DOI: 10.5061/dryad.2jm63xsmn
title: Endosymbionts facilitate rapid evolution in a polyphagous herbivore

description: Maternally transmitted bacterial symbionts can be important mediators of
 the interactions between insect herbivores and their foodplants. These
 symbionts are often facultative (present in some host individuals but not
 others) and can have large effects on their host’s phenotype, thus giving
 rise to heritable variation upon which selection can act. In the cowpea
 aphid (Aphis craccivora) it has been established that the facultative
 endosymbiont Arsenophonus improves aphid performance on black locust trees
 (Robinia pseudoacacia) but not on fava (Vicia faba). Here, we tested
 whether this fitness differential translated into contemporaneous
 evolution of aphid populations associated with the different plants. In a
 laboratory study lasting 16 weeks, we found that the frequency of
 Arsenophonus-infected individuals significantly increased over time for
 aphid populations on black locust but declined for aphid populations on
 fava. By the end of the experiment, Arsenophonus infection was &gt;3×
 more common on black locust than fava, which is comparable to previously
 described infection frequencies in natural field populations. Our results
 clearly demonstrate that aphid populations with mixed facultative symbiont
 infection status can rapidly evolve in response to the selective
 environments imposed by different host plants. This selection differential
 may be a sufficient explanation for the global association between
 Arsenophonus-infected cowpea aphids and black locust trees, without
 invoking additional assortative mechanisms. Because the aphid and plant
 originate from different parts of the world, we further hypothesize that
 Arsenophonus infection may have acted as a preadaptation that has promoted
 functional specialisation of infected aphids on a novel host plant.
description: All replicate aphid clones derived from a single Arsenophonus
 infected female, LW, originally described in Wagner et al. 2015 Functional
 Ecology 29: 1402-1410. The metadata for the accession and the curing
 process used to generate the Arsenophonus- isoline are described in that
 paper.  Twelve replicate populations were initiated, each with 20 Ars+ and
 20 Ars- aphids at time 0.  Half of these populations (1-6) were caged on
 fava (Vicia faba) and half (7-12) on locust (Robinia
 pseudoacacia).  Every two weeks (until week 16) one
 subset of 20 aphids per population were transferred to a fresh plant of
 the same species, and a second subset was transferred to 95% ethanol and
 had their DNA extracted.  Tab1 provides the data associated with each
 individual extraction. Extraction codes (column E) indicate the population
 (column A) and individual specimen (column C). Each extraction was checked
 for the presence (column F) or absence (column G) of Arsenophonus via
 diagnostic PCR for a segment of the Arsenophonus 23S gene, and were
 checked for extraction quality via diagnostic PCR for the aphid COI gene. 
 Any extractions  that did not yield COI product were considered failed
 extractions (column H). Each week's (column D) set of aphids per
 population represent 20 new aphids; aphid numbers (Column C) were randomly
 assigned and bear no particular significance, and no relationship to one
 another across populations, treatments or weeks.