Endosymbiosis

An endosymbiont or endobiont is any organism that lives within the body or cells of another organism in a mutualistic relationship with the host body or cell, often but not always to mutual benefit.The term endosymbiosis is from the Greek: ἔνδον endon 'within', σύν syn 'together' and βίωσις biosis 'living'). Examples are nitrogen-fixing bacteria (called rhizobia), which live in root nodules on legume roots, single-cell algae inside reef-building corals, and bacterial endosymbionts that provide essential nutrients to about 10–15% of insects. There are two types of symbiont transmissions. In horizontal transmission, each new generation acquires free living symbionts from the environment. An example is the nitrogen-fixing bacteria in certain plant roots. Vertical transmission takes place when the symbiont is transferred directly from parent to offspring. There is also a combination of these types, where symbionts are transferred vertically for some generation before a switch of host occurs and new symbionts are horizontally acquired from the environment. In vertical transmissions, the symbionts often have a reduced genomes and are no longer able to survive on their own. As a result, the symbiont depends on the host, resulting in a highly intimate co-dependent relationship. For instance, Pea Aphid symbionts have lost genes for essential molecules, now relying on the host to supply them with nturients. In return, the symbionts synthesize essential amino acids for the aphid host . Other examples include Wigglesworthia nutritional symbionts of tse-tse flies, or in sponges. When a symbiont reaches this stage, it begins to resemble a cellular organelle, similar to mitochondria or chloroplasts. Many instances of endosymbiosis are obligate; that is, either the endosymbiont or the host cannot survive without the other, such as the gutless marine worms of the genus Riftia, which get nutrition from their endosymbiotic bacteria. The most common examples of obligate endosymbioses are mitochondria and chloroplasts. Some human parasites, e.g. Wuchereria bancrofti and Mansonella perstans, thrive in their intermediate insect hosts because of an obligate endosymbiosis with Wolbachia spp. They can both be eliminated from said hosts by treatments that target this bacterium. However, not all endosymbioses are obligate and some endosymbioses can be harmful to either of the organisms involved. Two major types of organelle in eukaryotic cells, mitochondria and plastids such as chloroplasts, originated by as bacterial endosymbionts. This process is commonly referred to as symbiogenesis. Symbiogenesis explains the origins of eukaryotes, whose cells contain two major kinds of organelle: mitochondria and chloroplasts. The theory proposes that these organelles evolved from certain types of bacteria that eukaryotic cells engulfed through phagocytosis. These cells and the bacteria trapped inside them entered an endosymbiotic relationship, meaning that the bacteria took up residence and began living exclusively within the eukaryotic cells. Numerous insect species have endosymbionts at different stages of symbiogenesis. A common theme of symbiogenesis involves the reduction of the genome to only essential genes for the host and symbiont collective genome. An incredible example of this is the fractionation of the Hodgkinia genome of Magicicada cicadas. Because the cicada life cycle takes years underground, natural selection on endosymbiont populations is relaxed for many bacterial generations. This allows the symbiont genomes to diversify within the host for years with only punctuated periods of selection when the cicadas reproduce. As a result, the ancestral Hodgkinia genome has split into three groups of primary endosymbiont, each encoding only a fraction of the essential genes for the symbiosis. The host now requires all three sub-groups of symbiont, each with degraded genomes lacking most essential genes for bacterial viability. The best-studied examples of endosymbiosis are known from invertebrates. These symbioses affect organisms with global impact, including symbiodinium of corals, or Wolbachia of insects. Many insect agricultural pests and human disease vectors have intimate relationships with primary endosymbionts. Scientists classify insect endosymbionts in two broad categories, 'Primary' and 'Secondary'. Primary endosymbionts (sometimes referred to as P-endosymbionts) have been associated with their insect hosts for many millions of years (from 10 to several hundred million years in some cases). They form obligate associations (see below), and display cospeciation with their insect hosts. Secondary endosymbionts exhibit a more recently developed association, are sometimes horizontally transferred between hosts, live in the hemolymph of the insects (not specialized bacteriocytes, see below), and are not obligate.