291 INFLUENCE OF THE BREED OF BULL (BOS INDICUS × BOS TAURUS) IN TOLERANCE TO HEAT SHOCK IN BOVINE EMBRYOS PRODUCED IN VITRO

There is evidence that deleterious effects of heat shock (HS) on fertility are less pronounced in breeds tolerant to high temperatures, due mainly to differences in their thermoregulatory capacity. In vitro experiments have shown that Bos indicus embryos are more resistant to HS than Bos taurus. In order to better understand the differences related to HS resistance between Bos indicus and Bos taurus, the main objective of this study was to determine if tolerance to HS is caused by genetic contribution from the oocyte, spermatozoa, or both. Additionally, the influence of the time between collection of ovaries in the abattoir and oocyte aspiration in the laboratory on early embryo development was ascertained. In experiment 1, oocytes from Nellore and crossbreed Holstein cows (cHOL) were collected in a local abattoir, matured and fertilized using semen (n = 6 for each breed) from Nellore (NEL), Angus (ANG), Brahman (BRA,) and Gir (GIR) bulls. In experiment 2, oocytes from Nellore and Holstein (HOL) cows were collected in an abattoir and the oocytes were aspirated in the laboratory 4 (group 4 h) or 6.5 h (group 6.5 h) later, matured and fertilized using semen (n = 6 for each breed) from NEL, GIR, and HOL. In both experiments, 96 h post-insemination (hpi), embryos with > 16 cells were separated in 2 groups: control and HS. In the control group the embryos were cultured at 39°C, whereas in the HS group the embryos were submitted to 41°C for 12 h, and then returned to 39°C. In experiments 1 and 2 the results were analyzed by ANOVA (Proc MIXED, SAS Institute, Cary, NC, USA). In experiment 1, there was no effect of HS on blastocyst and hatched blastocyst rates in all breeds studied. The percentage of oocytes that cleaved and reached the morula stage was significantly lower (P < 0.05) in cHOL × GIR compared with the other breeds. Additionally, blastocyst rate was higher in cHOL × NEL than in cHOL × ANG and cHOL × GIR (P < 0.05). In experiment 2, cleavage, morula, and blastocyst rates in group 4 h were higher (P < 0.05) compared with group 6.5 h. The HS decreased blastocyst rates in all breeds (NEL × NEL, HOL × HOL, and HOL × GIR), and in both time intervals (4 and 6.5 h). The breed NEL × NEL had higher cleavage rate (P < 0.05) for both time intervals compared with HOL × HOL and HOL × GIR. In addition, Nellore oocytes fertilized with Nellore semen (NEL × NEL) originated higher blastocyst rates (P < 0.05) in control and HS group than the other breeds. We conclude that (a) embryos from Holstein are more susceptible to HS than embryos from crossbred Holstein; (b) the oocyte is more important than the spermatozoa for the development of thermotolerance, because the breed of the bull did not influence embryo development after HS; (c) in vitro early embryonic development was impaired by increasing (from 4 to 6.5 h) the time interval between ovary collection and oocyte aspiration. Fellowships to T. Nabhan from CAPES and to R. A. Satrapa, R. A. L. Simoes, and E. M. Razza from FAPESP. Funding from FAPESP (Sao Paulo, Brazil).