The transition from maternal to zygotic control of development occurs during the 4-cell stage in the domestic pig, Sus scrofa: quantitative and qualitative aspects of protein synthesis

A study was conducted to identify the embryonic stage when the zygotic genome begins to direct development and to characterize protein synthesis in pig oocytes and embryos. Reproductive tracts of gilts were flushed to obtain unfertilized oocytes (UFO), zygotes (Z), 2-. 4-, and 8-cell embryos, compact morulae (M), initial blastocysts (IB), blastocysts (B), and hatched blastocysts (HB). Pig eggs and embryos were cultured in medium containing 1 micromole L-[35S]methionine and evaluated for amino acid uptake, incorporation of the radiolabel into protein, and qualitative changes in protein profiles specific to each cleavage stage. Unfertilized oocytes sequestered 65.7 fmol methionine/4 h/embryo. Uptake of methionine decreased (p < 0.05) from the Z (49.4), 2-cell (41.8), and 4-cell (37.6) embryonic stages to the M (8.97 fmol/4 h/embryo) stage. This downward trend was reversed at the IB, B, and HB stages when uptake increased to 37.3, 50.3, and 84.2 fmol/4 h/embryo), respectively. Incorporation of methionine into protein followed a similar pattern, being relatively higher in the UFO (21.0), Z (20.5), and 2-cell stages (16.0); decreased (p < 0.05) at the 4-cell (6.67), 8-cell (6.84), and M (6.16) stages; and increased (p < 0.05) at the IB (28.0), B (41.5), and HB (69.6 fmol/4 h/embryo) stages. Differences in protein profiles were observed for UFO, Z, 4-cell, and M stages using lysates of single embryos, one-dimensional SDS-PAGE, and fluorography. A 24-kDa protein present in the UFO was absent in the Z and 2-cell stages. The late 4-cell embryo lacked proteins at 15-18 kDa, 34 kDa, and 200 kDa that were present in UFO, Z, and 2-cell embryos, but produced alpha-amanitin-sensitive proteins at 26 kDa and 50 kDa. Additional proteins were detected in M, IB, B, and HB at 42 kDa. These results suggest that, in pigs, the maternal genome controls development in the UFO, Z, 2-cell, and early 4-cell embryos, and the zygotic genome controls development beyond the 4-cell stage.