Endocrine modulators and the development of sexually dimorphic characters in weakly electric fish, Mormyrus rume proboscirostris.

摘要

The African weakly electric fish Mormyrus rume proboscirostris is an excellent preparation to study organization and activation of sexually dimorphic characters. These characters are readily observable and include the waveform of the fish's electric organ discharge (EOD) and the morphology of basal anal-fin rays. Fish used in this study were bred in our laboratory, and therefore allowed for accurate accounting of their developmental stage. This thesis investigated the organizational role of the non-aromatizable androgen, 17a-methyldihydrotestosterone (MDHT) in the development of sexually dimorphic characters in M. r. proboscirostris. The results served as a baseline for comparison with the effects of the organometal tributyltin (TBT), an aromatase inhibitor, on the expression of these characters. We tested the following predictions: (1) early-life exposure at 17-26 days post fertilization to MDHT (1.5-3 mg/L) will result in androgen-specific organization of sexually dimorphic characters at 4 and 24 months post exposure, (2) adult exposure to TBT (10 mug/L) will elicit androgen-specific activational transformations of these same characters, and (3) the aromatase inhibiting effects of TBT will adversely affect the fish's reproductive potential. The results revealed: (1) the female gonadal phenotype is organized prior to that of the male suggesting that mormyrids are undifferentiated gonochorists. (2) Activation of secondary characters (EOD duration and basal fin-ray expansion) likely occurred immediately following MDHT treatment as evident in juveniles. (3) Exposure of adults to TBT mimicked MDHT affects as shown in the expression of male-typical EOD duration and anal-fin morphology in females. In males, the purportedly stable first EOD phase lengthened, suggesting that TBT induced reduction of ATP synthesis affected membrane permeability. (4) The reproductive potential was adversely affected by TBT exposure, as both oocytic development and spermatogenesis appeared to be suppressed. The results strongly support the notions that the endocrine system in M. r. proboscirostris retains a high degree of plasticity into adulthood, and that organization and activation are not two independent processes.