Question

1. Two strains of E. coli, A and B, are inoculated into a chemostat in equal frequencies and undergo competition. After 10 generations, the frequency of the B strain is 75%.

(a) What is the fitness of strain B relative to strain A (under these particular experimental conditions)? __________

(b) What is the selection coefficient against strain A? __________

(Hint: Rescale so that the fitness of strain B equals one.)

Answer 1

Fitness (w) is a survival rate of a phenotype (bacterial strain 1) relative to the maximum survival rate of another phenotype (bacterial strain 2) in population.

The frequencies of E.coli strains (A & B ) in the input population are 50% for each strain. After 10 generation the freequencies of strains are 25% (strain A) and 75% (strain B).

Relative frequency after one generation are : Strain A = 25% / 50% = 0.5

strain B = 75%/ 50% = 1.5

The fitness of strain B relative to Strain A :

w = 1.5/0.5 = 3