Chapter 7 : Viruses, Land Plants, and Insects: a Trinity of Virus, Host, and Vector

Abstract:

This chapter examines the deep issues concerning the origin and evolution of land plants, insects, and their viruses together. Human activity, especially agricultural activity in the last 10,000 years, has generated large, closely spaced, and genetically homogeneous plant populations, which have frequently been introduced into new habitats. The chapter examines the relationship of algae to the evolution of higher green plants and considers the oceanic crustaceans along with their viruses and the evolution of terrestrial insects. The role that viruses have played in the evolution of their hosts has seldom been addressed in the context of either plant or insect evolution. The chapter presents the overall patterns of host plant evolution and then addresses the evolution of host insects. It is now accepted that a progenitor of land plants was the green microalgae, based on phylogenetic analysis of chloroplastic and mitochondrial DNA. Interestingly, in filamentous brown Fungi and other higher Fungi, zoospore formation is frequently associated with reactivation of species-specific persistent virus replication. Due to their enormous numbers, hexapods will be the main focus of this discussion of viruses and the origin of insects. The major groups of viruses of insects are clearly distinct from those found in both plants and animals. Most plant viruses need an insect vector for transmission, but in almost all cases the virus does not replicate in that insect. The most common vectors for plant viruses are homopterans, with their piercing and sucking mouthparts.