Native Regiella Endosymbionts Provide Strong Parasitoid Protection With Limited Impacts on Fitness and Virus Transmission in Myzus persicae.

A substantial literature has developed on facultative endosymbionts of insects, often portrayed as having large effects on their hosts, ranging from mutualistic to parasitic. The aphid endosymbiont Regiella insecticola occurs naturally in Myzus persicae and has potential biocontrol applications. Here, we examined the effects of native Regiella in two clones of M. persicae from Australia collected two decades apart, to assess whether these effects could explain the persistence of Regiella in natural populations. Genome sequencing revealed > 99.99% similarity between the Regiella strains from these clones. Regiella was stable in laboratory cultures and transmitted horizontally on excised leaves and intact plants. Fitness assays showed Regiella had modest costs to aphid reproduction but provided some benefits under heat stress, with no host-plant-specific effects. Regiella did not impact transmission of turnip yellows virus. Regiella provided strong protection against parasitism by two common parasitoid wasps, Diaeretiella rapae and Aphidius colemani. These findings indicate that while Regiella has limited fitness effects on M. persicae in the absence of parasitoids, its strong protection against parasitism underscores an important ecological function. However, these effects alone are not sufficient to explain the low incidence of Regiella in natural M. persicae populations, despite evidence for horizontal transfer.