Effects of compressed seasonally changing day-length cycles on spawning performance, production of viable eggs and levels of vitellogenin in plasma in female yellowtail snapper Lutjanus argentiventris.

Reproduction in yellowtail snapper Lutjanus argentiventris took place after compressing the seasonally changing day length into a 3 month period applied during two consecutive winters, with the longest and shortest days in December and February, respectively. During the first winter, there was no clear peak of days of spawning and the production of viable eggs was similar from the longest and throughout the decreasing day lengths until reproduction ceased. The level of plasma vitellogenin rose abruptly to a maximum concentration during the increasing day length and then decreased dramatically before the longest day length. During the second winter, a clear peak in the number of days of spawning and the highest production of viable eggs occurred around the longest day length. These results showed that it is feasible to synchronize day length between winter-induced and natural summer and autumn reproduction to produce eggs and larvae during the year.

Analysis of repetitive DNA sequences in the sex chromosomes of Oreochromis niloticus.

In the Nile tilapia, Oreochromis niloticus, sex determination is primarily genetic, with XX females and XY males. While the X and Y chromosomes (the largest pair) cannot be distinguished in mitotic chromosome spreads, analysis of comparative hybridization of X and Y chromosome derived probes (produced, by microdissection and DOP-PCR, from XX and YY genotypes, respectively) to different genotypes (XX, XY and YY) has demonstrated that sequence differences exist between the sex chromosomes. Here we report the characterization of these probes, showing that a significant proportion of the amplified sequences represent various transposable elements. We further demonstrate that concentrations of a number of these individual elements are found on the sex chromosomes and that the distribution of two such elements differs between the X and Y chromosomes. These findings are discussed in relation to sex chromosome differentiation in O. niloticus and to the changes expected during the early stages of sex chromosome evolution.

Karyotype evolution in Tilapia: mitotic and meiotic chromosome analysis of Oreochromis karongae and O. niloticus x O. karongae hybrids.

The karyotype of Oreochromis species is considered to be highly conserved, with a diploid chromosome complement of 2n = 44. Here we show, by analysis of mitotic and meiotic chromosomes, that the karyotype of O. karongae, one of the Lake Malawi 'chambo' species, is 2n = 38. This difference in chromosome number does not prevent the production of inter-specific hybrids between O. niloticus (2n = 44) and O. karongae (2n = 38). Analysis of the meiotic chromosomes of the O. niloticus x O. karongae hybrids indicates that three separate chromosome fusion events have occurred in O. karongae. Comparison of the O. karongae and O. niloticus karyotypes suggests that these consist of one Robertsonian fusion and two fusions of a more complex nature.

Early origins of the X and Y chromosomes: lessons from tilapia.

Differentiated sex chromosome pairs in diverse species display certain common characteristics, normally comprising one largely heterochromatic genetically inactive chromosome and one euchromatic genetically active chromosome (e.g. the mammalian Y and X respectively). It is widely accepted that dimorphic sex chromosomes evolved from homologous pairs of autosomes. Although the exact mechanisms through which the pair diverged are not fully understood, an initial suppression of recombination in the sex-determining region is required by all of the major theories. Here we address the question of the mechanism by which this initial suppression of recombination occurs. Our model postulates that the stochastic, de novo accumulation of heterochromatin in the sex determining region can delay pairing of the sex chromosomes in meiosis, resulting in a decrease in recombination. Data to support this model is presented from the cichlid fish, Oreochromis niloticus. Although such a decrease would in most circumstances be evolutionarily disadvantageous, if the region concerned included the major sex determining gene and other gene(s) with sex-specific functions, then this would be selectively advantageous and could trigger the process(es) which, ultimately, lead to the differentiation of the sex chromosomes.

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.