Evidence that sex chromosome asynapsis, rather than excess Y gene dosage, is responsible for the meiotic impairment of XYY mice

There is extensive evidence for the existence of a meiotic checkpoint that acts to eliminate spermatocytes that fail to achieve full sex chromosome sy napsis at the pachytene stage of the first meiotic prophase. XYY mice are n early always sterile, with clear signs of meiotic impairment, and sex chrom osome asynapsis has been proposed to underlie this impairment. However, a s tudy of XYY*(X) mice (mice having three sex chromosomes but only a single d ose of Y genes) revealed that these mice are fertile, and thus implicated Y gene dosage as a major factor in the sterility of XYY mice. To address thi s question further, sex chromosome synapsis and spermatogenic proficiency w ere compared between XYY*(X) and XYY mice generated in the same litters. Th is established that differences in spermatogenic proficiency within and bet ween the two genotypes correlated with the frequency of radial trivalent fo rmation (full sex chromosome synapsis); XYY*(X) males, as a group, had doub le the radial trivalent frequency of XYY males. This observation provides s trong support for the view that sex chromosome asynapsis (or some consequen ce thereof), rather than Y gene dosage, is the major factor leading to the meiotic impairment of XYY mice. Copyright(C)2000S.KargerAG,Basel.