Aphid facultative symbionts confer no protection against the fungal entomopathogen Batkoa apiculata.

Fungi in the family Entomophthoraceae are prevalent pathogens of aphids. Facultative symbiotic bacteria harbored by aphids, including Spiroplasma sp. and Regiella insecticola, have been shown to make their hosts more resistant to infection with the fungal pathogen Pandora neoaphidis. How far this protection extends against other species of fungi in the family Entomophthoraceae is unknown. Here we isolated a strain of the fungal pathogen Batkoa apiculata infecting a natural population of pea aphids (Acyrthosiphon pisum) and confirmed its identity by sequencing the 28S rRNA gene. We then infected a panel of aphids each harboring a different species or strain of endosymbiotic bacteria to test whether aphid symbionts protect against B. apiculata. We found no evidence of symbiont-mediated protection against this pathogen, and our data suggest that some symbionts make aphids more susceptible to infection. This finding is relevant to our understanding of this important model of host-microbe interactions, and we discuss our results in the context of aphid-microbe ecological and evolutionary dynamics.