Molecular-cytogenetic analysis reveals sequence differences between the sex chromosomes of <i>Oreochromis niloticus:</i> evidence for an early stage of sex-chromosome differentiation

Abstract

Sex determination in the Nile tilapia, Oreochromis niloticus, is primarily genetic, with XX females and XY males. A candidate sex-determining region in the terminal region of the largest chromosome pair has been identified by analysis of meiotic chromosomes. This region shows an inhibition of pairing and synapsis in the XY genotype, but not in XX or YY genotypes, suggesting that recombination is inhibited. Here we show that chromosome microdissection and subsequent amplification by degenerate oligonucleotide-primed PCR (DOP-PCR) can be used to produce in situ hybridization probes to this largest pair of O. niloticus chromosomes. Furthermore, analysis of the comparative hybridization of X and Y chromosome-derived probes to different genotypes provides the first demonstration that sequence differences exist between the sex chromosomes of O. niloticus. This provides further support for the theory that this chromosome pair is related to sex determination and further suggests that the sex chromosomes are at a very early stage of divergence.

Particularities of reproduction and oogenesis in teleost fish compared to mammals

“…However, sex-ratio results from the back crosses between F1 and parent species suggest that the genetic determination system might not be so simple [9,11]. Recent data actually confirm that sex chromosomes of O. niloticus, although identified by in situ hybridation techniques, are still weakly differentiated [12,13], and that sex determining genes can be localised on other chromosomes [14]. In salmonids, where the male is generally considered as the heterogametic sex, the determining locus can be situated on different chromosomes depending on the species [15], and other loci can also play a role [16].…”

Use of chromosome microdissection in fish molecular cytogenetics

“…The subsequent development of in situ hybridization procedures and establishment of DOP-PCR (Telenius et al, 1992) allowed the application of chromosomal probes obtained by microdissection in FISH (fluorescence in situ hybridization) procedures (sometimes referred to as microFISH). This technique has been applied in several studies involving fish species, and has been particularly useful in studies focused on sex-chromosomes (Reed et al, 1995;Harvey et al, 2002;Liu et al, 2002).…”

“…Despite being a numerous and highly diverse group, chromosome painting has been applied to only a handful of species (Phillips et al, 2001;Harvey et al, 2002;Liu et al, 2002). The possibility of developing paints from few, or even single microdissected chromosomes is promising, considering common characteristics of fish chromosomes such as uniformity in size and morphology.…”

Spermatogenesis in testis primary cell cultures of the tilapia (Oreochromis niloticus)

“…In this communication, we carried out the experiments in another species, tilapia (Oreochromis niloticus), which is economically an important fish in world aquaculture (Harvey et al, 2002). This species is in many ways ideally suited for studies using primary cultures, in particular for experiments that require more cells than can be provided by the small testes of other model teleosts, the medaka and the zebrafish.…”