在涉及混雜細菌菌落的實驗演化的實驗中,，病原體Pseudomonas aeruginosa的個體被發(fā)現(xiàn)彼此之間進行合作，以便能夠以鐵為食,。這種能力是細菌感染中生長和毒性的一個關鍵決定因素,，因為鐵對細菌生長來說經(jīng)常是限制其生長速度的成分,。寄主積極持留來自寄生蟲的鐵,，而這些寄生蟲可以通過合作的方式來克服寄主的防衛(wèi),。這種現(xiàn)象取決于相關細菌間的合作，是一種親情選擇,。病原體細菌間的合作很可能是決定細菌毒性的一個因素,，但也可能成為新的干預方法的一個目標。

Cooperation and competition in pathogenic bacteria

Explaining altruistic cooperation is one of the greatest challenges for evolutionary biology1-3. One solution to this problem is if costly cooperative behaviours are directed towards relatives4, 5. This idea of kin selection has been hugely influential and applied widely from microorganisms to vertebrates2-10. However, a problem arises if there is local competition for resources, because this leads to competition between relatives, reducing selection for cooperation3, 11-14. Here we use an experimental evolution approach to test the effect of the scale of competition, and how it interacts with relatedness. The cooperative trait that we examine is the production of siderophores, iron-scavenging agents, in the pathogenic bacterium Pseudomonas aeruginosa15-17. As expected, our results show that higher levels of cooperative siderophore production evolve in the higher relatedness treatments. However, our results also show that more local competition selects for lower levels of siderophore production and that there is a significant interaction between relatedness and the scale of competition, with relatedness having less effect when the scale of competition is more local. More generally, the scale of competition is likely to be of particular importance for the evolution of cooperation in microorganisms, and also the virulence of pathogenic microorganisms, because cooperative traits such as siderophore production have an important role in determining virulence6, 9, 17-19.

 
 
Figure 1 Scale of competition and kin selection theory. We have plotted the effect of the scale of competition on selection for an altruistic trait, from the incorporation of Frank's3 scale of competition parameter, a, into a classic tragedy of the commons formulation3,17. This allows a simple and general graphical representation of the theoretical predictions. The y axis gives the evolutionary stable allocation of resources to a cooperative trait that is costly, but provides a benefit locally3,25. The scale of competition varies from global (a = 0) to local (a = 1). The different lines represent relatively high (r = 0.75) and relatively low (r = 0.25) relatedness. Higher levels of cooperation are favoured when relatedness is higher (higher r), and competition is more global (lower a). Furthermore, there is an interaction between scale of competition and relatedness: as competition becomes more local, the influence of relatedness on selection for cooperation is reduced. In the extreme, if a = 1, then competition is completely local and so kin selection cannot favour altruism3,22. The same conclusions can be reached using Queller's11 approach of allowing for local competition, or with a model specifically developed for siderophore production17. 

 
 
Figure 2 Experimental design. We varied relatedness between interacting individuals by initiating each subpopulation with either a single bacterial clone (relatively high r) or two bacterial clones (relatively low r). We varied the scale of competition by either mixing the cultures from all of the subpopulations in a treatment before plating, and then transferring random colonies from this single plate to initiate new subpopulations (relatively global competition, lower a), or by allowing every subpopulation in a treatment to provide equal numbers of colonies to the next generation (relatively local competition, higher a). We use dark green to symbolize the siderophore-producing cooperator, white to symbolize a cheater that does not produce siderophores, and light green for a mixture of cooperators and cheaters. 

 
 
Figure 3 The evolution of cooperation in response to relatedness and the scale of competition. The proportion of cooperating individuals who produce pyoverdin siderophores is plotted against time. The different lines represent relatively high (solid line) and low (dashed line) relatedness. The different symbols represent relatively global (circle) and local (triangle) competition. Each of the four treatments was replicated four times, and standard errors are shown for the final time point. Time is measured as transfers, between which cultures were allowed to grow for 24 h. Cooperation is favoured by higher relatedness and more global competition.