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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 are using gene expression patterns called transcriptomics to investigate the ancient origins of an organ that is vital to pregnancy: the placenta.

“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 kind of placentas are 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 non-invasive 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, except marsupials and egg-laying monotremes, are eutherians that have long gestations in which the 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 might have functioned at different points during its evolution, using the gene expression profiles of extant animals to reconstruct the ancestors, is a really great approach and gives us more information about how the change of gene expression can contribute to the evolution of a new trait,” says Mika.

To perform the analysis, the team compared the genes active in the uterus of different mammals during pregnancy. After finding that these gene expression profiles correlated with the extent of placental invasiveness, the scientists used their data to predict what mammalian ancestral placentas 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 may also benefit 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. If these genes are not expressed in the right way, it 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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Courtesy of the University at Buffalo

Quote: Study: How Mammalian Placentas Evolved (2022, July 1) Retrieved July 2, 2022, from

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