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Background: Daphnia galeata is a suitable model organism for investigating predator-induced defense. Genes and pathways exhibiting differential expression between fish kairomone-treated and untreated groups in D. galeata have been identified. However, understanding of the significance of alternative splicing, a crucial process of the regulation of gene expression in eukaryotes, to this mechanism remains limited. This study measured life-history traits and conducted short-read RNA sequencing and long-read isoform sequencing of two Korean D. galeata genotypes (KB1 and KE2) to uncover the genetic mechanism underlying their phenotypic plasticity under predation stress. Results: KB1 exhibited strategies to enhance fertility and decrease body length when exposed to fish kairomones, while KE2 deployed an adaptive strategy to increase body length. Full-length transcriptomes from KB1 and KE2 yielded 65,736 and 57,437 transcripts, respectively, of which 32 differentially expressed transcripts (DETs) were shared under predation stress across both genotypes. Prominent DETs common to both genotypes were related to energy metabolism and the immune system. Additionally, differential alternative splicing (DAS) events were detected in both genotypes in response to fish kairomones. DAS genes shared between both genotypes may indicate their significant role in the post-transcriptional stress response to fish predation. Calpain-3, involved in digestion and nutrient absorption, was identified as a DAS gene in both genotypes when exposed to fish kairomones. In addition, the gene encoding thymosin beta, which is related to growth, was found to be a statistically significant DAS only in KB1, while that encoding ultraspiracle protein, also associated with growth, was only identified in KE2. Moreover, transcripts encoding proteins such as EGF-like domain-containing protein, vitellogenin fused with superoxide dismutase, and others were identified overlapping between DAS events and DETs and potentially elucidating their association with the observed phenotypic variation in each genotype. Conclusions: Our findings highlight the crucial role of alternative splicing in modulating transcriptome landscape under predation stress in D. galeata, emphasizing the requirement for integrating gene expression and splicing analyses in evolutionary adaptation studies. [ABSTRACT FROM AUTHOR]

The gut microbiota plays a crucial role in host physiology and the disruption of host-microbiota relationships caused by environmental stressors can impact host growth and survival. In this study, we used Daphnia galeata as a model organism to investigate the interactive effects of fish kairomones on the life-history traits and gut microbiota alterations of D. galeata, as well as the relationship between life-history traits and gut microbiota composition. The presence of fish kairomones enhanced fecundity, decreased growth, and altered gut microbiota, with significant changes in alpha diversity but not in beta diversity in the genotype KB5 of D. galeata. Statistical analysis revealed that the relative abundance of the Pseudomonadaceae family significantly increased upon exposure to fish kairomone, while the relative abundance of the Comamonadaceae family significantly decreased. The decreased growth in genotype KB5 may be associated with a significant increase in Pseudomonas, a member of the family Pseudomonadaceae, which is generally deficient in essential fatty acids, potentially negatively impacting growth. Meanwhile, it is speculated that the significant decrease in Limnohabitans belonging to the Comamonadaceae family is associated with the reduction of body size and increased fecundity of KB5 when exposed to fish kairomones. Furthermore, the genus Candidatus Protochlamydia was observed only under the fi sh kairomones-treated condition. These data suggest that variations in host life-history traits related to reproduction and growth are potentially associated with the relative abundance or presence of these microbial genera. Our research findings provide valuable insights into understanding the impact of biotic stress on the interaction between hosts and microbiota. [ABSTRACT FROM AUTHOR]