ISOLATION OF A SEX CHROMOSOME-SPECIFIC DNA-SEQUENCE IN THE MEDAKA, ORYZIAS-LATIPES

In the medaka, Oryzias latipes, which does not have cytologically reco gnizable sex chromosomes, the mechanism of sex determination (XX/XY) c an be revealed by genetic crosses using a particular pigment gene. Sin ce the only known sex-linked marker is this pigment gene, little infor mation is available on the genetic maps of the medaka's sex chromosome s. High resolution genetic maps of its sex chromosomes are necessary t o hunt for the sex-determining factor by the positional cloning method . In the present study, we isolated a sex-linked marker using the geno mic differences between inbred strains of medaka. We established a con genic strain whose sex-determining region is derived from the HNI stra in and whose genetic background is derived from the Hd-rR strain. Usin g differences between the genomes of the congenic and Hd-rR strains, w e isolated a sex-linked clone (pHO5.5) from a random genomic library c onstructed from the HO5 strain of medaka. The pHO5.5-related sequences were conserved among Oryzias species. A linkage analysis using backcr oss progeny from the Hd-rR and HNI strains demonstrated perfect linkag e between the pHO5.5-related sequence and the sex. We, hence, designat ed the locus of the pHO5.5-related sequence on sex chromosomes of meda ka as Sex-Linked 1 (SL1). Using this sequence, we could identify the s ex of inbred strains of medaka by PCR or Southern blotting. This seque nce may be useful for genetic mapping on the sex chromosomes. We also discuss the availability of the congenic strain established in this st udy for isolating other sex-linked clones.