A Cryptic Sex-Linked Locus Revealed by the Elimination of a Master Sex-Determining Locus in Medaka Fish

Sex chromosomes rapidly turn over in several taxo-nomic groups. Sex chromosome turnover is generally thought tostart with the appearance of a new sex-determining gene on anautosome while an old sex-determining gene still exists, followedby the fixation of the new one. However, we do not know how prev-alent the transient state is, where multiple sex-determining loci co-exist within natural populations. Here, we removed a Y chromo-some with a master male-determining gene DMY from medakafish using high temperature–induced sex-reversed males. After fourgenerations, the genomic characteristics of a sex chromosome werefound on one chromosome, which was an autosome in the originalpopulation. Thus, the elimination of a master sex-determining lo-cus can reveal a cryptic locus with a possible sex-determining effect,which can be the seed for sex chromosome turnover. Our resultssuggest that populations that seem to have a single-locus XY systemmay have other chromosomal regions with sex-determining effects.In conclusion, the coexistence of multiple sex-determining genesin a natural population may be more prevalent than previouslythought. Experimental elimination of a master sex-determining lo-cus may serve as a promising method for finding a locus that canbe a protosex chromosome.