The evolution of placentas in poeciliid fishes.

摘要

Organisms show abundant variation in whether embryos are provisioned before or after fertilization. The adaptive significance of these differences remains largely unexplained. Most available hypotheses for the evolution of placental reproduction are too specialized to have general explanatory power. Here, I address the evolution of the placenta in the fish family Poeciliidae. This family is amenable to field and laboratory studies, is host to multiple, independent origins of placentas, and contains closely related species that vary in the degree of placentotrophy. Using molecular phylogenetic hypotheses, I describe variation in placentotrophy and other life-history traits across 19 species (43 populations) from five genera. I performed common-garden experiments on four closely related species in the northern clade of the genus Poeciliopsis. One species lacks placentotrophy and three vary in the extent of placentation. I asked whether the degree of placentotrophy is a stable feature of the life-history and whether population differences have a genetic basis. I also asked whether there are life-history correlates of increased placentotrophy that could lead to a hypothesis to explain placental evolution that is consistent with life-history theory.; I report the discovery of two independent origins of extensive placentotrophy in the genera Micropoecilia and Pamphorichthys. I demonstrate that the degree of placentotrophy in the genus Poeciliopsis shows a small degree of plasticity in response to different levels of food availability but is otherwise a genetically-based, stable feature of the life-history that can be used to characterize populations and species. I document genetic variation among populations within a species in the degree of placentotrophy. Within the northern clade of Poeciliopsis, increased placentotrophy is associated with increased superfetation (number of simultaneous developing broods) and decreased interbrood interval, age at maturity, ova and offspring size, and reproductive allocation. I hypothesize that Poeciliopsis, thanks to the combined effects of superfetation and placentation, can adapt to high extrinsic mortality rates while circumventing trade-offs in locomotor performance that have been documented in P. reticulata, a non-placental poeciliid fish without superfetation. If the "extrinsic mortality hypothesis" is general, then it should be supported in studies of different poeciliid clades representing independent origins of the placenta.