Iwai Sosuke, Fujiwara Kenji, Tamura Takuro
Department of Biology, Faculty of Education, Hirosaki University, Hirosaki, 036-8560, Japan.
Environ Microbiol. 2016 Sep;18(8):2435-45. doi: 10.1111/1462-2920.13140. Epub 2016 Jan 18.
Algal endosymbiosis is widely distributed in eukaryotes including many protists and metazoans, and plays important roles in aquatic ecosystems, combining phagotrophy and phototrophy. To maintain a stable symbiotic relationship, endosymbiont population size in the host must be properly regulated and maintained at a constant level; however, the mechanisms underlying the maintenance of algal endosymbionts are still largely unknown. Here we investigate the population dynamics of the unicellular ciliate Paramecium bursaria and its Chlorella-like algal endosymbiont under various experimental conditions in a simple culture system. Our results suggest that endosymbiont population size in P. bursaria was not regulated by active processes such as cell division coupling between the two organisms, or partitioning of the endosymbionts at host cell division. Regardless, endosymbiont population size was eventually adjusted to a nearly constant level once cells were grown with light and nutrients. To explain this apparent regulation of population size, we propose a simple mechanism based on the different growth properties (specifically the nutrient requirements) of the two organisms, and based from this develop a mathematical model to describe the population dynamics of host and endosymbiont. The proposed mechanism and model may provide a basis for understanding the maintenance of algal endosymbionts.
藻类内共生在包括许多原生生物和后生动物在内的真核生物中广泛分布，在水生生态系统中发挥着重要作用，它结合了吞噬营养和光合营养。为了维持稳定的共生关系，宿主内共生体的种群大小必须得到适当调节并维持在恒定水平；然而，维持藻类内共生体的潜在机制在很大程度上仍然未知。在这里，我们在一个简单的培养系统中，研究了单细胞纤毛虫草履虫及其类小球藻藻类内共生体在各种实验条件下的种群动态。我们的结果表明，草履虫内共生体的种群大小不受诸如两种生物体之间的细胞分裂耦合或宿主细胞分裂时内共生体的分配等主动过程的调节。尽管如此，一旦细胞在光照和营养条件下生长，内共生体的种群大小最终会调整到几乎恒定的水平。为了解释这种明显的种群大小调节，我们基于两种生物体不同的生长特性（特别是营养需求）提出了一种简单的机制，并据此建立了一个数学模型来描述宿主和内共生体的种群动态。所提出的机制和模型可能为理解藻类内共生体的维持提供基础。
