Group selection as a novel tool to screen and improve biological pesticides

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In modern agriculture and horticulture there is more and importance placed on exploiting naturally occurring organisms, such as the pathogens of insects, for pest management. Microbes such as Bacillus thuringiensis (Bt) are currently the main active ingredients in organically licensed applications targeted at caterpillar, mosquito and beetle pests. In addition, the same bacterial species is an important source of proteins for genetically modified plants, as it produces many proteins that are highly toxic to a narrow range of insects but safe for humans and wildlife.Nevertheless, identifying novel virulent bacteria, and identifying new proteins that are toxic to insects, is a difficult and challenging task. The foundation of this proposal is the idea that if we can understand how investment in virulence factors (toxic proteins produced by microbes) increases the reproduction and fitness of microbes, then we can apply selection-based methods in the laboratory to isolate mutants or variants with high virulence. Prior to this proposal we made an important advance in understanding the selection pressure that maintains virulence. Pathogens such as Bt have to invest energy and resources in exporting virulence factors that help them infect insects. Investing in virulence is therefore costly for individual cells, but essential, as bacterial reproduction requires the death of the host. Individual level selection (within hosts) can favour mutants, known as 'cheaters', which rely on the proteins produced by their neighbours for successful infection. Conversely, selection at the group level (between infections) maintains virulence. Preliminary experiments have shown that group level selection (in combination with a supply of mutations) can lead to the emergence of mutants with increased levels of virulence, even if these mutations are costly to the individual.The group selection methods we have developed offer a new tool for: exploring existing microbial diversity and trying to improve microbial biocontrol agents. Moreover, for Bt, the critical proteins involved in killing host are the Cry toxins, and there is enormous applied interest in generating new and better Cry toxins that can kill hard-to-target pests. Using genetic engineering methods we can artificially construct a collection of Bt strains that expresses a diverse collection of these proteins, all with a subtly different make-up. Here we propose to use our new selection methods to pick out new and interesting proteins from this diverse mix.A major question for this proposal is to better understand when and how we might be able to apply these group selection methods. Evolutionary theory can help us refine our methods, for instance by predicting how the make up of groups affects the power of selection. Another consideration, that is important for understanding the applied potential of this method, is to understand whether we can only use selection to improve strains with low fitness, or whether we can also improve strains that are already quite effective at killing insect hosts.Another important prediction is that selection experiments might help us better understand how pathogens kill insect hosts, and identify the proteins responsible in Bt. When bacteria have evolved increases in virulence, we can use the change in DNA sequence, or the proteins expressed, to identify which molecules are responsible for increased virulence. Given our lack of knowledge on the function of the of the majority of proteins produced by Bt, we expect to gain significant extra knowledge regarding the means by which these bacteria can kill hosts and overcome their immune systems.