Quality over quantity: unraveling the contributions to cytoplasmic incompatibility caused by two coinfecting Cardinium symbionts

Cardinium hertigii is a common maternally-inherited bacterial endosymbiont
of arthropods. Some Cardinium strains spread by increasing female fitness
through the induction of cytoplasmic incompatibility (CI), which kills
offspring of crosses between infected males and uninfected female hosts.
CI is a two-step manipulation: first, sperm from male hosts are modified
by the symbiont in a manner that causes offspring death. Second, when
females are also infected, the symbiont reverses sperm modification in the
egg cytoplasm, allowing offspring of infected females to survive and
spread the symbiont. While Cardinium causes CI in many arthropod hosts,
most of what is known about Cardinium CI stems from the symbiosis between
the Cardinium strain cEper1 and its minute parasitoid host, Encarsia
suzannae. Here, we study a second Cardinium CI system in the wasp Encarsia
partenopea. Encarsia partenopea harbors two Cardinium strains, cEina2 and
cEina3, with the cEina3 present at a much lower density than cEina2. Using
antibiotic treatments and crossing assays, we find that the low-density
cEina3 strain is responsible for CI, and that cEina3 appears to modify
sperm during the pupal stage, like the better known cEper1. However,
cEina3 shows a markedly different localization pattern in male
reproductive tissues. Instead of infecting sperm cells, cEina3 is found in
somatic cells at the base of the testis and around the seminal vesicle.
This localization pattern suggests that cEina3 may use a different sperm
modification strategy from cEper1, and highlights variation between these
closely related symbioses.