Evidence for the presence of sex chromosomes in the Nile tilapia (Oreochromis niloticus) from synaptonemal complex analysis of XX, XY and YY genotypes

A cytogenetic analysis of chromosome synapsis was carried out during the fi rst meiotic prophase of the Nile tilapia, Oreochromis niloticus. Three diff erent genotypes were studied: XX sex-reversed males, 'wild-type' (XY) males and YY 'supermales'. TEM analysis of synaptonemal complex (SC) spreads rev ealed the presence of 22 fully paired bivalents during pachytene in both ho mogametic genotypes. In the heterogametic genotype, an incompletely paired segment was frequently observed during the process of meiotic synapsis in t he terminal region of the longest bivalent. The presence of this unpaired s egment, together with several features characteristic of sex-chromosome beh aviour during meiosis, suggests the existence of a non-homologous region in this chromosomal pair in the heterogametic genotype, and provides cytologi cal evidence for the chromosomal basis of sex determination in O. niloticus . The usefulness of SC analysis for the understanding of sex determination and its relevance in the management of species of aquacultural importance a re discussed. (C) 1999 Elsevier Science B.V. All rights reserved.