Transcripts involved in steroid biosynthesis and steroid receptor signaling are expressed early in development in the fathead minnow (Pimephales promelas).

Sex differentiation in organisms is correlated to sex steroid production and receptor signaling pathways involving androgens and estrogens. Timing of expression is critical, and characterization of sensitive windows is needed to determine how environmental stressors may perturb sex differentiation. The objectives of this study were to determine whether genes related to steroid biosynthesis, steroid receptor signaling, and those related to sex differentiation were expressed in pre-differentiated fathead minnow (FHM) embryos, an ecotoxicological model. Transcripts were measured over two weeks (1 day post fertilization (dpf) to 14 days), prior to sex differentiation. The first three time points investigated (1, 3, and 5 dpf) corresponded to the neurula stage, dorsal swim bladder pigmentation, and pre-hatch. The fourth time point (6 dpf) was collected immediately post-hatch and the fifth time point investigated was after 8 days of larval growth (14 dpf). The majority of transcripts investigated, for example estrogen, androgen, and thyroid receptors as well as steroid biosynthesis transcripts, were expressed within the first 72 hours of development; exceptions were star (steroidogenic acute regulatory protein) and cyp19a, which did not have detectable expression until 5 dpf (pre-hatch). Transcripts that increased in relative mRNA abundance over the first two weeks of development included ar, dax1, hsd11b2, hsd17b, cyp19a and thra. This study demonstrates that there is early expression of transcripts related to steroid biosynthesis, steroid receptor signaling, and sex differentiation in pre-hatch FHM embryos. Additional studies are required to determine their relative roles in male and female differentiation during these early developmental periods.