Fig. 1

Schematic outline of the laboratory cross. The parental species X. maculatus and X. hellerii, a female specimen of the first generation hybrid and the resulting offspring obtained after approximately 100 generation of backcrosses (see Methods for details) are shown. The dorsal red (Dr), the spotted dorsal (Sd) and the oncogene xmrk loci, closely linked on the X-chromosome of the platyfish (LG21), are also highlighted. A translocation event of a genomic region containing these loci accompanied by recombination suppression has likely contributed to the formation of a new sex chromosome, a neo W chromosome (LG2), during the backcross experiment. Fish images by M. Schartl