Pea Aphid

The summary provides information about pea aphids (Acyrthosiphon pisum), including their generalities and life cycle. Female pea aphids lay fertilized eggs in the autumn, which hatch in the spring. The nymphs undergo four moults before reaching sexual maturity and begin reproducing through viviparous parthenogenesis. Population densities peak in early summer but decrease due to predation and parasitism. Pea aphids can complete their reproductive cycle without changing host plants. The summary also discusses the use of chemical insecticides, natural predators, and resistant cultivars for protecting crops from pea aphids. Additionally, it mentions that pea aphids are considered a model organism for studying symbiosis, asexual reproduction, polyphenism, and gene duplication. The summary further highlights the pea aphid’s endosymbiotic relationship with Buchnera aphidicola bacteria and its vulnerability to parasitoid wasps and the fungal pathogen Pandora neoaphidis, which can potentially be used as biocontrols.

Generalities and life cycle

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In the autumn, female pea aphids lay fertilized eggs overwinter that 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.Inbreeding is avoided by the recognition of close kin. Mating between close kin has significantly lower egg hatching success and offspring survival than outbred mating.Several morphs exist 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 4 millimetres (5⁄32 in) 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 of 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 during the 1870s, and by 1900 it had become a serious pest species in the mid-Atlantic states. By the 1950s, it was widespread throughout the United States and Canada. Its host range in North America is very similar to that of the closely related blue alfalfa aphid (Acyrthosiphon kondoi).

Model organism

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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 a 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 millions of years. A. pisum also hosts a range of facultative bacterial symbionts (Hamiltonella defensa, Serratia symbiotica, Regiella insecticola) 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. (Specifically, Hamiltonella defensa and Serratia symbiotica retard the development of parasitoid wasps, and Regiella insecticola decreases mortality due to Pandora neoaphidis) 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 as one of few animals identified to synthesize carotenoids. Plants, fungi, and microorganisms can synthesize carotenoids, but torulene (3’,4’-didehydro-β,γ-carotene, specifically a hydrocarbon carotene) made by pea aphids, is one of few carotenoids known to be synthesized by animals. 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.

Endosymbiotic relationship with Buchnera aphidicola

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A. pisum participates in an obligate endosymbiotic relationship with the bacteria Buchnera aphidicola. A. pisum is the host and Buchnera is the primary endosymbiont. Together they form the holosymbiont. This is an obligate, symbiotic relationship and both partners are completely dependent on each other. When treated with antibiotics to remove the Buchnera bacteria, A. pisum growth and reproduction are interrupted or reduced. Buchnera lacks genes required for living independent of a host and is unculturable outside of the aphid host. The A. pisum and Buchnera holosymbiont is one of the most well studied symbiotic relationships both genetically and experimentally.

Pests, diseases, and biocontrols

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A. pisum faces threats from parasitoid wasps and the fungal pathogen Pandora neoaphidis. As such these are also promising potential biocontrols.

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