Open questions in the social lives of viruses

Abstract

Social interactions among viruses occur whenever multiple viral genomes infect the same cells, hosts, or populations of hosts. Viral social interactions range from cooperation to conflict, occur throughout the viral world, and affect every stage of the viral lifecycle. The ubiquity of these social interactions means that they can determine the population dynamics, evolutionary trajectory, and clinical progression of viral infections. At the same time, social interactions in viruses raise new questions for evolutionary theory, providing opportunities to test and extend existing frameworks within social evolution. Many opportunities exist at this interface: Insights into the evolution of viral social interactions have immediate implications for our understanding of the fundamental biology and clinical manifestation of viral diseases. However, these opportunities are currently limited because evolutionary biologists only rarely study social evolution in viruses. Here, we bridge this gap by (1) summarizing the ways in which viruses can interact socially, including consequences for social evolution and evolvability; (2) outlining some open questions raised by viruses that could challenge concepts within social evolution theory; and (3) providing some illustrative examples, data sources, and conceptual questions, for studying the natural history of social viruses.

Keywords: cheating; conflict; cooperation; evolutionary theory; natural history; population genetics, social evolution; virology; virus evolution.

FIGURE 1

Viral traits can influence group size. We provide some illustrative examples of viral traits that influence the rate of coinfection. On the one hand, many viruses have mechanisms of superinfection exclusion, which reduce the rate of coinfection: (a) in animals, viruses such as vaccinia, viruses trigger host cells to produce surface receptors that prevent further infection by vaccinia (Doceul et al., 2010); (b) in bacteria, phages such as T4 unleash a lysozyme enyme that degrades other phage genome copies that enter the cell (Shi et al., 2020); (c) in plants, virus such as Citrus Tristeva Virus release proteins that can prevent other infections at the level of the whole host (Bergua et al., 2014). On the other hand, many viruses have mechanisms of collective infection, that increase the rate of coinfection: (d) many plant viruses release proteins that allow viral genomes and gene products to stream directly into neighbouring host cells (Sanjuán, 2018); (e) in animal viruses, virions often aggregate after leaving a host cell (Andreu-Moreno & Sanjuán, 2018); (f) in a range of viruses including HIV and measles, virions can contain multiple copies of the viral genome (Rager et al., 2002).