Evidence for a trade-off between glyphosate resistance and anti-grazer defence in green algaChlamydomonas reinhardtii

Abstract

The widespread and persistent use of herbicides has both selected for dramatically increased levels of herbicide resistance in the weed populations they were designed to control, and increased contamination of non-target ecosystems. Experimental evolution using microbes offers the opportunity to explore basic evolutionary theory, including testing for the existence of intrinsic and extrinsic fitness trade-offs, connecting them to the patterns observed in both natural and agricultural populations. We used green alga Chlamydomonas reinhardtii adapted to high and moderate levels of glyphosate to test for an extrinsic cost to glyphosate resistance in the form of a trade-off with clumping, the inducible anti-grazer defence deployed against gape-limited micrograzers. Through exposing the algae to freshwater rotifer Brachionus calyciflorus as well as their isolated info-chemicals, we test whether glyphosate resistance affects ability to deploy defences as well as ability to withstand grazing compared to control populations. We find an increase in variation rather than uni-directional effect of glyphosate treatment, with treated populations exhibiting both lower and higher degree of anti-grazer defence compared to controls. Furthermore, our data suggests there are at least three different glyphosate resistant phenotypes present, with two conferring different extrinsic costs in the form of tradeoff with anti-grazer defence. Our study indicates that extrinsic costs to glyphosate resistance may be context dependent in this system, possibly only being present at particular stages in the adaptation process or with particular resistance mechanisms.