Vertical transmission at the pathogen-symbiont interface: Serratia symbiotica and aphids
Many insects possess beneficial bacterial symbionts that occupy specialized host cells and are maternally transmitted. As a consequence of their host-restricted lifestyle, these symbionts often possess reduced genomes and cannot be cultured outside hosts, limiting their study. The bacterial species Serratia symbiotica was originally described by noncultured strains that live as mutualistic symbionts of aphids and are vertically transmitted through transovarial endocytosis within the mothers body. More recently, culturable strains of S. symbiotica were discovered that retain a larger set of ancestral Serratia genes, are gut pathogens in aphid hosts, and are principally transmitted via a fecal-oral route. We find that these culturable strains, when injected into pea aphids, replicate in the hemolymph and are pathogenic. Unexpectedly, they are also capable of maternal transmission via transovarial endocytosis: using GFP-tagged strains, we observe that pathogenic S. symbiotica, but not Escherichia coli, are endocytosed into early embryos. Furthermore, pathogenic S. symbiotica strains are compartmentalized into specialized aphid cells in a similar fashion to mutualistic S. symbiotica strains during later stages of embryonic development. Thus, cultured, pathogenic strains of S. symbiotica have the latent capacity to transition to lifestyles as mutualistic symbionts of aphid hosts. This capacity is blocked by pathogenicity: their hosts die before infected progeny are born. To transition into stably inherited symbionts, culturable S. symbiotica strains may need to adapt to regulate their titer, limit their pathogenicity, and/or provide benefits to aphids that outweigh their cost. ImportanceInsects have evolved various mechanisms to reliably transmit their beneficial bacterial symbionts to the next generation. Sap-sucking insects, including aphids, transmit symbionts by endocytosis of the symbiont into cells of the early embryo within the mothers body. Experimental studies of this process are hampered by the inability to culture or genetically manipulate host-restricted, symbiotic bacteria. Serratia symbiotica is a bacterial species that includes strains ranging from obligate, heritable symbionts to culturable gut pathogens. We demonstrate that culturable S. symbiotica strains, that are aphid gut pathogens, can be maternally transmitted by endocytosis. Cultured S. symbiotica therefore possess a latent capacity for evolving a host-restricted lifestyle and can be used to understand the transition from pathogenicity to beneficial symbiosis.