Differences in heat tolerance between preimplantation embryos from Brahman, Romosinuano, and Angus breeds.

Exposure to 41 degrees C reduces development of embryos of heat-sensitive breeds (Holstein and Angus) more than for embryos of the heat-tolerant Brahman breed. Here it was tested whether embryonic resistance to heat shock occurs for a thermotolerant breed of different genetic origin than the Brahman. In particular, the thermal sensitivity of in vitro produced embryos of the Romosinuano, a Bos taurus, Criollo-derived breed, was compared to that for in vitro produced Brahman and Angus embryos. At d 4 after insemination, embryos > or = 8 cells were randomly assigned to control (38.5 degrees C) or heat shock (41 degrees C for 6 h) treatments. Heat shock reduced the proportion of embryos that developed to the blastocyst stage on d 8 after insemination. At 38.5 degrees C, there were no significant differences in development between breeds. Among embryos exposed to 41 degrees C, however, development was lower for Angus embryos than for Brahman and Romosinuano embryos. Furthermore, an Angus vs. (Brahman + Romosinuano) x temperature interaction occurred because heat shock reduced development more in Angus (30.3 +/- 4.6% at 38.5 degrees C vs. 4.9 +/- 4.6% at 41 degrees C) than in Brahman (25.1 +/- 4.6% vs. 13.6 +/- 4.6%) and Romosinuano (28.3 +/- 4.1% vs. 17.5 +/- 4.1%). Results demonstrate that embryos from Brahman and Romosinuano breeds are more resistant to elevated temperature than embryos from Angus. Thus, the process of adaptation of Brahman and Romosinuano breeds to hot environments resulted in both cases in selection of genes controlling thermotolerance at the cellular level.