Charlat S, Calmet C, Merçot H
Institut Jacques Monod, Laboratoire Dynamique du Génome et Evolution, CNRS, Université Paris 6 & 7, 75251 Paris, Cedex 05 France.
Genetics. 2001 Dec;159(4):1415-22. doi: 10.1093/genetics/159.4.1415.
Cytoplasmic incompatibility (CI) is induced by the endocellular bacterium Wolbachia. It results in an embryonic mortality occurring when infected males mate with uninfected females. The mechanism involved is currently unknown, but the mod resc model allows interpretation of all observations made so far. It postulates the existence of two bacterial functions: modification (mod) and rescue (resc). The mod function acts in the males' germline, before Wolbachia are shed from maturing sperm. If sperm is affected by mod, zygote development will fail unless resc is expressed in the egg. Interestingly, CI is also observed in crosses between infected males and infected females when the two partners bear different Wolbachia strains, demonstrating that mod and resc interact in a specific manner: Two Wolbachia strains are compatible with each other only if they harbor the same compatibility type. Here we focus on the evolutionary process involved in the emergence of new compatibility types from ancestral ones. We argue that new compatibility types are likely to evolve under a wider range of conditions than previously thought, through a two-step process. First, new mod variants can arise by mutation and spread by drift. This is possible because mod is expressed in males and Wolbachia is transmitted by females. Second, once such a mod variant achieves a certain frequency, it can create the conditions for the deterministic invasion of a new resc variant, allowing the invasion of a new mod resc pair. Furthermore, we show that a stable polymorphism might be maintained in natural populations, allowing the long-term existence of "suicidal" Wolbachia strains.
细胞质不亲和性(CI)由细胞内细菌沃尔巴克氏体诱导产生。当受感染的雄性与未受感染的雌性交配时,会导致胚胎死亡。目前尚不清楚其中涉及的机制,但mod resc模型能够解释迄今为止所做的所有观察结果。该模型假定存在两种细菌功能:修饰(mod)和拯救(resc)。mod功能在雄性生殖系中起作用,发生在沃尔巴克氏体从成熟精子中释放之前。如果精子受到mod的影响,除非卵子中表达resc,否则合子发育将会失败。有趣的是,当两个交配的受感染雄性和雌性携带不同的沃尔巴克氏体菌株时,也会观察到细胞质不亲和性,这表明mod和resc以一种特定方式相互作用:只有当两种沃尔巴克氏体菌株具有相同的亲和类型时,它们才相互兼容。在这里,我们关注从祖先亲和类型中产生新亲和类型所涉及的进化过程。我们认为,新的亲和类型可能通过两步过程在比之前认为的更广泛的条件下进化。首先,新的mod变体可能通过突变产生并通过漂变传播。这是可能的,因为mod在雄性中表达,而沃尔巴克氏体由雌性传播。其次,一旦这样的mod变体达到一定频率,它就可以为新的resc变体的确定性入侵创造条件,从而允许新的mod resc对的入侵。此外,我们表明自然种群中可能维持稳定的多态性,从而允许“自杀性”沃尔巴克氏体菌株长期存在。