An international team of scientists has identified large-scale genetic changes that marked the evolution of pregnancy in mammals.
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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.
The study, published online in Cell Reports on Jan. 29, 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," said study author Vincent Lynch, PhD, assistant professor of human genetics at the University of Chicago.
"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."
To study genetic changes during the evolution of pregnancy in mammals, Lynch and his colleagues used high-throughput sequencing to catalog genes expressed in the uterus of several types of living animals,- placental mammals (a human, monkey, mouse, dog, cow, pig, horse and armadillo), a marsupial (opossum), an egg-laying mammal (platypus), a bird, a reptile and a frog. Then they used computational and evolutionary methods to reconstruct which genes were expressed in ancestral mammals.
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 as 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 egg shell formation.
"We found lots of genes important for maintaining hormone signaling and mediating maternal-fetal communication, which are essential for pregnancy, evolved to be expressed in the uterus in early mammals," Lynch said.
"But immune suppression genes stand out. The fetus is genetically distinct from the mother. If these immune genes weren't expressed in the uterus, the fetus would be recognized by the mother's immune system as foreign and attacked like any other parasite."
The team found that this process was driven by ancient transposons, stretches of non-protein coding DNA that can change their position within the genome. Sometimes called "jumping genes," transposons are generally thought to be genomic parasites that serve only to replicate themselves. ■