Murder in their hearts: Sibling cannibalism and its consequences for cytoplasmic incompatibility in a spider host

Maternally-inherited bacterial symbionts often spread throughout their host populations via reproductive manipulations. The most common of these is cytoplasmic incompatibility (CI), in which pairings between uninfected females and infected males show reduced hatch rates, disadvantaging these uninfected females relative to infected females in the host population. However, in cannibalistic predators like spiders, unhatched eggs within a CI eggmass may serve as a nutritious resource for surviving siblings, giving them a fitness advantage over families without CI, partially counteracting the CI effect. We evaluated sibling cannibalism in the context of CI using Mermessus fradeorum (Linyphiidae), a common agricultural spider. Like many spiders, M. fradeorum spiderlings hatch within an eggmass, then undergo an additional molt and sclerotization before emergence, providing ample opportunity for unobserved pre-emergence cannibalism of unhatched siblings. We evaluated pre-emergence cannibalism in incompatible (CI) versus compatible crosses by dissecting a subset of eggmasses before spider sclerotization and counting hatched versus unhatched eggs. We compared these values to hatched spiderlings and unhatched eggs in unmanipulated eggmasses that were allowed to emerge naturally. In naturally-emerging eggmasses, we observed more evidence of cannibalized eggs in CI crosses compared to other types. Unhatched eggs found in early-dissected eggmasses were missing in naturally-emerged ones, likely due to unseen cannibalism. These findings suggest that spiderlings from CI crosses benefit from increased sibling cannibalism, while non-CI spiderlings do not, indicating that cannibalism could hinder the spread of CI bacteria in a host population. Overall, cannibalism may limit the spread of CI symbionts in hosts with siblicidal tendencies.