Godany, M; (2016) Evolution of chemotaxis in stochastic environments. Doctoral thesis , UCL (University College London).
Most of our understanding of bacterial chemotaxis comes from studies of Escherichia coli. However, recent evidence suggests significant departures from the E. coli paradigm in other bacterial species. In the first part of this work, we argue that the observed departures may stem from different species inhabiting distinct environments and thus adapting differently to specific environmental pressures. We therefore study the performance of various chemotactic strate¬gies under a range of stochastic time- and space-varying attractant distributions in silico. We describe a novel type of response in which the bacterium tumbles more when attractant concen¬tration is increasing, in contrast to the “adaptive” response of E. coli, and demonstrate how this response explains the behavior of aerobically-grown Rhodobacter sphaeroides. In this “specu¬lator” response, bacteria compare the current attractant concentration to the long-term average. By tumbling persistently when the current concentration is higher than the average, bacteria maintain their position in regions of high attractant concentration. If the current concentration is lower than the average, or is declining, bacteria swim away in search of more favorable con¬ditions. When the attractant distribution is spatially complex but slowly-changing, this response is as effective as that of E. coli. In the latter part of this work, we show that optimal response sensitivity is high for both adaptive and speculator responses. We argue that response sensi¬tivity would increase over long evolutionary timescales and show that increases in response sensitivity could drive the evolution of adaptive and speculator responses.