bucky ball mRNA is responsive to the total RNA content in the fertilized zebrafish oocyte

The DNA to cytoplasm ratio in the early stages of development is long held theoretically to be the determinant by which zygotic genome activation is synchronized with cell number. Through rapid cell divisions, the DNA increases, while the cytoplasm remains constant, causing a steady increase in the DNA: cytoplasm ratio until eventually transcription of the zygotic genome is initiated. However, the mechanism by which the embryo computes the DNA to cytoplasm ratio is not known. Here, we show that the mRNA for the gene bucky ball increases when the RNA pool in the oocyte is artificially increased at the single-cell stage. bucky ball mRNA is inherited in the unfertilized oocyte, rapidly degraded during the first few cell divisions, and never expressed again. Ectopic injection of multiple zebrafish lncRNAs, translation competent mRNA, irrespective of its capping status, and even a human lncRNA would all lead to the persistence of bucky ball mRNA at 11 hpf, well beyond its normal period of expression. We propose that degradation dynamics of bucky ball mRNA should be added to the growing list of factors including transcription factors, histones, and chromatin modifications that tightly regulate the timing of the maternal to zygotic transition in response to DNA: cytoplasm ratio.