Tissue Tropisms and Transstadial Transmission of a Endosymbiont in the Highland Midge, Culicoides impunctatus (Diptera: Ceratopogonidae).

is a genus of intracellular bacteria which can manipulate host reproduction and alter sensitivity to natural enemy attack in a diverse range of arthropods. The maintenance of endosymbionts in insect populations can be achieved through both vertical and horizontal transmission routes. For example, the presence of the symbiont in the follicle cells and salivary glands of whiteflies allows Belli group transmission via the germ line and plants, respectively. However, the transmission routes of other bacteria, such as those in the Torix group of the genus, remain underexplored. Through fluorescence hybridization (FISH) and transmission electron microscopy (TEM) screening, this study describes the pattern of Torix tissue tropisms in the highland midge, (Diptera: Ceratopogonidae). Of note is the high intensity of infection of the ovarian suspensory ligament, suggestive of a novel germ line targeting strategy. Additionally, localization of the symbiont in tissues of several developmental stages suggests transstadial transmission is a major route for ensuring maintenance of within populations. Aside from providing insights into transmission strategies, the presence of bacteria in the fat body of larvae indicates potential host fitness and vector capacity impacts to be investigated in the future. Microbial symbionts of disease vectors have garnered recent attention due to their ability to alter vectorial capacity. Their consideration as a means of arbovirus control depends on symbiont vertical transmission, which leads to spread of the bacteria through a population. Previous work has identified a symbiont present in several species of biting midges ( spp.), which transmit bluetongue and Schmallenberg arboviruses. However, symbiont transmission strategies and host effects remain underexplored. In this study, we describe the presence of in the ovarian suspensory ligament of Infection of this organ suggests the connective tissue surrounding developing eggs is important for ensuring vertical transmission of the symbiont in midges and possibly other insects. Additionally, our results indicate localization in the fat body of As the arboviruses spread by midges often replicate in the fat body, this location implies possible symbiont-virus interactions to be further investigated.