|Chase, Chadwick - Chad|
Submitted to: Journal of Dairy Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/28/2003
Publication Date: 1/15/2004
Citation: Hernandez-Ceron, J., Chase, C.C., Jr., Hansen, P.J. 2004. Differences in heat tolerance between preimplantation embryos from Brahman, Romosinuano, and Angus breeds. Journal of Dairy Science. 87:53-58. Interpretive Summary: Cows exposed to heat stress have a high incidence of early embryonic mortality and some of this effect is due to the direct effects of elevated temperature on the embryo. Certain heat-tolerant breeds of cattle appear to have acquired mechanisms to protect cells against damage from high temperature. In a recent study, it was observed that development of embryos from heat sensitive breeds (Holstein and Angus) exposed to high temperatures was reduced more than for embryos from the heat-tolerant Brahman breed. Senepol, another heat tolerant breed of cattle, exhibits heat tolerance like the brahman despite their different genetic origins. This suggests that either multiple genes for heat tolerance exist in cattle, the same gene was selected for in two different populations of cattle, or there was some introduction of Bos indicus genes in the Senepol population. The purpose of this study was to examine how widespread is the phenomenon of cellular heat tolerance for embryonic development before implantation. We compared tolerance of embryos from the Angus, a heat-sensitive breed, with embryos from Brahman and Romosinuano cattle, two heat-tolerant breeds. The results demonstrated 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 heat tolerance at the cellular level. Identification of these genes and their transfer into dairy breeds may reduce effects of heat stress on reproductive function in dairy cows.
Technical Abstract: In a previous study, exposure of embryos to 41 C reduced development of embryos of heat-sensitive breeds (Holstein and Angus) more than embryos of the heat-tolerant Brahman (Br) breed. Here it was tested whether the phenomenon of genetic control of embryonic resistance to heat shock occurs for other thermotolerant breeds of different genetic origin than the Brahman. In particular, the thermal sensitivity of embryos of the Romosinuano, a Bos taurus, Criollo-derived breed, was compared to that for Brahman and Angus embryos. Cows were slaughtered in groups of 2 or 3 per breed (5-6 replicates), oocytes collected and used to produce embryos by in vitro fertilization. At d 4 after insemination, embryos (at least 8 cells) were randomly assigned to control (38.5 C) or heat shock (41 C for 6 h) treatments. Development to the blastocyst stage was determined on d 8 after insemination. Heat shock caused a reduction in the proportion of embryos that developed to the blastocyst stage. At 38.5 C, there were no significant differences in development between breeds. Among embryos exposed to 41 C, however, development was lower for Angus embryos than for Brahman and Romosinuano embryos. Furthermore, there was a trend for an Angus vs (Brahman + Romosinuano) x temperature interaction that reflects the fact that heat shock reduced development more for Angus (30% at 38.5 C vs 4.9% at 41 C) than for Brahman (25.1 vs 13.6%) and Romosinuano (28.3 vs 17.5%). There was no effect of breed, temperature, or breed x temperature on the cell number of d 8 blastocysts. These 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. Identification of these genes and their transfer into dairy breeds may reduce effects of heat stress on reproductive function in lactating dairy cows.