Does Herbicide Resistance Have a Cost in Brassica rapa?

Abstract

The evolution of resistance to pesticides has become a classic example of natural selection at work (Palumbi, 2001). As humans chemically modify their environment, organisms with short generation times respond. Examples include resistance of bacteria to antibiotics, of insects to insecticides, and of weeds to herbicides. Here the focus is on resistance to herbicides in weedy plants. In particular, genetic strains of rapid-cycling Brassica rapa offer opportunities to study resistance to the herbicide atrazine, a triazine herbicide. In the presence of the herbicide, individuals of the resistant strain survive whereas individuals of the susceptible strain die. This experiment has been used in a college-level biology class but may, also be applicable to an Advanced Placement Biology class in high school. In addition to the biological content, students gain experience in hypothesis formation and testing, experimental design, and statistical analysis. Herbicide resistance may come with a cost to the organism. Resistance to triazine herbicides is often dependent on a mutation that also inhibits photosynthesis (Warwick, 1991; Holt et al., 1993). For atrazine to kill a plant, it first binds to the polypeptide psbA, located in the thylacoid membrane of chloroplasts. In resistant plants of Amaranthus hybridus, the herbicide fails to bind to psbA. One mutation, in the chloroplast DNA coding for psbA, accounts for the difference between susceptible and resistant plants (Hirschberg & McIntosh, 1983). The mutation that confers resistance also inhibits electron transfer in Photosystem II and under certain environmental conditions will inhibit the rate of photosynthesis (Dekker & Sharkey, 1992). Photosystem II is part of the energy-capturing apparatus of chloroplasts. When chlorophyll in Photosystem II absorbs a photon of light, it initiates a complex cascade of electron transport that eventually captures the energy in chemical form in the chloroplast. If electron transport is inhibited, less energy, can be captured (see Campbell & Reece, 2002, for a lucid and detailed explanation). In the absence of the herbicide, resistant organisms may be disadvantaged. Reduction of photosynthesis will lead to reduced biomass accumulation and may negatively affect competitive ability and fitness (Warwick, 1991; Warwick & Black, 1994). Williams et al. (1995) worked with two strains of jimsonweed (Datura stramonium) that were either susceptible or resistant to the triazine herbicides. They found that resistant jimsonweeds, when grown alone, had lower total biomass and lower reproductive biomass than susceptible jimsonweeds. When jimsonweed was grown in competition with maize, its total biomass and reproductive biomass were still lower than without competition. Resistant jimsonweeds were more negatively affected by competition than susceptible jimsonweeds. Cost of resistance is defined as the reduction in fitness of resistant plants grown in the absence of herbicide. Brassica rapa, in the form of Wisconsin Fast Plants[TM] (Hafner, 1990), is ideal for studying the cost of herbicide resistance. An atrazine-resistant strain (Aaa, Rci, zr, atrazine resistant) can be grown alone and in combination with an atrazine-susceptible strain without the use of herbicide. Cost of resistance can be measured as the reduction in growth and/or reproduction of the resistant strain relative to the susceptible strain. Competitive ability can be measured by comparing plants of one strain grown alone with plants of the same strain grown in combination with the second strain. When two plants of the same strain are grown together they experience intrastrain competition, whereas a resistant plant and a susceptible plant growing together experience inter-strain competition. Based on the experiment described below, students should be able to answer the questions: * Does herbicide resistance have a cost in terms of growth and reproduction in rapid-cycling Brassica rapa? …