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ABSTRACT The mummichog,Fundulus heteroclitus, an abundant estuarine fish broadly distributed along the eastern coast of North America, has repeatedly evolved tolerance to otherwise lethal levels of aromatic hydrocarbon exposure. This tolerance is linked to reduced activation of the aryl hydrocarbon receptor (AHR) signaling pathway. In other animals, the AHR has been shown to influence the gastrointestinal-associated microbial community, particularly when activated by the model toxic pollutant 3,3′,4,4′,5-pentachlorobiphenyl (PCB-126) and other dioxin-like compounds. To understand host population and PCB-126 exposure effects on mummichog gut microbiota, we sampled two populations of wild fish, one from a PCB-contaminated environment (New Bedford Harbor, MA, USA) and the other from a much less polluted location (Scorton Creek, MA, USA), as well as laboratory-reared F2 generation fish originating from each of these populations. We examined the microbes associated with the gut of these fish using amplicon sequencing of bacterial and archaeal small subunit ribosomal RNA genes. Fish living in the PCB-polluted site had high microbial alpha and beta diversity compared to fish from the low PCB site. These differences between wild fish were not present in laboratory-reared F2 fish that originated from the same populations. Microbial compositional differences existed between wild and lab-reared fish, with the wild fish dominated by Vibrionaceae and the lab-reared fish by Enterococceae. These results suggest that mummichog habitat and/or environmental conditions have a stronger influence on the mummichog gut microbiome compared to population or hereditary-based influences. Mummichog are important eco-evolutionary model organisms; this work reveals their importance for exploring host-environmental-microbiome dynamics. IMPORTANCEThe mummichog fish, a common resident of North America's east coast estuaries, has evolved the ability to survive in waters contaminated with toxic chemicals that would typically be deadly. Our study investigates how living in and adapting to these toxic environments may affect their gut microbiomes. We compared mummichogs from a polluted area in Massachusetts with those from a non-polluted site and found significant differences in their gut microbes. Interestingly, when we raised the next generation of these fish in a lab, these differences disappeared, suggesting that the environment plays a more crucial role in shaping the gut microbiome than genetics. Understanding these changes helps shed light on how animals and their associated microbiomes adapt to pollution, which can inform conservation efforts and our broader understanding of environmental impacts on host-microbe dynamics. 

Saxitoxin (STX) is a potent neurotoxin naturally produced by dinoflagellates and cyanobacteria. STX inhibits voltage-gated sodium channels (VGSCs), affecting the propagation of action potentials. Consumption of seafood contaminated with STX is responsible for paralytic shellfish poisoning (PSP). Humans are among the species most sensitive to PSP; neurological symptoms of exposure range from tingling of the extremities to severe paralysis. The objective of this study was to determine the effects of STX exposure on developmental processes during early embryogenesis. This study was designed to test the hypothesis that early developmental exposure to STX would disrupt key processes, particularly those related to neural development. Zebrafish embryos were exposed to STX (24 or 48 pg) or vehicle (0.3 mM HCl) at 6 hours post fertilization (hpf) via microinjection. There was no overt toxicity but starting at 36 hpf there was a temporary lack of pigmentation in STX-injected embryos, which resolved by 72 hpf. Using high performance liquid chromatography, we found that STX was retained in embryos up to 72 hpf in a dose-dependent manner. Temporal transcriptional profiling of embryos exposed to 48 pg STX per embryo revealed no differentially expressed genes (DEGs) at 24 hpf, but at 36 and 48 hpf, there were 3547 and 3356 DEGs, respectively. KEGG pathway
analysis revealed significant enrichment of genes related to focal adhesion, adherens junction and regulation of
actin cytoskeleton, suggesting that cell-cell and cell-extracellular matrix interactions were affected by STX. Genes affected are critical for axonal growth and the
development of functional neural
networks. We confirmed these findings by visualizing axonal defects in transgenic zebrafish with fluorescently labeled sensory neurons. In addition, our gene expression results suggest that STX exposure affects both canonical and noncanonical functions of VGSCs. Given the fundamental role of VGSCs in both physiology and development, these findings offer valuable insights into effects of exposure to neurotoxins. 

Abstract Chemical modifications of proteins, DNA, and RNA moieties play critical roles in regulating gene expression. Emerging evidence suggests the RNA modifications (epitranscriptomics) have substantive roles in basic biological processes. One of the most common modifications in mRNA and noncoding RNAs is N6-methyladenosine (m6A). In a subset of mRNAs, m6A sites are preferentially enriched near stop codons, in 3′ UTRs, and within exons, suggesting an important role in the regulation of mRNA processing and function including alternative splicing and gene expression. Very little is known about the effect of environmental chemical exposure on m6A modifications. As many of the commonly occurring environmental contaminants alter gene expression profiles and have detrimental effects on physiological processes, it is important to understand the effects of exposure on this important layer of gene regulation. Hence, the objective of this study was to characterize the acute effects of developmental exposure to PCB126, an environmentally relevant dioxin-like PCB, on m6A methylation patterns. We exposed zebrafish embryos to PCB126 for 6 h starting from 72 h post fertilization and profiled m6A RNA using methylated RNA immunoprecipitation followed by sequencing (MeRIP-seq). Our analysis revealed 117 and 217 m6A peaks in the DMSO and PCB126 samples (false discovery rate 5%), respectively. The majority of the peaks were preferentially located around the 3′ UTR and stop codons. Statistical analysis revealed 15 m6A marked transcripts to be differentially methylated by PCB126 exposure. These include transcripts that are known to be activated by AHR agonists (eg, ahrra, tiparp, nfe2l2b) as well as others that are important for normal development (vgf, cebpd, sned1). These results suggest that environmental chemicals such as dioxin-like PCBs could affect developmental gene expression patterns by altering m6A levels. Further studies are necessary to understand the functional consequences of exposure-associated alterations in m6A levels.