What's the News: A new analysis finds that many of the genes behind the development of modern mammalian pregnancy are controlled by mysterious genetic elements called transposons, long referred to as "junk DNA." The results suggest that the placental uterus did not evolve gradually but instead arose from a massive, transposon-driven genetic rewiring. How the Heck:
The research team looked at the DNA of uterine cells from the possum, a marsupial that gives birth two weeks after conception and shelters its developing young in a pouch, and compared them with cells from armadillos and humans, which both carry their children to term in a womb lined with a nutrient-rich placenta. The uterine cells of armadillos and humans shared more than 1,500 active genes that possums lacked.
Looking closer, the team saw that a number of these genes---about 13%---were very near on the genome to a particular kind of transposon specific to placental mammals. The origins of transposons, jumping genetic elements that copy and insert themselves in their host genomes seemingly at random, are still unclear, and their purpose has long been so opaque that they were called "junk DNA" until relatively recently. We now know that they play an important role in activating and disrupting the expression of genes.
The particular transposons the researchers observed turned out to make uterine cells sensitive to the hormone progesterone, encouraged the cells' development into the placenta, kept the genes from being turned off, and influenced a variety of other changes central to modern placental pregnancy.
The Future Holds: The fact that genes so crucial to modern mammalian pregnancy are controlled at so many levels by these mysterious jumping elements, which likely cut-and-pasted themselves into the genome years ago, is a new revelation, and the primary message the researchers take from this is that gradual modification of existing genes isn't the only way evolution can happen. Future work will likely investigate the different levels at which transposons control pregnancy, as well as further work on other areas where transposons have been involved in evolution. Reference: Lynch, et al. Transposon-mediated rewiring of gene regulatory networks contributed to the evolution of pregnancy in mammals. Nature Genetics (2011) doi:10.1038/ng.917 Published online 25 September 2011.