Costs, benefits and redundant mechanisms of adaption to chronic low-dose stress in yeast

Abstract

All organisms live in changeable, stressful environments. It has been reported that exposure to low-dose stresses or poisons can improve fitness. However, examining the effects of chronic low-dose chemical exposure is challenging. To address this issue we used temperature sensitive mutations affecting the yeast cell division cycle to induce low-dose stress for 40 generation times, or more. We examined cdc13-1 mutants, defective in telomere function, and cdc15-2 mutants, defective in mitotic kinase activity. We found that each stress induced similar adaptive responses. Stress-exposed cells became resistant to higher levels of stress but less fit, in comparison with unstressed cells, in conditions of low stress. The costs and benefits of adaptation to chronic stress were reversible. In the cdc13-1 context we tested the effects of Rad9, a central player in the response to telomere defects, Exo1, a nuclease that degrades defective telomeres, and Msn2 and Msn4, 2 transcription factors that contribute to the environmental stress response. We also observed, as expected, that Rad9 and Exo1 modulated the response of cells to stress. In addition we observed that adaptation to stress could still occur in these contexts, with associated costs and benefits. We conclude that functionally redundant cellular networks control the adaptive responses to low dose chronic stress. Our data suggests that if organisms adapt to low dose stress it is helpful if stress continues or increases but harmful should stress levels reduce.

Keywords: Adaptation; fitness; low-dose; stress; yeast.

Figure 1.

Hormetic response to mild stress in human cell lines. Human SW620 and SW620:8055R cell lines, the parental sensitive and a derived resistance line, were exposed to increasing concentrations of TOR kinase inhibitor, AZD8055, for 24 hours. Proliferation was assayed by [³H]thymidine incorporation. This figure is based on Figure 2 A of Cope et al.. We added a gray box to highlight the area of increased fitness.

Figure 2.

Passaging procedure. Freshly unfrozen strains were patched on YEPD agar plates and incubated at 23°C for 3 days, then passaged at 23°C or 25°C on YEPD agar plates as shown. 5–10 colonies from each genotype were pooled at each passage. Strains were: DDY81, 739, 738, 737, 736, 735, DLY1108, 1195, 3001. A) Genotypes of strains streaked on agar plates. B) Cells passaged for 2 passages at 25°C. C) Cells passaged for 2 passages at 23°C. DDY strains are diploids. DLY strains are haploids.

Figure 3.

The effects of adaptation to low-level of telomere stress in cdc13-1 mutants. Diploid strains with the genotypes indicated were grown on solid agar plates for 10 passages at 23°C, 10 passages at 25°C, or for 7 passages at 25°C followed by 3 passages at 23°C as indicated on the left of the Figure. Strains (DDY735, 736, 737, 738, 739 and DLY1108) were then inoculated into liquid and grown overnight at 23°C or 25°C. Fivefold dilution series of strains were set up in 96 well plates transferred to several independent solid YEPD agar plates using a pin tool. Individual plates were incubated at different temperatures, indicated across the top of the Figure, and photographs taken after 24, 48 and 72 hours of incubation. Arrows on right indicate haploid cultures.

Figure 4.

Adaptation to telomere stress in the absence of key stress response proteins. Diploid strains of different genotypes were analyzed as in Figure 3 except that strains were passaged twice at high temperature (25°C or 26°C), or once at high temperature followed by a single passage at low temperature, before being spotted onto agar plates. A) Strains as in Figure 3. Strains were DDY735, 736, 737, 738 and 739. B) Strains were DDY739; 868, 869, 860 and 861. C) Strains were DDY739, 992, 993, 994 and 995. D) Strains were DDY739, 974, 975, 972, 973. All strains shown in each subsection, at each temperature, were grown on single agar plates but images have been cut and pasted to make comparisons easier. Images were taken at 48 hours of incubation. Images taken after 24, 48 and 72 hours incubation are shown in Supplementary Figures: 2, 3 and 4, respectively.

Figure 5.

Adaptation to low-level kinase inhibition in cdc15-2 mutants. Diploid strains of different genotypes were analyzed as in Figure 3. Strains were grown for 3 passages at 23°C or 30°C on agar plates, or for 1 passage at 30°C and 2 at 23°C. A) Strains were DDY739; 839, 840, 841, 842. B) Strains were DDY739; 1036, 1037 1038, 1039. All strains shown in each subsection, at each temperature, were grown on single agar plates. Images taken after 24, 48 and 72 hours incubation are shown in Supplementary Figures: 5 and 6, respectively.

Figure 6.

The effects of adaptation to mild stress. Three curves are used to represent the fitness of yeast cells that were previously not stressed (black line), pre-stressed (gray line) or show a hormetic response (dashed line). A and B indicate low and high doses of stress.