Experimental evolution and characterisation of glyphosate resistance in green alga Chlamydomonas reinhardtii

Modern agriculture is completely dependent on chemical weed control to ensure crop yields, but the widespread and persistent use of herbicides has led to both a strong selective pressure for weeds to evolve resistance and pollution of non-target ecosystems. Understanding the evolutionary dynamics of herbicide resistance is pivotal to managing both aspects as they determine how weeds and organisms within non-target ecosystems will respond through time. Microbial model organisms like green alga Chlamydomonas reinhardtii provide the opportunity to control and monitor resistance evolution in action in the lab, as well as representing a major group of non-target organisms exposed to herbicide pollution. The dynamics of evolving glyphosate resistance in C. reinhardtii were thus here characterised by continuous flow-through cultures exposed to lethal and sublethal doses of glyphosate and continuous assaying through time to determine when resistance evolved, the extent of the increase in resistance, test for intrinsic and extrinsic fitness consequences, as well as creating a fingerprint of the changing cellular metabolic state. The results show rapid evolution to the lethal dose, with tentative evidence for the sublethal dose populations exhibiting the same pattern but at a delay. While intrinsic fitness consequences were indicated to be low or non-existent, an extrinsic fitness consequence was found in the form of increased variance in the ability to deploy anti-grazer defences. The metabolomics screen revealed extensive disruption of cellular machinery in response to glyphosate that was mitigated by the evolution of resistance, as well as strong evidence of a persisting secondary effect of glyphosate through oxidative damage after resistance evolved. This thesis thus emphasises the necessity of considering resistance evolution as a complex process with multiple components, the timing of which may matter to the outcome of management strategies aiming to prevent resistant weeds or conserve natural ecosystems.