The genome of the pea aphid Acyrthosiphon pisum, a parasite of leguminous plants that causes major crop infestations, has been sequenced by the International Aphid Genomics Consortium (IAGC), an international group of scientists that includes the University of Barcelona researchers Julio Rozas and Filipe G. Vieira from the Department of Genetics and the Biodiversity Research Institute. The results are published today in the journal PLoS Biology.
A. pisum is the first aphid to be investigated in genetic studies, and was selected for the speed with which it reproduces and its easy handling. It is the first of the Hemiptera with incomplete metamorphosis for which the complete genome sequence has been described. The pea aphid has a complex life cycle characterized by alternate sexual and asexual reproduction, and is an excellent example of high phenotypic plasticity in response to environmental conditions (polyphenism). The aphid also has a symbiont organism – the bacteria Buchnera aphidicola – with which it has coevolved, making it an ideal model for the study of interactions with microorganisms.
Professor Julio Rozas, a member of the Molecular Evolutionary Genetics Research Group at the UB, explains that, “The pea aphid is responsible for crop infestations that cause severe losses in the agriculture sector. In phylogenetic terms, it is very closely related to other highly damaging aphid species such as the green peach aphid and the Russian wheat aphid. A complete understanding of the genome will enable scientists to develop more effective strategies for preventing infestations and to carry out further research into models of insecticide resistance. The general findings will be applicable to other phylogenetically-related species such as the Phylloxera, which caused devastation to vines across Europe towards the end of the 19 th Century”.
A small genome, making sequencing a simpler task
The complete genome sequence has so far been obtained for twenty arthropod species from around the world. A. pisum has a small haploid genome (464 Mb), although this is larger than found in other insects studied to date, and presents four holocentric chromosomes. The research indicates that the pea aphid genome has a larger number of genes than those of other insects, and exhibits a high degree of gene duplication that could account for its high phenotypic plasticity. It has also been found that the relationship between the pea aphid and its symbiont bacteria B. aphidicola is a genetic one, and not simply a high level of metabolyte exchange between the two organisms. As Rozas explains, “This is the first complete genome sequence of an animal species with a symbiont organism, and this gives us a genetic model through which to study the coordination of gene products between host and symbiont”.
Genetics of the sensory processing of olfactory signals
The UB researchers involved in the project, who also contributed to sequencing the genome of the fruit fly Drosophila, used bioinformatic techniques to analyse the genes associated with the chemoreceptive system of the insect, focusing particularly on the multigene families encoding odour-binding proteins (OBPs) and chemosensory (CSPs). “The pea aphid's chemoreceptive system is fundamental to the recognition of the surrounding environment and in determing reactions to chemical signals, finding food, reproduction and nutrition”, says Rozas, explaining that, “The OBPs play a vital role in the reception of olfactory signals, as they are the first point of contact between the insect and its surroundings and regulate its relationship with the environment.” The UB team, which is investigating the evolutionary dynamics of this gene family in various organisms, has discovered that the pea aphid has fewer genes encoding OBPs than other insect species.
The research has also revealed that the evolution of this type of gene family conforms to the “ birth-and-death” model, which describes a highly dynamic evolutionary process in which genes are generated and lost continuously. Rozas explains that, “One of the most significant discoveries of this study is this high degree of gene exchange, which could have a major biological impact on the adaptation of the species. [...] The birth-and-death model, in which genes are born through duplication and lost through genome deletion, could be a far more important phenomenon that we thought only a few years ago”. Experts believe that new understanding the pea aphid genome will make it easier to develop specific insecticides and to define more effective strategies for preventing crop infestations, as well as providing a basis for evolutionary studies of other species.
A genome deciphered by an international consortium
The International Aphid Genomics Consortium (IAGC), created in Paris in 2003 by a collection of international research groups, aims to unlock the potential of aphids for use as molecular, cellular and biological models genetic and evolutionary studies (similar to the case of Drosophila, etc). The consortium is directed by a team from the Human Genome Sequencing Center at Baylor College of Medicine (Houston), and other members include the University of Barcelona and teams of experts from Center for Genomic Regulation (CRG-UPF), the University of Valencia, and the Department of Health of the Valencian government.
The article can be viewed at: Genome Sequence of the Pea Aphid Acyrthosiphon pisum.
The International Aphid Genomics Consortium. PLoS Biology, February 2010 issue.
