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

Abstract

The pattern of chromosomal synapsis after treatment with 17α-methyltestosterone (MT), a testosterone analogue routinely used for the reversal of phenotypic sex in aquaculture, was investigated using the Nile tilapia ( Oreochromis niloticus) as a model teleost species. Progeny-tested, monosex diploid (2 n=44) individuals were orally administered with diets containing 50 mg/kg MT for 30 days after first feeding (XX MT neomales and XY MT males) and compared to controls (XY males). The formation and structure of the synaptonemal complex (SC) and the nature of chromosomal synapsis were investigated in control and treated groups by computer-assisted image analysis of transmission electron microscope (TEM) microphotographs taken from SC spreads. Nuclei at the pachytene stage were first observed in XX MT neomales, indicating an earlier commitment of genetically female spermatocytes to enter the first meiotic prophase. Administration of MT did not result in obvious SC lesions, breakage, asynapsis or formation of multivalents in genotypic females (XX MT neomales). Administration of MT resulted in a significant increase in the SC lengths in XY MT males, although it did not significantly alter the pattern of synapsis (SC structure and number and morphology of bivalents) in comparison to XY controls. The significance of the effects and the putative mode(s) of action of MT on chromosomal synapsis in teleosts is discussed.