Pea aphid
Encyclopedia
The pea aphid, Acyrthosiphon pisum is a sap-sucking insect in the Aphididae
family. It feeds on severals species of legumes (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
whose genome
has been sequenced and annotated
.
that hatch from these eggs are all females which undergo four moults before reaching sexual maturity
. It will then begin to reproduce by viviparous
parthenogenesis
, like most aphids. Each adult female gives birth to four to twelve 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 thirty days.
Population densities are at their highest in early summer, then will 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 aphid 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. As opposed to many aphid species, pea aphids do not tend to form dense colonies where individuals would stay where they were born during their whole lifetime. Pea aphids are not known to be farmed by ants that feed on honeydews
.
More than twenty legume genera
are known to host pea aphids, though the complete host range remains undetermined. On crops such as pea 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 vector viral
diseases to the plants it visits. Protection against pea aphids include 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.
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:
Aphididae
Aphididae is a very large insect family in the aphid superfamily , of the order Hemiptera. There are several thousand species in this family, many of which are well known for being serious plant pests...
family. It feeds on severals species of legumes (Fabaceae
Fabaceae
The Fabaceae or Leguminosae, commonly known as the legume, pea, or bean family, is a large and economically important family of flowering plants. The group is the third largest land plant family, behind only the Orchidaceae and Asteraceae, with 730 genera and over 19,400 species...
) worldwide, including forage crops such as pea
Pea
A pea is most commonly the small spherical seed or the seed-pod of the pod fruit Pisum sativum. Each pod contains several peas. Peapods are botanically a fruit, since they contain seeds developed from the ovary of a flower. However, peas are considered to be a vegetable in cooking...
, clover
Clover
Clover , or trefoil, is a genus of about 300 species of plants in the leguminous pea family Fabaceae. The genus has a cosmopolitan distribution; the highest diversity is found in the temperate Northern Hemisphere, but many species also occur in South America and Africa, including at high altitudes...
, alfalfa
Alfalfa
Alfalfa is a flowering plant in the pea family Fabaceae cultivated as an important forage crop in the US, Canada, Argentina, France, Australia, the Middle East, South Africa, and many other countries. It is known as lucerne in the UK, France, Australia, South Africa and New Zealand, and known as...
, and broad bean, and ranks among the aphid
Aphid
Aphids, also known as plant lice and in Britain and the Commonwealth as greenflies, blackflies or whiteflies, are small sap sucking insects, and members of the superfamily Aphidoidea. Aphids are among the most destructive insect pests on cultivated plants in temperate regions...
species of major agronomical importance.
The pea aphid is a model organism
Model organism
A model organism is a non-human species that is extensively studied to understand particular biological phenomena, with the expectation that discoveries made in the organism model will provide insight into the workings of other organisms. Model organisms are in vivo models and are widely used to...
whose genome
Genome
In modern molecular biology and genetics, the genome is the entirety of an organism's hereditary information. It is encoded either in DNA or, for many types of virus, in RNA. The genome includes both the genes and the non-coding sequences of the DNA/RNA....
has been sequenced and annotated
Genome project
Genome projects are scientific endeavours that ultimately aim to determine the complete genome sequence of an organism and to annotate protein-coding genes and other important genome-encoded features...
.
Generalities and Life cycle
In the autumn, female pea aphids lay fertilized eggs that overwinter and hatch the following spring. The nymphsNymph (biology)
In biology, a nymph is the immature form of some invertebrates, particularly insects, which undergoes gradual metamorphosis before reaching its adult stage. Unlike a typical larva, a nymph's overall form already resembles that of the adult. In addition, while a nymph moults it never enters a...
that hatch from these eggs are all females which undergo four moults before reaching sexual maturity
Sexual maturity
Sexual maturity is the age or stage when an organism can reproduce. It is sometimes considered synonymous with adulthood, though the two are distinct...
. It will then begin to reproduce by viviparous
Vivipary
Vivipary has two different meanings. In animals, it means development of the embryo inside the body of the mother, eventually leading to live birth, as opposed to laying eggs...
parthenogenesis
Parthenogenesis
Parthenogenesis is a form of asexual reproduction found in females, where growth and development of embryos occur without fertilization by a male...
, like most aphids. Each adult female gives birth to four to twelve 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 thirty days.
Population densities are at their highest in early summer, then will decrease through predation and parasitism
Parasitism
Parasitism is a type of symbiotic relationship between organisms of different species where one organism, the parasite, benefits at the expense of the other, the host. Traditionally parasite referred to organisms with lifestages that needed more than one host . These are now called macroparasites...
. 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
Insemination
Insemination is the deliberate introduction of sperm into the uterus of a mammal or the oviduct of an oviparous animal for the objective of impregnating a female for reproduction...
sexual females will lay overwintering eggs
Egg (biology)
An egg is an organic vessel in which an embryo first begins to develop. In most birds, reptiles, insects, molluscs, fish, and monotremes, an egg is the zygote, resulting from fertilization of the ovum, which is expelled from the body and permitted to develop outside the body until the developing...
