| Cladocerans are not only the main predators of the phytoplankton,but also the main food source of higher vertebrates or invertebrates.Therefore,they play a top-down role in the aquatic ecosystem and an extremely important role in the energy flow and information transmission of the ecosystem.On this basis,the inducible defense of Daphnia to the vertebrate and invertebrate predators,which has become an important subject of ecological research.The predator-induced defense response of cladocerans mainly includes life history,morphology,behavior,etc.These anti-predator defense responses can effectively reduce the predation efficiency of predators,reduce the loss of biomass caused by predation,and help maintain the stability of the prey population.Due to the limitations of cost and resource allocation,the anti-predator defense response of cladocerans is extremely susceptible to various environmental factors.Against the background of global warming and eutrophication of water bodies,cyanobacteria blooms frequently occur in freshwater ecosystems.The large-scale outbreak of cyanobacteria blooms has become a worldwide problem.It will not only directly affect cladocerans who eat it,but also further affect the interspecies relationship between cladocerans and higher trophic levels through the food chain and eventually lead to ecological imbalance.As one of the dominant species of cyanobacteria blooms,Microcystis has two types: toxic and non-toxic.Because of the difference in toxins,the two types of Microcystis have different ways in influencing inducible defense.In order to comprehensively study the effects of Microcystis on the inducible defense of cladocerans against predators,this paper explored the effects of toxic and non-toxic Microcystis on Daphnia pulex's defense response to fish predation.1.The effect of toxic Microcystis on resource allocation in Daphnia pulex's inducible defense process against fish predationWe exposed two clones of D.pulex(Shanghai and Zhenjiang clones)to different combinations of fish cues and a diet containing toxic Microcystis(0%,10%,20%,30%).Under the fish cues,the body length at maturation became smaller,the relative tail spine length became longer,and the somatic growth rate decreased.As the concentration of Microcystis increased,the content of microcystin in the individual's body increased and the body length at maturation and the number of offspring were significantly reduced.The morphological defense of D.pulex was not affected by the Microcystis at a concentration of 30% and below.However,the reproduction rate induced by fish cues in different clones tended to increase or not change,which was related to the decreased or unchanged response of somatic growth rate under the increasing Microcystis concentration.2.The life history and morphological defense of Daphnia pulex under fish predation changes with the increasing concentration of non-toxic MicrocystisIn order to explore the effect of increasing non-toxic Microcystis concentration on the inducible defense of Cladocerans against fish predation during the cyanobacteria bloom,we exposed D.pulex to seven non-toxic Microcystis concentrations(0%,10%,20%,40%,60%,80%,100%)combined with fish cues.As the concentration of non-toxic Microcystis increased,individuals became smaller,the number of offspring decreased,and mature time delayed.The adverse effects of non-toxic Microcystis increased with the exposure time.Non-toxic Microcystis inhibited the defense response of D.pulex to fish,which was mainly manifested in inhibiting the expression of phenotypic defense(tail spine length,relative tail spine length)and reproduction.On the other hand,fish predation also had a positive effect on D.pulex under the non-toxic Microcystis.Fish cues shorted the embryonic development time under the stress of non-toxic Microcystis.3.Transcriptomics analysis of Daphnia pulex's inducible defense against fish predation under toxic and non-toxic Microcystis stressIn order to compare the effect of toxic and non-toxic Microcystis on D.pulex's inducible defense against fish predation,and to reveal the possible mechanism about the influence of Microcystis on the inducible defense of cladocerans,we exposed D.pulex to 100% Chlorella,30% toxic Microcystis+70% Chlorella,30% non-toxic Microcystis+70% Chlorella with or without fish cues to conduct the life history experiments and transcriptome analysis.Combined with the analysis of life history and transcriptome analysis,it is found that toxic Microcystis and non-toxic Microcystis had multiple identical regulatory pathways in the influence of D.pulex's inducible defense of fish predation,mainly affected the individual growth by affecting the expression of chitin-related epidermal genes,regulated the growth-related genes by affecting the expression of digestion-related genes,and reduced the number of offspring by affecting the expression of genes related to sterol metabolism.The difference between two types of Microcystis was mainly manifested in the down-regulation of neuronal activity.The nervous system participates in the regulation of various activities.The decreased neuronal activity under the toxic Microcystis led to significantly stronger changes in body length and tail spine length than that under the non-toxic Microcystis.In summary,Microcystis have an adverse effect on D.pulex's induced defense against fish predation.This adverse effect is manifested in life history and morphological defense,which includes changes in the expression of related pathways.This study will not only reveal the influence of Microcystis on the resource allocation in the inducible defense process of cladocerans,but also will reveal the gradient effect of Microcystis on cladocerans in the water body during the cyanobacteria blooms and the possible mechanism of the effect of Microcystis.This study will help to deepen the understanding of the changes in the relationship between different trophic levels in the context of cyanobacteria blooms. |
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