Meiotic segregation analysis in cow heterozygotes for the t(1;29) Robertsonian translocation

Heterozygotes for Robertsonian translocations have usually a normal phenotype but show variable decrease in fertility. In humans, meiotic segregation studies carried out in male carriers reveal a majority of balanced spermatozoa. The rate of unbalanced segregation products appears higher in females than in males, but the number of human oocytes analysed so far remains limited (about 100). The occurrence of the t(1;29) translocation in several cattle breeds makes bovines an interesting animal model to study the meiotic segregation in female carrier of Robertsonian translocations. The aim of our project is thus to assess the rates of unbalanced oocytes produced by four cows and to compare these values with those obtained in bulls carriers of the same translocation. The superstimulated cows were subjected to twenty um pick up (OPU) sessions. Dual colour whole chromosome painting probes for BTA1 and 29 were hybridized on oocytes preparations. To date, 649 oocytes have been matured in vitro and 500 successfully spread. Among the 266 oocytes analysed by FISH so far: 201 (75,5%) are balanced, including 105 translocated and 96 normal, 55 are diploid (20.7%), and 10 unbalanced (3.8%). The proportion of balanced oocytes is lower than the average proportion of balanced spermatozoa estimated for two heterozygous bulls (97.1%). This difference is essentially due to a higher rate of diploid oocytes (20.7% vs 0.04% diploid spermatozoa). The origin of observed differences in the segregation profiles of the two sexes, as well as the practical implications of these results for genetic counselling and cattle breeding are discussed.