Document Type

Thesis

Degree Name

Master of Science (MSc)

Department

Biology

Program Name/Specialization

Integrative Biology

Faculty/School

Faculty of Science

First Advisor

Dr. Deborah MacLatchy

Advisor Role

Mentor

Second Advisor

Dr. Glen Van Der Kraak

Abstract

The mummichog (Fundulus heteroclitus) is a small-bodied estuarine fish that inhabits the North American east coast and is often used as a model adult saltwater fish in environmental bioassays. This study aimed to describe gonadal development and gene expression levels in five-week-old mummichog under control conditions and following 17α-ethinylestradiol (EE2) treatment in order to better understand the susceptibility of sex determination (SD) and gonadal differentiation (GD) processes following contaminant exposure. In the first experiment, eggs were fertilized, collected and incubated in the laboratory for three weeks, then yolk-sac larvae were grown out for five weeks and sampled for gonadal development and gene expression levels in ovaries and testes. In a second experiment, yolk-sac larvae were reared for five weeks in one of three static (daily renewal) treatments [0, 2 and 10ng/L EE2 (actual)] and then sampled for gonadal histology and gene expression analysis. In controls, the sex ratio approached 50/50 and EE2 treatment did not change the sex ratios. However, altered gametogenesis and increased degeneration were observed in EE2-treated ovaries and testes, indicating a possible downregulation of the HPG (hypothalamus-pituitary-gonadal) axis, thus, halting normal gonadal development. Several GD genes were identified for the first time in the mummichog that may be influencing female- and male- promoting networks. EE2 treatment did not change ovarian gene expression, however, testis gene expression was severely altered by increased expression of female-promoting genes and anti-Mullerian hormone (amh) which may influence self-renewal in early-staged germ cells. The lack of effect of EE2 on sex ratio was surprising and may have been a result of treating the fish after hatching instead of during embryogenesis. These results demonstrate that male mummichog are more sensitive to EE2 after hatching than females. This study also demonstrates that GD mechanisms in the mummichog can be altered by environmentally-relevant concentrations of EE2 when exposed after hatching. This study suggests that in mummichog SD is fixed prior to hatching but that GD mechanisms are susceptible to environmental estrogen exposure during early development.

Convocation Year

2022

Convocation Season

Spring

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