Effects of chemical activation and season on birth efficiency of cloned pigs.

The effects of chemical activation on birth efficiency of cloned pigs were studied by investigating the developmental process from porcine oocyte activation to birth of cloned pigs. Three different activation methods were used: (i) Electroporation (Ele); (ii) Ele followed by incubation with 6-dimethylaminopurine (6-DMAP); and (iii) Ele followed by a treatment with cycloheximide (CHX). In experiment 1, the rates of cleavage, developmental rates and cell number of porcine parthenogenetic (PA) embryos were investigated in the three treatment groups. In experiment 2, NT embryos produced by the three different activation treatments were compared for the rates of cleavage, development and cell number. Finally, the effects of Ele and Ele+CHX activation methods on birth efficiency of cloned pigs were compared. The activated oocytes treated by combination activation generally showed a higher (P<0.05) blastocyst rate and produced more expanded blastocysts than oocytes activated with Ele. The rates of cleavage and total cell number of parthenotes were not significantly different. Parthenogenetic embryos activated with 6-DMAP developed into blastocyst and expanded blastocyst stages at a significantly (P<0.05) higher rate than those treated with Ele, but the developmental capability was dramatically decreased in NT embryos. With the CHX activation method, the NT embryo blastocyst rate was substantially (P<0.05) increased although the production of expanded blastocysts was not significantly different from that by the other two methods. The birth rate of cloned pigs increased in the CHX group, though the rate was not significantly different from Ele. The effects of season on developmental rate of the porcine PA embryos and birth rate of cloned pigs were also examined in our study. Porcine oocytes collected in the spring had higher developmental capabilities than those collected in the winter. However, no difference in birth rate of the cloned pigs was found between the oocytes collected in the two seasons. The results obtained from PA and NT embryos, following different activation methods, were inconsistent, suggesting that activation mechanisms are dissimilar in PA and NT embryos. Although the chemical activation in our study leads to an elevation of the blastocyst rate, it does not improve the oocyte's molecular programming and so does not significantly improve the efficiency of producing cloned pig births.