The effects of oral administration with 17 alpha-methyltestosterone on chromosomal synapsis in Oreochromis niloticus (Pisces, Cichlidae)

The pattern of chromosomal synapsis after treatment with 17 alpha-methylles tosterone (MT), a testosterone analogue routinely used for the reversal of phenotypic sex in aquaculture, was investigated using the Nile tilapia (Ore ochromis niloticus) as a model teleost species. Progeny-tested, monosex dip loid (2n = 44) individuals were orally administered with diets containing 5 0 mg/kg MT for 30 days after first feeding (XXMT neomales and XYMT males) a nd compared to controls (XY males). The formation and structure of the syna ptonemal complex (SC) and the nature of chromosomal synapsis were investiga ted in control and treated groups by computer-assisted image analysis of tr ansmission electron microscope (TEM) microphotographs taken from SC spreads . Nuclei at the pachytene stage were first observed in XXMT neomales, indicat ing an earlier commitment of genetically female spermatocytes to enter the first meiotic prophase. Administration of MT did not result in obvious SC l esions, breakage, asynapsis or formation of multivalents in genotypic femal es (XXMT neomales). Administration of MT resulted in a significant increase in the SC lengths in XYMT males, although it did not significantly alter t he pattern of synapsis (SC structure and number and morphology of bivalents ) in comparison to XY controls. The significance of the effects and the put ative mode(s) of action of MT on chromosomal synapsis in teleosts is discus sed. (C) 1999 Elsevier Science B.V. All rights reserved.