Strain variation shapes endosymbiont community structure and, possibly, transmission rates in the pea aphid

Host-associated microbes live as members of multi-species symbiotic communities. Interactions among co-infecting microbes are, hence, expected, with a potential to shape the stable maintenance and spread of symbiotic associates. The pea aphid hosts a fairly simple complement of endosymbionts, including one obligate and seven non-essential bacterial species, with known defensive roles against natural enemies. Previous work has shown nearly perfect maternal transmission of such bacteria in the lab. But recent findings suggest rates are lower for field-caught aphids. Intriguingly, field-caught aphids with common pairings of endosymbiont species transmit their endosymbionts at high efficiency; those harboring rarely partnered endosymbiont species transmit them with lower efficiency. In an effort to better quantify this phenomenon, we characterized strain diversity of all eight endosymbiont species in North American pea aphids, by genotyping bacteria at several protein-coding loci. Most species are comprised of genetically distinct strains. The most prevalent endosymbiont species, Hamiltonella defensa, is comprised of five common strains, which show variable tendencies to co-infect with other endosymbionts. In an initial study of the mechanism driving this pattern, we genotyped endosymbionts from aphids obtained in a prior study of field transmission. Our preliminary findings argue that strain-specific patterns of endosymbiont community structure are shaped by context- and strain-dependent rates of vertical transmission. Combined, this work points to within-host bacterial interactions as likely drivers of endosymbiont community stability, raising important questions on the potential for competition and cooperation among non-essential, defensive endosymbionts found so commonly across the world’s insects.