Crossing experiments suggest a simple genetic basis of complex male self-sacrifice phenotypes in widow spiders.

Sexual cannibalism is among the most extreme outcomes of sexual conflict, and the Australian widow spider Latrodectus hasselti is well known for this behaviour. Curiously, males of L. hasselti actively facilitate cannibalism through self-sacrificial mating traits. During copulation, the male somersaults his abdomen onto the female's fangs and develops an abdominal constriction that prevents his premature death, allowing him to monopolize paternity. The closely related sister species L. katipo, however, lacks self-sacrificial mating traits. Given the close relatedness of the two species, the differences in mating traits must have evolved rapidly and are expected to have a rather simple genetic basis. We hypothesize that these two complex traits are tightly linked on the X chromosome and show a single-locus Mendelian inheritance. To test this hypothesis, we backcrossed the two species and scored F2 males for the presence of copulatory somersault and abdominal constriction during mating. Our results suggest that the two traits are not genetically linked, and that the self-sacrificial somersault follows a single-locus X-linked Mendelian inheritance pattern, while the abdominal constriction has a more complex genetic basis. Our work shows that even extremely complex mating traits can evolve rapidly and have a simple genetic basis.