Acyrthosiphon pisum

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. Mating between close kin has significantly lower egg hatching success and offspring survival than outbred mating. == Model organism ==

- this red morph shows the reddish/dark markings due to carotenoids that some individuals produce. 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 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) 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. 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 ==

A pisum can survive only through its relationship with the bacterium Buchnera aphidicola, which also depends on this relationship for its own survival. A. pisum is the host, and Buchnera is the primary endosymbiont. Together, they form a holosymbiont, an entity that, formed by different species, becomes a single ecological unit. Treatment with antibiotics to remove the bacterium interrupts or reduces A. pisum 's reproduction and growth. Their relationship allows the aphid to use the bacterium to overcome the nutritional deficiencies of phloem sap that makes up A. pisum 's diet, while providing the bacterium with the genes that Buchnera lacks but which are needed for its survival. In both genetic research and as subjects of experiments, theirs is the most well-studied such relationship. Evolution of the endosymbiotic relationship This relationship likely evolved 160 to 280 million years ago. Their evolutionary history suggests that the bacterium originated from a common ancestor. It is thus likely that the original Buchnera infection of the aphid's common ancestor and their consequential coevolution caused the formation of one symbiotic partner as a new species, thereby dictating the formation of the other as also a new species. Buchnera, which is related to Enterobacteriaceae, including Escheriachia coli, Nutritional symbiosis Like other insects of the order Hemiptera, A.pisum utilizes an endosymbiotic bacterium to overcome the nutritional deficiencies of phloem sap. A. pisum feeds on phloem sap of host plants including Medicago sativa (alfalfa), Pisum sativa (pea), Trifolium pretense (red clover), and Vicia faba (broad bean). The phloem saps of these plants are nutritionally rich in carbohydrates but poor in terms of nitrogen. The ratio of essential amino acids to nonessential amino acids in these phloem saps ranges from 1:4-1:20. This ratio of essential to nonessential amino acids is severely disproportional compared to the 1:1 ratio present in animal tissues and necessary for survival. The endosymbiotic relationship with Buchnera allows A. pisum to overcome this lack of essential amino acids in the phloem sap When provided with nonessential amino acids, Buchnera converts nonessential amino acids into essential amino acids to be returned to A. pisum. This nutritional provisioning has been examined genomically (metabolic complementary, discussed below) and experimentally. Isolated bacteriocytes containing Buchnera have been shown to actively take up 14C labeled glutamine (a nonessential amino acid) where it is then converted into glutamic acid. Bacteriocytes are located near the ovariole cluster and Buchnera cells are vertically transferred from the mother's ovaries through transovarial transmission. This has provided researchers with a great deal of information about the evolutionary and molecular interactions of this endosymbiosis. These duplications are likely associated with the genetic establishment and maintenance of the endosymbiotic relationship. No lateral gene transfer has been detected between A. pisum and Buchnera. It was previously believed that lateral gene transfer was responsible for the severe gene reduction in the Buchnera genome but sequencing has shown that this has not occurred. The ancestral partners of this symbiosis are likely to have had complete metabolic pathways, however pressure to maintain these pathway genes was reduced due to redundancy as a result of the presence of the other partner's genome. The A. pisum genome lacks IMS, dFADD, Dredd and Retish genes that are a part of the IMD (immunodeficiency) pathway and present in other related insects. Also missing are peptidoglycan recognition proteins (PGRPs) that detect pathogens and alert the IMD pathway as well as antimicrobial peptide (AMP) genes which are produced once the immune pathway has been activated. A reduced immune system may have facilitated the establishment and sustained maintenance of the symbiotic relationship between the Buchnera bacterium and A. pisum. Also, phloem sap is a diet with reduced amounts of microbes which may have lower the evolutionary pressure of A. pisum to maintain the immune response pathway genes. ==Pests, diseases, and biocontrols==

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