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.