The synaptonemal complex and analysis of sex chromosomes in the genus Oreochromis

Four species of fish from the genus Oreochromis (Cichlidae) were studied cytogenetically through meiotic prophase I using TEM. The female synaptonemal complex complement length (SCCL) in these species was around 1.4 times longer than that of the males. The SCCL of neomales (hormonally masculinized genetic females) was reduced significantly compared to control females, while the SCCL of genetic males given the standard hormonal masculinization treatment was increased slightly compared to control males. Unpaired lateral elements were observed in the heterogametic sex of O. niloticus (XY: unsynapsed sub-terminal and terminal region of bivalent 1). In O. aureus, two distinct regions of restricted pairing were present only in the heterogametic sex. The first, a subterminal region of the largest bivalent, was located near to the region of unpairing found in the closely related O. niloticus, while the second was in a small bivalent, most of which was unpaired. These results suggest that O. aureus has two pairs of sex chromosomes. In O. mossambicus, unpaired lateral elements were not observed, nevertheless, a region of bivalent 1 with diffuse chromatin and differential staining was observed in one XY male, perhaps near to the kinetochore. A similar pattern was observed in female O. niloticus x male O. mossambicus hybrids. The recombination rates between the centromere and sex loci in O. mossambicus also suggested that sex genes in this species are located close to the centromere. An autosomal influence on sex determination was also observed. During meiosis in the female O. niloticus (2n = 44) x male O. karongae (2n = 38) hybrid (2n = 41), three trivalents were observed in some spreads. In other spreads failures in chromosome synapsis were observed as aberrant pairing and multivalents. The O. karongae karyotype appears to have arisen from the typical Oreochromis one by three fusions.