I've been fascinated by the placenta ever since I wrote about Lucile Packard Children's Hospital neonataologist Anna Penn, MD, PhD, and her quest to find out more about this 'most mysterious organ.' The recent work of Penn and others have shown that the placenta is much more than a mere housekeeper moderating the ongoing biological conversation between mother and fetus. It also differs markedly among species, which suggests a history of rapid evolution.
Now, geneticist Julie Baker, PhD, and graduate student Edward Chuong have published an intriguing article (.pdf) in Nature Genetics suggesting that species-specific differences are due to the activity of viral sequences that have been incorporated into the mammalian genome over time. (My colleague, Bruce Goldman, has written an elegant description of how these sequences, called endogenous retroviruses, are constantly accumulating in our DNA.) As Chuong explained in an e-mail to me:
Endogenous retroviruses, or ERVs, are genomic "parasites" that occupy 8 to 10 percent of mammalian genomes. They must be aggressively silenced for the embryo to develop properly. In contrast, ERVs are highly active in the placenta, although their functional role - if any - has largely remained a mystery. In this study, we show that these ERVs function as a genome-wide source of enhancers in the placenta. Our findings point to ERV enhancer activity as a potentially significant evolutionary mechanism driving the rapid evolution of the placenta.
Why might the placenta need to evolve so quickly? Well, as any pregnant woman will tell you, with pregnancy comes many indignities. Some are purely physical in nature, others are less obvious. Specifically, the maternal immune system has to be kept from recognizing the fetal cells as foreign and mounting a fatal attack. The placenta protects the fetus by, among other things, secreting molecules to dampen the mother's immune response; the immune system, conversely, tries to evade this suppression. Says Chuong:
Intriguingly, the fact that the placenta is remarkably different across species may reflect an ongoing co-evolutionary arms races between parent and offspring.