Endogenosymbiosis

Endogenosymbiosis (from the combination of the words: endo-geno-symbiosis) is the capacity of “gene carriers” (viruses, retrovirus and bacteriophages) to share parts of their genome in an endogenous symbiotic relationship with their hosts.^[1]

This evolutionary process was proposed in 2015 by Roberto Cazzolla Gatti, Ph.D., associate professor in ecology and biodiversity at the Tomsk State University (Russia), in a paper.^[2] uploaded to bioRXiv^[3] and published in March of this year in the journal Biologia.^[4]

History

This hypothesis came after the idea of “endosymbiosis” proposed by Lynn Margulis in 1967 and was argued to explain another process (apart from the classical mutation-adaptation evolutionary model and the symbiogenensis) that drives the expansion of the diversity of life on our planet.

Genetic symbiosis from parasites

Dr. Cazzolla Gatti argued in his hypothesis that “the main likely cause of the evolution of sexual reproduction, the parasitism, 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 Dr. 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 and Niles Eldredge.

Confirmation of the original hypothesis

Two independent studies recently published on eLife empirically confirmed Cazzolla Gatti’s hypothesis and gave it the status of a new evolutionary theory.

Firstly, Dr. Jamie E. Henzy and Dr. Welkin E. Johnson from the Boston College (US) 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 mammalian hosts and their many viruses^[5]

Then, Dr. David Enard and colleagues from the Stanford University (US) 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^[6]

Previously, it was estimated that about 7-8% percent of the entire human genome carry about 100,000 pieces of DNA that came from retroviruses known as endogenous retroviruses. But currently we have more evidences showing that this can be an underestimation and that the quantity of the endogenous genetic material coming from external sources could be much higher and very significant for the evolution of species.

In October 2016 the biologists Sarah R. and Seth R. Bordestein, from the Vanderbilt University, Nashville, published a paper^[7] that further proved Gatti's original idea. In this study the authors confirmed that "eukaryotic genes are typically co-opted by eukaryotic viruses and bacterial genes are commonly found in bacteriophages. The presence of bacteriophages in symbiotic bacteria that obligately reside in eukaryotes may promote eukayotic DNA transfers to bacteriophages".

References