Gagneux and colleagues say this strong immune reaction to Neu5Gc likely had a profound effect upon early human reproduction. In all mammals, the biological costs of pregnancy for the female can be huge, sometimes even life-threatening, and so it behooves females to ensure only the best-matching sperm successfully fertilize an egg. The scientists hypothesized that anti-Neu5Gc antibodies would target Neu5Gc-positive sperm or fetal tissues in early humans, kill them and thus reduce the chances of reproductive success.
The researchers tested the idea by exposing chimpanzee sperm, whose cell surface sugars are more than half non-human sialic acids, to human anti-Neu5Gc antibodies. The antibodies bound and killed the ape sperm in vitro. The scientists then mated female mice genetically altered to lack Neu5Gc and immunized to produce anti-Neu5Gc antibodies with Neu5Gc-positive males. The fertility rate for the females was measurably lower due to pre-zygotic incompatibility the anti-sperm effects of female antibodies.
"Over time, this incompatibility would reduce and then eliminate individuals with Neu5Gc," said Gagneux. "Oddly enough, based on our theoretical model, the process works faster when the fertility rate is only slightly decreased, rather than producing 100 percent infertility."
Gagneux noted that the findings add further weight to the concept of "speciation by infection," in which a combination of infectious diseases suffered by a particular population could predispose that population to diverge from other populations due to reproductive incompatibility. In the case of early humans, one driver may have been female immunity to Neu5Gc.
Previous studies (http://ucsdnews.ucsd.edu/newsrel/health/Varki%208%2022.htm) have shown that the loss of Neu5Gc occurred about two to thre
|Contact: Scott LaFee|
University of California - San Diego