Prochlorococcus（原綠球藻）依靠其他微生物清楚壞境中的過氧化氫
微生物有時(shí)候會(huì)失去看起來對(duì)生存必不可少的功能,，但它們?nèi)匀灰阅撤N方式存活了下來并且進(jìn)行增殖。這是如何做到的呢,？一個(gè)發(fā)表在美國(guó)微生物協(xié)會(huì)的在線雜志上的理論對(duì)此進(jìn)行了解釋,。該理論認(rèn)為，失去必要生存功能的微生物讓其他微生物為自己來完成那些艱難的任務(wù),，這種適應(yīng)機(jī)制使微生物在一個(gè)合作性的群落中生存下來,。
該理論的提出者把它叫做“黑桃皇后理論”，該理論認(rèn)為,，一些微生物的需求可以由其他的微生物來滿足,，使得微生物可以減少所必須攜帶的基因并互相依賴，實(shí)現(xiàn)更高效的生存,。在這些情況下,，這對(duì)于微生物具有進(jìn)化上的意義，可讓它們舍棄累贅基因,。研究的作者,，來自密歇根州立大學(xué)的Richard Lenski及J. Jeffrey Morris以及田納西大學(xué)的Erik Zinser以紅心大戰(zhàn)游戲中的黑桃皇后為該理論命名，在這個(gè)游戲中,，通常的策略是要避免拿到黑桃皇后這張牌的,。
“這是一個(gè)全面的理論，解釋了自由生存的微生物如何進(jìn)化為互為依賴地生存,，”負(fù)責(zé)編輯修改論文的哈佛大學(xué)的Richard Losick說,。“這個(gè)理論的核心就是，許多功能基因產(chǎn)物從細(xì)胞中分泌出來成為共有的,，”他說道,。
為了說明該理論,，作者們將該理論應(yīng)用到一個(gè)已經(jīng)引發(fā)了一些困擾的特定微生物系統(tǒng)中：在海洋中一種最為普通的浮游生物Prochlorococcus，其基因組要比想象的小的多,?？茖W(xué)家一直困惑，這種生物如何在失去用于抵消過氧化氫（一種可以損傷甚至殺死細(xì)胞的化合物）的過氧化氫酶-過氧化物酶基因后還能如此成功地生存下來,。作者認(rèn)為,，Prochlorococcus是依靠它們周圍的其他微生物來清除環(huán)境中的過氧化氫，它們將自己的責(zé)任扔給了自己周圍拿了“差牌”的玩家,。這是一個(gè)某些物種減少自己基因攜帶量而依靠群落中其他物種解決問題的例子,。
Losick說，“黑桃皇后理論”提供了一種新的方式來研究復(fù)雜的微生物群落內(nèi)部的依賴現(xiàn)象,。“我對(duì)微生物如何形成生物膜這種通常由許多不同中細(xì)菌組成的復(fù)雜自然群落有特別的興趣,。黑桃皇后理論提供很有價(jià)值的新方法來研究思考生物膜群落中的成員的共進(jìn)化。”（生物谷 bioon.com）
doi:10.1128/mBio.00036-12
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The Black Queen Hypothesis: Evolution of Dependencies through Adaptive Gene Loss
J. Jeffrey Morris, Richard E. Lenski, Erik R. Zinser
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.
