Physiological perturbations of bovine follicle-enclosed oocytes during the lengthy period of follicular development can lead to reduced oocyte developmental competence. It is suggested that heat stress-induced alterations in germinal vesicle (GV)-stage oocytes are further expressed in the transcriptional levels of genes involved in oocyte maturation and early embryonic development. Bovine oocytes were collected during cold (December–April) and hot (May–November) seasons, matured, fertilized, and cultured in vitro. The percentage of fertilized oocytes cleaving to the 2- to 4-cell stage was higher in the cold vs. hot season (89.0% ± 2.63% vs. 75% ± 2.63%, respectively; P < 0.05), as was the percentage of cleaved embryos further developing to blastocysts (26.6% ± 0.9% vs. 10.1% ± 1.8%, respectively; P < 0.05). Total RNA and poly(A) mRNA of oocytes and developing embryos were isolated and subjected to semiquantitative and real-time PCR for MOS, GDF9, and POU5F1 genes. In GV-stage oocytes, their mRNA levels did not differ between the seasons. However, following maturation, mRNA levels were higher in oocytes collected in the cold season (P < 0.05). In 4-cell-stage embryos, GDF9 and POU5F1 showed opposite mRNA patterns between seasons (higher and lower levels, respectively) in the hot season (P < 0.05). In both 8-cell-stage embryos and blastocysts, POU5F1 expression was lower during the hot season (P < 0.05). Exposing the ovarian pool of oocytes to environmental stress appears to impair maternal mRNA storage and/or the mechanism of transcription renewal, in turn affecting embryo gene expression before and after embryonic genome activation. Such impairment might partially explain the carry-over effect of summer heat stress on dairy cow conception rates.
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Vol. 86 • No. 1