A Rickettsiella transinfection in Rhopalosiphum padi reduces fitness and alate production but not plant virus transmission

BACKGROUND Native bacterial endosymbionts in aphids have been studied for many years but it is only recently that transinfections across species are being investigated from an applied perspective. Here we consider the impact of a Rickettsiella viridis transinfection originally from the pea aphid Acyrthosiphon pisum , in an important pest of cereals, Rhopalosiphum padi , that causes feeding damage and transmits barley yellow dwarf virus (BYDV). Our main aims were to quantify the fitness and dispersal consequences of the transinfection, assess its transmission dynamics, and determine whether Rickettsiella influences BYDV acquisition or transmission. RESULTS The transinfected strain had fitness costs in its new aphid host, with an intrinsic rate of increase ( r m ) value around 20% lower, and showed horizontal transmission. Rickettsiella did not transmit vertically with complete fidelity, although it persisted in population cages for at least 11 weeks. Although Rickettsiella did not affect transmission of BYDV, it reduced the production of alates by 10% or more depending on aphid density. Aphids carrying Rickettsiella showed a slower rate of movement to adjacent plants compared with those without Rickettsiella . The body colour of aphids with Rickettsiella was also darker. CONCLUSION This Rickettsiella transinfection imposes deleterious host effects, while retaining the capacity to persist in populations through horizontal transmission. Although it does not influence BYDV transmission, the reduced alate formation and slower movement suggest potential impacts on pest spread and population structure. These findings advance our understanding of symbiont–host interactions and highlight the potential for endosymbiont manipulations to influence aphid ecology and management.