FERTILIZATION RATE AND PREIMPLANTATION EMBRYONIC-DEVELOPMENT IN 2 RABBIT STRAINS OF DIFFERENT FECUNDITY, IN PUREBREEDING AND CROSSBREEDING

Does (n = 289) and males (n = 82) from two INRA strains selected for l itter size, A2066 and A1077, were mated in purebreeding and crossbreed ing systems, in a diallel design. Females were killed 28 h (D1), 72 h (D3) or 114 h (D5) post-coitum to assess the fertilisation rate of ova shed and preimplantation embryonic survival. A2066 does shed signific antly more ova than A1077 does (+ 1.2). The overall pregnancy rate of ovulating females was 92%, without any effect of dam strain and an unf avourable effect of A2066 sires. Among pregnant females (with at least one live embryo), the mating between A2066 males and females gave a f ertilisation rate of ova shed significantly lower (82%) than the three other types of matings, which were not significantly different (93%). In contrast, there were no significant differences between the four t ypes of matings in the survival rate of fertilised eggs at D3 (93%) an d D5 (87%), although embryos from A2066 dams at D5 were significantly smaller than those from A1077 dams, whatever the sire. The mucin coat was significantly larger in the eggs of A2066 dams, whatever the sire and the stage of pregnancy. Thus, the superiority of A2066 females in the number of ova shed was completely lost in purebreeding by D1, and was retained only in crossbreeding. This was not due to a heterosis ef fect on embryo viability, but to some antagonism between A2066 male an d female gametes in purebreeding. The reasons for this antagonism have to be studied further.