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Illegitimate pairing of the X and Y chromosomes in Sxr mice

Published online by Cambridge University Press:  14 April 2009

B. M. Cattanach*
M.R.C. Radiobiology Unit, Chiltan, Didcot, Oxon OX11 ORD
C. Rasberry
M.R.C. Radiobiology Unit, Chiltan, Didcot, Oxon OX11 ORD
M. D. Burtenshaw
Sir William Dunn School of Pathology, University of Oxford, South Parks Road, Oxford OX1 3RE
E. P. Evans
Sir William Dunn School of Pathology, University of Oxford, South Parks Road, Oxford OX1 3RE
Corresponding author.


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X/Y male mice carrying the sex reversal factor, Sxr, on their Y chromosomes typically produce 4 classes of progeny (recombinant X/X Sxr ♂♂ and X/Y non-Sxr ♂♂, and non-recombinant X/X ♀♀ and X/Y Sxr ♂♂) 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. The data support the concept that Y short arm-Sxr pairing and crossing over may be the mechanism responsible for the occurrence of the Sxr variants reported in the literature.

Research Article
Copyright © Cambridge University Press 1990


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