Identification of the sex chromosomes of the medaka, Oryzias latipes, by fluorescence in situ hybridization

In the medaka, Oryzias latipes, which does not have cytologically recognizable sex chromosomes, the sex is genetically determined and the mechanism of sex determination (XX/XY) can be revealed by genetic crosses using a particular pigment gene. In a previous study, we isolated a sex-linked DNA marker (SL1) using the genomic differences between inbred strains of medaka. In the present paper, we further isolated another sex-linked clone (pHO5.110). The pHO5. 110-related sequences were tightly linked to sex in O. latipes. We designated the locus of the pHO5.110-related sequence on sex chromosomes of medaka as Sex-Linked 2 (SL2). Southern blot analyses suggested that the pHO5.110-related sequence was tandemly repetitive in the medaka genome. Using the clone as a probe for FISH analysis, strong hybridization signals were obtained in a couple of chromosomes that formed one of two large submetacentric chromosome pairs. The pHO5.110-related sequences were repetitive in the genomes of other species of Oryzias (O. curvinotus, O. luzonensis and O. mekongensis) that are karyologically related to O. latipes (all are members of the so-called biarmed group). By contrast, the sequences were not detected as repetitive in other Oryzias species. Hence, it is thought that pHO5.110-related sequences were amplified in the genome of a common ancestor of the biarmed group.