1. Background

Studying the origin and evolution of vertebrate organs is difficult because many have ancient origins and have only evolved once in the history of vertebrates. The placenta is a great model to study the evolution of novel organs because it has evolved repeatedly in many vertebrate clades, it has evolved relatively recently in some lineages, and exists in intermediate forms in extant taxa.

 2. Methods

By studying pregnancy in both reptiles and mammals, I investigated the evolution of maternal-fetal interactions in the placenta. In terrestrial vertebrates, placentas form following the interactions of two distinct tissues, the luminal surface of the uterus, and the epithelial surface of an embryonic membrane.

 3. Results

Using transcriptomics of the uterus and embryonic membranes from oviparous and viviparous vertebrates I show that the chorioallantoic membrane of amniotes (reptiles, birds, and mammals) was ancestrally an endocrine organ. I then show the uterus of oviparous reptiles expresses receptors for the hormones and signalling molecules produced by the chorioallantoic membrane. Then using a novel tissue culturing system, I show that the apposition of maternal and fetal tissues is sufficient to induce changes to the uterus consistent with pregnancy.

 4. Conclusion

I argue that the novel apposition of uterine tissues with embryonic membranes is sufficient for the generation of maternal-fetal signalling networks. This model proposes that the interaction of placental tissues, is not merely a consequence of placenta formation, but that novel interactions form the basis of new placental regulatory networks, functions, and patterning mechanisms.