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A secondary endosymbiosis event involving an ancestral red alga and a heterotrophic eukaryote resulted in the evolution and diversification of several other photosynthetic lineages including Cryptophyta, Haptophyta, Stramenopiles (or Heterokontophyta), and Alveolata.
Endogenosymbiosis is an evolutionary process, proposed by the evolutionary and environmental biologist Roberto Cazzolla Gatti, in which "gene carriers" (viruses, retroviruses and bacteriophages) and symbiotic prokaryotic cells (bacteria or archaea) could share parts or all of their genomes in an endogenous symbiotic relationship with their hosts.
Fungi living below the ground amidst plant roots are known as mycorrhiza, but are further categorized based on their location inside the root, with prefixes such as ecto, endo, arbuscular, ericoid, etc. Fungal endosymbionts that live in the roots and extend their extraradical hyphae into the outer rhizosphere are known as ectendosymbionts.
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.
The endosymbiosis theory of organogenesis became widely accepted in the early 1980s, after the genetic material of mitochondria and chloroplasts had been found to be significantly different from that of the symbiont's nuclear DNA. [24] In 1995, English evolutionary biologist Richard Dawkins had this to say about Lynn Margulis and her work:
The biologist Lynn Margulis, famous for her work on endosymbiosis, contended that symbiosis is a major driving force behind evolution. She considered Darwin's notion of evolution, driven by competition, to be incomplete and claimed that evolution is strongly based on co-operation, interaction, and mutual dependence among organisms.
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]