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Multiple sex-chromosome system in a loach fish.

This study shows an X1X1X2X2/X1X2Y type of multiple sex-chromosome system in the genus Cobitis (Pisces, Ostariophysi). Observation of C-banded male meioses revealed that one arm of the neo-Y always associates end-to-end with one of the X chromosomes. This finding implies that a mechanism might be present that has prevented crossing-over between the ancestral X and Y chromosomes without morphological differentiation. This is the first reported case of multiple sex chromosomes among cyprinoid fish.