Pregnancy sounds like the ultimate form of animal cooperation – mothers share their own bodies to grow and support their children’s prenatal development. But in reality, embryos use every trick in the book to take more than their fair share. Mothers, in turn, marshal their best defensive tactics.
Ultimately, it’s an evolutionary arms race. Offspring continually evolve strategies to steal resources, while mothers evolve strategies to defend their resources. Natural selection will favor embryos that are able to steal resources, but this will impose costs on the mother.
My colleagues and I are interested in how the mechanisms of this battle could have evolved. We recently investigated some differences between closely related animals that carry their young and others that lay eggs to figure out how hormones evolved to be expressed in the placenta. By understanding the processes that support conflict, we can identify how this conflict arose, and the impacts that it might hold for human health.
Placenta as a combat zone
During pregnancy, mothers support their offspring by providing nourishment across a placenta. Formed from both the embryo’s and mother’s tissue, this organ facilitates the exchange of materials between the two. The placenta is responsible for transferring oxygen and nutrients to the baby, while taking away waste products like carbon dioxide and urea.
By secreting hormone signals across the placenta to be received by the mother’s body, embryos can alter the amount of food they’re provided. In a truly cooperative world, offspring would release these “gimme more!” hormones only if they were undernourished. But embryos actually produce these hormones demanding more of the mother almost constantly throughout pregnancy.
Mothers’ bodies fend off these hormonal demands with defenses including the development of physical barriers between the embryo and the maternal blood supply, and the production of enzymes that can break down excessive levels of embryo-produced hormones.
But where did the tools embryos use to wage this battle come from? That’s the question my colleagues and I recently investigated.
Hunting for the origins of the conflict
In recent research published in the journal General and Comparative Endocrinology, my collaborators and I aimed to identify how an animal species evolves a placenta.
We know that live-bearing animals evolve from egg-laying ones, but we were curious about the role of parent-offspring conflict in this process. Did placental control of pregnancy evolve via novel hormones? Or did it rely on genes that were already present in the ancestral populations?
Our first step was examining the hormones produced by the placental tissue of three animal species: the horse, the southern grass skink lizard and a live-bearing population of the southeastern slider lizard.
We know each of these groups evolved pregnancy independently, because each is more closely related to an egg-laying species than they are to each other. For example, the first mammals were egg-laying and some of them are still around today – Australia’s platypus, for instance. Similarly, each of the live-bearing lizard species we studied has closely related egg-laying relatives.
By studying both the live-bearing and egg-laying relatives of these animals we can understand the things that are necessary for the transition.
We compared the list of hormones produced by these animals’ placental tissues to a similar tissue from two egg-laying animals: the chicken and an egg-laying population of the southeastern slider lizard. These species don’t have placentas because they lay eggs rather than carrying their unborn young internally. But placentas evolved from a membrane that lines the internal surface of developing eggs. This embryonic membrane supports the exchange of gasses between the embryo and the world outside its egg.
When we compared the genes found in the embryonic membrane of species with and without a placenta, the lists largely matched. This finding shows that the hormones used by embryos to manipulate their mothers evolved a long time ago, in an ancestor of both reptiles and mammals. When pregnancy evolved, the mechanisms to initiate conflict between the mother and embryo were already in place.
While we don’t know the function of these hormones in egg-laying species, we can speculate. The embryonic membrane is the first living point of contact between an embryo and the outside world. These hormones may alter the development of embryos in response to some environmental stimulus, such as temperature or disease.
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