UC Davis

Epidemiological studies continue to reveal associations between the organochlorine pesticide, dichlorodiphenyltrichloroethane (DDT) and reproductive diseases across species including urogenital carcinoma in sea lions, eggshell thinning in birds, and premature ovarian failure and cancer in humans. Advancing understanding of endocrine disruption at the molecular and cellular level is essential for determining possible developmental origins of adult-onset reproductive diseases. However, identifying mechanisms by which DDT affects developing gonads is a logistical and ethical challenge in long-lived, free-ranging species. The Japanese medaka (Oryzias latipes) is a valuable model for sex-specific toxicological studies due to its chromosomal sex determination, short generation time, and highly mapped genome. Medaka exposed to the estrogenic isomer, o,p’-DDT, at the part per billion (ppb) level experience ovarian abnormalities, reduced fertility, and reduced offspring survival. Yet, molecular mechanisms remain unclear despite decades of research and ongoing public health concerns surrounding DDT’s environmental persistence. This dissertation research investigated how early developmental exposures to o,p’-DDT alter gene regulation networks in the ovary using both traditional and innovative sequencing and molecular technologies. Because o,p’-DDT is a potent estrogen receptor agonist, the central hypothesis was that cell types involved in estrogen signaling would have significantly altered expression of steroidogenesis-related genes following DDT exposure. Chapter 1 discussed the reproductive toxicity of DDT and its impacts on ovarian development and health in the Japanese medaka. Chapter 2 defined long-term reproductive consequences of environmentally relevant, low dose o,p’-DDT exposure during an early window of ovarian differentiation using targeted quantification of estrogen- and steroidogenesis-related genes, hormone quantification, and histopathology. Chapter 3 used cutting-edge single cell transcriptomics (scRNA-seq) to gain a comprehensive view of which ovarian somatic cell types and gene regulatory networks (both steroid hormone- dependent and hormone-independent) are permanently altered by early-life exposure to o,p’-DDT. Contrary to the original hypothesis, the most significantly altered genes and signaling pathways were not steroidogenesis-related, but rather corresponded to upregulation of pro-inflammatory signaling and oxidative stress. This finding revealed an unexpected plausible mechanism of DDT-induced decreases in fertility in the medaka, prioritizing gene targets for further mechanistic studies in aquatic species impacted by persistent DDT exposure. Furthermore, it paired medaka with scRNA-seq to explore a potential high-throughput comprehensive pipeline for suspect endocrine disruptor screening, enhancing predictive toxicology to meet public and environmental health needs.