Introduction
Aphids (Hemiptera: Aphididae) are plant sap-feeding insects that
interact with a range of microbial mutualists and pathogens. Almost all
aphid species carry the obligate nutritional symbiont, Buchnera
aphidicola, which is maternally transmitted and upgrades the
nutritional profile of plant phloem (Douglas 2009). Nine facultative,
heritable symbionts have also been identified in different species that
can confer conditional benefits including defense against parasitic
wasps and fungal pathogens (Oliver et al. 2010, Oliver and Martinez
2014, Vorburger 2014, Guo et al. 2017). Aphids frequently encounter
viruses in the environment, including economically-important plant
viruses that they vector (Brault et al. 2010) and pathogenic viruses
specialized on aphids. The latter include single-stranded DNA viruses in
the family Parvoviridae (Piccovirales) and positive-sense ssRNA viruses
in the families Picornaviridae and Dicistroviridae (Picornavirales) (Van
der Wilk et al. 1997, Moon et al. 1998, van Munster et al. 2003, Ryabov
2007, Ryabov et al. 2009, Asgari and Johnson 2010, Liu et al. 2016).
However, little is known about the interactions among the many
protective symbionts in aphids and pathogenic viruses.
Acyrthosiphon pisum virus (APV), is a picorna-like virus that
persistently infects the pea aphid, Acyrthosiphon pisum (Van den
Heuvel et al. 1997). The 10 kb APV genome contains two open reading
frames (P1, P2) encoding a protease, helicase, RNA-dependent RNA
polymerase and capsid protein (Van der Wilk et al. 1997). While
primarily detected in epithelial cells of the gut and salivary glands,
APV has also been weakly detected in the ovaries of pea aphids (Van den
Heuvel et al. 1997, Lu et al. 2020). APV is vertically transmitted at
moderate rates, and can be horizontally transferred from aphids to
plants (Van den Heuvel et al. 1997, Lu et al. 2020), but no studies have
shown aphid acquisition of APV from plants. APV exerts variable effects
on aphid growth, survival and reproduction (Van den Heuvel et al. 1997,
Lu et al. 2020), but how pea aphid genotype and facultative symbionts
influence APV infection has not previously been investigated.
Hamiltonella defensa (Yersiniaceae: γ-Proteobacteria) is one of
the most studied facultative symbionts in pea aphids because certain
strains confer high levels of resistance against parasitoid wasps likeAphidius ervi (Hymenoptera: Braconidae) (Oliver and Higashi
2019). Protective strains of H. defensa further host specific
variants of a bacteriophage named APSE which as a provirus expresses
virulence genes that have been implicated in disabling parasitoid
development (Oliver et al. 2009, Brandt et al. 2017, Lynn-Bell et al.
2019, Rouïl et al. 2020, Boyd et al. 2021). H. defensa strains
infected by APSE-3 confer high levels of protection (> 85%
of parasitized aphids survive) while strains infected by APSE-2 or
APSE-8 provide moderate protection (40-60%) (Weldon et al. 2013, Oliver
and Higashi 2019). Aphid genotype also contributes to resistance to
parasitoids (Martinez et al. 2018). Another facultative symbiont
associated with pea aphids, Regiella insecticola , is closely
related to H. defensa (Patel et al. 2019) and confers protection
against specialist entomopathogenic fungi like Pandora neoaphidis(Entomophthorales) (Scarborough et al. 2005, Parker et al. 2013). Levels
of protection conferred by R. insecticola also vary with symbiont
strain and host genotype (Parker et al. 2017).
We recently discovered an APV in certain pea aphid lines maintained in
our laboratory. In this study, we assembled the genome of this APV and
conducted assays that examined transmission and fitness effects on
aphids by generating genetically homogeneous lines that controlled aphid
and Buchnera genotypes while manipulating facultative symbiont
presence and APV infection. We report that aphid fitness was adversely
affected in aphids without facultative symbionts, effects which were
worsened in aphid carrying H. defensa . In contrast, negative
fitness impacts were ameliorated in aphids hosting R.
insecticola . Overall, our results identify heretofore unknown
interactions between facultative symbionts and a pathogenic virus that
strongly influence host fitness.