Absence of selection against aneuploid mouse sperm at fertilization

Is there selection against aneuploid sperm during spermatogenesis and ferti lization? To address this question, we used male mice doubly heterozygous f or the Robertsonian (Rb) translocations Rb(6.16)24Lub and Rb(16.17)7Bnr, wh ich produce high levels of sperm aneuploid for chromosome 16, the mouse cou nterpart of human chromosome 21. The frequencies of aneuploid male gametes before and after fertilization were compared by analyzing similar to 500 me iosis II spermatocytes and similar to 500 first-cleavage zygotes using fluo rescence in situ hybridization with a DNA painting probe mixture containing three biotin-labeled probes specific for chromosomes 8, 16, and 17 plus a digoxigenin-labeled probe specific for chromosome Y. Hyperhaploidy for chro mosome 16 occurred in 20.0% of spermatocytes and in 21.8% of zygotes. Hypoh aploidy for chromosome 16 occurred in 17.0% and 16.7% of spermatocytes and zygotes, respectively. In addition, there was no preferential association b etween chromosome 16 aneuploidy and either of the sex chromosomes, nor was there an elevation in aneuploidy for chromosomes not involved in the Rb tra nslocations. These findings provide direct evidence that there is no select ion against aneuploid sperm during spermiogenesis, fertilization, and the f irst cell cycle of zygotic development.