Intra- and inter-specific competition and the evolution of cooperation in Bacillus thuringiensis

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Cooperation is a common feature of bacterial lifestyles. This may be particularly true of bacteria that cause diseases (pathogenic bacteria) and of beneficial bacteria that live in close association with larger hosts (symbiotic bacteria). Many of the essential tools that enable bacteria to exploit hosts are based on what can be called 'public goods'. These are enzymes or toxins and other compounds that bacteria must export outside the cell in order to break open host cells and harvest the resources. Bacteria also export chemical signals that communicate information about their abundance within hosts to other bacteria and may thereby coordinate attack. Efficient use of hosts therefore requires bacteria to act collectively, if a low proportion of bacteria fail to cooperate bacterial infections should to be less successful and produce fewer infections in new hosts. These public goods are expensive to make in terms of resources. In evolutionary terms cooperation can be unstable because bacteria may leave more offspring within hosts if they 'cheat' and fail to contribute to these expensive cooperative products. Evolutionary theory has made predictions about how cooperation could be maintained. If most infections are established by close relatives with similar strategies, metabolically expensive cooperation will benefit their relative and they, in turn will then spread the genes for cooperation. In addition, while competition within hosts can lead to selection for cheating, competition between groups of bacteria inhabiting different hosts will select for groups that exploit their host more efficiently, and which therefore cooperate. The evolutionary forces that can maintain cooperation between hosts and symbiotic bacteria are diverse. However, one possible mechanism is that host can discriminate between bacteria that are exploitative or not and produce increased immune responses against symbionts that are not cooperative. I propose to test these evolutionary ideas on cooperation, in relation to the production of toxins, antibiotics and chemical signals. Prelimary data also indicate that the exploitation of hosts by Bt is strongly affect by competition with symbionts such as P. agglomerans. I will test how competition with symbionts affects the expression of cooperative toxins. Conversely, these symbionts can cooperate with Bt rather than continue to cooperate with hosts as gut symbionts. I will test how host insects react to infections with 'cheating' symbionts. I will use a study system which is familiar to me and also of environmental and medical importance. This system is the insect-killing bacteria Baccillus thuringiensis, a caterpillar host (the larvae of the diamondback moth) and the gut symbiont Pantoea agglomerans. B. thuringiensis (Bt) is used as a biological pesticide. It is applied against pests in horticulture, forestry and fruit productionan and against mosquito larvae. It has an excellent safety record, it does not harm humans, animals or beneficial insect predators and is licensed as an organic spray. While Bt pesticides are efficient at killing pests they are relatively poor at being transmitted as a disease from pest to pest after spraying. Improved transmission would have many benefits for the ability of Bt to control pests. Preliminary data in my laboratory suggests that cooperative traits are vital for efficient transmission between hosts, as the above theory predicts. An understanding of how cooperation maintains efficient transmission and transmission maintains cooperation could therefore be vital to understanding how to improve its use. Bt is closely related to the bacteria that causes anthrax, Bacillus anthracis and to Bacillus cereus, several strains of which cause food-poisoning in humans. These human pathogens use very similar biochemical machinery to Bt and a understanding of how these bacteria cooperate to exploit hosts may eventually be of medical significance.