Horizontal Gene Transfer Responsible for Carotenoid Production in Aphids Horizontal Gene Transfer Horizontal gene transfer (referred to as HGT for the rest of the paper) is said to have occurred when an organism successfully incorporates genetic information from another organism into its own genetic makeup when the first organism is not the offspring of the other organism. HGT, or lateral gene transfer (LGT), is used to describe both the artificial and natural transfers of genetic information from one organism to another.
The flow of genetic information is thought to occur relatively frequently between microorganisms. Current evidence suggests that roughly 2% of genetic information in microorganisms is acquired though HGT. While significant it is is not widely believed to be enough to require changes to the current organization of phylogenetic trees. The transfer of genetic information is not limited by species, kingdom or even domain and can occur between species that are very different. Within the kingdom of Bacteria HGT has been observed to function in three ways.
The first is referred to as bacterial transformation and is caused by the alteration of a cell, which results in the uptake and expression of foreign DNA. Transduction is the process by which DNA from one bacterium is transferred to another bacterium through a virus, which infects one taking genetic information and then the other, depositing the genetic information. The final way HGT occurs is though a process known as conjugation. Conjugation is a form of bacterial “mating” which results in the sharing of genes.
This process is common among bacteria of the same species but occurs with less frequency between bacteria of different species. While evidence for HGT in microbes is abundant, evidence for HGT in higher order multicellular organisms is uncommon. As such the mechanisms by which HGT occurs from microbes to plants and microbes to animals has not been verified. However, it is speculated that HGT occurs in multicellular organisms in much the same way as it does between microbes.
Horizontal gene transfer has not been observed first hand in multicellular organisms. However, there is strong evidence to support that it has happened in the past. The most widely accepted occurrence of HGT in eukaryotic cells occurred after an early eukaryotic cell engulfed an alpha-protobacteria and maintained it as a symbiont. Eventually the symbiotic relationship proved so successful that the bacteria shed much of its genetic information, along with its ability to live on its own, and transferred segments of genetic information to the host cell.
Recently researchers have found strong evidence suggesting that Acyrthosiphon pisum, commonly known as the pea aphid, is utilizing genes most likely taken from an ancient fungal symbiont for the production of carotenoids. Acyrthosiphon pisum, the pea aphid Kingdom| Animalia| Phylum| Arthropoda| Class| Insecta| Order| Hemipter| Suborder| Sternorrhyncha| Superfamily| Aphidoidea| Family| Aphididae| Genus| Acyrthosiphon| Species| A. pisum| Scientific classification| Acyrthosiphon pisum, or the pea aphid, belongs to the family Aphididae.
The pea aphid is of significant agricultural importance because of the annual destruction it causes to crops. A. pisum is also of biological importance because its genome has been completely mapped and it is bred easily in laboratory conditions, making it a model organism. Acyrthosiphon pisum is found through out the world on all continents with the exception of Antarctica. Aphids live on the plants they choose for food, typically on the underside of the leaves. Aphids are incredible host specific and speciation has occurred in relation to the host plants of choice.
A. isum prefer the pea plant, thus its common name, and relatives of the pea plant such as, alfalfa, clover, broad bean and other plants. In total there are around twenty known host plants for the pea aphid subspecies though it is believed that not all of the pea aphid hosts are known. The diet of the pea aphid is very specific. They feed exclusively on plant sap from pholem and xylem vessels. Pholem vessels are conductive tissue in plants used to transport sucrose and glucose from the leaves to other parts of the plant. The xylem is also conductive tissue but is used primarily to transport water from the roots to the leaves.
The aphids pierce the pholem vessels and sap is forced into the aphids food canal by hydrostatic pressure. The aphid’s sap diet is incomplete as sap is lacking in many essential amino acids that the aphid is incapable of synthesizing. Rather than giving up its preferred diet aphids had developed successful endosymbiotic relationships with bacteria who are capable of synthesizing the missing amino acids for the aphids. This relationship is mutualistic as the bacteria are provided sugars that the aphids take from their host plants.
There are many species of bacteria that aphids have formed symbiotic relationships with. Their primary endosymbiont is the bacteria known as Buchnera aphidicola. The success of A. pisum as an organism is due in large part to its complex reproduction cycle. The pea aphid has the capability of switching between sexual and asexual reproduction and does so in response to seasonal changes and availability of food. The cycle begins in the spring with a viviparous (live birth giving) female who produces Viviparous parthenogenetic females.
These viviparous parthenogenetic females reproduce asexually giving live birth to what are known as nymphs. Nymphs mature with in ten days then begin reproduce asexually and quite rapidly. They can produce up to twelve nymphs per day. Once the colony has reach a point of over population a secondary polymorph of winged females are produced and leave the colony in search of new host plants. In the fall the sexual stage of the aphid reproductive cycle begins. The viviparous parthenogenetic females produce viviparous females who produce the sexual forms of the species.
The females produced at this time are oviparous (egg producing) and after mating with the smaller males being produced at this time lay diapause eggs that hibernate through the winter months and hatch in the spring. Some sources make the mistake of referring to the aphid reproductive cycle as alternation of generations. However, since there is not a time when a haploid version of the species exists, the term alternation of generations does not apply. The genome of the pea aphid has been both sequenced and annotated. The entire genome consists of 525 Megabases and 34,000 genes on 8 diploid chromosomes.
This has lead to it use as a model organism and an increase interest in research involving pea Aphids. Perhaps the most unusual characteristic of the pea aphid, and one that no other family in the animal kingdom shares, is their ability to produce their own carotenoid proteins. While all animals require carotenoids and must obtain them either by eating plants or by eating animals that eat plants, it is the aphid alone that has gained the ability to produce its own carotenoids. To researchers that discovered the aphid’s ability to produce carotenoids it seemed unlikely that aphids evolved the ability independently.
This set in motion research aimed at discovering how pea aphids came to posses this unique ability among animals. Carotenoids Carotenoids are yellow and red pigments. Produced by many plants, fungi, and microorganisms, carotenoids are always accompanied by chlorophyll and assist in photosynthesis, auxiliary light absorption and phototaxis. Carotenes are highly reactive which is a result of their structure. “Carotenoids are long aliphatic, conjugated double bond systems,” known as polyenes. Part of the carotenoid molecule is hydrocarbon structures that contain eight isoprene units that have a formula C40H56.
As indicated by their pigment, carotenoids assist in the absorption of light in the yellow and red spectrum and are responsible for the color change of leaves in the summer. This occurs because the decreasing angle of the sun results in higher availability of yellow and red light. This triggers the plants to respond by producing more carotenoids and less chlorophyll. Members of the animal kingdom require carotenoids but are incapable of producing them. Animals must ingest carotenoids from plants for a few reasons. Animals use carotenoids for their color properties.
Though carotenoids are red and yellow by their nature animals are capable of using them for blues, brown and purples by combining them with proteins. Carotenoids also serve several important physiological functions in animals. Because of their highly reactive nature carotenoids are “potent free radical quenchers, singlet oxygen scavengers and lipid antioxidants (some of them are vitamin A precursors).