Ubiquitous selfish toxin-antidote elements inCaenorhabditisspecies

Toxin-antidote elements (TAs) are selfish genetic dyads that spread in populations by selectively killing non-carriers. TAs are common in prokaryotes, but few examples are known in animals. We discovered five maternal-effect TAs in the nematode Caenorhabditis tropicalis and one in C. briggsae . Unlike previously reported TAs, five of these novel toxins do not kill embryos but instead cause larval arrest or developmental delay. We identified the genes underlying a TA causing developmental delay, slow-1/grow-1 , and found that the toxin, slow-1, is homologous to nuclear hormone receptors. Last, we found that balancing selection of conflicting TAs hampers their ability to drive in populations, leading to more stable genetic incompatibilities. Our results show that TAs are common in Caenorhabditis species, target a wide range of developmental processes, and may act as barriers preventing gene flow.