EFFECT OF HEAT STRESS ON BOVINE FOLLICULAR DEVELOPMENT AND EPIGENETIC CHANGES

Abstract

Climate is one of the most important limiting factors in animal production. Cattle under the effect of heat stress have reduced fertility. Negative effects during gamete maturation and embryo development have been observed at the morphological, biochemical, transcriptional and developmental levels. Epigenetic mechanisms play a fundamental role in the regulation of gamete and embryo development. Several genes activated during gamete maturation and early embryo development are controlled by epigenetic mechanisms. There are no studies evaluating the effect of heat-stress on the epigenetic profile of bovine oocytes and embryos. Similarly, there is no information on the effect of heat stress on Anti-Mullerian Hormone (AMH) secretion and antral follicle count. In this regard, the objective of this study is to evaluate the effect of in vivo and in vitro heat stress on the DNA methylation and DNA hydroxymethylation of bovine oocytes and embryos. Additionally, to evaluate the effect of heat stress on AMH secretion and antral follicle count. The objective of the first experiment was to assess the effect of heat stress on developmental rate, DNA methylation and DNA hydroxymethylation of bovine oocytes at the germinal vesicle and metaphase II stage. Ten Bos taurus non-lactating non-pregnant crossbred beef cows and heifers were used as oocyte donors. Oocytes were collected once monthly from April to August through Ovum Pick-Up. DNA methylation and DNA hydroxymethylation of GV and MII oocytes was assessed by fluorescence immunohistochemistry. Results of the experiment showed there was a significantly higher number of oocytes collected during spring compared to summer. Heat stress significantly reduced the number of grade 1 oocytes and significantly increased the number of grade 3 oocytes. There was no difference in maturation rates of oocytes subjected to in vitro maturation between treatments. No effect of heat stress on DNA methylation and DNA hydroxymethylation of germinal vesicle and metaphase II oocytes was detected. The objective of experiment two was to assess the effect of in vivo and in vitro heat stress on developmental rate, DNA methylation and DNA hydroxymethylation of bovine oocytes and embryos. A group of Bos taurus non-lactating non-pregnant crossbred beef cows and heifers were used as oocyte donors. A total of 5 repetitions were performed in the experiment. Samples for this experiment were collected during the summer (August) and winter (February).Three treatments were utilized: in vivo heat stress (August samples), in vitro heat stress (February samples subjected to 41°C during the first 12 hours of