Chapter 54 - Hypotheses of virus origin and evolutionary patterns of plant viruses

Abstract

Viruses are the most abundant living entity in the world, and they have an obligatory parasitic relationship with other living beings. There is a continuing tug-of-war among biologists on the concept of the origin of viruses; there are two broad hypotheses, “cell-first” and “virus-first.” Recent comparative genomics and metagenomic studies have favored the virus-first hypothesis, arguing that viruses were the relics of precellular life. It is more convincing that the ancient viruses have originated from the primordial RNA pool much earlier than the ancestor of common cellular life forms. Initially, they became a parasite on prokaryotic cells and subsequently adopted the eukaryotic cells. In the long evolutionary time scale, they evolved and emerged as the modern-day viruses through extensive genetic modifications. It is not yet clear at what point viruses adopted plants and began a new evolutionary path. However, the evolutionary process and pattern of plant viruses are of no exception as compared with other viruses. As there is no fossil evidence for viruses, the evolutionary relationships of viruses are traced through the significant homology in hallmark genes such as RNA-dependent RNA polymerase, helicase, and jelly-roll capsid. Plant viruses evolve through mutations, recombination, and pseudo-recombination. This chapter presents an update of the views on the origin of viruses in general and of the recent understanding on the evolution of plant viruses with reference to the other eukaryotic viruses.