Resonating circadian clocks enhance fitness in cyanobacteria

Abstract

In some organisms longevity, growth, and developmental rate are improved when they are maintained on a light/dark cycle, the period of which "resonates" optimally with the period of the endogenous circadian clock. However, to our knowledge no studies have demonstrated that reproductive fitness per se is improved by resonance between the endogenous clock and the environmental cycle. We tested the adaptive significance of circadian programming by measuring the relative fitness under competition between various strains of cyanobacteria expressing different circadian periods. Strains that had a circadian period similar to that of the light/dark cycle were favored under competition in a manner that indicates the action of soft selection.

Figure 1

Characteristics of the strains used in the competition experiments. (A) The circadian phenotypes of different strains as determined with the CCD camera/turntable device described (13). All strains have a luciferase construct that reports the promoter activity of the psbAI gene (16, 22). AMC343 has almost the same phenotype as AMC149; therefore, it is not shown here. FRPs of the different strains at 30°C are approximately: 23 h (SP22), 25 h (AMC149 and AMC343), and 30 h (P28). P28R is the P28 strain that has been genetically rescued to an FRP ≈25 h. (B and C) Growth curves of the individual strains in A. On average, the doubling times for LD were one division per 16.1 h and for LL the doubling times were one division per 6.3 h.

Figure 2

Growth of various cyanobacterial strains in competition with each other in batch cultures. Pairs of strains were mixed together and an aliquot was plated to form single colonies to determine the initial composition of the culture. The strains were grown for 27 days in batch culture under either a 22-h LD cycle (LD 11:11) or a 30-h LD cycle (LD 15:15), and then another aliquot of the mixed culture was plated to determine the 27-day composition of the culture. Each horizontal sequence shows the results for the pairs of strains listed on the right. The composition of the cultures was determined by observing the luminescence patterns of the colonies and counting the number of colonies on each plate exhibiting each of the strain-specific phenotypes by using a CCD camera/turntable apparatus (13). In the experiments depicted in this figure, wild type was strain AMC149. The experiment was repeated three times with essentially the same outcomes.

Figure 3

Kinetics of competition between wild-type and mutant/rescued strains (batch cultures). (A) Competition between wild type (AMC343) and P28 in LD 12:12, LD 15:15, and LL plotted as a function of estimated number of generations. The results for two experiments (of five independent experiments) are shown for each LD treatment. Ordinate, Percentage of colonies in the population that are P28. (B) Competition between wild type (AMC343) and SP22 in LD 12:12 and LL plotted as in A. The results for two independent LD experiments are shown. Ordinate, Percentage of colonies in the population that are SP22. (C) Comparison of the open-symbol raw data from A (points and heavy lines) with the results of modeling (thin lines) by using the equation shown, where w = relative fitness [w_wt for AMC343 (wild type), w_P28 for P28] and p = fraction of a given strain in the mixed population [p_t for generation (t), p_t+1 for generation (t + 1)]. (D) Competition between wild type (AMC343) and either P28 (FRP ≅ 30 h) or P28R (FRP ≅ 25 h) in LD 12:12. Two independent experiments are shown (open and closed symbols). Ordinate, Percentage of colonies in the population that are P28 or P28R. For all panels, the horizontal lines are the predictions if the initial population compositions are maintained.

Figure 4

Phase relationships of the rhythms of wild type (AMC149) and P28 after entrainment to LD cycles. These are composites showing the average trace for 60–200 colonies. Colonies were entrained to LD 11:11, LD 12:12 or LD 15:15 for four to five cycles and then transferred to LL at time 0; their circadian rhythms were monitored for the first cycle in LL. Data shown are after entrainment to LD 11:11 (A), LD 12:12 (B), and LD 15:15 (C). All data are from LL; shaded areas are the extrapolated night phases of the prior LD cycles. Ordinates are the luminescence intensities (left, for AMC149; right, for P28).