Many insects harbor bacterial symbionts that are passed from mother to offspring. LIttle is known about whether such symbionts can boost the host insects' resistance to natural enemies.

As part of his doctoral research in UA's Interdisciplinary Program in Insect Science, Oliver investigated the pea aphid, Acyrthosiphon pisum, an aphid known to vary in resistance to one of its major enemies, the parasitoid wasp Aphidius ervi. The aphids can also host one of several bacterial symbionts known as secondary symbionts.

One of his previous experiments indicated the symbiotic bacteria contribute at least some of the observed variation in resistance to the wasps.

"But we still thought most of the variation in resistance was due to the aphids' genotype," he said.

To tease out how much of the resistance was from the aphids' genes and how much from their symbionts, Oliver did another series of experiments. He took advantage of the fact that in the lab, aphids reproduce asexually -- female aphids produce more female aphids who produce more female aphids -- which means the offspring are identical genetically, generation after generation after generation.

Oliver injected several uninfected lineages of aphids with a secondary symbiont called Hamiltonella defensa. As a result, he had five genetically distinct colonies of pea aphids that all carried Hamiltonella defensa.

To test resistance to the wasps, he caged the different groups of infected aphids on individual fava bean plants by inverting plastic drink cups over the potted plants with aphids. Each plant had 30 aphids, all with the same bacteria inside.

Then he introduced a female wasp into each enclosure. Each wasp set about laying one egg per aphid in as many aphids as she could manage in the time allotted. Oliver removed the wasps after 6 hours.

Ten days later, he looked for evidence of wasp parasitism by counting the "mummies" -- the golden, hard
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Contact: Mari N. Jensen
mnjensen@email.arizona.edu
520-626-9635
University of Arizona
25-Aug-2005