Aphids are among the first insects to appear in early spring on crops. They possess incredible adaptive abilities, and under optimal conditions, reproduce very rapidly. They live by sucking the juices of plants, and if left untreated they can cause plant death. All this means trouble for growers. Annual global crop losses associated with aphids run at hundreds of millions of dollars.

"The most important direct benefits to society from this project will come from the way the project increases our ability to understand the ways that aphids interact with their host plants, the plant viruses they transmit, and their bacterial symbionts. These aspects of aphid biology directly impact food supply and pesticide use," says Alex Wilson, assistant professor of biology at the University of Miami College of Arts and Sciences, and representative for the Americas on the Board of the International Aphid Genomics Consortium, who also serves as member of the project leadership team.

The new study by the International Aphid Genomics Consortium uncovers some of the best-kept secrets of this amazing creature. The study entitled "Genome Sequence of the Pea Aphid Acyrthosiphon pisum" was published online by PLoS Biology. Genomic analysis of the pea aphid and its bacterial symbiont, Buchnera aphidicola, implicates extensive collaboration between the two partners.

"We found that the interaction of the pea aphid with its bacterial symbiont is far more intimate than anyone had previously envisioned," says Wilson. "We hypothesize, based on the genome sequence that they each compensate for the evolutionary loss of genes by shuffling essential metabolic products between them. Gene loss between the two partners is so extensive that neither one can live without the other."

Pea aphids are small, with adult sizes ranging from 4-5 mm, and are green or pink in color. They have amazing plasticity. Whenever overcrowding occurs; an aphid colony will produce winged females that migrate to establish new colonies in other areas.

Able to reproduce both sexually and asexually, aphid populations can increase in size at exponential rates: when asexual females are pregnant, their embryos are also pregnant, so females can carry both their daughters and granddaughters, a condition called telescoping of generations.

The sequencing of the pea aphid genome, funded by the National Human Genome Research Institute, resulted from a collaboration between a team at the Baylor College of Medicine led by Dr. Stephen Richards and the International Aphid Genomics Consortium, a diverse group of investigators who work together to advance understanding of the genome biology of the aphid. This study has engaged 200 scientists from 16 countries in advancing research on an insect of scientific, economic and agricultural importance.

"Having a genome opens up our world. Anything is now possible," says Wilson. "The genome provides the foundation. Now the hard work begins."

earlier related report
Successful Genome Sequencing Of Pea Aphid Is A Breakthrough For Ecology And Agricultural Research

A special issue of Insect Molecular Biology reports the detailed analyses of specific aspects of the genome of the important plant pest, the Pea Aphid. The analyses are based on the publication of the aphid genome sequence in PLoS Biology and is a major step in enhancing our understanding of insect ecology and evolution with important implications for controlling these significant plant pests.

The sequencing of the Pea Aphid, Acyrthosiphon pisum, genome is a major milestone for insect scientists. To date all insect genomes that have been sequenced have been holometabolus species, such as flies, bees, ants, butterflies and wasps. The Pea Aphid is a member of a group of insects that are more ancient than flies and bees etc. and are closely related to the wingless insects which are thought to have evolved more directly from the first insects. This unique position of the Pea Aphid within the insect tree of life will provide important keys to understanding insect biology and evolution.

"Aphids are economically very important insects as they contain a host of agricultural and forestry pests as well as some medically important species," said Professor Charles Godfrey from the University of Oxford, in the editorial of the special issue. "The pea aphid is, as the name would suggest, a pest of peas and other legumes though does not cause the major economic damage of related species such as the peach-potato aphid."

The Pea Aphid is of particular interest to ecologists as aphid populations can develop to specialise in different food plants. When a population selects a new plant to colonise the association with the plant leads to balanced gene flow which prevents further divergence and speciation to occur in the aphid population. This process of specialisation means the Pea Aphid has become a model organism for evolutionists studying specialisation and ecological speciation with the sequencing of the genome now allowing new testing of speciation theories and models.

The sequencing also allows scientists studying the spread of agricultural diseases to further understand the relationship between a virus, the host insect and the plant. Aphids are also major vectors of viral plant disease, which cause severe economic damage for agriculture. Some viruses have evolved to facilitate their transmission by aphids and the newly published genome sequence will allow scientists to understand the physiological, genetic and molecular basis of this critical interaction.

Aphids feed on plant juices which they obtain from the phloem tissue of leaves and stems using long piercing mouthparts. Phloem is rich in carbohydrates, but low in the nitrogenous compounds which complex organisms need to make proteins to survive. Aphids have a highly developed gut and the genome sequence reveals many genes for sugar transporter proteins but oddly are missing common genes involved in making some amino acids. Remarkably, symbiotic bacteria living inside the aphid provide these missing proteins.

One of the most curious findings of this sequencing project is the absence of many genes involved in defending the insect from pathogens, parasites and predators. A large part of the typical insect immune system which is well studied in other insects, is absent from the Pea Aphid. This is surprising as Pea Aphids are attacked by a variety of natural enemies ranging from fungal diseases to parasitoid wasps.

"It is likely that aphids are selected for extremely high rates of reproduction, they have to colonise a plant and produce offspring before their enemies find and exterminate them," said Godfrey. "We know there are tradeoffs between defence and other fitness components and in Aphids natural selection may have favoured reproduction over defence."

"Biologists working on the pea aphid now have a valuable new set of tools to attack novel questions," concludes Godfrey. "Studies on the pea aphid will inform our understanding of aphid biology and of insects more generally, with clear economic benefits at a time of increasing food security."

"At some point, perhaps in the near future, the publication of another insect genome may not warrant special notice, and this, no doubt, will be a reflection of how advanced our technical capabilities as molecular biologists have become," said journal editors David O'Brochta and Lin Field. "Presently, at least for Insect Molecular Biology, a new insect genome remains an exciting and significant event in which we are pleased to play a small role."

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