Submitted to: Journal of Molecular Reproduction and Development
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: November 20, 2002
Publication Date: December 20, 2002
Citation: Block, J., Chase, C.C., Hansen, P.J. 2002. Inheritance of resistance of bovine preimplantation embryos to heat shock: relative importance of the maternal versus paternal contribution. Journal of Molecular Reproduction and Development. 63:32-37.
Interpretive Summary: Heat stress in cattle has an adverse impact on reproduction. Heat stress compromises the competence of the oocyte (or egg), and reduces the development of embryos. There are tropically adapted breeds of cattle (e.g., Brahman) that have been shown to be more tolerant to heat and humidity than are temperate-adapted breeds (e.g., Angus and Holstein). A previous study indicated that high temperature and humidity was associated with a decline in oocyte quality and embryo development from unadapted but not adapted breeds. For thermotolerance of embryos, however, it is not known whether contributions of the spermatozoa or of the egg are most important. This study was conducted to determine whether the ability of Brahman embryos to resist the deleterious effects of heat shock is due primarily to contributions of the spermatozoa, the oocyte or a combination of both. Results indicate that the contribution of the oocyte plays a more crucial role in the ability of Brahman embryos to resist effects of heat shock than the contribution of the spermatozoa. Embryos were more affected by heat shock if produced using Holstein oocytes than if using Brahman oocytes. In contrast, use of Angus or Brahman semen did not affect thermal resistance of embryos. These results have relevance for understanding protective responses of embryos and for practical approaches to increase the pregnancy of heat-stressed cows.
Two experiments were conducted to test whether the ability of Brahman embryos to resist the deleterious effects of heat shock was a result of the genetic and cellular contributions from the oocyte, spermatozoa, or a combination of both. In the first experiment, Brahman and Holstein oocytes were collected from slaughterhouse ovaries and fertilized with spermatozoa from an Angus bull. A different bull was used for each replicate to eliminate bull effects. On day 4 after fertilization, embryos (at least 9 cell stage) were collected and randomly assigned to control (38.5 C) or heat shock (41 C for 6 h) treatments. The proportion of embryos developing to the blastocyst and advanced blastocyst stages was recorded on day 8. Heat shock reduced the number of embryos produced from Holstein oocytes that developed to the blastocyst and advanced blastocyst stages on day 8, but did not affect the development of embryos developed from Brahman oocytes. In the second experiment, oocytes from Holstein cows were fertilized with semen from bulls of either Brahman or Angus breeds. Heat shock reduced development when applied to embryos (at least 9 cell stage) but there was no breed x temperature interactions. The current findings suggest that the oocyte plays a more significant role in the resistance of Brahman embryos to the deleterious effects of heat shock than the spermatozoa. Further research is necessary to discover whether the oocyte's contribution to embryonic resistance to heat shock is cytoplasmic or genetic and, if genetic, whether failure to observe effects of breed of sire represents imprinting.