Ancient parasites spurred evolution.

• Position: Mammals

Large-scale genetic changes that marked the evolution of pregnancy in mammals have been identified by an international team of scientists. They found thousands of genes that evolved to be expressed in the uterus in early mammals, including many that are important for maternal-fetal communication and suppression of the immune system. Surprisingly, these genes appear to have been recruited and repurposed from other tissue types by transposons--ancient mobile genetic elements sometimes thought of as genomic parasites.

This research sheds light on how organisms evolve new morphological structures and functions. "For the first time, we have a good understanding of how something completely novel evolves in nature, of how this new way of reproducing came to be," says study author Vincent Lynch, assistant professor of human genetics at the University of Chicago (III.).

"Most remarkably, we found the genetic changes that likely underlie the evolution of pregnancy are linked to domesticated transposable elements that invaded the genome in early mammals. So, I guess we owe the evolution of pregnancy to what are effectively genomic parasites."

The researchers found that, as the first mammals evolved--and resources for fetal development began to come more from the mother and less from a yolk--hundreds of genes that are important for cellular signaling, metabolism, and uterine development started to be expressed in the uterus. As the eggshell was lost and live birth evolved in the common ancestor to marsupials and placental mammals, more than 1,000 genes were turned on, many of which were strongly linked to the establishment of maternal-fetal communication. As prolonged pregnancy evolved in placental mammals, hundreds of genes began to be expressed that greatly strengthened and elaborated maternal-fetal communication, as well locally suppressing the maternal immune system in the uterus --thus protecting the developing fetus.

The team also identified hundreds of genes that were turned off as these lineages evolved, many of which had been involved in eggshell formation.

"We found lots of genes important for maintaining hormone signaling and mediating maternal-fetal communication, which are...