The murine Rb(6.16) translocation: evidence for sperm selection and a modulating effect of aging.

The segregation products of the Rb(6.16) translocation were studied at first cleavage metaphase. Male mice heterozygous for the translocation mated at 3- and 14-day intervals to superovulated random-bred ICR females. Chromosome preparations of the recovered one-cell embryos were sequentially G- and C-banded and male and female complements analyzed cytogenetically. Of the 309 zygotes analyzed from both mating groups, no unbalanced segregants of the translocation were observed. In the 3-day group there was a highly significant difference (P less than 0.001) from the expected 1:1 ratio between sperm with normal (70.5%) and balanced segregants (26.2%) of alternate segregation. In the 14-day group the level of significance for the difference was ten times lower (P less than 0.01) as normal segregants were observed in 56.4% of the sperm and balanced ones in 36.5%. A comparison of the two groups using a 2 x 2 contingency table showed that segregant type was related to mating frequency (P less than 0.05). There was a highly significant distortion (P less than 0.01) of the sex ratio, with 178 Y-bearing and 131 X-bearing sperm in the combined populations. When sex ratio was analyzed according to mating intervals, the distortion was significant (P less than 0.01) only for the 3-day group. An analysis of the sex ratio according to the segregant type showed no significant deviation from 1:1 for type 1 segregants, which had normal chromosomes, while type 2 segregants, with the translocation, had a deficiency of X-bearing sperm. This deficiency was significant (P less than 0.05) only for the 3-day population. Analysis of meiotic preparations showed no association between the translocation trivalent and the X-Y bivalent. Our results, obtained under physiological conditions, provide definitive evidence for sperm selection and support previous findings that aging of sperm can modify the effect of selection.