- Email: [email protected]
1985 Coevolution of bacteria and phage
CHAPTER FIFTY TWO
1985 Coevolution of bacteria and phage The concept The simple prediction of the Red Queen hypothesis (Chapter 36) is that species engaged in antagonistic interactions (such as predators and prey or host and parasites) will engage in an endless arms race fueled by evolution. Lenski and Levin (1985) sought to experimentally test this with laboratory communities of bacteria and phage. They found that evolution is constrained and that endless responses by phage and their host bacteria are not what is observed.
The explanation Phage require bacteria to reproduce. Infected bacteria are ultimately killed by phage and thus the interaction between bacteria and phage is exclusively antagonistic. Bacteria (Escherichia coli) and phage (T2, T4, T5, and T7) reproduce quickly and thus their evolution can be experimentally studied. Bacteria can become resistant to phage through single mutations that prevent the adsorption of phage. Phage accomplish adsorption via the recognition of specific lipopolysaccharides or proteins in the bacteria cell wall. Phage may also experience host-range mutations that allow the phage to infect both susceptible and resistant bacteria. Mutant bacteria resistant to all four phage were observed. Mutant bacteria resistant to T2, T4, and T7 showed a competitive disadvantage relative to the parental, susceptible bacteria. Mutants resistant to T5 did not show a competitive disadvantage. The cultures with bacteria resistant to T2, T4, and T7 did not drive the phage to extinction. In these cultures, both resistant and susceptible bacteria coexisted with a population of unmutated phage. The competitive disadvantage of the resistant bacteria prevented them from displacing the susceptible bacteria and these susceptible bacteria prevented the phage from being driven to extinction. Cultures with T5 showed different results. Once a resistant bacterial mutant appeared the phage were driven to extinction. Conceptual Breakthroughs in Evolutionary Ecology ISBN: 978-0-12-816013-8 https://doi.org/10.1016/B978-0-12-816013-8.00052-1
© 2020 Elsevier Inc. All rights reserved.
123
j
124
Conceptual Breakthroughs in Evolutionary Ecology
Lenski and Levin didn’t observe any host-range mutants. By estimating an upper bound for the mutation rate of host-range mutants, Lenski and Levin conclude that it would take about 7 years for such a mutant to appear whereas the bacterial mutants appeared in about 100 h. Lenski and Levin suggest that genetic changes that produce a resistant bacterium are much simpler than the changes that would be required on the part of phage to extend their host range. Thus, the lack of an arms race is due to architectural constraints on the evolution of phage despite the strong selective advantage that would accrue to such a mutant. Lenski and Levin (1985) conclude that “This general asymmetry in the coevolutionary potential of bacteria and phage occurs because mutations conferring resistance may arise by either the loss or alteration of gene function, while host-range mutations depend on specific alterations of gene function. These constraints preclude observing endless arms races between bacteria and virulent phage.”
Impact: 9 These experiments provided a powerful demonstration of the complications in the coevolutionary process. It is not enough to understand the selective forces in play but also evolutionary constraints and fitness trade-offs (see also Chapter 44).
Reference Lenski, R.E., Levin, B.R., 1985. Constraints on the coevolution of bacteria and virulent phage: a model, some experiments, and predictions for natural communities. Am. Nat. 125, 585e602.
1985 Coevolution of bacteria and phage The concept The simple prediction of the Red Queen hypothesis (Chapter 36) is that species engaged in antagonistic interactions (such as predators and prey or host and parasites) will engage in an endless arms race fueled by evolution. Lenski and Levin (1985) sought to experimentally test this with laboratory communities of bacteria and phage. They found that evolution is constrained and that endless responses by phage and their host bacteria are not what is observed.
The explanation Phage require bacteria to reproduce. Infected bacteria are ultimately killed by phage and thus the interaction between bacteria and phage is exclusively antagonistic. Bacteria (Escherichia coli) and phage (T2, T4, T5, and T7) reproduce quickly and thus their evolution can be experimentally studied. Bacteria can become resistant to phage through single mutations that prevent the adsorption of phage. Phage accomplish adsorption via the recognition of specific lipopolysaccharides or proteins in the bacteria cell wall. Phage may also experience host-range mutations that allow the phage to infect both susceptible and resistant bacteria. Mutant bacteria resistant to all four phage were observed. Mutant bacteria resistant to T2, T4, and T7 showed a competitive disadvantage relative to the parental, susceptible bacteria. Mutants resistant to T5 did not show a competitive disadvantage. The cultures with bacteria resistant to T2, T4, and T7 did not drive the phage to extinction. In these cultures, both resistant and susceptible bacteria coexisted with a population of unmutated phage. The competitive disadvantage of the resistant bacteria prevented them from displacing the susceptible bacteria and these susceptible bacteria prevented the phage from being driven to extinction. Cultures with T5 showed different results. Once a resistant bacterial mutant appeared the phage were driven to extinction. Conceptual Breakthroughs in Evolutionary Ecology ISBN: 978-0-12-816013-8 https://doi.org/10.1016/B978-0-12-816013-8.00052-1
© 2020 Elsevier Inc. All rights reserved.
123
j
124
Conceptual Breakthroughs in Evolutionary Ecology
Lenski and Levin didn’t observe any host-range mutants. By estimating an upper bound for the mutation rate of host-range mutants, Lenski and Levin conclude that it would take about 7 years for such a mutant to appear whereas the bacterial mutants appeared in about 100 h. Lenski and Levin suggest that genetic changes that produce a resistant bacterium are much simpler than the changes that would be required on the part of phage to extend their host range. Thus, the lack of an arms race is due to architectural constraints on the evolution of phage despite the strong selective advantage that would accrue to such a mutant. Lenski and Levin (1985) conclude that “This general asymmetry in the coevolutionary potential of bacteria and phage occurs because mutations conferring resistance may arise by either the loss or alteration of gene function, while host-range mutations depend on specific alterations of gene function. These constraints preclude observing endless arms races between bacteria and virulent phage.”
Impact: 9 These experiments provided a powerful demonstration of the complications in the coevolutionary process. It is not enough to understand the selective forces in play but also evolutionary constraints and fitness trade-offs (see also Chapter 44).
Reference Lenski, R.E., Levin, B.R., 1985. Constraints on the coevolution of bacteria and virulent phage: a model, some experiments, and predictions for natural communities. Am. Nat. 125, 585e602.