Marsupial moms express placental genes in milk

September 12, 2017

Modern mothers, whether they be human or mouse, might be forgiven for envying marsupial mamas. Rather than enduring a long pregnancy and the birth of a relatively well-developed—and comparatively large—baby, kangaroos, wallabies and their ilk blithely pop out offspring after pregnancies measured in days rather than months.

These tiny, almost formless creatures then make their own intrepid way up to the mother's pouch to nestle politely and nurse for sometimes as long as a year.

For decades, researchers assumed that this premature eviction from the womb left little or no role for the placenta, which in most mammals tightly links the physiological processes of the mother and the fetus to support the fetus's many stages of development. These mammals are called eutherian mammals to distinguish them from the evolutionarily distant marsupials. In the past decade or so, however, it has become apparent that marsupials do sport their own, rudimentary version of this important organ.

Now researchers at the Stanford University School of Medicine and the University of Melbourne in Australia have collaborated to learn that marsupials have evolved a clever trick to support their need for a shortened pregnancy and a long lactation period. In short, female marsupials express genes important for fetal development that are normally found in the later stages of the eutherian placenta in their mammary glands instead—a kind of handoff of the developmental baton from womb to teat that suits their unique, savanna-hopping lifestyle.

"This research basically shows that the placenta, while really different-looking in the marsupial, has many of the functions of the eutherian," said Julie Baker, PhD, professor of genetics at Stanford. "Each animal has come up with their own unique strategies for delivering the functions of the placenta that takes into account where they live, how many offspring they have and what they eat, for example. But the actual function is very well-conserved."

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