Frequent intertrophic transmission ofWolbachiaby parasitism but not predation

Wolbachiais one of the most pervasive symbionts, estimated to infect ∼50% of arthropod species. It is primarily transmitted vertically, inducing a variety of fascinating reproductive manipulations to promote its spread within host populations. However, incongruences between host andWolbachiaphylogenies indicate substantial horizontal transmissions, the mechanisms of which are largely unexplored. By systematically analyzingWolbachiasurface protein (wsp) sequences on NCBI, we found that parasitism, not predation, is the primary route of intertropicalWolbachiatransmission. This conclusion held after accounting sampling bias. One example of frequentWolbachiatransfers is between egg parasitoid wasps,Trichogramma, and their lepidopteran hosts. Moreover, both bioinformatics and experimental results showed thatWolbachiafrom the parasitoid waspEncarsia formosacan be transmitted to its whitefly hostBemisia tabaci, through unsuccessful parasitism. OnceEn. formosa Wolbachiais transferred to whiteflies, it can be vertically transmitted within whiteflies and induce fitness costs. To our knowledge, this is the first compelling evidence thatWolbachiacan be transmitted from parasitoid wasps to their hosts, revealing the bidirectional nature ofWolbachiatransfers between parasitoids and their hosts. Overall, our findings enrich the current understanding of the horizontal transmission ofWolbachiaand shed new light on its ecology and evolution.