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THE FREQUENCY OF ANEUPLOIDY IN THE SECONDARY SPERMATOCYTES OF NORMAL AND ROBERTSONIAN TRANSLOCATION-CARRYING RAMS

Summary.: A detailed analysis was made of the chromosomes in 1008 M II figures from three different types of heterozygous Robertsonian translocation-carrying rams (53,xy, t₁; 53,xy,t₂ and 53,xy,t₃) and 225 M II figures from homozygous Robertsonian translocation-carrying rams (52,xy,t₁t₁; 52,xy,t₃t₃) and rams of normal karotype (54,xy). No hypermodal cells were recorded in either the normal or the homozygous rams, but from 4·5% to 9·2% of M II cells from the heterozygous rams were hypermodal. The heterozygous rams also produced a significantly higher level of hypomodal cells suggesting that, in addition to nondisjunction, lagging at anaphase I may have occurred. There were also distinct differences in M II aneuploid spermatocyte frequency between heterozygous versus normal and homozygous rams.

Fewer balanced translocation X-carrying M II cells were recorded than expected in three of the four 53,xy,t₂ rams. This coincides with mating data which suggest that 26,x,t₂ gametes may occur less frequently than expected.

Since ewes of normal karotype mated to 53,xy,t rams conceive to first service at a rate equal to or better than normal mating groups, and because no blastocysts with unbalanced karotypes associated with the t₁ translocation have been recorded, it is suggested that only euploid spermatozoa are involved in fertilization. In the sheep, aneuploid spermatocytes probably degenerate before sperm maturation.