Abstract
‘Candidatus Cardinium’, a recently described bacterium from the Bacteroidetes group, causes diverse reproductive alterations of its arthropod hosts, including cytoplasmic incompatibility (CI), parthenogenesis and feminization. We detected severe CI expression induced by Cardinium in the carmine spider mite Tetranychus cinnabarinus (Boisduval). CI was expressed as a reduction in egg hatchability and a male-biased sex ratio in crosses between uninfected females and infected males. To determine whether male age affects Cardinium-induced CI expression, uninfected females were crossed with infected virgin males at different ages. Cardinium density in males, as measured by quantitative PCR using the 16S rDNA gene, declined rapidly with increasing age. The CI level also declined rapidly with increasing age. Cardinium lost the ability to induce CI by day 12, and the Cardinium density declined to 3.0×106 copies per ml. Cardinium density was positively correlated with the CI level, which is consistent with the bacterial density model of CI. Our results suggest that both male age and Cardinium density are key factors in Cardinium-induced CI expression in T. cinnabarinus, and a threshold level of Cardinium density may be required for induction of CI.
