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RNA Bacteriophage Φ6Evolution of sex and its consequences Evolution of sex and its consequencesWe use the RNA bacteriophage Φ6 as a model to study the costs and benefits of genetic exchange (sex). When multiple viruses co-infect the same host cell, sex produces hybrid progeny containing a mixture of genetic information from the co-infecting parents (Fig. 1).
Sex may benefit viruses by promoting genetic variation, which might allow sexual populations to evolve faster than asexual ones. In contrast, sex requires co-infection and may exert a cost because it increases competition between viruses for limited intracellular resources. One experiment allowed replicate populations of Φ6 to evolve in the presence and absence of sex for hundreds of viral generations (Turner and Chao 1998, Genetics 150:523-532). Unexpectedly, sex was costly because sexually-evolved viruses became attenuated (weakened) in their ability to infect the host alone. This indicates that the cost of intra-host competition can outweigh any of the potential benefits associated with sex. Ongoing projects examine whether sex is advantageous in purging epistatic (interacting) mutations from the virus genome, whether sex is costly in terms of breaking apart co-adapted loci, and how reproductive system (sexuality versus asexuality) influences the rate of molecular evolution and prevalence for epistasis to evolve.
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We use phage Φ6 to study the evolution of viral conflicts. One study showed that viruses cultured under high levels of co-infection evolve selfish strategies, but their fixation in the population causes mean fitness to decline (Turner and Chao 1999, Nature 398:441-443). These data conform to the prisoner’s dilemma of game theory (Fig. 2), where selfishness evolves despite the greater fitness payoff if all players cooperate.
Ongoing projects examine the generality of the prisoner’s dilemma result in Φ6; for instance, evolved cooperator viruses can re-invade populations of defectors, allowing the two strategies to coexist in a stable polymorphism (Turner and Chao 2003, American Naturalist 161:497-505). Current research examines the molecular mechanism(s) that may afford selfish genotypes an advantage during intra-host competition. We are also examining whether viruses evolve specific mechanisms to exclude large numbers of viruses from co-infecting the same cell, in order to reduce intracellular competition (Turner et al. 1999, Journal of Virology 73:2420-2424).
Evolutionary ecology of host shifts
A virus’s ecological niche is governed in part by its host
range, the hosts in which a virus can produce viable progeny. Viruses
may expand their host range through mutations that facilitate entry
into new host environments. Although host shifts allow a virus (or
any other parasite) to expand its ecological niche, traits governing
the infection of multiple host types can decrease fitness in the
original or alternate host environments.
We use phage Φ6 and Pseudomonas bacteria as a model to test basic
questions relating to evolutionary ecology of host shifts. Ongoing
projects include measuring the mutation spectrum associated with
expanded host range, growth tradeoffs for host range mutants across
host environments, and strength of selection resulting from simultaneous
adaptation of viruses to multiple habitats.