Date of Award

2021

Degree Type

Thesis

Degree Name

Master of Science in Biological and Environmental Sciences (MSBES)

Department

Biological Sciences

First Advisor

Hollie Putnam

Abstract

Prior to fertilization, mothers provision their oocytes with mRNA that regulates the early stages of development and may additionally include transcripts for proteins that support embryonic stress response early on. At some point during embryogenesis, however, these maternal transcripts are degraded as zygotic transcription activates and intensifies during a phenomenon known as the maternal-to-zygotic transition (MZT). Some evidence suggests that as the MZT progresses, and the effects of maternal transcripts are waning while the zygotic expression is being established, offspring of marine broadcast spawners become more vulnerable to environmental perturbations. In light of escalating threats to marine broadcast spawners, it is critical to understand their reproduction and development, which are essential processes for species resilience by repopulating and replenishing existing populations. Reef building corals, in particular, are under threat from multiple stressors at the local and global scales. Mass mortality has occurred in recent years due to a series of marine heatwaves. In addition, there is chronic stress occurring in the form of ocean acidification, or the decline in pH in surface waters due to the uptake of atmospheric carbon dioxide of anthropogenic origin. Here, we characterize the function of maternal mRNAs, the timeline of the MZT, and sensitivity of gene expression to ocean acidification (OA) in the reef- building coral, Montipora capitata to investigate role of the MZT in embryonic stress response in reef-building corals.

In Manuscript 1, we examine gene expression over nine developmental stages in Montipora capitata from eggs and embryos at 1, 4, 9, 14, 22, and 36 hours-post-fertilization (hpf), as well as swimming larvae (9d), and adult colonies. Weighted Gene Coexpression Network Analysis revealed four expression peaks, identifying the maternal complement, two waves of the MZT, and adult expression. Gene ontology enrichment revealed maternal mRNAs are dominated by cell division, methylation, biosynthesis, metabolism, and protein/RNA processing and transport functions. The first MZT wave occurs from ∼4-14 hpf and is enriched in terms related to biosynthesis, methylation, cell division, and transcription. In contrast, functional enrichment in the second MZT wave, or ZGA, from 22 hpf-9dpf, includes ion/peptide transport and cell signaling. Finally, adult expression is enriched for functions related to signaling, metabolism, and ion/peptide transport. Our proposed MZT timing is further supported by expression of enzymes involved in zygotic transcriptional repression (Kaiso) and activation (Sox2), which peak at 14 hpf and 22 hpf, respectively. Further, DNA methylation writing (DNMT3a) and removing enzymes (TET1) peak and remain stable past ∼4 hpf, indicating that methylome programming occurs before 4 hpf.

In Manuscript 2, we quantify gene expression sensitivity to ocean acidification across a set of developmental stages in Montipora capitata. Developing embryos were exposed to three pH treatments ranging from 7.8 (Ambient), 7.6 (Low) and 7.3 (Xlow) from fertilization to 9 days post-fertilization. Embryo and planula volume and gene expression were compared between treatments at each developmental stage to determine the effects of acidified seawater on early development. While there was no measurable size differentiation between fertilized eggs and the prawn chip stage (9 hpf) exposed to ambient, low, and extreme low pH, early gastrula and planula raised in low and extreme low pH treatments were significantly smaller than those raised in ambient seawater, suggesting an energetic cost to developing under low pH. However, no differentially expressed genes emerged between treatments at any time point, except swimming larvae (9 dpf). In a global analysis, principal components analysis shows stronger variation due to life stage, with PC1 showing separation of the life stages and representing 83% of the variance in expression. This suggests that either ocean acidification is not very stressful to organisms not actively calcifying, or that the maternal-to-zygotic transition is robust to pH stress. Planula, however, showed a strong response to reduced pH. Notably, larvae developing at pH 7.8 (Ambient) and pH 7.3 (Xlow) were more similar than those developing at pH 7.6 (Low), which may be due to high CO2 stimulation of symbiont function at pH 7.3. There was not enough expression data available from the symbiont to test this hypothesis here. Larvae from pH 7.6 showed upregulation of genes involved in cell division, regulation of transcription, lipid metabolism, and oxidative stress in comparison to the other two treatments, supported by smallest sizes in this treatment. While low pH appears to increase energetic demands and trigger oxidative stress, the developmental process is robust to this at a molecular level.

Our high-resolution insight into the coral maternal mRNA and MZT provides essential information regarding setting the stage for, and the sensitivity of, developmental success and parental carryover effects under increasing environmental stress. Additionally, identifying the genes, pathways, and functions underlying the response capacity of embryonic and larval corals to future conditions such as ocean acidification is critical to forecasting coral population resistance and resilience to disturbance.

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