Abstract
A cytogenetic analysis of chromosome synapsis was carried out during the first meiotic prophase of the Nile tilapia, Oreochromis niloticus. Three different genotypes were studied: XX sex-reversed males, `wild-type' (XY) males and YY `supermales'. TEM analysis of synaptonemal complex (SC) spreads revealed the presence of 22 fully paired bivalents during pachytene in both homogametic genotypes. In the heterogametic genotype, an incompletely paired segment was frequently observed during the process of meiotic synapsis in the terminal region of the longest bivalent. The presence of this unpaired segment, together with several features characteristic of sex-chromosome behaviour during meiosis, suggests the existence of a non-homologous region in this chromosomal pair in the heterogametic genotype, and provides cytological 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 are discussed.
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