But I’m not a molecular biologist. Today’s sophisticated DNA-sequencing technologies show that the genetic materials of the two species, which diverged only 5 million or so years ago (an eye-blink in evolutionary time), are about 98 percent identical. Think about that next time you eat a banana.
One major exception to that parallelism: a set of three genes collectively called the major histocompatibility complex, or MHC. These genes code for proteins that sit on the surfaces of each cell in your body, where they serve as jewel cases that display bits of proteins that were once inside that cell but have since been chopped into pieces by molecular garbage disposals, transported to the cell surface and encased in one or another of the MHC proteins. That makes the protein bits highly visible to roving immune cells patrolling our tissues to see if any of the cells within are harboring any funny-looking proteins. If those roving sentry cells spot a foreign-looking protein bit, they flag the cell on whose surface it’s displayed as possibly having been infected by a virus or begun to become cancerous.
Viruses replicate frequently and furiously, so they evolve super-rapidly. If they can evade immune detection, that’s groovy from their perspective. So our MHC has to evolve rapidly, too, and as a result, different species’ MHC genes diverge relatively quickly. To the extent they don’t, there’s probably a good reason.
Stanford immunologist and evolutionary theorist Peter Parham, PhD, pays a lot of attention to the MHC genes. In a new study in PLOS Biology, he and his colleagues have made a discovery that may prove relevant to AIDS research, by analyzing genetic material found in chimp feces. Not zoo chimps. Wild Tanzanian chimps. As I noted in a news release about the study:
The wild chimps inhabit Gombe Stream National Park, a 13.5-square-mile preserve where they have been continuously observed from afar since famed primatologist Jane Goodall, PhD, began monitoring them more than 50 years ago.
The study’s lead author, postdoc Emily Wroblewski, PhD, set up shop in a corner of Parham’s lab and extracted DNA from fecal samples legally obtained by other researchers (close contact with the animals is prohibited). Each sample could be tied to a particular Gombe-resident chimp. RNA extracted from the same sample indicated that chimp’s infection status.
Parham, Wroblewski and their colleagues found that one particular MHC gene came in 11 different varieties – astounding diversity for such a small collection of chimps (fewer than 125 of them in the entire Gombe). Surprisingly, one small part of one of those 11 gene variants was nearly identical to a piece of a protective version of its human counterpart gene, a version that seems to protect HIV- infected people slowing HIV progression to full-blown AIDS.
Why is that important? Because any piece of an MHC gene that has maintained its sequence in the face of 5 million years of intense evolutionary pressure must be worth something.
Sure enough, fecal samples from chimps with that MHC gene variant, so strikingly analogous to the protective human variant, had lower counts of virus that those from infected chimps carrying other versions of the gene.
You can believe that scientists will be closely examining the DNA sequence contained in both the human and chimp gene variant, as well as the part of the MHC protein that DNA sequence codes for. Because it must be doing something right.
Previously: Revealed: Epic evolutionary struggle between reproduction and immunity to infectious disease, Our species’ twisted family tree and Humans share history – and a fair amount of genetic material – with Neanderthals
Photo by Emily Wroblewski