The "Kill the Winner" hypothesis (KtW) is an ecological model of population growth involving prokaryotes, viruses and protozoans that links trophic interactions to biogeochemistry. The model is related to the Lotka–Volterra equations. It assumes that prokaryotes adopt one of two strategies when competing for limited resources: priority is either given to population growth ("winners") or survival ("defenders"). As "winners" become more abundant and active in their environment, their contact with host-specific viruses (also known as phages) increases, making them more susceptible to viral infection and lysis. Thus, viruses moderate the population size of "winners" and allow multiple species (both "winners" and "defenders") to coexist. Current understanding of KtW primarily stems from studies of lytic viruses and their host populations. KtW provides a possible solution to the paradox of the plankton. It provides a mechanism for species coexistence despite resource limitations. Some investigations into virus-bacteria interactions in laboratory settings have suggested viruses play a major role in maintaining microbial diversity and provided more evidence in support of KtW. Competition specialists, or "winners", are often the fastest growing populations. Their abundance and activity increases when they outcompete other species for a shared limiting resource (e.g. phosphate). The resource can exist as a free form or as something that needs to be sequestered from biomass. Competition specialists (predators, grazers, parasites) are expected to dominate in oligotrophic environments where competition is a large ecological constraint. When competition specialists are found at uncharacteristically low abundances in oligotrophic environments, viruses may be responsible for moderating their population size. Defence specialists invest resources in strategies to avoid viral infection, but these strategies may result in reduced growth. Hence, the "defender" does not increase viral predation. Defence specialists are expected to dominate in eutrophic environments where competition pressure is reduced. While the KtW model is widely applicable to different trophic levels and complex microbial systems, it has many limitations. The KtW model represents an idealized microbial food web with mathematical parameters that only account for viral predation studied in vitro. Because it assumes environmental conditions are stable, it can only predict population dynamics over a small time frame relative to a microbial community's history. It also fails to account for the fact that a prokaryotic species can be attacked by multiple viruses at once. The KtW model may be modified as other models that assess its limitations (e.g. CKTW) are developed.
History
Paradox of the plankton The "Kill the Winner" hypothesis is related to the paradox of the plankton, which is an observation that many planktonic species exist despite having similar resource requirements. This paradox was first noted by G.E. Hutchinson in 1961 in relation to phytoplankton. He noticed that many distinct species coexisted despite filling the same niche. In a well-mixed pelagic environment, with conditions being roughly constant, prior biological theories (e.g. the competitive exclusion principle) suggested one species should eventually dominate. Selective predation, symbiotic interactions, and variations in environmental conditions over space and time were initially proposed as solutions to the paradox.
Virus-bacteria interactions Early modelling of viral infections in bacterial populations assumed a predator-prey relationship between viruses and bacteria following the Lotka-Volterra equations. Viruses and bacteria were thought to coexist stably in cycles of high and low population. These theoretical models of virus-bacteria interactions were supported by studies of Escherichia coli and bacteriophages in laboratory settings. It was also observed that multiple strains of E.coli could coexist if nutrients were limited and phages were introduced to the culture. Growth-oriented, phage-susceptible E.coli could coexist stably with slower-growing strains that were more resistant to infection. However, these experiments could not replicate the high diversity, grazing, and environmental conditions of marine ecosystems.
"Kill the Winner" The "Kill the Winner" hypothesis was first raised in a 1997 study of theoretical models for marine bacterial populations. In this study, T. Frede Thingstad and Risto Lignell found that the total size of a bacterial population was controlled by grazing and that lytic viruses had no impact on bacterial abundance in any of their nutrient-limited models. Instead, viruses promoted diversity by preferentially infecting more abundant and active bacteria. Thingstad later found that bacteria with varying growth rates could coexist stably, with faster-growing bacterial species maintaining a higher abundance of viruses. In this way, viruses can prevent the dominance of one species in any particular niche, which maintains microbial diversity and presents a solution to the paradox of the plankton. More recent discoveries of the significant role viruses play in cellular turnover also support the idea that viruses play a major role in maintaining planktonic diversity.
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