A toxin-antidote selfish element increases fitness of its host.

Selfish genetic elements can promote their transmission at the expense of individual survival, creating conflict between the element and the rest of the genome. Recently, a large number of toxin-antidote (TA) post-segregation distorters have been identified in non-obligate outcrossing nematodes. Their origin and the evolutionary forces that keep them at intermediate population frequencies are poorly understood. Here, we study a TA element in called . Two major haplotypes of this locus, with and without the selfish element, segregate in . We evaluate the fitness consequences of the element outside of its role in gene drive in non-outcrossing animals and demonstrate that loss of the toxin decreased fitness of hermaphrodites and resulted in reductions in fecundity and body size. These findings suggest a biological role for beyond toxin lethality. This work demonstrates that a TA element can provide a fitness benefit to its hosts either during their initial evolution or by being co-opted by the animals following their selfish spread. These findings guide our understanding on how TA elements can remain in a population where gene drive is minimized, helping resolve the mystery of prevalent TA elements in selfing animals.