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Bacteria are an aphid's best friend, say researchers

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Under embargo for
19.00 GMT / 14.00 EST
Thursday 15 December 2005

An aphid's best weapon for survival is a bacterium that lives within its body, according to new research published today.

Writing in the journal Science, biologists report that the presence within the pea aphid of the bacterium Regiella insecticola can dramatically enhance its resistance to one of its major enemies, a fungal pathogen that, under the right conditions, can wipe out 80% of a population.

Pea aphids killed by the fungus Pandora neoaphidis

The fungus Pandora neoaphidis causes death in the 3 millimetre long pea aphid within a few days of infection by invading and filling its body cavity. The fungus' infectious spores also germinate on the aphids' outer body shell, enabling it to move quickly through a population.

The research team at Imperial College London tested whether the presence of Regiella enhances resistance by injecting pea aphid clones free of the bacterium with body fluids from insects carrying it. After several generations, the aphids were tested for resistance to the fungus and were found to have greatly increased their ability to survive exposure to the pathogen.

Researchers also found that the presence of the bacterium greatly decreased Pandora's ability to successfully develop its spores on aphids it had killed, further diminishing its ability to decimate a population. Lead author Dr Julia Ferrari comments:

"Preventing the fungus from sporulating improves the survival chances of an aphid's family, even though it doesnt help the individual that has succumbed to the pathogen. Its important from a communal point of view."

Having proved the benefits of carrying Regiella to pea aphids, the research team remains mystified as to why all aphids do not have a relationship with this bacterium. One possibility, Dr Ferrari speculates, is that the bacterium may be costly to its host in other ways.

"We have not yet identified any costs to the aphid, but it's possible that the advantage of disease resistance varies over time and in the absence of Pandora, there is no benefit to hosting the bacterium and potentially some disadvantages," she says.

The team now expects to find that this symbiotic relationship is replicated in other organisms. Dr Ferrari adds:

"Over the past few years, it has been shown that many aphid species have formed intimate associations with bacteria which influence their ability to defend themselves against natural enemies. These relationships are proving to be far more complex than previously thought. If we find similar associations in a wide range of other organisms as expected, we will be able to build a very in depth understanding of the dynamics of populations."

For further information contact:

Abigail Smith
Imperial College London Press Office
Tel: 020 7594 6701

Notes to editors:

'Aphid Protected from Pathogen by Endosymbiont', Claire Scarborough, Julia Ferrari, H C J Godfray, Centre for Population Biology, Imperial College London.
Science, 16 December 2005

About the pea aphid

The pea aphid is common in Europe and also the United States and Canada in areas where legumes are cultivated. It infests garden and field peas, sweet peas, sweet clover, alfalfa and some leguminous weeds.

Adults are soft bodied and low-moving, and grow to about 3 mm long and 1.5 mm wide. It has a complex life cycle, reproducing asexually during spring and summer, while in autumn increasing night length and decreasing temperatures induce the production of sexual morphs. The sexually produced eggs overwinter and in spring the fundatrices (first generation of asexual females) of the next generation emerge.

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