Siddiq Akbar

Host‐associated microbiota is an essential mediator of host adaptation to changeable environmental conditions. However, whether fish predation risk alters the gut microbiota composition of Daphnia while also of inducing Daphnia's life‐history defence traits remains unclear. Examination of the effect of predation risk on prey‐associated microbiota may help in improving our understanding of induced anti‐predation defence from a microbial perspective. In this study, Daphnia magna was used as the model organism to investigate the changes in the microbiota in the gut of Daphnia as well as in the whole Daphnia and the culture medium during D. magna life‐history anti‐predation defence. The relationship of these changes with the development stage was also assessed. The results showed that predation risk can simultaneously induce anti‐predation defences and reshape the associated microbiota composition of Daphnia. The gut microbiota of D. magna responded to both developmental stages and predation risk. The Shannon diversity of gut microbiota under predation risk decreased significantly with developmental stage. The abundance of dominant bacteria such as Burkholderiaceae and Flavobacteriaceae decreased under predation risk and was further reduced with developmental stage. Functional prediction results showed that the presence of predation risk mainly weakened overall microbial function, especially metabolic function, in the gut of D. magna. The weakening of microbial function was most obvious during sexual maturity. In addition, the number of unique operational taxonomic units in the whole Daphnia and dissected guts under predation risk was significantly higher than those in control. Based on the screening results, the Pantoea genus (Erwiniaceae family) can clearly distinguish dissected gut samples with or without predation risk. The present study provided more insights into the structural features and functional roles of the Daphnia microbiota and improved our understanding of induced anti‐predation defence from a microbial perspective. Furthermore, this study highlights the possible complex correlations that exist between induced defensive traits and altered microbiota, which warrants further investigation.

Kairomone‐induced anti‐predation defences in zooplankton mediate trophic interactions in aquatic ecosystems. Changes in food quality greatly affect induced defences because of the potential influence on resource allocation. Understanding how zooplankton respond to predation risk under poor nutrition conditions is critical for predicting ecosystem function. The present study focused on the fish kairomone‐induced defences in Daphnia pulex fed unicellular non‐toxic Microcystis aeruginosa. Transcriptions of key genes were detected to interpret changes in morphological and life history traits. Additionally, the degree of responsiveness of traits to increasing proportions of M. aeruginosa in the diet was compared at different endpoints with varied exposure duration. Daphnia pulex decreased body size but showed an elongated tail spine as morphological adaptations to the presence of fish kairomones, supported by enhanced transcriptions of chitin deacetylase and cuticle protein genes. However, with suppressed expression of the related genes, tail spine length was linearly reduced by increasing proportions of M. aeruginosa in the diet, accompanied by decreased somatic growth and reproduction. Presence of fish kairomones interactively affected the inhibitive effects of increasing proportions of M. aeruginosa in the diet on Daphnia growth, reproduction, and tail spine defence but not body size. In addition, the magnitude of effects of M. aeruginosa on Daphnia growth and morphological responses under fish kairomone exposure was increased with the increasing length of exposure time from 3–7 days (reaching maturity) to 14 days (by the end of tests), indicating an aggravated inhibitive effect of M. aeruginosa on induced defences over time. The present study provides references for evaluating zooplankton‐fish interactions under poor nutrition conditions. Since a cyanobacterial bloom may last from a few weeks to several months, experiments involving the whole life span of zooplankton are thought to fully reveal the actual impact of cyanobacteria on interspecific interactions related to zooplankton. In addition, unifying the exposure time to cyanobacteria for comparative studies is recommended in future research.

Inducible defences of prey are evolved under diverse and variable predation risks. However, during the co‐evolution of prey and multiple predators, the responses of prey to antagonistic predation risks, which may put the prey into a dilemma of responding to predators, remain unclear. Based on antagonistic predation pressure from an invertebrate (Chaoborus larvae) and a vertebrate (Rhodeus ocellatus) predator, we studied the responses of multiple traits and transcriptomes of the freshwater crustacean Ceriodaphnia cornuta under multiple predation risks. Chaoborus predation risk altered the expression of genes encoding cuticle proteins and modulated the biosynthesis of steroid hormones, cutin, suberine, and wax, leading to the development of horns and increase in size at the late developmental stage. Meanwhile, fish predation risk primarily triggered genes encoding ribosomes and those involved in unsaturated fatty acid biosynthesis and cysteine and methionine metabolism, resulting in smaller individual size and earlier reproduction. Inducible responses of both transcriptome and individual traits revealed that predator‐dependent unique responses were dominant and the dilemma of antagonistic responses was relatively limited. However, the unique individual traits in response to invertebrate predation could be significantly impaired by vertebrate predation risk, even though the unique responses to different predators were extremely weakly correlated and could be elicited simultaneously. These results indicate that diverse predator‐dependent unique responses are favoured by Ceriodaphnia during its co‐evolution with multiple predators. Nonetheless, Ceriodaphnia is not a generalist that can fully adopt all predator‐dependent unique responses simultaneously under multiple predation risks.

Host genotype and environment are considered crucial factors in shaping Daphnia gut microbiome composition. Among the environmental factors, diet is an important factor that regulates Daphnia microbiome. Most of the studies only focused on the use of axenic diet and non-sterile medium to investigate their effects on Daphnia microbiome. However, in natural environment, Daphnia diets such as phytoplankton are associated with microbes and could affect Daphnia microbiome composition and fitness, but remain relatively poorly understood compared to that of axenic diet. To test this, we cultured two Daphnia magna genotypes (genotype-1 and genotype-2) in sterile medium and fed with axenic diet. To check the effects of algal diet-associated microbes versus free water-related microbes, Daphnia were respectively inoculated with three different inoculums: medium microbial inoculum, diet-associated microbial inoculum, and medium and diet-mixed microbial inoculum. Daphnia were cultured for 3 weeks and their gut microbiome and life history traits were recorded. Results showed that Daphnia inoculated with medium microbial inoculum were dominated by Comamonadaceae in both genotypes. In Daphnia inoculated with mixed inoculum, genotype-1 microbiome was highly changed, whereas genotype-2 microbiome was slightly altered. Daphnia inoculated with diet microbial inoculum has almost the same microbiome in both genotypes. The total number of neonates and body size were significantly reduced in Daphnia inoculated with diet microbial inoculum regardless of genotype compared to all other treatments. Overall, this study shows that the microbiome of Daphnia is flexible and varies with genotype and diet- and medium-associated microbes, but not every bacteria is beneficial to Daphnia, and only symbionts can increase Daphnia performance.