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Project Progress

This project analyzes the evolutionary adaptation of experimental lineages of the bacterium Escherichia coli to low temperatures. We have established thirty replicate lineages (having diverse thermal histories) that have now been adapted to 20^ºC 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 found that adaptation to 20^ºC very frequently involves a loss of competitive fitness at 40^ºC, indicating a frequent tradeoff associated with low-temperature adaptation. Genetic analysis indicates that gene deletion events are more common than gene duplication events during the adaptation to low temperature. Both genetic and adaptive tradeoff analyses will continue next year.

We piloted three different types of experiments on further low-temperature selection in these lineages. These include cyclic freeze-thaw exposure, static exposure to 4^ºC, and serial propagation at 12-14^ºC. We will undertake all three different types of selection experiments during the coming year to establish new lineages of cold-adapted bacteria for further study.

With respect to work on tolerance to prolonged freezing and freeze-thaw cycles, we found that most of the mortality is associated with the latter in experiments with E. coli. In the absence of glycerol as a cryoprotectant, each daily round of freeze-thaw cycles causes about 10-fold more mortality than an equal time spent at constant -80^ºC. We also found that lines of E. coli that have adapted for 20,000 generations to growth at 37^ºC have become even more sensitive to the harmful effects of freeze-thaw cycles. Thus, there is genetic variation in this trait, and therefore potential for a response to selection in the experiments we will begin this coming year.