Endogenosymbiosis is an evolutionary process, proposed by the evolutionary and environmental biologist Roberto Cazzolla Gatti, in which "gene carriers" ( viruses,

retrovirus A retrovirus is a type of virus that inserts a DNA copy of its RNA genome into the DNA of a host cell that it invades, thus changing the genome of that cell. Once inside the host cell's cytoplasm, the virus uses its own reverse transcriptase ...

es and

bacteriophage A bacteriophage (), also known informally as a ''phage'' (), is a duplodnaviria virus that infects and replicates within bacteria and archaea. The term was derived from "bacteria" and the Greek φαγεῖν ('), meaning "to devour". Bacteri ...

s) and symbiotic prokaryotic cells ( bacteria or

archaea Archaea ( ; singular archaeon ) is a domain of single-celled organisms. These microorganisms lack cell nuclei and are therefore prokaryotes. Archaea were initially classified as bacteria, receiving the name archaebacteria (in the Archaebac ...

) could share parts or all of their genomes in an endogenous symbiotic relationship with their hosts.

Context

The related process of symbiogenesis or endosymbiosis was proposed by Lynn Margulis in 1967. She argued that the internal symbiosis of bacteria-like organisms had formed organelles like

chloroplast A chloroplast () is a type of membrane-bound organelle known as a plastid that conducts photosynthesis mostly in plant and algal cells. The photosynthetic pigment chlorophyll captures the energy from sunlight, converts it, and stores it in ...

s and

mitochondria A mitochondrion (; ) is an organelle found in the Cell (biology), cells of most Eukaryotes, such as animals, plants and Fungus, fungi. Mitochondria have a double lipid bilayer, membrane structure and use aerobic respiration to generate adenosi ...

. She proposed that this had created the

eukaryote Eukaryotes () are organisms whose cells have a nucleus. All animals, plants, fungi, and many unicellular organisms, are Eukaryotes. They belong to the group of organisms Eukaryota or Eukarya, which is one of the three domains of life. Bacte ...

s, and thus driven the expansion of life on Earth. She had argued that this process of symbiotic collaboration had run alongside the classical Darwinian cycle of mutation, natural selection and

adaptation In biology, adaptation has three related meanings. Firstly, it is the dynamic evolutionary process of natural selection that fits organisms to their environment, enhancing their evolutionary fitness. Secondly, it is a state reached by the po ...

Genetic symbiosis from parasites

Roberto Cazzolla Gatti, Ph.D., associate professor at Tomsk State University (Russia), argued in his hypothesis that "the main likely cause of the evolution of sexual reproduction, the

parasitism Parasitism is a Symbiosis, close relationship between species, where one organism, the parasite, lives on or inside another organism, the Host (biology), host, causing it some harm, and is Adaptation, adapted structurally to this way of lif ...

, also represents the origin of biodiversity". In other terms, this theory suggests that sexual reproduction acts as a conservative system against the inclusion of new genetic variations into cells' DNA (supported by the DNA repair systems) and, instead, the evolution of species can take place only when this preservative system fails to contrast the inclusion, within the host genome, of hexogen parts of DNA (and RNA) coming from obliged "parasitic" elements (viruses and phages) that establish a symbiosis with their hosts. "As two parallel evolutionary lines – Cazzolla Gatti wrote in his original paper – sexual reproduction seems to preserve what the endogenosymbiosis moves to diversify. Following the former process, the species can adapt slowly and indefinitely to the external factors, adjusting themselves, but not 'creating' novelty. The latter process, instead, leads to the speciation due to sudden changes in genes sequences. Not only organelles can be symbiotic with other cells, as suggested Lynn Margulis, but entire pieces of genetic material coming from symbiotic parasites, can be included in the host DNA, changing the gene expression and addressing the speciation process". This idea challenges the canonical natural selection models based on the gradualism of the mutation-adaptation pattern, providing more support to the punctuated equilibrium theory proposed by

Stephen Jay Gould Stephen Jay Gould (; September 10, 1941 – May 20, 2002) was an American paleontologist, evolutionary biologist, and historian of science. He was one of the most influential and widely read authors of popular science of his generation. Gould sp ...

and Niles Eldredge.

Evidence

Two independent studies provide support for the hypothesis. Jamie E. Henzy and Welkin E. Johnson demonstrated that the complex evolutionary history of the IFIT (Interferon Induced proteins with Tetratricopeptide repeats) family of antiviral genes has been shaped by continuous interactions between

mammal Mammals () are a group of vertebrate animals constituting the class Mammalia (), characterized by the presence of mammary glands which in females produce milk for feeding (nursing) their young, a neocortex (a region of the brain), fur or ...

ian hosts and their many viruses. David Enard and colleagues estimated that viruses have driven close to 30% of all adaptive amino acid changes in the part of the human proteome conserved within mammals. Their results suggest that viruses are one of the most dominant drivers of evolutionary change across mammalian and human proteomes. Previously, it was estimated that about 7–8% percent of the entire human genome carry about 100,000 pieces of DNA that came from endogenous

es. This may be an underestimate. In 2016 the biologists Sarah R. Bordestein and Seth R. Bordestein reported that genes are frequently transferred between hosts and parasites. Eukaryotic genes are often co-opted by viruses and bacterial genes are commonly found in

s. The presence of bacteriophages in symbiotic bacteria that obligately reside in eukaryotes may promote eukaryotic DNA transfers to bacteriophages.Sarah R. Bordenstein & Seth R. Bordenstein, Eukaryotic association module in phage WO genomes from Wolbachia, Nature Communications 7, Article number: 13155 (2016) doi:10.1038/ncomms13155

References

{{reflist, 30em Symbiosis Mutualism (biology) Endosymbiotic events Evolution