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The best-studied examples of endosymbiosis are in invertebrates. These symbioses affect organisms with global impact, including Symbiodinium (corals), or Wolbachia (insects). Many insect agricultural pests and human disease vectors have intimate relationships with primary endosymbionts.
The theory of endosymbiosis, as known as symbiogenesis, provides an explanation for the evolution of eukaryotic organisms. According to the theory of endosymbiosis for the origin of eukaryotic cells, scientists believe that eukaryotes originated from the relationship between two or more prokaryotic cells approximately 2.7 billion years ago.
Secondary endosymbiosis has occurred several times and has given rise to extremely diverse groups of algae and other eukaryotes. Some organisms can take opportunistic advantage of a similar process, where they engulf an alga and use the products of its photosynthesis, but once the prey item dies (or is lost) the host returns to a free living state.
The definition of symbiosis was a matter of debate for 130 years. [7] In 1877, Albert Bernhard Frank used the term symbiosis to describe the mutualistic relationship in lichens . [ 8 ] [ 9 ] In 1878, the German mycologist Heinrich Anton de Bary defined it as "the living together of unlike organisms".
Fungal-bacterial endosymbiosis encompasses the mutualistic relationship between a fungus and intracellular bacteria species residing within the fungus. Many examples of endosymbiotic relationships between bacteria and plants, algae and insects exist and have been well characterized, however fungal-bacteria endosymbiosis has been less well described.
A symbiosome is formed as a result of a complex and coordinated interaction between the symbiont host and the endosymbiont. [5] At the point of entry into a symbiont host cell, part of the cell's membrane envelops the endosymbiont and breaks off into the cytoplasm as a discrete unit, an organelle-like vacuole called the symbiosome.
Secondary endosymbiosis results in the engulfment of an organism that has already performed primary endosymbiosis. Thus, four plasma membranes are formed. The first originating from the cyanobacteria, the second from the eukaryote that engulfed the cyanobacteria, and the third from the eukaryote who engulfed the primary endosymbiotic eukaryote. [11]
Most tertiary endosymbiosis events end up with only the plastid retained. However, in the case of dinotoms (i.e. those having diatom endosymbionts), the symbiont's nucleus appears to be of normal size with a large amount of DNA, surrounded by plenty of cytoplasm. The symbiont even has its own DNA-containing mitochondria.