Abstract
Early embryos must rapidly generate large numbers of cells to form an organism. Many species accomplish this through a series of rapid, reductive, and transcriptionally silent cleavage divisions. Previous work has demonstrated that the number of divisions before both cell cycle elongation and zygotic genome activation (ZGA) is regulated by the ratio of nuclear content to cytoplasm (N/C). To understand how the N/C ratio affects the timing of ZGA, we directly assayed the behavior of several previously identified N/C-ratio-dependent genes using the MS2-MCP reporter system in living Drosophila embryos with altered ploidy and cell cycle durations. For every gene that we examined, we found that nascent RNA output per cycle is delayed in haploid embryos. Moreover, we found that the N/C ratio influences transcription through three separate modes of action. For some genes (knirps and snail) the effect of ploidy can be entirely accounted for by changes in cell cycle duration. However, for other genes (giant, bottleneck and fruhstart) the N/C ratio directly affects ZGA. For giant and bottleneck, the N/C ratio regulates the kinetics of transcription activation, while for fruhstart it controls the probability of transcription initiation. Our data demonstrate that the regulatory elements of N/C-ratio-dependent genes respond directly to the N/C ratio, through multiple modes of regulation, independent of interphase length.
Competing Interest Statement
The authors have declared no competing interest.
Footnotes
Analysis of new gene (bottleneck) and reexamination of the gene giant revealed an additional mode of N/C ratio sensitive regulation. These two genes regulate the slope of transcription initiation in response to the N/C ratio rather than the probability of activation as frs. Therefore, there are at least three distinct categories of N/C ratio response for cis-regulatory regions during zygotic genome activation in flies.