Biodiversity Science ›› 2016, Vol. 24 ›› Issue (9): 1056-1061.doi: 10.17520/biods.2016143

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Frequency dependent fitness in different evolved Escherichia coli lines

Chuan Ni, Biru Zhu, Dayong Zhang*()   

  1. Key Laboratory for Biodiversity Science and Ecological Engineering of Ministry of Education, Beijing Normal University, Beijing 100875
  • Online:2016-10-09
  • Zhang Dayong

Differences in fitness between two species or genotypes is usually assumed to be constant when competition experiments are used to measure relative fitness in evolutionary experiments. However, interactions between competitors may lead to frequency-dependence in fitness. We measured the relative fitness of two types of evolved lines of Escherichia coli under different initial relative frequencies to analyze the effects of initial relative frequency on relative fitness. Competed with the low nitrogen evolved lines, the high nitrogen evolved lines displayed increased relative fitness with decreased initial relative frequency, which suggests negative frequency dependence. Both types did not grow in the filtrate from high nitrogen evolved lines, but grew in the filtrate from low nitrogen evolved lines. However, the number of cell doublings of the high nitrogen evolved lines was three times higher than that of the low nitrogen evolved lines. One probable explanation for the negative frequency dependent fitness was that the low nitrogen evolved lines had weaker resource competitive ability and could not sufficiently use resources. Another explanation was that the high nitrogen evolved lines could use some metabolites produced by the low nitrogen evolved lines, which suggests the existence of cross-feeding interaction. Different interactions may lead to different relationships between relative fitness and initial relative frequency. Therefore, we need to account for the effects of initial relative frequency on relative fitness to more accurately measure fitness in evolutionary experiments.

Key words: competition experiment, initial relative frequency, cross-feeding interaction, allelopathic interaction

Table 1

The relative fitness of N+ lines with different initial relative frequencies when comparing with N- lines. One sample t-tests were used and the null hypothesis was the relative fitness equaled to one. Tukey HSD test was used to do multiple comparisons for the relative fitness with different initial relative frequencies, and the different letters on the upper right indicate statistically significant difference."

Initial relative frequency of N+
Relative fitness (mean±SE, n = 10)
t 统计量
0.01 1.5176 ± 0.0760a 6.812 9 <0.001
0.1 1.2992 ± 0.0345b 8.681 9 <0.001
0.5 1.0829 ± 0.0188c 4.414 9 0.002
0.9 1.0814 ± 0.0241c 3.384 9 0.008
0.99 1.0750 ± 0.0282c 2.658 9 0.026

Fig. 1

The relationship between relative fitness and initial relative frequency of N+ lines (A), and the number of cell doublings (D) of two types of evolved Escherichia coli lines in two different filtrates (B). (A) The black dots show the mean relative fitness of N+ lines comparing with N- lines, and the error bars show SE (n =10). The curve indicates the regression model, whose equation and statistical significance are showed upper right. (B) Data show mean ± SE (n = 10). N+F and N-F indicate filtrate prepared from 24 h cultures of N+ lines and N- lines, respectively. *** P < 0.001, and NS indicates P > 0.05 (two independent sample t-tests)."

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