzed the relationship between retrotransposable elements and the KRAB zinc finger (KZNF) protein family, which regulate genetic expression and prevent transposable elements from moving. Roughly 170 of the 400+ genes in the KZNF family arose after primates diverged from other mammals. When KZNF is in action, it binds the DNA and prevents retrotransposons from replicating and moving. In addition to preventing these genes from moving, it also inhibits the genes next to it.
"There have been successive waves of retrotransposon activity in primate evolution, when a transposable element changed to become expressed and replicated itself throughout the genome until something turned it off," Salama said in a press release. "We've discovered a major mechanism by which the genome is able to shut down these mobile DNA elements.”
In order to eliminate any chance of the other 170 primate KZNF genes interfering with the study, the researchers inserted one human chromosome into mice. Though two of the KZNF proteins did a pretty good job of silencing transposable elements, there was a subclass of retrotransposons that was able to evade the attempts. When that subclass was put into a mouse without those proteins present, it was shown to be very adept at jumping. The transposable elements and regulatory genes were in a consistent battle to outdo one another.
"That's kind of the icing on the cake for aficionados of molecular evolution, because it demonstrates that this is a never-ending race," Salama explained. "KRAB zinc finger proteins are a rare class of proteins that is rapidly expanding and evolving in mammalian genomes, which makes sense because the transposable elements are themselves continually evolving to escape repression.”