Giving birth to live, free-living young has evolved many times in many animal groups. In a subset of these cases, genuine matrotrophy has evolved. “Matrotrophy” refers to pre-natal development in which most of the energy for embryonic growth and development is provided directly by the mother after the ovum has been fertilized. In a subset of these cases, a placenta connects the vascular systems of mother and offspring for provision of nutrients. Placentae have evolved several times in the family Poeciliidae, known as “live bearers,” familiar to many people through their popularity as aquarium fish (guppies, mollies, platys, swordtails, etc.).

In the Poeciliidae, the placenta is actually the ovarian follicle within which offspring develop. The heavily vascularized follicle is connected to the embryo via the “heart tube,” which forms from the pericardial sac of the embryo. The embryo and mother communicate chemically through this connection.
“Placentation,” or the evolution of offspring provisioning through a placenta, has several consequences in these fish. At the most immediate level, mothers can regulate their investment in offspring based on their own access to resources and, apparently, the sire’s genotype. Offspring can coerce resources from mothers. At a second level, the interactions between mother and embryo, and the conflicts of interest that emerge, can create patterns of asymmetric reproductive isolation. At the macro-evolutionary level, placentation is associated with dramatic changes in levels of sexual dimorphism, mating system, and patterns of male trait evolution.