Margarida Cardoso Moreira: The evolution of new vertebrate cell types and organs

How do new cells, new tissues, and entire new organs arise during evolution? Our lab investigates these questions using an organ that has evolved independently many times and is remarkably diverse: the placenta. The placenta forms through the fusion of embryonic and maternal tissues to enable the transfer of nutrients, waste, and more during gestation. Placentas have evolved more than 100 times independently among vertebrates, including once in mammals. Within a family of small live-bearing fishes known as Poeciliidae, at least nine independent origins of the placenta have occurred. This makes them excellent models for studying how new organs emerge during evolution. We combined whole-genome sequencing, single-cell RNA and ATAC sequencing, and imaging to uncover the molecular, cellular, and developmental basis of five independent origins of a placenta in Poeciliids. We found that the evolution of a novel cell type unique to Poeciliids allowed this family to transition from egg-laying to live-bearing through egg retention. This cell type was subsequently and independently co-opted five times to form the placenta in five species. Across these independently evolved placentas, we observe strong convergence at the molecular level, with specific genes and pathways repeatedly recruited during placental evolution. Together, our findings provide empirical support for a model in which cell-type innovation drives the emergence of new organs and demonstrate that major life history transitions can follow predictable developmental and molecular trajectories.