| Recently as cyanobacteria blooms has become one of most common environmental problem, the cyanobacteria-zooplankton interactions attract more and more attention. Research shows that zooplankton can adapt to cyanobacteria or predation through various mechanisms, including rapid evolution and phenotypic plasticity, we used Simocephalus vetulus and Daphnia pulex as the test subject to explore the effects of mosquito fish (Gambusia affinis) predation and Microcystis aeruginosa on these two adaption modes.First, to determine how G. affinis modulates the resistance of D. pulex and S. vetulus to toxic M. aeruginosa, we divided a population of 50 clones to four groups which then were reared under different conditions for two months. Control group was fed with 100% Chlorella pyrenoidosa as a good food. One group was exposed to G. affinis every four days while being fed with 100% C. pyrenoidosa (later was referred as Gambusia group). One group was exposed to 25% M. aeruginosa only (later was referred as Microcystis group). And another group was exposed to G. affinis every four days while being fed with 25% M. aeruginosa and 75% C. pyrenoidosa (later was referred as Gambusia+Microcystis group). Then life table experiment was performed to evaluate the resistance against M. aeruginosa for the four groups. We found that the resistance of the three test groups is significantly higher than that of control group, and that the resistance of Gambusia+Microcystis group is significantly higher than that of both Gambusia group and Microcystis group. After genetically identifying D. pulex clone with 11 microsatellite primers, we found that the genetic diversity of three test groups were lower than control group. At the end of two months artificial selection we evaluated the resistance of four groups of D. pulex to G. affinis using predation rate as an index. We found no significant differences among four groups. All these suggest that D. pulex can adapt to M. aeruginosa via evolutionary responses. And being exposed toG. affinis predation risk can enhance the resistance of both zooplankton against M. aeruginosa. This can be evidence of evolutionary adaption of zooplankton to unfavored environment and facilitates understanding of adaption mechanisms.Second, to explore the correlation between the resistance of zooplankton against G. affinis predation and against toxic M. aeruginosa, we isolated 45 S. vetulus clones and 45 D. pulex clones from the same location, using population growth rate and predation rate to evaluate the ability to resist M. aeruginosa and fish predation. We found significant variation of population growth rate, resistance and predation rate among 45 clones of both S. vetulus and D. pulex. Bivariate correlation analysis results showed no correlation between population growth rate or resistance and predation rate. This indicates that both the resistance against M. aeruginosa and the ability of resisting fish predation vary among clones. But there may be no significant correlation between these two strategies. Third, we investigated how toxic M. aeruginosa modulate the phenotypic plasticity in D. pulex responding to fish kairomones. Neonates(FO) from the same clone were reared under three different conditions. Control group was reared in tap water and fed with 100% C. pyrenoidosa. Kairomone group was reared in G:affinis kairomone medium while being fed with 100% C. pyrenoidosa. Another group was also reared in G. affinis kairomone medium while being fed with 50% M. aeruginosa and 50% C. pyrenoidosa (later was referred as Kairomone+ Microcystis group). We found no significant difference between the tail spine length of first-instar juvenile(F1) in control group and that in Kairomone group. Keeping F1 exposed to kairomone, we found that the tail spine length, body length and relative tail spine length were all significantly increased comparing to control group. And F0 D. pelux in Kairomone group had a high reproductive rate than control group. Besides the relative expression levels of Hox3 andMet were down-regulated, while the relative expression levels of exd, esg, InR and IRS-1 were up-regulated in first-instar juveniles. As for Kairomone+M/crocystis group, results showed that the tail spine length, body length and relative tail spine length were significantly decreased comparing to control group. The relative expression level of Hox3, exd, esg, JHAMT, InR and IRS-1 were all up-regulated, whereas the relative expression level of Met was down-regulated. In addition, the relative expression levels of these seven genes were all higher than that of first-instar juveniles in Kairomone group. These suggest that D. pelux shows inducible defenses to G. affinis kairomone. And being exposed to toxic M. aeruginosa simultaneously may weaken this phenotypic changes. This implicates that there is energy allocation in D. pulex when facing with multiple stresses. And it helps us to comprehend the adaption mechanism of zooplankton when facing multiple changes.After two months artificial selection, we performed life table experiment to evaluate the resistance of two aquatic grazers against toxic cyanobacteria and provided evidence for rapid evolution through genetically identifying D. pulex clones with 11 microsatellite primers. Then we used population growth rate and predation rate to investigate whether there is correlation between resistance to toxic M. aeruginosa and fish predation. Finally, we treated D. pulex with G. affinis kairomone and 50% M. aeruginosa to see how toxic c modulates phenotypic plasticity induced by fish kairomone. We explored how fish predation and toxic cyanobacteria interfere with the adaption of zooplankton to environmental cues. Further study can focus on the mechanisms underlying phenotypic plasticity and rapid evolution in other. |
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