Epigenomes and reprogramming of zebrafish gametes and early embryos

Sperm and egg are differentiated haploid germ cells that upon fertilization give rise to a totipotent embryo with a potential to differentiate into all cell types. Proper embryonic development requires the correct execution of developmental events which involves the repackaging of sperm with maternal histones, zygotic genome activation and lineage specification. Genes involved in early development have to resist repression by chromatin and also be permissive to gene activation at the correct time in development. Understanding the chromatin dynamics during early embryo development provides insight into how totipotency is established, and the strategy of how developmental programs are executed. Zebrafish has emerged as a powerful vertebrate model system to study development, gene functions and human disease. We used zebrafish as a model system to study the epigenetic changes accompanying early embryo development. Our study provides a comprehensive analysis of the DNA methylation dynamics in the context of histone modifications and gene expression in germ cells, during early embryo development and in somatic tissue. From a DNA methylation stand point, we find that in zebrafish totipotency is achieved through the maintenance of the paternal genome and the reprogramming of a maternal genome. Our study and others reveal both the similarities and differences of how totipotency can be achieved between animals with different reproductive strategies. Lately, this work also provides potential factors and approaches to investigate the mechanism of DNA methylation reprogramming in zebrafish.