导读：微生物生存在复杂的环境中游刃有余，尽管它们并没有携带所有的应付各种环境条件的功能基因。一些微生物会依赖自己周围的其他微生物来处理环境压力，从而打好生存这副牌。
Prochlorococcus（原绿球藻）依靠其他微生物清楚坏境中的过氧化氢
微生物有时候会失去看起来对生存必不可少的功能，但它们仍然以某种方式存活了下来并且进行增殖。这是如何做到的呢？一个发表在美国微生物协会的在线杂志上的理论对此进行了解释。该理论认为，失去必要生存功能的微生物让其他微生物为自己来完成那些艰难的任务，这种适应机制使微生物在一个合作性的群落中生存下来。
该理论的提出者把它叫做“黑桃皇后理论”，该理论认为，一些微生物的需求可以由其他的微生物来满足，使得微生物可以减少所必须携带的基因并互相依赖，实现更高效的生存。在这些情况下，这对于微生物具有进化上的意义，可让它们舍弃累赘基因。研究的作者，来自密歇根州立大学的Richard Lenski及J. Jeffrey Morris以及田纳西大学的Erik Zinser以红心大战游戏中的黑桃皇后为该理论命名，在这个游戏中，通常的策略是要避免拿到黑桃皇后这张牌的。
“这是一个全面的理论，解释了自由生存的微生物如何进化为互为依赖地生存，”负责编辑修改论文的哈佛大学的Richard Losick说。“这个理论的核心就是，许多功能基因产物从细胞中分泌出来成为共有的，”他说道。
为了说明该理论，作者们将该理论应用到一个已经引发了一些困扰的特定微生物系统中：在海洋中一种最为普通的浮游生物Prochlorococcus，其基因组要比想象的小的多。科学家一直困惑，这种生物如何在失去用于抵消过氧化氢（一种可以损伤甚至杀死细胞的化合物）的过氧化氢酶-过氧化物酶基因后还能如此成功地生存下来。作者认为，Prochlorococcus是依靠它们周围的其他微生物来清除环境中的过氧化氢，它们将自己的责任扔给了自己周围拿了“差牌”的玩家。这是一个某些物种减少自己基因携带量而依靠群落中其他物种解决问题的例子。
Losick说，“黑桃皇后理论”提供了一种新的方式来研究复杂的微生物群落内部的依赖现象。“我对微生物如何形成生物膜这种通常由许多不同中细菌组成的复杂自然群落有特别的兴趣。黑桃皇后理论提供很有价值的新方法来研究思考生物膜群落中的成员的共进化。”
The Black Queen Hypothesis: Evolution of Dependencies through Adaptive Gene Loss
J. Jeffrey Morris, Richard E. Lenski, and Erik R. Zinserc
Reductive genomic evolution, driven by genetic drift, is common in endosymbiotic bacteria. Genome reduction is less common in free-living organisms, but it has occurred in the numerically dominant open-ocean bacterioplankton Prochlorococcus and “Candidatus Pelagibacter,” and in these cases the reduction appears to be driven by natural selection rather than drift. Gene loss in free-living organisms may leave them dependent on cooccurring microbes for lost metabolic functions. We present the Black Queen Hypothesis (BQH), a novel theory of reductive evolution that explains how selection leads to such dependencies; its name refers to the queen of spades in the game Hearts, where the usual strategy is to avoid taking this card. Gene loss can provide a selective advantage by conserving an organism’s limiting resources, provided the gene’s function is dispensable. Many vital genetic functions are leaky, thereby unavoidably producing public goods that are available to the entire community. Such leaky functions are thus dispensable for individuals, provided they are not lost entirely from the community. The BQH predicts that the loss of a costly, leaky function is selectively favored at the individual level and will proceed until the production of public goods is just sufficient to support the equilibrium community; at that point, the benefit of any further loss would be offset by the cost. Evolution in accordance with the BQH thus generates “beneficiaries” of reduced genomic content that are dependent on leaky “helpers,” and it may explain the observed nonuniversality of prototrophy, stress resistance, and other cellular functions in the microbial world.
文献链接：https://mbio.asm.org/content/3/2/e00036-12.abstract?sid=5573c3d8-4e0a-49ab-bc41-cab2b433e1c7
