The persistence of genetic variation in master regulators of gene expression, such as quorum-sensing systems, is hard to explain. Here, we investigated two alternative hypotheses for the prevalence of polymorphic quorum sensing in Gram-positive bacteria, i.e., the use of different signal/receptor pairs (‘pherotypes’) to regulate the same functions. First, social interactions between pherotypes or ‘facultative cheating’ may favor rare variants that exploit the signals of others. Second, different pherotypes may increase fitness in different environments. We evaluated these hypotheses in the invertebrate pathogen Bacillus thuringiensis, using three pherotypes expressed in a common genetic background. Facultative cheating could occur in well-mixed host homogenates provided there was minimal cross talk between competing pherotypes. However, facultative cheating did not occur when spatial structure was increased in static cultures or in naturalistic oral infections, where common pherotypes had higher fitness. There was clear support for environment-dependent fitness; pherotypes varied in responsiveness to signals and in mean competitive fitness. Notably, competitive fitness varied with group size. In contrast to typical social evolution models of quorum sensing which predict higher response to signal at larger group size, the pherotype with highest responsiveness to signals performed best in smaller hosts where infections have a lower pathogen group size. In this system, low signal abundance appears to limit fitness in hosts, while the optimal level of response to signals varies in different host environments.

中文翻译：

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在具有不同病原体组大小的不同宿主环境中，对群体感应信号的最佳响应会有所不同。

基因表达的主要调控因子（例如群体感应系统）中遗传变异的持久性很难解释。在这里，我们调查了革兰氏阳性细菌中多态群体感应的普遍性的两个替代假设，即使用不同的信号/受体对（“外型”）来调节相同的功能。首先，个体之间的社交互动或“兼作弊”可能会偏rare利用他人信号的稀有变异。其次，不同的表型可能会增加在不同环境中的适应性。我们在无脊椎动物病原体苏云金芽孢杆菌中评估了这些假设，使用在共同的遗传背景下表达的三种表型。只要竞争型之间的串扰最小，在充分混合的宿主匀浆中可能发生兼性作弊。但是，当静态培养中的空间结构增加或自然型口腔感染（通常的表型具有较高的适应性）时，当空间结构增加时，不会发生兼职作弊。明确支持环境依赖性健身；表型在对信号的响应和平均竞争适应性上各不相同。值得注意的是，竞争适应度随组的大小而变化。与群体感知的典型社会进化模型预测群体更大时对信号的响应更高相比，对信号具有最高响应的表型在病原体群体较小的较小宿主中表现最佳。在这个系统中