Mechanisms of Horizontal Cell-to-Cell Transfer of Wolbachia spp. in Drosophila melanogaster

Abstract

Wolbachia is an intracellular endosymbiont present in most arthropod and filarial nematode species. Transmission between hosts is primarily vertical, taking place exclusively through the female germ line, although horizontal transmission has also been documented. The results of several studies indicate that Wolbachia spp. can undergo transfer between somatic and germ line cells during nematode development and in adult flies. However, the mechanisms underlying horizontal cellto-cell transfer remain largely unexplored. Here, we establish a tractable system for probing horizontal transfer of Wolbachia cells between Drosophila melanogaster cells in culture using fluorescence in situ hybridization (FISH). First, we show that horizontal transfer is independent of cell-to-cell contact and can efficiently take place through the culture medium within hours. Further, we demonstrate that efficient transfer utilizes host cell phagocytic and clathrin/dynamin-dependent endocytic machinery. Lastly, we provide evidence that this process is conserved between species, showing that horizontal transfer from mosquito to Drosophila cells takes place in a similar fashion. Altogether, our results indicate that Wolbachia utilizes host internalization machinery during infection, and this mechanism is conserved across insect species. IMPORTANCE Our work has broad implications for the control and treatment of tropical diseases. Wolbachia can confer resistance against a variety of human pathogens in mosquito vectors. Elucidating the mechanisms of horizontal transfer will be useful for efforts to more efficiently infect nonnatural insect hosts with Wolbachia as a biological control agent. Further, as Wolbachia is essential for the survival of filarial nematodes, understanding horizontal transfer might provide new approaches to treating human infections by targeting Wolbachia. Finally, this work provides a key first step toward the genetic manipulation of Wolbachia.KEYWORDS Drosophila, Wolbachia, endocytosis, entry, horizontal, invasion, phagocytosis, transfer, transmission W olbachia spp. are intracellular bacteria that are transmitted through the female germ lines of arthropods and filarial nematodes (1, 2). In arthropods, Wolbachia spp. function as either a mutualist or a parasite, while in filarial nematodes, Wolbachia spp. are essential for host survival. Efficient maternal transmission of Wolbachia cells in Drosophila melanogaster requires their localization to the posterior cortex of the developing embryo, as this is the future site of the germ line (3). In filarial nematodes, Wolbachia cells undergo a precise pattern of migration during host development that involves not only asymmetric mitotic segregation but also the invasion of germ line precursors from somatic cells (4). Thus, the ability of Wolbachia spp. to undergo cell-to-cell transfer plays an important role in maintaining vertical transmission (5).

Horizontal Transmission of Intracellular Insect Symbionts via Plants

“…Intriguingly, electron microscopy revealed a Wolbachia -like bacterium within the vacuole of the phloem cell of the plant ( Li et al, 2017 ). Wolbachia has been shown before to be able to invade insect cells in culture ( White et al, 2017 ), but an invasion of a plant cell in vivo would likely require different machinery. Whether this is a singular case of intracellular localization or a common Wolbachia niche in plants remains to be determined.…”

Large-Scale Identification of Wolbachia pipientis Effectors

“…Wolbachia infected flies produce a larger number of eggs due to increased mitotic activity in infected germ line stem cells ( Fast et al 2011 ). Finally, Wolbachia rely on host clathrin/dynamin-dependent endocytosis for cell to cell transfer ( White et al 2017 ). However, to date, only two Wolbachia proteins have been conclusively shown to interact with specific host cell components: TomO ( Ote et al 2016 ) and WalE1 ( Sheehan et al 2016 ).…”

Cytonuclear Epistasis Controls the Density of SymbiontWolbachia pipientisin Nongonadal Tissues of MosquitoCulex quinquefasciatus

“…Identifying the genes affected by the causal QTL polymorphisms, and ultimately knowing the molecular pathways involved, would provide insight into Wolbachia –host interactions that determine Wolbachia density in host tissues. A wide variety of possible molecular pathways for such interactions can be envisioned, from host innate immunity or metabolic pathways that directly impact bacterial density to different aspects of host cell biology that might indirectly modulate Wolbachia density, such as rates of autophagy, proteolysis, or pathways involved in movement of Wolbachia between tissues ( Frydman et al 2006 ; Voronin et al 2012 ; White et al 2017a , b ).…”