Adaptive Response to DNA-Damaging Agents in Natural Saccharomyces cerevisiae Populations from Evolution Canyon'', Mt. Carmel, Israel

Background: Natural populations of most organisms, especially unicellular microorganisms, are constantly exposed to harsh environmental factors which affect their growth. UV radiation is one of the most important physical parameters which influences yeast growth in nature. Here we used 46 natural strains of Saccharomyces cerevisiae isolated from several natural populations at the Evolution Canyon'' microsite (Nahal Oren, Mt. Carmel, Israel). The opposing slopes of this canyon share the same geology, soil, and macroclimate, but they differ in microclimatic conditions. The interslope differences in solar radiation (200%-800% more on the African'' slope) caused the development of two distinct biomes. The south-facing slope is sunnier and has xeric, savannoid African'' environment while the north-facing slope is represented by temperate, European'' forested environment. Here we studied the phenotypic response of the S. cerevisiae strains to UVA and UVC radiations and to methyl methanesulfonate (MMS) in order to evaluate the interslope effect on the strains' ability to withstand DNA-damaging agents. Methodology/Principal Findings: We exposed our strains to the different DNA-damaging agents and measured survival by counting colony forming units. The strains from the African'' slope were more resilient to both UVA and MMS than the strains from the European'' slope. In contrast, we found that there was almost no difference between strains (with similar ploidy) from the opposite slopes, in their sensitivity to UVC radiation. These results suggest that the African'' strains are more adapted to higher solar radiation than the European'' strains. We also found that the tetraploids strains were more tolerant to all DNA-damaging agents than their neighboring diploid strains, which suggest that high ploidy level might be a mechanism of adaptation to high solar radiation. Conclusions/Significance: Our results and the results of parallel studies with several other organisms, suggest that natural selection appears to select, at a microscale, for adaptive complexes that can tolerate the higher UV radiation on the African'' slope.