Dean Andrew D, Minter Ewan J A, Sørensen Megan E S, Lowe Christopher D, Cameron Duncan D, Brockhurst Michael A, Jamie Wood A
Department of Biology, University of York, Wentworth Way, York YO10 5DD, UK; Department of Mathematics, University of York, York YO10 5DD, UK.
Department of Biology, University of York, Wentworth Way, York YO10 5DD, UK.
J Theor Biol. 2016 Sep 21;405:82-93. doi: 10.1016/j.jtbi.2016.02.021. Epub 2016 Feb 27.
Photosymbiosis is one of the most important evolutionary trajectories, resulting in the chloroplast and the subsequent development of all complex photosynthetic organisms. The ciliate Paramecium bursaria and the alga Chlorella have a well established and well studied light dependent endosymbiotic relationship. Despite its prominence, there remain many unanswered questions regarding the exact mechanisms of the photosymbiosis. Of particular interest is how a host maintains and manages its symbiont load in response to the allocation of nutrients between itself and its symbionts. Here we construct a detailed mathematical model, parameterised from the literature, that explicitly incorporates nutrient trading within a deterministic model of both partners. The model demonstrates how the symbiotic relationship can manifest as parasitism of the host by the symbionts, mutualism, wherein both partners benefit, or exploitation of the symbionts by the hosts. We show that the precise nature of the photosymbiosis is determined by both environmental conditions (how much light is available for photosynthesis) and the level of control a host has over its symbiont load. Our model provides a framework within which it is possible to pose detailed questions regarding the evolutionary behaviour of this important example of an established light dependent endosymbiosis; we focus on one question in particular, namely the evolution of host control, and show using an adaptive dynamics approach that a moderate level of host control may evolve provided the associated costs are not prohibitive.
光合共生是最重要的进化轨迹之一，它导致了叶绿体的形成以及所有复杂光合生物的后续发展。纤毛虫草履虫和藻类小球藻之间存在着一种已确立且经过充分研究的光依赖型内共生关系。尽管其很突出，但关于光合共生的确切机制仍有许多未解决的问题。特别令人感兴趣的是宿主如何根据自身与共生体之间的营养分配来维持和管理其共生体负载。在此，我们构建了一个详细的数学模型，该模型根据文献进行参数化，在一个确定性的双方模型中明确纳入了营养交换。该模型展示了共生关系如何表现为共生体对宿主的寄生、互利共生（即双方都受益）或宿主对共生体的剥削。我们表明，光合共生的确切性质由环境条件（有多少光可用于光合作用）以及宿主对其共生体负载的控制水平共同决定。我们的模型提供了一个框架，在此框架内可以就这种已确立的光依赖型内共生的重要例子的进化行为提出详细问题；我们特别关注一个问题，即宿主控制的进化，并使用自适应动力学方法表明，只要相关成本不是过高，适度水平的宿主控制可能会进化。
