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.