Physiologically aged and unaged sperm from each of 12 sexually mature B6SJLF1/J mice were used to fertilize oocytes from females of the same strain, with each male serving as its own control. Male genomes in 323 and 307 first-cleavage metaphases obtained by in vivo and in vitro fertilization, respectively, were analyzed cytogenetically, using C-banding for detection of the Y chromosome. The sex (X:Y) ratio among all zygotes resulting from in vivo fertilization was 1.18; in zygotes resulting from in vivo fertilization by aged (14-d mating intervals) sperm, however, the ratio was 1.53, which differed significantly (χ2 = 6.72, P 0.01) from the theoretical value of 1.00. Comparison of the sex ratio in zygotes resulting from in vivo fertilization by unaged sperm (3-d mating intervals), 0.94, with that in zygotes resulting from fertilization by aged sperm (using a 2 × 2 contingency table) showed a significant (χc2 = 4.19, P c2 = 5.74, P 0.01). All hyperhaploid zygotes had a complement of n + 1 chromosomes, except the 14-d in vitro group, where complements of n + 2 and n + 3 chromosomes were seen. Sperm-derived polyploidy, which was observed only in the in vitro group, was independent of sperm age and occurred in 6.8 of the zygotes. These data provide support for the sperm-aging hypothesis and indicate, for the first time, an influence of sperm aging in the male genital tract on the X:Y ratio of conceptuses resulting from natural matings of chromosomally normal ma
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