How placentas evolved in mammals?

BUFFALO, NY – The fossil record tells us about ancient life through the preserved remains of body parts such as bones, teeth, and turtles. But how do you study the history of soft tissues and organs, which can decay quickly and leave little evidence behind?

In a new study, scientists are using gene expression patterns called transcriptomics to investigate the ancient origin of one organ: the placenta, which is essential for pregnancy.

“In some mammals, like humans, the placenta is really invasive, so it penetrates all the way through the uterine wall into the maternal tissue. In other mammals, the placenta just touches the uterine wall. And then there’s everything in between,” said senior author Vincent J. LynchPhD, associate professor of biological sciences at the University at Buffalo College of Arts and Sciences.

“So what kind of placentas were early placentas?” he says. “We use gene expression patterns to reconstruct the evolution of the placenta and predict what the placenta of the last common ancestor of eutherian mammals looked like. Our data tells us that this placenta was invasive and non-invasive placentas have evolved multiple times in mammals. This solves a 150-year-old mystery: People have been debating what kind of placenta came first ever since.

As Lynch explains, all living mammals except marsupials and oviparous monotremes are eutherians, who have long pregnancies in which the developing fetus elicits a strong physiological response in the mother.

The investigation was: published on June 30 in eLife† Lynch led the study with first author Katelyn Mika, PhD, a University of Chicago postdoctoral researcher in human genetics and in organismal biology and anatomy. Camilla M. Whittington, PhD, and Bronwyn M. McAllan, PhD, both at the University of Sydney, are also co-authors.

“Our ability to ask how the placenta functioned at various points during its evolution by using the gene expression profiles of currently existing animals to reconstruct the ancestors is a really cool approach and gives us more information about how altering gene expression might contribute to the evolution of a new trait,” says Mika.

To conduct the analysis, the team compared the genes active in the wombs of different mammals during pregnancy. After finding that these gene expression profiles correlated with the degree of placental invasiveness, the scientists used their data to predict what the placentas of ancestral mammals looked like.

The study included about 20 species, such as the egg-laying platypus, marsupial marsupials and a range of eutherian mammals that give birth to live young.

The small subset is a limitation of the analysis: The authors write in eLife that research on a larger number of species is needed to help determine the strength of the findings.

Nevertheless, the study makes an important contribution to understanding how pregnancy evolved, Lynch says. The results may also benefit modern medicine.

“Knowing which genes are active in different species during pregnancy tells us how evolution works,” he says. “But it also tells us what makes a healthy pregnancy and how things can go wrong. We find the genes that create the right environment for healthy human pregnancies. If those genes are not expressed in the right way, it can cause problems.”

This study was supported by grants from the March of Dimes and the Burroughs Wellcome Fund Preterm Birth Initiative.