Rohrlack Thomas, Haande Sigrid, Molversmyr Åge, Kyle Marcia
Norwegian University of Life Sciences, Institute for Environmental Studies, Postbox 5003, NO-1432 Ås, Norway.
Norwegian Institute for Water Research, Gaustadalléen 21, NO-0349 Oslo, Norway.
PLoS One. 2015 Dec 29;10(12):e0145559. doi: 10.1371/journal.pone.0145559. eCollection 2015.
Chytrid fungi are highly potent parasites of phytoplankton. They are thought to force phytoplankton organisms into an evolutionary arms race with high population diversity as the outcome. The underlying selection regime is known as Red Queen dynamics. However, our study suggests a more complex picture for chytrid parasitism in the cyanobacterium Planktothrix. Laboratory experiments identified a "cold thermal refuge", inside which Planktothrix can grow without chytrid infection. A field study in two Norwegian lakes underlined the ecological significance of this finding. The study utilized sediment DNA as a biological archive in combination with existing monitoring data. In one lake, temperature and light conditions forced Planktothrix outside the thermal refuge for most of the growing season. This probably resulted in Red Queen dynamics as suggested by a high parasitic pressure exerted by chytrids, an increase in Planktothrix genotype diversity over time, and a correlation between Planktothrix genotype diversity and duration of bloom events. In the second lake, a colder climate allowed Planktothrix to largely stay inside the thermal refuge. The parasitic pressure exerted by chytrids and Planktothrix genotype diversity remained low, indicating that Planktothrix successfully evaded the Red Queen dynamics. Episodic Planktothrix blooms were observed during spring and autumn circulation, in the metalimnion or under the ice. Interestingly, both lakes were dominated by the same or related Planktothrix genotypes. Taken together, our data suggest that, depending on environmental conditions, chytrid parasitism can impose distinct selection regimes on conspecific phytoplankton populations with similar genotype composition, causing these populations to behave and perhaps to evolve differently.
壶菌是浮游植物的高效寄生虫。人们认为它们迫使浮游植物生物进入一场进化军备竞赛，最终导致高种群多样性。潜在的选择机制被称为红皇后动态。然而，我们的研究表明，壶菌对蓝藻浮游丝体的寄生情况更为复杂。实验室实验发现了一个“冷温避难所”，浮游丝体在其中可以不受壶菌感染地生长。在挪威两个湖泊进行的一项实地研究强调了这一发现的生态意义。该研究将沉积物DNA作为生物档案，结合现有的监测数据。在一个湖泊中，温度和光照条件在生长季节的大部分时间里迫使浮游丝体离开温避难所。这可能导致了红皇后动态，表现为壶菌施加的高寄生压力、浮游丝体基因型多样性随时间增加，以及浮游丝体基因型多样性与水华事件持续时间之间的相关性。在第二个湖泊中，较冷的气候使浮游丝体基本停留在温避难所内。壶菌施加的寄生压力和浮游丝体基因型多样性仍然较低，这表明浮游丝体成功规避了红皇后动态。在春季和秋季环流期间、在温跃层或冰层下观察到了间歇性的浮游丝体水华。有趣的是，两个湖泊都以相同或相关的浮游丝体基因型为主。综合来看，我们的数据表明，根据环境条件，壶菌寄生可以对具有相似基因型组成的同种浮游植物种群施加不同的选择机制，导致这些种群表现不同，甚至可能进化方向不同。
