Meiosis and fertility in XYY mice

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Abstract
Four XYY male mice that showed no evidence of somatic-cell mosaicism are described. All had much reduced testis weights and considerable impairment of spermatogenesis, so that few spermatocytes survived to the second division of meiosis. In each mouse the primary spermatocytes observed were predominantly XYY, although, in two of the males, small populations of XY spermatocytes and spermatogonia were seen. However, unlike the situation in man, these XY components had failed to become dominant at the expense of the XYY cells. At diakinesis/first metaphase in the XYY spermatocytes the observed frequencies of the four possible kinds of sex-chromosome associations showed highly significant differences between males. These differences may be real or may in part be artifactual. It is suggested that conclusions drawn concerning pairing preferences of the three sex chromosomes and of their consequences as well should be treated with caution. In view of the small testis size and the few germ cells of all kinds seen in our males, we consider that in most mice the detrimental effect of the XYY constitution starts early and progresses through the seminiferous cycle rather than having a sudden effect at first anaphase, as has been previously suggested. The four males all produced some sperm, but only one gave an above-zero epididymal sperm count, and this was only 1 % of normal. This male, unlike all the previously tested XYY males, proved fertile early in life but soon became sterile. When the observations from all seven XYY males that have now been described are considered together, it is concluded that there is usually a severe impairment of spermatogenesis leading to sterility at an early age. However, the condition may have degrees of manifestation, as in man, and some XYY mice may be subfertile for a limited period rather than completely sterile.

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