CEB Committe on Evolutionary Biology

Jumping DNA and the Evolution of Pregnancy

Vincent Lynch is an assistant professor in the Department of Human Genetics and the Committee on Evolutionary Biology at the University of Chicago.

CEB faculty member Vincent Lynch’s research on the evolution of pregnancy in mammals was recently highlighted in National Geographic.

The article, written by Ed Yong, can be read in its entirety here. An excerpt is included below.

Jumping DNA and the Evolution of Pregnancy
by Ed Yong

About a decade ago, Vincent Lynch emailed Frank Grutzner to ask for a tissue sample from a pregnant platypus. He got a polite brush-off instead.

Then, around eight years later, Grutzner got back in touch. His team had collected tissues from a platypus that had been killed by someone’s dog. They had some uterus. Did Lynch still want some?

“Hell yes!”

The platypus was the final critical part of a project that Lynch, now at the University of Chicago, had longed to do since he was a graduate student. He wanted to study the evolution of pregnancy in mammals, and specifically the genetic changes that transformed egg-laying creatures (like platypuses) into those that give birth to live young (like us).

The platypus enjoys a short pregnancy. Its embryo sits in the uterus for just 2-3 weeks, surrounded by a thin eggshell, and nourished by a primitive placenta. It then emerges as an egg. Marsupials, like kangaroos and koalas, also have short pregnancies. But mothers give birth to live young, which live in a pouch until they’re big enough. Other mammals—the placentals, or eutherians—keep their babies in the uterus for as long as possible, nourishing them through a complex placenta. Their pregnancies can be marathons—up to two years in an elephant.

The move from egg-laying to live-bearing was huge. Mammals had to go from holding a shell-covered embryo for weeks to nourishing one for months. To understand how they made the leap, Lynch compared 13 different animals, including egg-layers like the platypus, marsupials like the short-tailed opossum, and eutherians like the dog, cow, and armadillo. He catalogued all the genes that each species switches on in its uterus during pregnancy. He then compared these different sets to work out when mammals started (or stopped) using those genes during reproduction.

He found thousands of differences, many more than he anticipated. For example, hundreds of genes are involved in making eggshell minerals; they’re active in the uterus of a platypus but silent in those of other live-bearing mammals. Conversely, the marsupials and eutherians started activating hundreds of genes involved in suppressing the immune system, and in passing hormonal signals between the mother and foetus.

This all makes sense. A platypus embryo, during its brief stay in the uterus, is separated from its mother—and its mother’s immune system—by a shell. “It’s like the embryo has a cloak,” says Lynch. When mammals evolved live births, the cloak disappeared and a problem arose. Every foetus shares only half of its genes with its mother, so mum’s immune system should recognise this lump of growing tissue as a potential threat. To dispense with eggs, early marsupials and eutherians had to evolve ways of tamping down their immune responses, and only in the uterus. They also needed ways of exchanging signals with their embryos. “The foetus needs to say, Hey I’m here, and the mum needs to say, Oh, that’s okay,” says Lynch.

His study shows that they did so by repurposing a vast array of genes that already had roles in other organs, like the guts, brains, and bloodstream. But how? How does an animal deploy a gene—or thousands of genes—in a different organ?