Mouse chromosome 6 in Rb translocations: consequences in singly and doubly heterozygous males

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Abstract
Previously it has been shown that male mice heterozygous for the Rb(6.16) translocation show a transmission distortion of the segregants in resulting zygotes, particularly for sexually active animals (Aranha and Martin-DeLeon, 1991). Sperm bearing the translocation have an impaired fertilizing ability, compared to chromosomally normal sperm, and this dysfunction is potentiated in X-bearing cells leading to a sex ratio distortion. Since earlier data suggested that a deficiency of Chromosome 6 was associated with sperm dysfunction (Zackowski and Martin-DeLeon, 1989), the present study was designed to determine the uniqueness of the coupling of the distortions for the Rb(6.16) translocation and the effect of 0 and 2 copies of Chr 6 on sperm function. First cleavage metaphases, resulting from chromosomally normal females that were mated to sexually active or rested Rb(6.15) or Rb(2.8) heterozygous males, were sequentially G-and C-banded. For unaged sperm from sexually active Rb(6.15) males, cytogenetic analysis showed that the ratio of normal (90) to balanced (37) zygotes deviated significantly (P < 0.001) from 1:1, as did the sex ratio, 46X:81Y (P < 0.01). The latter distortion was related to a deficiency of X-bearing sperm with the translocation (8X:29Y). Both segregation and sex ratio distortions disappeared for aged sperm (69:50, 51X:68Y) and a contingency table showed that segregant type was related to sperm age (P < 0.05). For the Rb(2.8) the ratio of normal to balanced zygotes (65:46) was not significantly different from 1:1 for unaged sperm, and although the ratio from aged sperm was 62:39, there was no relationship between segregant type and sperm age (P > 0.05). There was also no sex ratio distortion for either unaged or aged sperm (P > 0.05). To generate zygotes with sperm complements containing 0 and 2 copies of Chr 6, males doubly heterozygous for Rb(6.16) and Rb(6.15) were mated to chromosomally normal females. No zygotes were recovered and the males were shown histologically to have azoospermia and spermatogenic arrest. The results support the conclusion that the presence of Chr 6 in Rb translocations specifically affects segregant and sex ratio distributions. They also indicate, that as the common arm in compound heterozygotes, this chromosome grossly affects spermatogenesis.

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