Acyrthosiphon pisum, commonly known as the pea aphid, is a sap-sucking insect in the Aphididae family. It feeds on several species of legumes (plant family Fabaceae) worldwide, including forage crops, such as pea, clover, alfalfa, and broad bean, and ranks among the aphid species of major agronomical importance. The pea aphid is a model organism for biological study whose genome has been sequenced and annotated.
Generalities and life cycle
In the autumn, female pea aphids lay fertilized eggs that overwinter and hatch the following spring. The nymphs that hatch from these eggs are all females, which undergo four moults before reaching sexual maturity. They will then begin to reproduce by viviparous parthenogenesis, like most aphids. Each adult female gives birth to four to 12 female nymphs per day, around a hundred in her lifetime. These develop into mature females in about seven to ten days. The life span of an adult is about 30 days.
Population densities are at their highest in early summer, then decrease through predation and parasitism. In autumn, the lengthening of the night triggers the production of a single generation of sexual individuals (males and oviparous females) by the same parthenogenetic parent females. Inseminated sexual females will lay overwintering eggs, from which new parthenogenetic females will emerge in early spring. When the colony begins to become overcrowded, some winged females are produced. These disperse to infest other plants, where they continue to reproduce asexually. When temperatures become colder and day lengths shorter, sexual winged females and males appear. These mate, the females lay diapausing eggs and the life cycle starts again. Pea aphids can complete their whole reproductive cycle without shifting host plant.
Several morphs exists in pea aphids. Besides differences between sexual and parthenogenetic morphs, winged and wingless morphs exist. Overcrowding and poor food quality may trigger the development of winged individuals in subsequent generations. Winged aphids can then colonize other host plants. Pea aphids also show hereditary body color variations of green or red/pink. The green morphs are generally more frequent in natural populations.
Acyrthosiphon pisum is a rather large aphid whose body can reach 4mm in adults. It generally feeds on the lower sides of leaves, buds and pods of legumes, ingesting phloem sap through its stylets. Unlike many aphid species, pea aphids do not tend to form dense colonies where individuals would stay where they were born during their whole lifetimes. Pea aphids are not known to be farmed by ants that feed on honeydews.
More than 20 legume genera are known to host pea aphids, though the complete host range remains undetermined. On crops such as peas and alfalfa, A. pisum is considered among the aphid species or major agronomical importance. Yields can be affected by the sap intake that directly weakens plants, although pea aphids seldom reach densities that might significantly reduce crop production. However, like many aphid species, A. pisum can be a vector of viral diseases to the plants it visits. Protection against pea aphids includes the use of chemical insecticides, natural predators and parasitoids, and the selection of resistant cultivars. No insecticide resistance is documented in A. pisum, as opposed to many aphid pests.
Pea aphids, although collectively designated by the single scientific name A. pisum, encompass several biotypes described as cryptic species, subspecies or races, which are specialized on different host species. Therefore, the pea aphid is more accurately described as a species complex.
The pea aphid is thought to be of Palearctic origin, but it is now commonly found worldwide under temperate climate. The spread of A. pisum probably resulted from the introduction of some of its host plants for agriculture. Such an introduction likely occurred into North America near the 1870s.
A. pisum is considered as the model aphid species. Its reproductive cycle, including the sexual phase and the overwintering of eggs, can be easily completed on host plants under laboratory conditions, and the relatively large size of individuals facilitates physiological studies. In 2010, the International Aphid Genomics Consortium published an annotated draft sequence of the pea aphid genome  composed of approximately 525 megabases and 34000 predicted genes in 2n=8 chromosomes. This constitutes the first genome of an hemimetabolous insect to have been published. The pea aphid genome and other of its features are the focus of studies covering the following areas:
- Symbiosis with bacteria - As all aphididae, A. pisum hosts the primary endosymbiont Buchnera aphidicola, which provides essential amino acids and is necessary for aphid reproduction. Buchnera is transmitted from mothers to offspring, and it has coevolved with aphids for dozens of million of years. A. pisum also hosts a range of facultative bacterial symbionts that can be transmitted maternally and horizontally, and which affect ecologically important traits in aphids, such as body color, resistance to abiotic and biotic stress, and nutrition.
- Polyphenism (the production of several discrete morphs by the same genotype) - Studies on pea aphids have helped to establish the environmental and genetic components controlling the production of sexual and winged morphs, among other features.
- Asexual reproduction - Pea aphid lineages include parthenogenesis in their life cycles, and some have even lost the sexual phase. Pea aphids are models for deciphering the origin and consequences of asexual reproduction, an important question in evolutionary biology.
- Polymorphism and physiology explaining phenotypic variations in aphids - Loci and physiological mechanisms underlying body color, reproductive cycle and the presence of wings in males (which is genetically based) have been identified in pea aphids or are being investigated. A. pisum is notable for being the only animal organism so-far identified that has the ability to synthesize a carotenoid. Plants, fungi, and microorganisms can synthesize carotenoids, but torulene (3',4'-didehydro-β,γ-carotene, specifically a hydrocarbon carotene) made by pea aphids, is the only carotenoid known to be synthesized by an organism in the animal kingdom. Torulene imparts natural, red-colored patches to some aphids, which possibly aid in their camouflage and escape from predation by wasps. The aphids have gained the ability to synthesize torulene by horizontal gene transfer of a number of genes for carotenoid synthesis, apparently from fungi.
- Gene duplication and expansion of gene families - The pea aphid genome presents high levels of gene duplication compared to other insect genomes, such as Drosophila, with the notable expansion of some gene families.
- Interaction with host plants and speciation - As most phloem feeders, the pea aphid is adapted to feeding on a limited set of plants. Studies on pea aphids have identified candidate loci, molecular and physiological mechanisms that are involved in host nutrition and virulence. Genetic, molecular and physiological studies have also evidenced specialization to different host species as a motor of ecological speciation between pea aphid biotypes.
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