Date of Award

Summer 2022

Document Type

Open Access Dissertation


Biological Sciences

First Advisor

Jeffry L Dudycha


Daphnia are ecologically important organisms that have been well studied in the context of evolution, ecology, ecotoxicity, and genomics. Daphnia have strong maternal effects in which the environment experienced by the mother can alter offspring disease resistance, life history traits, and morphology, as well as gene expression and methylation for multiple generations. Coupled with their ability to reduce genetic differences via parthenogenesis, Daphnia are an ideal system for epigenetic studies involving the transmission of maternal effects. Using two clones of Daphnia pulex, we investigated the plasticity of life history and DNA methyltransferase (Dnmt) gene expression with respect to food limitation within- and across-generations. We then addressed how these same traits would respond to methyltransferase gene knockdowns.

We found strong evidence of genotypic variation of responses of life history and Dnmt expression to low food diets, both within- and across-generations. In general, effects of offspring diet were larger than either the direct maternal effect or offspring maternal environment interactions, but the direction of the maternal effect was usually in the opposite direction of the within-generation effect. For both life history and Dnmt expression, we also found that when offspring had low food, effects of the maternal environment were stronger than when offspring had high food.

We used an RNAi inducing bacterial feeding regime to target DNA methyltransferase genes in two Daphnia pulex clones. We found a strong genotypic effect between the clones that resulted in increased mortality, decreased or halted reproduction, and morphological deformities. Offspring born into the environment were similarly affected. Gene expression results show that the presence of dsRNA significantly increases Dnmt expression of all three methyltransferase genes. RNAi treatments targeting specific methyltransferase mRNA transcripts significantly reduced gene expression of their target gene as well as reduced expression of the other two genes. RNAi treated animals experienced premature ecdysis events and increased levels of hemoglobin, but no significant differences were observed in morality or offspring production between treatments. We propose that dsRNA and bacteria induced an innate immune response, elevating methyltransferase expression levels. Loss or reduction of Dnmt transcripts through RNAi treatments limited the Daphnia’s innate immunity responses, triggering a generic stress response resulting in the molting and increased hemoglobin production.


© 2022, Trenton C Agrelius

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