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."