In: Contemporary Trends in Bacteriophage Research Editor: Horace T. Adams, pp. 191-215 Chapter 7 ISBN: 978-1-60692-181-4 © 2009 Nova Science Publishers, Inc. BACTERIOPHAGE INTRASPECIFIC COOPERATION AND DEFECTION Stephen T. Abedon 1 Department of Microbiology, The Ohio State University, Mansfield, OH 44906, USA Abstract Bacteriophages (phages) are the viruses of bacteria. Though typically studied as populations of minimally interacting individuals, in fact there also exists in phage research a tradition of analysis of the interactions of phages which have multiply adsorbed individual bacteria. Most commonly such interactions involve recombination between coinfecting phages and the study of phage genetics was founded upon such analyses. However, there exist additional phage- phage "social" interactions including superinfection exclusion, superinfection immunity, lysis inhibition, and biases in the genetic output of infections given coinfection. These interactions may be viewed generically as behaviors of microbial cooperation or defection. That is, cooperative behaviors at a minimum are those that result in an elevated (or simply not depressed) Darwinian fitness among the individuals a cooperator interacts with. Defection behaviors, by contrast, result in a reduced (or simply not elevated) Darwinian fitness among the individuals that a defector interacts with and are often engaged in to elevate the fitness of self. Among sophisticated organisms, e.g., such as within human societies, we can consider situations in which cooperators strive to mutually interact. Absent such sophistication, however, it is more reasonable to expect that organisms display behaviors which serve, at best, simply to reduce the potential for other individuals to successfully defect. Consistent with their relative simplicity, most phage social interactions can be viewed as mechanisms of phage-phage defection. As discussed, there also exists a phage behavior, called lysis from without, which seemingly can serve to counter at least some phage defection. A glossary of terms relevant to an understanding of phage cooperation and defection is provided. Introduction The characteristics that impact an organism's Darwinian fitness include, at a basic level, its ability to survive, obtain resources, and replicate. Each of these core facets of organismal I E-mail address: abedon.l@osu.edu. Tel.: + I 4197554343; fax: +I 4197554327. Corresponding author.