Asymmetry and niche partitioning shape the infection dynamics of co-transmitted Wolbachia symbionts

Wolbachia is an incredibly widespread maternally transmitted bacterium in arthropods that can alter host physiology, nutrition, reproduction, and immunity. In some cases, multiple Wolbachia strains infect the same host and are stably transmitted alongside each other. This raises the question of how multiple intracellular symbionts interact with each another and with the host to ensure stable transmission. Here, we use fluorescence in situ hybridizations and confocal microscopy to investigate co-transmission in a naturally occurring co-infection of two Wolbachia strains in Drosophila simulans : w Ha and w No. We find significant differences in spatial occupancy and abundance between the co-transmitted strains across stages of oogenesis and embryogenesis. We show that w Ha and w No have biases for different niches during oogenesis, and their strain-specific abundance is driven by egg chamber development, mating status, and their interaction. After differential curing of the co-infection, we find that w No is dependent on w Ha for vertical transmission, but not vice versa. Additionally, while w Ha localization patterns are unchanged by loss of co-infection, abundance of w Ha in the ovaries increases when w No is removed. Understanding how symbiont co-infections achieve stability has important implications for the ongoing use of Wolbachia as a tool for insect management programs, but also for our understanding of the ecology of intracellular communities more broadly.