Horizontal Transmission of Microbial Symbionts Within a Guild of Fly Parasitoids

Many insects harbor facultative microbial symbionts which affect the ecology of their hosts in diverse ways. Most symbionts are transmitted vertically with high fidelity, whereas horizontal transmission occurs rarely. Parasitoid larvae feed on a single host and are in close physical contact with it, providing an ecological opportunity for symbionts' horizontal transmission, but there is little empirical evidence documenting this. Here we studied horizontal transmission of three bacterial symbionts-Rickettsia, Sodalis, and Wolbachia-between three fly pupal ectoparasitoid species: Spalangia cameroni, S. endius, and Muscidifurax raptor. Muscidifurax raptor readily parasitized and successfully developed on the Spalangia spp., while the inverse did not happen. The two Spalangia spp. attacked each other and conspecifics in very low rates. Symbiont horizontal transmissions followed by stable vertical transmission in the recipient species were achieved, in low percentages, only between conspecifics: Wolbachia from infected to uninfected M. raptor, Rickettsia in S. endius, and Sodalis in S. cameroni. Low frequency of horizontal transmissions occurred in the interspecific combinations, but none of them persisted in the recipient species beyond F₄, at most. Our study is one of few to demonstrate symbionts' horizontal transmission between hosts within the same trophic level and guild and highlights the rarity of such events.