Annual sex steroid profiles and effects of gender and season on cytochrome P450 mRNA induction in atlantic tomcod (Microgadus tomcod)

Abstract

AbstractAs a preliminary step in a 4‐year biomonitoring program, sex steroid levels (testosterone and 11‐ketotestosterone in males, and testosterone and estradiol in females), gonad weights, and diameter of vitellogenic oocytes were measured in tomcod collected bimonthly from the Miramichi (industrialized site) and Kouchibouguac (reference site) rivers from September 1993 to September 1994. As well as the reproductive indices, hepatic levels of cytochrome P4501A mRNA (CYP1A mRNA) were also measured. The preparatory period for spawning began in September, with maximal steroid levels in November, and spawning took place from late December to January. The CYP1A mRNA levels in female tomcod appeared inversely related to plasma steroids, with the lowest amounts of CYP1A mRNA coinciding with maximal steroids. The CYP1A mRNA levels in male tomcod did not exhibit this relationship. River–river comparisons of female tomcod showed significantly smaller vitellogenic oocytes in the Miramichi, along with lower plasma testosterone, estradiol, and relative gonad weight. Miramichi CYP1A mRNA levels were higher than Kouchibouguac in the fall but lower in the spring sample. The CYP1A mRNA–sex steroid relationship observed in this study will facilitate meaningful interpretation of data collected during the full 4‐year study.