CLASSICAL AND NONCLASSICAL REPRODUCTIVE STEROIDS IN FISH

The nature of the steroids secreted by gonads of fish differs significantly from those of the mammals. The 'classical teleost steroids', oestradiol, testosterone, 11-ketotestosterone and 17,20βP, have been measured in a wide variety of teleosts but do not always reflect biological activities. There is increasing evidence that teleost gonads may produce a number of 'non-classical steroids' which may play an important role in their reproductive biology. Such products include metabolites reduced at C5 to 5α or 5β, at C3 to 3α or 3β, or hydroxylated at 6-, 7-, or 21. Conjugation, as either glucuronides or sulphates, may contribute either to localization of action within the gonad or to production of pheromones during the immediate pre-spawning period. Although it is often possible only to measure the steroids for which assays are readily available, it is important to recognize that there may be production of steroids that are not detected by such assays, but which nevertheless play a major role in reproductive activity. In evolutionary terms, the teleost hydroxylases probably originated in the very early fishes and show parallels with their analogues in amphibians and mammals. Gene sequencing may reveal a possible common ancestor for all vertebrate 6, 7 and 11-hydroxylases. © 1993 Chapman & Hall.