Sachs Joel L, Wilcox Thomas P
Section of Integrative Biology, Patterson Laboratories, 1 University Station C0930, University of Texas, Austin, TX 78712-0253, USA.
Proc Biol Sci. 2006 Feb 22;273(1585):425-9. doi: 10.1098/rspb.2005.3346.
One of the outstanding and poorly understood examples of cooperation between species is found in corals, hydras and jellyfish that form symbioses with algae. These mutualistic algae are mostly acquired infectiously from the seawater and, according to models of virulence evolution, should be selected to parasitize their hosts. We altered algal transmission between jellyfish hosts in the laboratory to examine the potential for virulence evolution in this widespread symbiosis. In one experimental treatment, vertical transmission of algae (parent to offspring) selected for symbiont cooperation, because symbiont fitness was tied to host reproduction. In the other treatment, horizontal transmission (infectious spread) decoupled symbiont fitness from the host, potentially allowing parasitic symbionts to spread. Fitness estimates revealed a striking shift to parasitism in the horizontal treatment. The horizontally transmitted algae proliferated faster within hosts and had higher dispersal rates from hosts compared to the vertical treatment, while reducing host reproduction and growth. However, a trade-off was detected between harm caused to hosts and symbiont fitness. Virulence trade-offs have been modelled for pathogens and may be critical in stabilising 'infectious' symbioses. Our results demonstrate the dynamic nature of this symbiosis and illustrate the potential ease with which beneficial symbionts can evolve into parasites.
物种间合作的一个显著且鲜为人知的例子存在于与藻类形成共生关系的珊瑚、水螅和水母中。这些互利共生的藻类大多是通过感染从海水中获取的，根据毒力进化模型，它们应该会被选择来寄生其宿主。我们在实验室中改变了水母宿主之间藻类的传播方式，以研究这种广泛存在的共生关系中毒力进化的可能性。在一种实验处理中，藻类的垂直传播（从亲代到子代）选择了共生体的合作，因为共生体的适应性与宿主繁殖相关。在另一种处理中，水平传播（感染性传播）使共生体的适应性与宿主脱钩，这可能会使寄生性共生体得以传播。适应性估计显示，在水平处理中出现了向寄生状态的显著转变。与垂直处理相比，水平传播的藻类在宿主体内增殖更快，从宿主中扩散的速率更高，同时降低了宿主的繁殖和生长。然而，在对宿主造成的伤害和共生体适应性之间发现了一种权衡。毒力权衡已针对病原体进行了建模，可能对稳定“感染性”共生关系至关重要。我们的结果证明了这种共生关系的动态性质，并说明了有益共生体演变为寄生虫的潜在易发性。
