NEW CODING SEQUENCES FORMATION BY VIRUSES WITH THE HELP OF HORIZONTAL TRANSFER AND GENE DUPLICATION

Besides the gene co-optation discusses in the previous article, viruses are able to form the cellular genome through horizontal gene transfer and by the copy number increase of the cellular genes with the help of amplification or retrocopies production. Previously horizontal transfer was thought to be a rare event. But genome sequence data for a wide range of organisms together with new analytical tools enable to detect a large number of horizontal gene transfer events across eukaryotes species. Many of these transfers have generated evolutionary novelties. Therefore it has been suggested that horizontal transfer of DNA may be an important evolutionary force shaping eukaryote genomes.  However, the vectors involved in horizontal transfer of DNA between eukaryotes are poorly understood. Viruses have been proposed as candidate vectors for DNA transfer. Really they are transmitted horizontally, infect a variety of taxa, replicate inside host cells, some have a host genome integration stage and some viruses have germ cells tropism which allows transmission to the progeny. Despite these only limited but growing number of evidence argues that viruses could serve as horizontal transfer vectors between species. In this review we summarized these data.  Many genes of large double-stranded DNA viruses have a cellular origin, suggesting the host-to-virus horizontal transfer of DNA. Indeed it has been identified a continuum influx of cabbage looper genetic material in baculoviruses genome. Furthermore at least 21 of the cabbage looper transposable elements integrated into baculoviruses genomes underwent repeated horizontal transfers between various insect species. These data identify potential recurrent gene flow both in virus-to-host and host-to-virus direction. Another example host RNAs encapsidation in the virions of single-strand RNA viruses.  Gene duplication is a major driver of organism evolution. Gene retroposition is a mechanism of gene duplication whereby a gene’s transcript is used as a template to generate retroposed gene copies. A genome-wide study across a fruit fly, mosquito, zebrafish, chicken, mouse, and human shows that LTR retrotransposons capable of creating retrocopies across a wide range of eukaryotes, which could subsequently evolve to be neofunctionalized retrogenes. Moreover it has been found a significant association between two retroviruses and lineage-specific gene family expansions in the human and mouse genomes. Altogether these data indicate that viruses, once considered as purely junk and selfish sequences, have repeatedly been used as a source of novel protein-coding genes during the evolution of eukaryote.