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

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.

Identification of putative sex chromosomes in the blue tilapia, Oreochromis aureus, through synaptonemal complex and FISH analysis.

Sex determination in the blue tilapia, Oreochromis aureus, is primarily a ZW female-ZZ male system. Here, by analysis of the pachytene meiotic chromosomes of O. aureus, we demonstrate the presence of two distinct regions of restricted pairing present only in heterogametic fish. The first, a subterminal region of the largest bivalent is located near to the region of unpairing found in the closely related species O. niloticus, while the second is in a small bivalent, most of which was unpaired. These results suggest that O. aureus has two separate pairs of sex chromosomes.

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-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 (2n = 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.

Long-term, quantitative analysis of gametogenesis in autotriploid rainbow trout, Oncorhynchus mykiss.

A long-term, quantitative analysis was conducted on the gametogenesis of autotriploid rainbow trout (Oncorhynchus mykiss) to quantify their degree of germline development and reproductive potential. Triploid and diploid (control) trout siblings were raised separately under identical conditions and sampled randomly for histological analysis. Triploid males underwent testicular development and proliferation of germ cells by mitosis and meiosis, progressing through initial phases of spermatogenesis at a similar pace to diploid controls. The effects of triploidy on males were most evident during the final stages of spermatogenesis, when all diploid males contained free spermatozoa in the lumen of most tubules (average relative frequency, ARF = 68.5%), whereas triploid males contained predominantly spermatocytes (ARF = 36.3%) and morphologically abnormal spermatozoa (ARF = 31.8%). In contrast, the gonadal development of triploid females was affected during its early stages; the major patterns observed were the arrest of the oogonia within oogonial clusters (ARF = 30.4-71.1%), the appearance of small numbers (ARF = 1.5-6.0%) of previtellogenic and early vitellogenic follicles, and the proliferation of non-follicular elements (vascular lacunae, fibrosis and tubular adenomas). In agreement with previous reports on the ovarian development of chromosomally female (3A:ZZW) triploid chickens, male-differentiating areas (ARF = 0.2-12.2%) were observed in most triploid females examined, which by the end of the sampling period appeared as gonadal hermaphrodites. It is hypothesized that the lack of proper somatic-to-germ cell interactions prevents the segregation of the oocytes from the gonial clusters and may explain the early blockage observed during the gonadal morphogenesis of autotriploid female rainbow trout.