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Project Progress

This project analyzes the adaptation of experimental lineages of the bacterium Escherichia coli to low temperatures. We have established thirty replicate lineages of diverse thermal history that have now been adapted to 20°C for 2000 generations. We are in the process of analyzing both the adaptive consequences of that evolution and its genetic basis. We are also preparing further selection experiments at even colder temperatures to determine the limits of cold tolerance and its consequences.

We have demonstrated that the rate of adaptation to low temperature (20°C) is independent of previous thermal adaptive history; that is, change in relative fitness or fitness after 2000 generations is no different if the bacteria were previously adapted to 32, 37, or 42°C or an alternation of 32 & 42°C. We have further found that adaptation to 20°C is associated with a significant decrease in fitness at high (40°C) temperature. Not only is this general effect significant, it was also demonstrated in 15 of 24 replicate lineages of diverse thermal history.

Genomic analysis of the 20°C adapted lineages has found that several deletions of small portions of the chromosome have occurred independently in many of the lineages. Most remarkably, genes at approximately 1.85Mb on the circular chromosome were independently deleted in 6 of the 30 lines examined. We are in the process of mapping exactly which genes were involved in each lineage and will determine the genes in common that were deleted.

We previously demonstrated significant mortality of bacteria during repeated freeze-thaw cycles, and that populations that had grown and evolved for 20,000 generations at 37°C were more sensitive to this mortality than their ancestors. Based on these findings, we began an experiment to investigate the potential for evolutionary adaptation to cycles consisting of freeze, thaw, and growth at 37°C.