Abstract
The experimental evolution of microorganisms exposed to extreme conditions can provide insight into cellular adaptation to stress. Typically, stress-sensitive species are exposed to stress over many generations and then examined for improvements in their stress tolerance. In contrast, when starting with an already stress-tolerant progenitor there may be less room for further improvement, it may still be able to tweak its cellular machinery to increase extremotolerance, perhaps at the cost of poorer performance under non-extreme conditions. To investigate these possibilities, a strain of extremely halotolerant black yeast Hortaea werneckii was grown for over seven years through at least 800 generations in a medium containing 4.3 M NaCl. Although this salinity is well above the optimum (0.8–1.7 M) for the species, the growth rate of the evolved H. werneckii did not change in the absence of salt or at high concentrations of NaCl, KCl, sorbitol, or glycerol. Other phenotypic traits did change during the course of the experimental evolution, including fewer multicellular chains in the evolved strains, significantly narrower cells, increased resistance to caspofungin, and altered melanisation. Whole-genome sequencing revealed the occurrence of multiple aneuploidies during the experimental evolution of the otherwise diploid H. werneckii. A significant overrepresentation of several gene groups was observed in aneuploid regions. Taken together, these changes suggest that long-term growth at extreme salinity led to alterations in cell wall and morphology, signalling pathways, and the pentose phosphate cycle. Although there is currently limited evidence for the adaptive value of these changes, they offer promising starting points for future studies of fungal halotolerance.
Highlights
With their short generation times and large populations, microorganisms adapt so rapidly that their evolution can be observed on timescales practically achievable in laboratory settings
A single strain of H. werneckii was inoculated into a high-salinity liquid medium
Because melanin is an important component of H. werneckii cell walls, we looked for qualitative differences in the melanisation of colonies on MEA plates supplemented with different osmolytes (Figure 2B)
Summary
With their short generation times and large populations, microorganisms adapt so rapidly that their evolution can be observed on timescales practically achievable in laboratory settings. Microbial cells do not have the protection enjoyed by the cells of multicellular organisms. Combined with their limited ability to ameliorate or escape hostile environmental conditions, rapid adaptation is one of the most important strategies for microbial fitness, competitiveness, and survival. The most famous experiment, which has been running for over three decades, was started by Lenski et al [2]. The experiment showed how quickly microorganisms can adapt and diversify even under rather homogenous, spatially unstructured, and modestly stressful conditions [3]. The fitness of these evolved strains increased initially in large steps, more slowly. Populations diversified, most lineages evolved towards extinction during the experiment, some evolved unexpected metabolic traits, and some even found complementary niches
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