Evolution: Wallaby placental activity (Introduction)

by David Turell , Monday, September 18, 2017, 22:24 (2621 days ago) @ David Turell

The fetus is protected from the mother's antibodies by the mother's milk:

https://www.scientificamerican.com/article/wallaby-milk-acts-as-a-placenta-for-babies/?...

"'Wallabies are kicking over scientific conventions surrounding mammalian placentas, the organ responsible for protecting and nourishing a developing fetus. A study finds that contrary to what scientists thought previously, mother tammar wallabies (Macropus eugenii) have both a functioning internal placenta and milk that performs some of the organ’s usual roles.

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"'marsupials develop simple, placenta-like structures during the end of pregnancy, just before the underdeveloped baby crawls from the uterus into the mother’s pouch. These placental structures, just two cell layers thick, provide oxygen, nutrients and molecular signals that drive development to the fetus while protecting it from the mother’s immune system.

"It shouldn't be surprising that marsupial placentas look different from those of other animals since even closely related species can have very different-looking placentas that perform the same functions, says Derek Wildman, an evolutionary biologist at the University of Illinois in Urbana-Champaign. “It is the most variable organ in mammals in terms of anatomy and physiology,” he says.

"Marsupial pregnancy is remarkably short for mammals of their size. Tammar wallabies, which can grow to between 6 and 9 kilograms, are pregnant for just 26.5 days — barely longer than rats. Yet the baby, or joey, spends nearly a year continuing to develop and nurse inside the mother’s pouch: a long time compared to other mammals. This developmental mismatch led researchers to suspect that the majority of a joey’s development is driven by specialized features of the mother’s milk.

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"Baker thinks that the rapid evolution could be necessary for the placenta to effectively shield the fetus from the mother’s immune system, which treats the offspring as a foreign invader. “The placenta is evolving, trying to evade the mom, and comes up with these really bizarre strategies” — including taking liquid form in the mother marsupial’s milk, she says.

"Wildman says that the finding suggests that lactation may have evolved before eutherian placentas, as egg-laying mammals such as platypuses and echidnas lactate but do not have placentas. The egg-laying group came before marsupials and eutherians. He praises the paper, but says that the researchers could compare gene expresssion across more species than just mice and humans. A paper he published of 14 animals found that placentas did express different genes depending on the species."

Comment: These are transitional forms of mammals, but the transitions are giant gaps in physiology again unsupportive of the gradualism proposed by Darwin.