Revisiting suppression of interspecies hybrid malelethality in Caenorhabditis nematodes

Within the nematode genus Caenorhabditis, C. briggsae and C. nigoni are among the most closely related species known. They differ in sexual mode, with C. nigoni retaining the ancestral XO male-XX female outcrossing system, while C. briggsae females recently evolved self-fertility and an XX-biased sex ratio. Wild-type C. briggsae and C. nigoni can produce fertile hybrid XX female progeny, but XO progeny are either 100% inviable (when C. briggsae is the mother) or viable but sterile (when C. nigoni is the mother). A recent study provided evidence suggesting that loss of the Cbr-him-8 meiotic regulator in C. briggsae hermaphrodites allowed them to produce viable and fertile hybrid XO male progeny when mated to C. nigoni. Because such males would be useful for a variety of genetic experiments, we sought to verify this result. Preliminary crosses with wild-type C. briggsae hermaphrodites occasionally produced fertile males, but they could not be confirmed to be interspecies hybrids. Using an RNA interference protocol that eliminates any possibility of self-progeny in Cbr-him-8 hermaphrodites, we find sterile males bearing the C. nigoni X chromosome, but no fertile males bearing the C. briggsae X, as in wild-type crosses. Our results suggest that the apparent rescue of XO hybrid viability and fertility is due to incomplete purging of self-sperm prior to mating.