How placentas evolved in mammals

the placenta

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Fossils tell us about ancient life through preserved remains of body parts such as bones, teeth and turtle shells. But how do we study the history of soft tissues and organs that can decay quickly, leaving little evidence behind?

In a new study, scientists use gene expression models, called transcriptomics, to explore the ancient origins of an organ: the placenta, which is vital for pregnancy.

“In some mammals, like humans, the placenta is really invasive, so it invades all the way through the wall of the uterus, into the tissue of the mother. In other mammals, the placenta simply touches the wall of the uterus. And then there’s everything in between,” says senior author Vincent J. Lynch, Ph.D., associate professor of biological sciences in the College of Arts and Sciences at the University at Buffalo.

“So what were the early placentas?” he says. “We use gene expression patterns to reconstruct the evolution of the placenta and predict what the placenta looked like in the last common ancestor of eutherian mammals. Our data tell us that this placenta was invasive and that noninvasive placentas have evolved many times in mammals. This addresses a 150-year-old mystery: ever since, people have debated what kind of placenta came first.”

As Lynch explains, all living mammals, other than marsupials and egg-laying marsupials, are eutherians that have long gestations in which a developing fetus elicits a strong physiological response in the mother.

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

“Our ability to ask how the placenta may have functioned at different times during its evolution, using the gene expression profiles of extant animals to reconstruct the ancestors is a really cool approach and gives us more information about how changing gene expression can contribute to the evolution of a new trait,” Micah says.

To perform the analysis, the team compared the genes active in the uterus of different mammals during pregnancy. Having established that these gene expression profiles correlated with the degree of invasiveness of the placenta, the scientists used their data to predict what the placentas of mammalian ancestors looked like.

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

The small subset is one of the limitations of the analysis: the authors write in eLife that studies on a larger number of species are needed to determine the strength of the findings.

Still, the study makes an important contribution to understanding pregnancy development, Lynch says. The results can also be helpful modern medicine.

“Knowing which genes are active among 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 are finding the genes that create the right kind of environment for a healthy human pregnancy. gen are not expressed in the right way, which can lead to problems.”

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More info:
Katelyn Mika et al, Gene expression phylogenies and ancestral transcriptome reconstruction resolve major transitions in the origin of pregnancy, eLife (2022). DOI: 10.7554/eLife.74297

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Quote: Study: How placentas evolved in mammals (2022, July 1) Retrieved July 1, 2022, from

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