Institut de Biologie de l'École Normale Supérieur, Paris, France.
PLoS One. 2020 Jul 30;15(7):e0235811. doi: 10.1371/journal.pone.0235811. eCollection 2020.
Obligate symbiosis evolved from free-living individuals most likely via the intermediate stage of facultative symbiosis. However, why should facultative symbionts, who can live independently but also benefit from their partners if these are available, give up this best of both worlds? Using the adaptive dynamics approach, we analyse a simple model, focusing on one partner of the symbiosis, to gain more insight into the selective forces that make individuals forgo the ability to reproduce in the free-living state. Our results suggest that, similar to the parasitism-mutualism continuum, the free-living way of life and obligate symbiosis are two extremes of a continuum of the ability to reproduce independently of a partner. More importantly, facultative symbiosis should be the rule as for many parameter combinations completely giving up independent reproduction or adopting a pure free-living strategy is not so easy. We also show that if host encounter comes at a cost, individuals that put more effort into increasing the chances to meet with their partners are more likely to give up the ability to reproduce independently. Finally, our model does not specify the ecological interactions between hosts and symbionts but we discuss briefly how the ecological nature of an interaction can influence the transition from facultative to obligate symbiosis.
专性共生很可能是从自由生活的个体通过兼性共生的中间阶段进化而来的。然而，为什么兼性共生体（如果共生体存在，它们可以独立生活，但也可以从共生体中受益）要放弃这种两全其美的生活方式呢？我们使用适应性动态方法分析了一个简单的模型，重点关注共生关系的一个合作伙伴，以更深入地了解使个体放弃在自由生活状态下繁殖能力的选择压力。我们的研究结果表明，与寄生-共生连续体类似，自由生活方式和专性共生是独立于合作伙伴繁殖能力的连续体的两个极端。更重要的是，兼性共生应该是规则，因为对于许多参数组合，完全放弃独立繁殖或采用纯自由生活策略并不容易。我们还表明，如果宿主遭遇有代价，那么那些投入更多努力增加与伴侣相遇机会的个体更有可能放弃独立繁殖的能力。最后，我们的模型没有具体说明宿主和共生体之间的生态相互作用，但我们简要讨论了相互作用的生态性质如何影响从兼性共生到专性共生的转变。