, 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
Asexual reproduction
Asexual reproduction is a mode of reproduction by which offspring arise from a single parent, and inherit the genes of that parent only, it is reproduction which does not involve meiosis, ploidy reduction, or fertilization. A more stringent definition is agamogenesis which is reproduction without...
. When temperatures become colder and day lengths shorter, sexual winged females and males appear. These mate, the females lay diapausing
Diapause
Diapause is the delay in development in response to regularly and recurring periods of adverse environmental conditions. It is considered to be a physiological state of dormancy with very specific initiating and inhibiting conditions...
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 aphid can then colonize other host plants. Pea aphids also show hereditary
Heredity
Heredity is the passing of traits to offspring . This is the process by which an offspring cell or organism acquires or becomes predisposed to the characteristics of its parent cell or organism. Through heredity, variations exhibited by individuals can accumulate and cause some species to evolve...
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
Plant sap
Sap is a fluid transported in xylem cells or phloem sieve tube elements of a plant. It transports water and nutrients throughout the plant....
through its stylets. As opposed to many aphid species, pea aphids do not tend to form dense colonies where individuals would stay where they were born during their whole lifetime. Pea aphids are not known to be farmed by ants that feed on honeydews
Honeydew (secretion)
Honeydew is a sugar-rich sticky liquid, secreted by aphids and some scale insects as they feed on plant sap. When their mouthpart penetrates the phloem, the sugary, high-pressure liquid is forced out of the gut's terminal opening. Honeydew is particularly common as a secretion in the Hemipteran...
.
More than twenty legume genera
Genera
Genera is a commercial operating system and development environment for Lisp machines developed by Symbolics. It is essentially a fork of an earlier operating system originating on the MIT AI Lab's Lisp machines which Symbolics had used in common with LMI and Texas Instruments...
are known to host pea aphids, though the complete host range remains undetermined. On crops such as pea 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 vector viral
Virus
A virus is a small infectious agent that can replicate only inside the living cells of organisms. Viruses infect all types of organisms, from animals and plants to bacteria and archaea...
diseases to the plants it visits. Protection against pea aphids include 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
Binomial nomenclature
Binomial nomenclature is a formal system of naming species of living things by giving each a name composed of two parts, both of which use Latin grammatical forms, although they can be based on words from other languages...
A. pisum, encompass several biotypes described as cryptic species, subspecies
Subspecies
Subspecies in biological classification, is either a taxonomic rank subordinate to species, ora taxonomic unit in that rank . A subspecies cannot be recognized in isolation: a species will either be recognized as having no subspecies at all or two or more, never just one...
or races
Race (biology)
In biology, races are distinct genetically divergent populations within the same species with relatively small morphological and genetic differences. The populations can be described as ecological races if they arise from adaptation to different local habitats or geographic races when they are...
, which are specialized on different host species. Therefore, the pea aphid is more accurately described as a species complex
Species complex
A species complex is a group of closely related species, where the exact demarcation between species is often unclear or cryptic owing to their recent and usually still incomplete reproductive isolation. Ring species, superspecies and cryptic species complex are example of species complex...
.
The pea aphid is thought to be of Palearctic
Palearctic
The Palearctic or Palaearctic is one of the eight ecozones dividing the Earth's surface.Physically, the Palearctic is the largest ecozone...
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
North America
North America is a continent wholly within the Northern Hemisphere and almost wholly within the Western Hemisphere. It is also considered a northern subcontinent of the Americas...
near the 1870s.
Model organism
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 . The pea aphid genome is composed of approximately 525 megabases and 34000 predictedGene prediction
In computational biology gene prediction or gene finding refers to the process of identifying the regions of genomic DNA that encode genes. This includes protein-coding genes as well as RNA genes, but may also include prediction of other functional elements such as regulatory regions...
genes
Gênes
Gênes is the name of a département of the First French Empire in present Italy, named after the city of Genoa. It was formed in 1805, when Napoleon Bonaparte occupied the Republic of Genoa. Its capital was Genoa, and it was divided in the arrondissements of Genoa, Bobbio, Novi Ligure, Tortona and...
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:
- SymbiosisSymbiosisSymbiosis is close and often long-term interaction between different biological species. In 1877 Bennett used the word symbiosis to describe the mutualistic relationship in lichens...
with bacteriaBacteriaBacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...
. As all aphididaeAphididaeAphididae is a very large insect family in the aphid superfamily , of the order Hemiptera. There are several thousand species in this family, many of which are well known for being serious plant pests...
, A. pisum hosts the primary endosymbiontEndosymbiontAn endosymbiont is any organism that lives within the body or cells of another organism, i.e. forming an endosymbiosis...
