Analysis of Early Embryogenesis in Rainbow and Banded Darters (Percidae: Etheostoma) Reveals Asymmetric Postmating Barrier

This study examines early embryogenesis in two species of darters, Etheostoma caeruleum and E. zonale (Teleostei: Percidae), and their hybrids. Results document the course of ontogeny from fertilization until the onset of pigmented eyes. Comparing fertilization and developmental success of conspecific versus heterospecific crosses revealed an asymmetric postmating reproductive barrier. E. caeruleum eggs treated with E. zonale sperm exhibited fertilization and developmental success similar to that of conspecific crosses. In contrast, E. zonale eggs treated with E. caeruleum sperm exhibited reduced fertilization relative to conspecific crosses and abnormal development. Development in this latter cross was compromised at all stages, but appeared to be concentrated around epiboly, or cell migration. As epiboly represents the stage of ontogeny when zygotic genes of both species jointly contribute to embryogenesis, results provide insight into the genetic mechanisms underlying postmating barriers in Etheostoma. Finally, the observed asymmetry in fertilization success is consistent with predictions based on the behavioral ecology of these species. Etheostoma zonale spawn in the open water column, whereas E.␣caeruleum bury their eggs under the substrate. The observed fertilization barrier may have therefore resulted from selection favoring increased fertilization specificity in E. zonale.