Data from: Facultative endosymbionts mediate dietary breadth in a polyphagous herbivore

1. Intraspecific variation in dietary breadth can influence important
ecological and evolutionary processes, yet the mechanisms generating this
variation are usually unknown. Maternally-transmitted bacterial symbionts
frequently infect insect herbivores, and many have been shown to mediate
key ecological interactions. For polyphagous herbivores, infection with
particular symbionts is often strongly correlated with feeding on
particular plant species, suggesting that facultative symbionts might
directly determine herbivore food plant specificity. However, previous
tests of this hypothesis have returned inconsistent results, providing
little empirical support for a causal relationship between facultative
symbiont infection and dietary breadth. 2. Here, we investigate whether
heritable facultative symbionts mediate dietary breadth in the polyphagous
aphid, Aphis craccivora. We first determined that asexual clones of the
aphid differ dramatically in performance across two leguminous food
plants, locust and alfalfa, and could be considered biotypes with distinct
ecological characteristics. The heritable symbiont Arsenophonus is
strongly associated with locust-origin aphids. 3. We created experimental
lines that share aphid genotypes but differed with respect to Arsenophonus
infection status, and compared performance across three food plant
species. Naturally Arsenophonus-infected clones performed 2-4× better on
locust and up to 75% worse on two alternate plant species than uninfected
controls, clearly demonstrating that Arsenophonus promotes specialization
on locust. In both laboratory and field experiments, uninfected locust-
and alfalfa-origin clones exhibited similar and modest performance on
locust, indicating that the “locust-associated biotype” wouldn't
exist without Arsenophonus. 4. We also hypothesized that moving
Arsenophonus, via transinfection, to an alfalfa-origin lineage would
improve performance on locust and serve to expand dietary breadth. Indeed,
transinfection doubled aphid performance on locust, and halved aphid
performance on alfalfa. However, because this aphid lineage naturally
performs better on alfalfa, the transinfected symbiont functionally
equalized aphid performance between locust and alfalfa, making the alfalfa
biotype more generalized. Thus, the same symbiont can either reduce or
expand dietary breadth, depending on host genotype. 5. Our results
unequivocally demonstrate that symbiont gain or loss can instantaneously
and substantially change the topology of food plant use in a polyphagous
insect, modifying diet in ways that potentially influence the
insect's ecological niche, evolutionary trajectory and pest status.