Is mom in charge? Implications of resource provisioning on the evolution of the placenta.

The Trexler-DeAngelis model shows that placentas are most likely to evolve in environments with consistent, high levels of resource availability. An assumption imperative to the model is that placental species abort embryos in low food conditions. However, a previous experimental test of this assumption using the northern clade of Poeciliopsis showed no evidence for abortion. To distinguish between the alternatives that placental species either sacrifice body condition to maintain reproduction when resources are restricted, or that the previously documented pattern of resource allocation is a function of other life-history correlates of placentation rather than placentation alone, we perform a similar experiment on the southern clade of Poeciliopsis. The southern clade has the opposite relationship between life-history traits and placentation as seen in the northern clade. Our results mirror those from the northern clade, indicating that reproductive mode, rather than life history, dictates the pattern of resource allocation. These results add to the difficulties of explaining placental evolution within the constraints of the Trexler-DeAngelis model by restricting the range of resource conditions in which placental species can outcompete nonplacental species. They also lend support to hypotheses that suggest parent-offspring conflict in utero drives the evolution of the placenta.