Pregnancy in mammals evolved with help from roving DNA

'Jumping genes' changed uterus to stop laying eggs


HOLD THE EGGS  Roving DNA helped program mammals like this dog to give birth to live young instead of laying eggs.


Roving pieces of DNA helped early mammals ditch egg-laying in favor of giving birth to live young. These “jumping genes,” or transposable elements, flipped the switch on thousands of genes, turning off ones that build hard eggshells and turning on ones that allow a fetus to develop in the uterus. Researchers report the finding in the Feb. 3 Cell Reports.

“Transposable elements … rewired when and where genes are expressed by giving them new regulatory information,” says study coauthor Vincent Lynch, an evolutionary biologist at the University of Chicago.

In early mammals, traveling DNA moved to new spots in the genome and carried machinery that allowed certain genes to be activated in the presence of a hormone called progesterone, which controls many aspects of pregnancy. These moves flipped on or off genes in the uterus. 

Lynch and his team used RNA sequencing to identify which genes were expressed in the uteruses of pregnant mammals such as dogs and pigs and in egg-laying animals, including chickens and frogs. The researchers then determined when and how in mammals’ evolutionary history these genes probably were switched on or off.

Many of the genes that turned on allow the mother’s body to recognize that she is pregnant and suppress her immune system so her body doesn’t sense the foreign DNA in the fetus and reject it. Animals that become pregnant have the advantage of carrying their developing young with them, instead of laying eggs in one spot, leaving them vulnerable to predators or unpredictable weather.

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