Endocrine regulation of reproduction in Tilapia

Abstract

Tilapia, the dithiocarbamoylhydrazine derivative Methallibure (I. C. I. 33828), added as a suspension in 6 daily doses of 1 mg/liter of aquarium water each week, effectively suppresses gonadotrophin production after 5 weeks, leading to the disappearance of all spermatocytes and spermatids. Using this compound, it has been possible to demonstrate stimulatory effects of both Tilapia pituitary extracts as well as HCG at relatively low doses in the male Tilapia. While testosterone propionate can (in very high doses) induce spermiation, it has no demonstrable spermatokinetic effects. In the female Tilapia Methallibure administration depresses ovarian weight, G. S. I., vitellogenesis and thecal cell proliferation and resembles in other species the effects of surgical hypophysectomy. In Methallibure-treated females, FSH produces at a dosage of 50 IU a slight increase in ovarian weight and in some (but not all) a clear increase in oocyte growth, deposition of smooth yolk and thecal cell hypertrophy similar to but very much less than those induced almost uniformly by the administration of low dosages of Tilapia pituitary extracts. HCG given to Methallibure-treated females even in high does has no effect on the Tilapia ovary. The results are discussed in relation to the pituitary-gonadal axis in Tilapia and it is suggested that the results could be explained on the basis of either two separate gonadotrophins or two differentially active subunits of one gonadotrophin