Tetraploidy and early development: effects on developmental timing and embryonic metabolism

The effect of balanced gene dosage changes on the timing of cavitation and on the timing of appearance of a stage-specific embryonic cell surface antigen was studied in preimplantation mouse embryos. Gene dosage was increased by creating tetraploid embryos at the 4-cell stage, either by blastomere fusion with polyethylene glycol (PEG) or by incubation in cytochalasin B (cytB) to block cell division. Removal of the zona pellucida with Pronase from diploid embryos caused a 7 h delay in cavitation. Further manipulations, either with PEG or cytB to induce tetraploidy, did not produce a statistically significant additional delay in cavitation timing. Likewise, PEG-induced tetraploidy did not affect the timing of appearance or disappearance of the embryonic cell surface antigen as compared with diploid control embryos. In analysing the metabolic effects of tetraploidy, we found that in tetraploid embryos with cell number equivalent to intact diploid embryos, MDH activity did not double with the doubling of the genome, being only 50% greater than diploid levels in cytB-induced tetraploid embryos and only 20% greater than diploid levels in PEG-induced tetraploid embryos. However, in tetraploid embryos with one-half normal cell number, enzyme activity was equal to that in whole diploid embryos, suggesting that in such embryos, MDH activity increased in parallel with increases in gene dosage. Further studies showed that levels of RNA synthesis in PEGinduced etraploid embryos also did not increase in parallel with the doubling of the genome. Rather, these results suggested that in tetraploid embryos, compensation was made for at least part of the excess genetic material.