Genetic manipulation allows in vivo tracking of the life cycle of the son‐killer symbiont, Arsenophonus nasoniae, and reveals patterns of host invasion, tropism and pathology

Abstract

Summary Maternally heritable symbionts are common in arthropods and represent important partners and antagonists. A major impediment to understanding the mechanistic basis of these symbioses has been lack of genetic manipulation tools, for instance, those enabling transgenic GFP expression systems for in vivo visualization. Here, we transform the ‘son‐killer’ reproductive parasite Arsenophonus nasoniae that infects the parasitic wasp Nasonia vitripennis with the plasmid pOM1‐gfp, re‐introduce this strain to N. vitripennis and then used this system to track symbiont life history in vivo. These data revealed transfer of the symbiont into the fly pupa by N. vitripennis during oviposition and N. vitripennis larvae developing infection over time through feeding. A strong tropism of A. nasoniae to the N. vitripennis ovipositor developed during wasp pupation, which aids onward transmission. The symbiont was also visualized in diapause larvae. Occasional necrotic diapause larvae were observed which displayed intense systemic infection alongside widespread melanotic nodules indicative of an active but failed immune response. Our results provide the foundation for the study of this symbiosis through in vivo tracking of the fate of symbionts through host development, which is rarely achieved in heritable microbe/insect interactions.

Transitions in symbiosis: evidence for environmental acquisition & social transmission within a clade of heritable symbionts

“…First, the absence of the bacterium in eggs of infected colonies corroborates previous findings that Arsenophonus is not transovarially transmitted in honey bees [43], while very low prevalence in early host life stages provides new evidence that no alternative routes of VT are operating in the association. Secondly, the pronounced seasonal dynamics we observed for symbiont that also has phases outside of the wasp host [68], is culturable [28], and is also maintained by a combination of vertical and infectious transmission [13]. Arsenophonus nasoniae and the bee Arsenophonus also share the capacity to establish through the gut [68], indicating this pathway is preserved in the clade.…”

“…Secondly, the pronounced seasonal dynamics we observed for symbiont that also has phases outside of the wasp host [68], is culturable [28], and is also maintained by a combination of vertical and infectious transmission [13]. Arsenophonus nasoniae and the bee Arsenophonus also share the capacity to establish through the gut [68], indicating this pathway is preserved in the clade. The genome degradation process that occurs in the obligate symbionts in the genus imply that the bee Arsenophonus/A.nasonaie lineage within the clade has retained the extracellular and environmental growth capacities found in a free-living ancestor.…”

Vertical transmission at the pathogen-symbiont interface:Serratia symbioticaand aphids

“…Focusing on secondary symbionts may be more useful for understanding these early transitions, as some secondary symbionts or their close relatives are culturable, genetically tractable, and can be removed from or introduced to hosts without dramatically compromising host fitness (48)(49)(50). Cultured S. symbiotica strains are close relatives to non-culturable, vertically transmitted mutualists.…”

Engineering a culturableSerratia symbioticastrain for aphid paratransgenesis

“…This feature simplifies large assays where colonization is required, such as mortality or transmission experiments. Similar work engineering Arsenophonus nasoniae, a parasitoid wasp symbiont, to express GFP allowed researchers to better understand how it is vertically transmitted during oviposition (36). In the future, the ability to engineer CWBI-2.3 T and deliver it back to aphids could be combined with engineering tools for gene knockout.…”