Paternal influence on the time of first embryonic cleavage post insemination and the implications for subsequent bovine embryo development in vitro and fertility in vivo

The objectives of this study were: (1) to evaluate the effect of sire on th e time from insemination to first cleavage following insemination in vitro and the relationship of this parameter to field fertility and (2) to establ ish the relationship between the kinetics of cleavage in vitro and oocyte d evelopmental competence for bulls of known field fertility. Frozen semen fr om six bulls with 150-day non-return rates ranging from 57-78% was used. In experiment 1, after insemination with semen from one of the six bulls, pre sumptive zygotes were transferred to IVC in droplets of synthetic oviduct f luid. Droplets were examined at 24, 27, 30, 33, 36, 42, and 48 hr after ins emination and the number of cleaved oocytes was recorded. Blastocyst yield was recorded on Days 6-, 7-, and 8-post insemination. In experiment 2, cult ure droplets were examined at 30, 36, and 48 hr after insemination. At each time point, the number of cleaved embryos was recorded and these embryos w ere transferred into new droplets and were cultured separately for the dura tion of the experiment. The proportion of embryos developing to the blastoc yst stage was recorded for each of the groups for each bull. The best predi ctor of field fertility was a model containing 33-hpi-cleavage percentage o nly (r=0.689, P < 0.0001). There was also a significant correlation between blastocyst yield and non-return rate, with Day 7 blastocyst yield having t he highest correlation (r=0.356), although this was relatively low in compa rison. In experiment 2, irrespective of sire, a significantly higher propor tion of those early-cleaving oocytes (before 30 hpi) developed to blastocys ts than those cleaving later. In most cases, a higher proportion of blastoc ysts derived from early-cleaving oocytes hatched from the zona pellucida su ggesting that such blastocysts are of superior quality to those derived fro m late-cleaving oocytes. In conclusion these data confirm our earlier obser vations that earliest cleaving zygotes are more competent in terms of devel opment to the blastocyst stage than those that cleave later. This phenomeno n is independent of the sire used. However, we have demonstrated that the k inetics of early embryonic development as measured by the timing of the fir st cleavage division post insemination vary between different bulls and tha t these differences can be used to discriminate between bulls of high and l ow bull field fertility.