Male Age and Wolbachia Dynamics: Investigating How Fast and Why Bacterial Densities and Cytoplasmic Incompatibility Strengths Vary

Endosymbionts can influence host reproduction and fitness to favor their maternal transmission. For example, endosymbiotic bacteria often cause cytoplasmic incompatibility (CI) that kills uninfected embryos fertilized by -modified sperm. Infected females can rescue CI, providing them a relative fitness advantage. -induced CI strength varies widely and tends to decrease as host males age. Since strong CI drives to high equilibrium frequencies, understanding how fast and why CI strength declines with male age is crucial to explaining age-dependent CI's influence on prevalence. Here, we investigate if densities and/or CI gene ( ) expression covary with CI-strength variation and explore covariates of age-dependent -density variation in two classic CI systems. Ri CI strength decreases slowly with Drosophila simulans male age (6%/day), but Mel CI strength decreases very rapidly (19%/day), yielding statistically insignificant CI after only 3 days of Drosophila melanogaster adult emergence. densities and expression in testes decrease as Ri-infected males age, but both surprisingly increase as Mel-infected males age, and CI strength declines. We then tested if phage lysis, Octomom copy number (which impacts Mel density), or host immune expression covary with age-dependent Mel densities. Only host immune expression correlated with density. Together, our results identify how fast CI strength declines with male age in two model systems and reveal unique relationships between male age, densities, expression, and host immunity. We discuss new hypotheses about the basis of age-dependent CI strength and its contributions to prevalence. bacteria are the most common animal-associated endosymbionts due in large part to their manipulation of host reproduction. Many cause cytoplasmic incompatibility (CI) that kills uninfected host eggs. Infected eggs are protected from CI, favoring spread in natural systems and in transinfected mosquito populations where vector-control groups use strong CI to maintain pathogen-blocking at high frequencies for biocontrol of arboviruses. CI strength varies considerably in nature and declines as males age for unknown reasons. Here, we determine that CI strength weakens at different rates with age in two model symbioses. density and CI gene expression covary with Ri-induced CI strength in Drosophila simulans, but neither explain rapidly declining Mel-induced CI in aging D. melanogaster males. Patterns of host immune gene expression suggest a candi