In mature human sperm, genes of importance for embryo development (i.e., transcription factors) lack DNA methylation and bear nucleosomes with distinctive histone modifications, suggesting the specialized packaging of these developmental genes in the germline. Here, we explored the tractable zebrafish model and found conceptual conservation as well as several new features. Biochemical and mass spectrometric approaches reveal the zebrafish sperm genome packaged in nucleosomes and histone variants (and not protamine), and we find linker histones high and H4K16ac absent,

The fate and function of epigenetic marks during the germline-to-embryo transition is a key issue in developmental biology, with relevance to stem cell programming and transgenerational inheritance. In zebrafish, DNA methylation patterns are programmed in transcriptionally quiescent cleavage embryos; paternally inherited patterns are maintained, whereas maternal patterns are reprogrammed to match the paternal. Here, we provide the mechanism by demonstrating that "Placeholder" nucleosomes, containing histone H2A variant H2A.Z(FV) and H3K4me1, virtually occupy all regions lacking DNA methylation in both sperm and cleavage embryos and reside at promoters encoding housekeeping and early embryonic transcription factors. Upon genome-wide transcriptional onset, genes with Placeholder become either active (H3K4me3) or silent (H3K4me3/K27me3). Notably, perturbations causing Placeholder loss confer DNA methylation accumulation, whereas acquisition/expansion of Placeholder confers DNA hypomethylation and improper gene activation. Thus, during transcriptionally quiescent gametic and embryonic stages, an H2A.Z(FV)/H3K4me1-containing Placeholder nucleosome deters DNA methylation, poising parental genes for either gene-specific activation or facultative repression.

Although the functional importance of the hypothalamus has been demonstrated throughout vertebrates, the mechanisms controlling neurogenesis in this forebrain structure are poorly understood. We report that canonical Wnt signaling acts through Lef1 to regulate neurogenesis in the zebrafish hypothalamus. We show that Lef1 is required for proneural and neuronal gene expression, and for neuronal differentiation in the posterior hypothalamus. Furthermore, we find that this process is dependent on Wnt8b, a ligand of the canonical pathway expressed in the posterior hypothalamus, and that both Wnt8b and Lef1 act to mediate ␤-catenin-dependent transcription in this region. Finally, we show that Lef1 associates in vivo with the promoter of sox3, which depends on Lef1 for its expression and can rescue neurogenesis in the absence of Lef1. The conserved presence of this pathway in other vertebrates suggests a common mechanism for regulating hypothalamic neurogenesis.