In higher vertebrates, sex chromosomes cytologically differ from autosomes, and the sex chromosomes are either male heterogamety (XX-XY systems) or female heterogamety (ZZ-ZW systems). However, recognizable sex chromosomes are not observed in many taxa in which sexes are generally separated. Most fish also has no cytologically differentiated sex chromosomes. There are a few reports of sex determining systems and sex chromosome systems in fishes. However, the reports indicate that fishes have a variety of sex chromosome type and sex determining system (reviewed in Solari, 1994). Furthermore, such sex chromosome types are found among many separate families and genus. In addition, some fishes show the only instances of natural hermaphroditism (either simultaneous or sequential) in vertebrates. Therefore, the study of sex determination in fishes may reveal important clues about the origin and function of sex chromosomes of other vertebrates.
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