sept-1/zina-1 is an ancient toxin-antidote system in Caenorhabditis elegans

The toxin-antidote (TA) systems, consisting of two tightly linked genes acting as a postzygotic distorter, are found in several eukaryotic organisms, including three androdioecious Caenorhabditis species. However, the evolutionary history of such TA systems remains poorly understood. Here, we report an ancient C. elegans TA system identified in the highly divergent Hawaiian strain XZ1516. The maternal toxin SEPT-1 induces a rod-like larval arrest phenotype in progeny that do not carry the antidote ZINA-1, which has 10 zinc finger motifs. Interestingly, zina-1 is pseudogenized in most non-Hawaiian strains, while sept-1 evolves from a toxic to a non-toxic form through sequence divergence. Phylogenetic studies found that among the three known TA systems sept-1/zina-1 is the most prevalent one in the Hawaiian population that carries the ancestral variants, but it is no longer active in the more derived non-Hawaiian population. The loss of sept-1/zina-1 coincides with the rise of sup-35/pha-1 and peel-1/zeel-1 in non-Hawaiian strains, suggesting dynamic evolution of the TA systems.