Analysis of sex-chromosome aneuploidy in interspecific backcross progeny between the laboratory mouse strain C57BL/6 and Mus spretus.

Sex-chromosomal aneuploidy was identified in four female progeny of 200 interspecific backcrosses between laboratory mice (C57BL/6Ros) and Mus spretus. The progeny included two 39,XO monosomy mice resulting from a backcross with M. spretus, as well as a 41,XXX trisomic mouse and a 40,XX/41,XXX mosaic mouse resulting from two separate backcrosses with C57BL/6 mice. The parental origin and meiotic stage of the aneuploidies was determined for each of the mice using a series of markers that identified allelic differences in the parental X-chromosome genes present in the hybrid female. Two of the probes identified differences in repeated elements between the M. spretus and laboratory mouse X chromosomes, whereas the remaining sites involved restriction fragment length differences of single-copy genes detectable by Southern analysis. These markers indicated that the aneuploidies were most likely of maternal origin and that the trisomy resulted from a nondisjunction at the second meiotic division. In contrast, the mosaic female could have originated either from a trisomic embryo that had lost a single X in a portion of its cells or from a mitotic nondisjunction during early embryogenesis that resulted in XXX and XO daughter cells, with subsequent loss of the XO cells.