Evolution of the mammalian maternal-fetal cell-communication

Viviparity and placentation are key characteristics of placental mammals, and yet also some of their most variable traits. An important dimension of placental variation is the degree to which fetal tissue invades the maternal uterine tissue. This trait varies from the least invasive in marsupials to highly invasive in primates or rodents. The degree of invasiveness is a result of negotiation between fetal and maternal cells during early pregnancy, with maternal cells playing no lesser role then fetal cells in promoting and controlling the invasion. For example, some species have evolved maternal means to actively minimize invasiveness; such is the case in hoofed animals like cow or pig. It is clear that in all placental mammals, the communication between mother and fetus during development is central to successful pregnancy, and its failure is a common cause of pregnancy complications. It is less clear, how this communication is established during development and evolution, and how it is maintained in spite of the wide variation in the extent of maternal-fetal communication interface. The present study focuses on the communication between maternal and fetal tissue during pregnancy, and on the evolution of this communication. Thereby we use modern single-cell technology which enables us to identify genes active in specific maternal and fetal cell types of the interface. As the general signaling molecules and their receptors involved in cell communication are well known, gene activation can help us reconstruct the pathways active in the cross-talk between mother and fetus. The study of different pregnancy stages, and comparison between species with different degree of invasiveness, furthermore enables us to trace the changes during development and evolution. The species are chosen to be most informative about the transitions of placental invasiveness. Our work complements the existing approaches: while comparative genomic studies have identified many candidate genes which have changed sequence simultaneously with changes in invasiveness, the nature of their involvement in pregnancy remains unclear. By reconstructing cell communication at the maternal-fetal interface, our work characterizes the mechanistic aspect of the interface. With a single experiment per species, we expect to substantially enhance our cellular-level understanding of development and evolution of this important biological communication interface.

Placentation is a key characteristic of placental mammals, and also their most variable trait. It is clear that the communication between mother and fetus throughout the development is central to successful pregnancy in all mammals. The failure of this communication is the main cause of pregnancy complications. It is less clear, what this communication entails, how it is established in development and evolution, and how it is maintained across species, despite the wide variation in the maternal-fetal interface. The present study focused on the origin and evolution of the maternal-fetal communication during mammalian pregnancy. We applied single-cell technology to identify genes active in maternal and fetal cell types at the interface. As the signaling molecules and their receptors are well known, expression of both can reconstruct putative pathways active in the crosstalk between mother and fetus. The study of different pregnancy stages, and comparison between species with different reproductive strategies, enable us to trace the changes during development and evolution. Prior to empirical work, we reviewed mammalian pregnancy in the broader context of vertebrate reproduction. This enabled us to identify aspects aspects of mammalian pregnancy are specific to mammals, rather than being recruited into pregnancy. For example, ovarian progesterone, crucial for mammalian pregnancy, originated many milions of years before mammals. In contrast, embryo implantation into the uterine wall and long pregnancy, are key innovations of placental mammals. From the review of mechanisms underlying pregnancy prolongation across mammals, we conclude that implantation is shared but the pregnancy was prolonged independently in different mammalian lineages. An important consequence of this insight is that the pregnancy length in mammals is not a homologous trait, therefore it cannot be directly compared. Apart from this insight, our review contributes an important rare compilation of literature on pregnancy regulation to the research community. By the empirical work, we asked which maternal and fetal cell types reside at the interface, how they negotiate their coexistence, and how the interface changes during pregnancy and across species. To do that, we first studied the implantation stage, and compared it between opossum (no implantation), mouse and guinea pig (both with implantation). In summary, we find that the origin of implantation involved the re-wiring of existing maternal signaling to naswer to the embryo which already ancestrally posessed invasive potential. This suggests an essential role of maternal innovations in the evolution of mammalian placentation. Finally, a six-species atlas of single-cell gene expression at midgestation reveals a shared lineage of invasive fetal cells and the stepwise evolution of the maternal decidual cells. Reconstructed cell communication networks show strong participation of fetal cells in maternal signaling network. Together, our results reveal a dynamic history of cell type and signalling evolution.