Intraspecies variation in cytoplasmic incompatibility intensity in the bean beetle Callosobruchus analis

Wolbachia, a prevalent endosymbiont amongst arthropods, can effectively invade the host population by inducing cytoplasmic incompatibility (CI). CI occurs when a female parent lacks the Wolbachia strain that is possessed by its male mate, resulting in embryonic death. In the bean beetle, Callosobruchus analis, two distinct Wolbachia strains have been identified: the non-CI-inducing wCana1 and the CI-inducing wCana2. Field-collected C. analis individuals were either singly infected with wCana1 or doubly infected with wCana1 and wCana2. The higher prevalence of wCana1 over wCana2 in C. analis raises the question of why CI-inducing wCana2 is not more widespread. To address this, we measured the egg hatch rates in all the cross combinations between seven C. analis lines differing in Wolbachia infection status. We found that the intensity of CI was highly variable between combinations, which can be attributed to either the host genetic background or intra-strain genetic variation of wCana2. These findings may suggest that CI is susceptible to change and emphasize the need to consider the adaptive nature of host manipulation. Understanding the genetic and environmental factors underlying the CI variation is crucial for predicting the long-term stability of Wolbachia-host associations.