Hypothesis on the Origin of Viruses from Transposons

Abstract

In this review, the role of transposons in the origin of viruses, losing the traits of their evolutionary precursors due to their high mutability, is considered. However, there are a number of common properties of viruses and transposons suggesting their phylogenetic relationship, including the ability to integrate into the host genome, specific activation in certain tissues, high degree of mutability, the existence of virophages propagating only in the presence of another virus (which is similar to non-autonomous transposons, for which the expression products of autonomous ones are required). Ideas about the emergence of viruses from transposons in evolution are discussed. The genomic elements exhibiting a dual nature of existence as mobile genetic elements and viruses are found: polintons, Tlr elements, and PLV viruses. It is assumed that a horizontal transfer (HT) of transposons, during the natural selection of which the elements possessing the virulence properties are preserved (while the genes required for the integration can mutate), is a key event required for turning into viruses. The horizontal transposon transfer, which is common in all representatives of living organisms, is accompanied by their variability required for the acquisition of new adaptive traits. In the course of evolution, the mechanisms of protection against viruses and transposons, including RNA interference, DNA methylation, and histone modification, began to be used to control the operation of the genomes, providing intercellular interactions; this explains the emergence of multicellular organisms. In the evolution of eukaryotes, the transposons have been used successfully for transformations of regulatory gene networks, as well as possible sources of new genes encoding both proteins and non-coding RNA, the fragmentary translation of which can produce short functional peptides. Thus, the products of transposon transcription and translation are the most important sources of evolutionary transformations; these mechanisms could be the basis for the evolution of viruses and the emergence of the fundamental properties of living organisms when they appear.