Native endosymbiont Regiella insecticola in Myzus persicae is stable, horizontally transmitted, and provides protection against parasitoids, but has limited fitness impacts and no effect on virus transmission

A substantial literature has developed on facultative endosymbiotic bacteria of insects, particularly aphids. These endosymbionts are often portrayed as having large effects on their hosts, and mostly seen as mutualistic but also occasionally as parasitic. The aphid endosymbiont, Regiella insecticola (hereafter referred to as Regiella), occurs naturally in Myzus persicae and has potential biocontrol applications due to its virus inhibiting properties and fitness effects on aphids. Here, we examined the effects of native Regiella in two clones of M. persicae from Australia collected two decades apart. Whole-genome sequencing revealed >99.99% similarity between the Regiella strains from these clones. Fitness assays showed Regiella to have modest costs to aphid reproduction but to provide some fitness benefit under heat stress. Across multiple host plant species, the effects of Regiella were not specific to the aphids’ host plant, and the infection did not impact transmission of Turnip yellows virus in canola (Brassica rapa). Regiella was stable in laboratory cultures and transmitted horizontally on both excised leaves and intact plants of bok choy (Brassica rapa subsp. chinensis). Consistent with other published results, 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 parasitioids, its strong protection against parasitism underscores a potentially important ecological function. Why Regiella remains at a very low incidence in natural M. persicae populations despite such this effect as well as horizontal and stable vertical transmission remains an open question.