Reproductive isolation in Caenorhabditis: terminal phenotypes of hybrid embryos.

Several interspecific combinations of the "elegans" group of Caenorhabditis species are cross-fertile. Most F1 hybrids from these crosses arrest during embryogenesis. Developmental defects observed in hybrid embryos include defects in gastrulation initiation, defects in embryonic compaction, and defects in embryonic elongation. These reproductive barriers have arisen multiple times in the evolution of Caenorhabditis.