The Right Hitchhiker Can Save An Aphid's Life

Peaaphids are often under attack by wasps seeking to lay their eggs insideaphids, turning them into an all-you-can-eat buffet for the larvalwasps.

The aphids vary in their resistance to the wasps, which scientists had chalked up to genetic differences between aphids.

Butit’s not in their genes at all -- the wasp-resistant aphids owe theirlives to the symbiotic bacteria carried inside them, according to newresearch.

“We knew there was a tremendous amount of variation inresistance to parasitism among different aphid lineages,” said Kerry M.Oliver, lead author on the report. “We were definitely surprised – weassumed the bulk of the resistance was due to the aphids’ genotype.”

Figuringout what makes insects resistant to natural enemies is important tofarmers. Pea aphids can be agricultural pests on plants in the peafamily including lentils and peas.

The new finding suggests why some lineages of aphids readily succumb to the wasps and others don't.

“Thiswork shows the difference can be attributed to the symbionts, not tothe aphid genotype,” said Oliver, a postdoctoral research associate inThe University of Arizona’s department of entomology and a member ofUA's Center for Insect Science.

Oliver added that this impliesaphids could acquire resistance to natural enemies by picking upbacterial symbionts, rather than having changes in the aphids' genes.Such a newly acquired resistance is heritable, because the bacteria getpassed down from mother to her offspring.

The article by Oliverand his co-authors Nancy A. Moran, UA Regents' Professor of ecology andevolutionary biology, and Martha S. Hunter, UA associate professor ofentomology, is published this week in the online early edition of theProceedings of the National Academy of Sciences. The U.S. Department ofAgriculture funded the research.

Many insects harbor bacterialsymbionts that are passed from mother to offspring. LIttle is knownabout whether such symbionts can boost the host insects' resistance tonatural enemies.

As part of his doctoral research in UA'sInterdisciplinary Program in Insect Science, Oliver investigated thepea aphid, Acyrthosiphon pisum, an aphid known to vary in resistance toone of its major enemies, the parasitoid wasp Aphidius ervi. The aphidscan also host one of several bacterial symbionts known as secondarysymbionts.

One of his previous experiments indicated thesymbiotic bacteria contribute at least some of the observed variationin resistance to the wasps.

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

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

Oliver injectedseveral uninfected lineages of aphids with a secondary symbiont calledHamiltonella defensa. As a result, he had five genetically distinctcolonies of pea aphids that all carried Hamiltonella defensa.

Totest resistance to the wasps, he caged the different groups of infectedaphids on individual fava bean plants by inverting plastic drink cupsover the potted plants with aphids. Each plant had 30 aphids, all withthe same bacteria inside.

Then he introduced a female wasp intoeach enclosure. Each wasp set about laying one egg per aphid in as manyaphids as she could manage in the time allotted. Oliver removed thewasps after 6 hours.

Ten days later, he looked for evidence ofwasp parasitism by counting the "mummies" -- the golden, hardenedcarcasses of aphids that had become homes for wasp larvae.

Olivercompared the rates of parasitism in uninfected aphids to infectedaphids and found that carrying the H. defensa symbiont reducedparasitism by about 40 percent, no matter what the aphid's genotype.

Healso did another experiment by taking uninfected aphids and injectedthem with various different secondary symbionts, so he had fivecolonies of pea aphids that were genetically identical but harboreddifferent bacteria.

In this case, the infected aphids varied from 19% to almost 100% resistance, depending on which symbiont they carried.

The experiments show that aphids' variation in resistance to wasp parasitism stems from their bacteria, not their genes.

"Aphidsare just completely nailed by natural enemies. Having these symbioticdefenders is really important to the aphids," Oliver said.

"Nowwe know symbionts confer resistance, but we don't know the mechanism ofresistance -- we don't know the how." He said figuring that out is theteam's next step.