Infection Dynamics of Cotransmitted Reproductive Symbionts Are Mediated by Sex, Tissue, and Development.

One of the most prevalent intracellular infections on earth is with , a bacterium in the that infects a range of insects, crustaceans, chelicerates, and nematodes. is maternally transmitted to offspring and has profound effects on the reproduction and physiology of its hosts, which can result in reproductive isolation, altered vectorial capacity, mitochondrial sweeps, and even host speciation. Some populations stably harbor multiple strains, which can further contribute to reproductive isolation and altered host physiology. However, almost nothing is known about the requirements for multiple intracellular microbes to be stably maintained across generations while they likely compete for space and resources. Here, we use a coinfection of two strains ("Ha" and "No") in Drosophila simulans to define the infection and transmission dynamics of an evolutionarily stable double infection. We find that a combination of sex, tissue, and host development contributes to the infection dynamics of the two microbes and that these infections exhibit a degree of niche partitioning across host tissues. Ha is present at a significantly higher titer than No in most tissues and developmental stages, but No is uniquely dominant in ovaries. Unexpectedly, the ratio of Ha to No in embryos does not reflect those observed in the ovaries, indicative of strain-specific transmission dynamics. Understanding how strains interact to establish and maintain stable infections has important implications for the development and effective implementation of -based vector biocontrol strategies, as well as more broadly defining how cooperation and conflict shape intracellular communities. is a maternally transmitted intracellular bacterium that manipulates the reproduction and physiology of arthropods, resulting in drastic effects on the fitness, evolution, and even speciation of its hosts. Some hosts naturally harbor multiple strains of that are stably transmitted across generations, but almost nothing is known about the factors that limit or promote these coinfections, which can have profound effects on the host's biology and evolution and are under consideration as an insect-management tool. Here, we define the infection dynamics of a known stably transmitted double infection in Drosophila simulans with an eye toward understanding the patterns of infection that might facilitate compatibility between the two microbes. We find that a combination of sex, tissue, and development all contributes to infection dynamics of the coinfection.