Buchnera aphidicola, which provides essential aminoacids and is necessary for aphid reproduction. Buchnera is transmitted from mothers to offspring and it has co-evolved with aphids for dozens of million of years. A. pisum also hosts a range of facultative bacterial symbionts that can be transmitted maternally and horizontallyHorizontal gene transferHorizontal gene transfer , also lateral gene transfer , is any process in which an organism incorporates genetic material from another organism without being the offspring of that organism...
, and which affect ecologically important traits in aphids, such as body color, resistance to abiotic and bioticBiotic componentBiotic components are the living things that shape an ecosystem. A biotic factor is any living component that affects another organism, including animals that consume the organism in question, and the living food that the organism consumes. Each biotic factor needs energy to do work and food for...
stressStress (biology)Stress is a term in psychology and biology, borrowed from physics and engineering and first used in the biological context in the 1930s, which has in more recent decades become commonly used in popular parlance...
, and nutrition. - PolyphenismPolyphenismA polyphenic trait is a trait for which multiple, discrete phenotypes can arise from a single genotype as a result of differing environmental conditions.-Definition:A polyphenism is a biological mechanism that causes a trait to be polyphenic...
(the production of several discrete morphs by the same genotypeGenotypeThe genotype is the genetic makeup of a cell, an organism, or an individual usually with reference to a specific character under consideration...
). Studies on pea aphids have helped to establish the environmental and geneticGeneticsGenetics , a discipline of biology, is the science of genes, heredity, and variation in living organisms....
components controlling the production of sexuals and winged morphs, among other features. - Asexual reproductionAsexual reproductionAsexual reproduction is a mode of reproduction by which offspring arise from a single parent, and inherit the genes of that parent only, it is reproduction which does not involve meiosis, ploidy reduction, or fertilization. A more stringent definition is agamogenesis which is reproduction without...
. Pea aphid lineages include parthenogenesis in their life cycle 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. - PolymorphismPolymorphism (biology)Polymorphism in biology occurs when two or more clearly different phenotypes exist in the same population of a species — in other words, the occurrence of more than one form or morph...
and physiology explaining phenotypic variations in aphids. LociLocus (genetics)In the fields of genetics and genetic computation, a locus is the specific location of a gene or DNA sequence on a chromosome. A variant of the DNA sequence at a given locus is called an allele. The ordered list of loci known for a particular genome is called a genetic map...
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 carotenoidCarotenoidCarotenoids are tetraterpenoid organic pigments that are naturally occurring in the chloroplasts and chromoplasts of plants and some other photosynthetic organisms like algae, some bacteria, and some types of fungus. Carotenoids can be synthesized fats and other basic organic metabolic building...
. Plants, fungi, and microorganisms can synthesize carotenoids, but toruleneToruleneTorulene is a carotene which is notable for being synthesized by red pea aphids , imparting the natural red color to the aphids, which aids in their camouflage and escape from predation...
(3',4'-didehydro-β,γ-carotene, specifically a hydrocarbon caroteneCaroteneThe term carotene is used for several related unsaturated hydrocarbon substances having the formula C40Hx, which are synthesized by plants but cannot be made by animals. Carotene is an orange photosynthetic pigment important for photosynthesis. Carotenes are all coloured to the human eye...
) 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 transferHorizontal gene transferHorizontal gene transfer , also lateral gene transfer , is any process in which an organism incorporates genetic material from another organism without being the offspring of that organism...
of a number of genes for carotenoid synthesis, apparently from fungi . - Gene duplicationGene duplicationGene duplication is any duplication of a region of DNA that contains a gene; it may occur as an error in homologous recombination, a retrotransposition event, or duplication of an entire chromosome.The second copy of the gene is often free from selective pressure — that is, mutations of it have no...
and expansion of gene families. The pea aphid genome presents high levels of gene duplication compared to other insect genomes, such as DrosophilaDrosophilaDrosophila is a genus of small flies, belonging to the family Drosophilidae, whose members are often called "fruit flies" or more appropriately pomace flies, vinegar flies, or wine flies, a reference to the characteristic of many species to linger around overripe or rotting fruit...
, with the notable expansion of some gene families . - Interaction with host plants and speciationSpeciationSpeciation is the evolutionary process by which new biological species arise. The biologist Orator F. Cook seems to have been the first to coin the term 'speciation' for the splitting of lineages or 'cladogenesis,' as opposed to 'anagenesis' or 'phyletic evolution' occurring within lineages...
. 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 speciationSympatric speciationSympatric speciation is the process through which new species evolve from a single ancestral species while inhabiting the same geographic region. In evolutionary biology and biogeography, sympatric and sympatry are terms referring to organisms whose ranges overlap or are even identical, so that...
between pea aphid biotypes .