In all animals, the initial events of embryogenesis are controlled by maternal gene products that are deposited into the developing oocyte. At some point after fertilization, control of embryogenesis is transferred to the zygotic genome in a process called the maternal-to-zygotic transition. During this time, many maternal RNAs are degraded and transcription of zygotic RNAs ensues1. There is a long-standing question as to which factors regulate these events. The recent findings that microRNAs2, 3 and Smaug4 mediate maternal transcript degradation have shed new light on this aspect of the problem. However, the transcription factor(s) that activate the zygotic genome remain elusive. The discovery that many of the early transcribed genes in Drosophila share a cis-regulatory heptamer motif, CAGGTAG and related sequences5, 6, collectively referred to as TAGteam sites5 raised the possibility that a dedicated transcription factor could interact with these sites to activate transcription. Here we report that the zinc-finger protein Zelda (Zld; Zinc-finger early Drosophila activator) binds specifically to these sites and is capable of activating transcription in transient transfection assays. Mutant embryos lacking zld are defective in cellular blastoderm formation, and fail to activate many genes essential for cellularization, sex determination and pattern formation. Global expression profiling confirmed that Zld has an important role in the activation of the early zygotic genome and suggests that Zld may also regulate maternal RNA degradation during the maternal-to-zygotic transition.

Nature 456, 400-403 (20 November 2008) | doi:10.1038/nature07388; Received 20 June 2008; Accepted 29 August 2008; Published online 19 October 2008

A long-standing goal of biology is to map the behavior of all cells during vertebrate embryogenesis. We developed digital scanned laser light sheet fluorescence microscopy and recorded nuclei localization and movement in entire wild-type and mutant zebrafish embryos over the first 24 hours of development. Multiview in vivo imaging at 1.5 billion voxels per minute provides "digital embryos," that is, comprehensive databases of cell positions, divisions, and migratory tracks. Our analysis of global cell division patterns reveals a maternally defined initial morphodynamic symmetry break, which identifies the embryonic body axis. We further derive a model of germ layer formation and show that the mesendoderm forms from one-third of the embryo's cells in a single event. Our digital embryos, with 55 million nucleus entries, are provided as a resource.

Originally published in Science Express on 9 October 2008
Science 14 November 2008:
Vol. 322. no. 5904, pp. 1065 - 1069

Changes in gene regulation are thought to have contributed to the evolution of human development. However, in vivo evidence for uniquely human developmental regulatory function has remained elusive. In transgenic mice, a conserved noncoding sequence (HACNS1) that evolved extremely rapidly in humans acted as an enhancer of gene expression that has gained a strong limb expression domain relative to the orthologous elements from chimpanzee and rhesus macaque. This gain of function was consistent across two developmental stages in the mouse and included the presumptive anterior wrist and proximal thumb. In vivo analyses with synthetic enhancers, in which human-specific substitutions were introduced into the chimpanzee enhancer sequence or reverted in the human enhancer to the ancestral state, indicated that 13 substitutions clustered in an 81–base pair module otherwise highly constrained among terrestrial vertebrates were sufficient to confer the human-specific limb expression domain.

Science 5 September 2008: Vol. 321. no. 5894, pp. 1346 - 1350
DOI: 10.1126/science.1159974

Engineered mice play an ever-increasing role in defining connections between genotype and phenotypic expression. The potential of magnetic resonance microscopy (MRM) for morphologic phenotyping in the mouse has previously been demonstrated; however, applications have been limited by long scan times, availability of the technology, and a foundation of normative data. This article describes an integrated environment for high-resolution study of normal, transgenic, and mutant mouse models at embryonic and neonatal stages. Three-dimensional images are shown at an isotropic resolution of 19.5 μm (voxel volumes of 8 pL), acquired in 3 h at embryonic days 10.5–19.5 (10 stages) and postnatal days 0–32 (6 stages). A web-accessible atlas encompassing this data was developed, and for critical stages of embryonic development (prenatal days 14.5–18.5), >200 anatomical structures have been identified and labeled. Also, matching optical histology and analysis tools are provided to compare multiple specimens at multiple developmental stages. The utility of the approach is demonstrated in characterizing cardiac septal defects in conditional mutant embryos lacking the Smoothened receptor gene. Finally, a collaborative paradigm is presented that allows sharing of data across the scientific community. This work makes magnetic resonance microscopy of the mouse embryo and neonate broadly available with carefully annotated normative data and an extensive environment for collaborations.

PNAS 2008 105:12331-12336; published ahead of print August 19, 2008, doi:10.1073/pnas.0805747105

The E2F family is conserved from Caenorhabditis elegans to mammals, with some family members having transcription activation functions and others having repressor functions1, 2. Whereas C. elegans 3 and Drosophila melanogaster 4, 5 have a single E2F activator protein and repressor protein, mammals have at least three activator and five repressor proteins1, 2, 6. Why such genetic complexity evolved in mammals is not known. To begin to evaluate this genetic complexity, we targeted the inactivation of the entire subset of activators, E2f1, E2f2, E2f3a and E2f3b, singly or in combination in mice. We demonstrate that E2f3a is sufficient to support mouse embryonic and postnatal development. Remarkably, expression of E2f3b or E2f1 from the E2f3a locus (E2f3a3bki or E2f3a1ki, respectively) suppressed all the postnatal phenotypes associated with the inactivation of E2f3a. We conclude that there is significant functional redundancy among activators and that the specific requirement for E2f3a during postnatal development is dictated by regulatory sequences governing its selective spatiotemporal expression and not by its intrinsic protein functions. These findings provide a molecular basis for the observed specificity among E2F activators during development.

Nature 454, 1137-1141 (28 August 2008) | doi:10.1038/nature07066

DNA methylation is essential for normal development1, 2, 3 and has been implicated in many pathologies including cancer4, 5. Our knowledge about the genome-wide distribution of DNA methylation, how it changes during cellular differentiation and how it relates to histone methylation and other chromatin modifications in mammals remains limited. Here we report the generation and analysis of genome-scale DNA methylation profiles at nucleotide resolution in mammalian cells. Using high-throughput reduced representation bisulphite sequencing6 and single-molecule-based sequencing, we generated DNA methylation maps covering most CpG islands, and a representative sampling of conserved non-coding elements, transposons and other genomic features, for mouse embryonic stem cells, embryonic-stem-cell-derived and primary neural cells, and eight other primary tissues. Several key findings emerge from the data. First, DNA methylation patterns are better correlated with histone methylation patterns than with the underlying genome sequence context. Second, methylation of CpGs are dynamic epigenetic marks that undergo extensive changes during cellular differentiation, particularly in regulatory regions outside of core promoters. Third, analysis of embryonic-stem-cell-derived and primary cells reveals that 'weak' CpG islands associated with a specific set of developmentally regulated genes undergo aberrant hypermethylation during extended proliferation in vitro, in a pattern reminiscent of that reported in some primary tumours. More generally, the results establish reduced representation bisulphite sequencing as a powerful technology for epigenetic profiling of cell populations relevant to developmental biology, cancer and regenerative medicine.

Nature 454, 766-770 (7 August 2008) | doi:10.1038/nature07107; Received 24 March 2008; Accepted 21 May 2008; Published online 6 July 2008