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.
X/Y male mice carrying the sex reversal factor, Sxr, on their Y chromosomes typically produce 4 classes of progeny (recombinant X/X Sxr male male and X/Y non-Sxr male male, and non-recombinant X/X female female and X/Y Sxr male male) in equal frequencies, these deriving from obligatory crossing over between the chromatids of the X and Y during meiosis. Here we show that X/Y males that, exceptionally, carry Sxr on their X chromosome, rather than their Y, produce fewer recombinants than expected. Cytological studies confirmed that X-Y univalence is frequent (58%) at diakinesis as in X/Y Sxr males, but among those cells with X-Y bivalents only 38% showed normal X-Y pseudo-autosomal pairing. The majority of such cells (62%) instead showed an illegitimate pairing between the short arms of the Y and the Sxr region located at the distal end of the X, and this can be understood in terms of the known homology between the testis-determining region of the Y short arm and that of the Sxr region. This pairing was sufficiently tenacious to suggest that crossing over took place between the 2 regions, and misalignment and unequal exchange were suggested by indications of bivalent asymmetry. Metaphase II cells deriving from meiosis I divisions in which the normal X-Y exchange had not occurred were also found. The cytological data are therefore consistent with the breeding results and suggest that normal pseudo-autosomal pairing and crossing over is not a prerequisite for functional germ cell formation.(ABSTRACT TRUNCATED AT 250 WORDS)
Probes for loci situated near one end of the proximal (Tcp-1) and distal (Qa-2, 3) inversions of the mouse t complex have been hybridized to chromosomes of mice with and without t complexes and with morphologically distinguishable chromosome 17s. Both the probe for Tcp-1 and that for Qa-2, 3 hybridized to clearly different positions on t and non-t chromosomes, thus making visible the extent of the two inversions. The proximal inversion extends from roughly the junction of bands A1 and A2 to band A3, and the distal inversion from band A3 to band C. Thus, the whole t complex extends from the band A1-A2 junction to band C, and is therefore somewhat larger than previously thought, and occupies about 1.2% of the genome. A probe for complement component 3 (C3-1), genetically known to be several cM distal to the t complex, was found by in situ hybridization to lie in band E1. The proximal part of chromosome 17 is one of the best known parts of the mouse genome, at both the genetic and molecular levels. It may soon be possible to correlate the length of the t complex in terms of chromosomal distance with its physical length in megabases.
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