Onset Of Zygotic Transcription Research Articles

Temporal Reciprocity of miRNAs and Their Targets during the Maternal-to-Zygotic Transition in Drosophila

During oogenesis, female animals load their eggs with messenger RNAs (mRNAs) that will be translated to produce new proteins in the developing embryo. Some of these maternally provided mRNAs are stable and continue to contribute to development long after the onset of transcription of the embryonic (zygotic) genome. However, a subset of maternal mRNAs are degraded during the transition from purely maternal to mixed maternal-zygotic gene expression. In Drosophila, two independent RNA degradation pathways are used to promote turnover of maternal transcripts during the maternal-to-zygotic transition [1]. The first is driven by maternally encoded factors, including SMAUG [2], whereas the second is activated about 2 hr after fertilization, coinciding with the onset of zygotic transcription. Here, we report that a cluster of zygotically expressed microRNAs (miRNAs) targets maternal mRNAs for turnover, as part of the zygotic degradation pathway. miRNAs are small noncoding RNAs that silence gene expression by repressing translation of their target mRNAs and by promoting mRNA turnover. Intriguingly, use of miRNAs to promote mRNA turnover during the maternal-to-zygotic transition appears to be a conserved phenomenon because a comparable role was reported for miR-430 in zebrafish [3]. The finding that unrelated miRNAs regulate the maternal to zygotic transition in different animals suggests convergent evolution.

Genes important for early development display promoter hypomethylation in zebrafish sperm

Fertilization involves the joining of sperm and egg to form a pluripotent zygote. One key question for development is whether the promoters of genes important for pluripotency, or for guiding early development (transcription and signaling factors), bear chromatin marks or composition that facilitate their expression at the onset of zygotic transcription. As mature sperm are reported to be transcriptionally inactive and have much higher levels of DNA methylation than somatic cells (or eggs), it has been assumed that the sperm genome is essentially inert, is largely reprogrammed by the egg chromatin, and makes only a few unique contributions to development: the sex chromosome, paternal imprints and certain RNA molecules. To test the status of sperm chromatin, we performed genome‐wide DNA methylation profiling on zebrafish sperm and adult tissues. Surprisingly, although zebrafish sperm is globally hypermethylated, the promoters of many genes encoding important developmental transcription factors and signaling proteins are distinctly hypomethylated. This raises the interesting possibility that certain genes are poised in sperm by hypomethylation to assist in their expression in the developing embryo. Our current efforts to understand how DNA methylation levels are correlated with, and related to, histone modifications will also be presented.

Osteogenic transcription factor Runx2 is a maternal determinant of dorsoventral patterning in zebrafish

The maternal genome greatly influences vertebrate embryogenesis before activation of zygotic transcription. Dorsoventral patterning is initiated by maternal factors, but the molecular pathways involved are incompletely understood. In frogs and fish, localized zygotic domains are induced whereby cells either express dorsal or ventral genes. Wnt/beta-catenin signalling promotes expression of the earliest dorsal zygotic genes. Among key zygotic ventralizing factors are the bone morphogenetic proteins (Bmps) and the Vent homeodomain family; the latter act as repressors of dorsal organizer gene transcription. Here we show that Runx2, a transcription factor essential for bone formation, is an important maternal determinant of ventral zygotic genes in zebrafish. Depletion of maternal Runx2b type2 strongly dorsalizes embryos, due to loss of the earliest zygotic expression of vox, vent and ved, resulting in expansion of dorsal gene expression. To date, Runx2b is the only known regulator of vox, vent and ved at the onset of zygotic transcription; we show that this regulation is direct. Runx2 transcripts are processed in mature mouse oocytes and we show that murine Runx2 type2 can substitute for the zebrafish orthologue in its ventralizing function, suggesting that Runx2 may have an evolutionarily conserved role in axis formation.

Tissue-dependent subcellular localization of Drosophila arginine methyl-transferase 4 (DART4), a coactivator whose overexpression affects neither viability nor differentiation

Drosophila arginine methyl- transferase 4 (DART4) belongs to the type I class of arginine methyltransferases. It catalyzes the methylation of arginine residues to monomethylarginines and asymmetrical dimethylarginines. The DART4 sequence is highly similar to mammalian PRMT4/CARM1, and DART4 substrate specificity has been conserved, too. Recently it was suggested that DART4/Carmer functions in ecdysone receptor mediated apoptosis of the polytene larval salivary glands and an apparent up-regulation of DART4/ Carmer mRNA levels before tissue histolysis was reported. Here we show that in Drosophila larvae, DART4 is mainly expressed in the imaginal disks and in larval brains, and to a much lesser degree in the polytene larval tissue such as salivary glands. In glands, DART4 protein is present in the cytoplasm and the nucleus. The nuclear signal emanates from the extrachromosomal domain and gets progressively restricted to the region of the nuclear lamina upon pupariation. Surprisingly, DART4 levels do not increase in salivary glands during pupariation, and overexpression of DART4 does not cause precautious cell death in the glands. Furthermore, over- and misexpression of DART4 under the control of the α tubulin promoter do not lead to any major problem in the life of a fly. This suggests that DART4 activity is regulated at the posttranslational level and/or that it acts as a true cofactor in vivo. We present evidence that nuclear localization of DART4 may contribute to its function because DART4 accumulation changes from a distribution with a strong cytoplasmic component during the transcriptional quiescence of the young embryo to a predominantly nuclear one at the onset of zygotic transcription.

Rapid ATP-dependent Deadenylation of nanos mRNA in a Cell-free System from Drosophila Embryos

Shortening of the poly(A) tail (deadenylation) is the first and often rate-limiting step in the degradation pathway of most eukaryotic mRNAs and is also used as a means of translational repression, in particular in early embryonic development. The nanos mRNA is translationally repressed by the protein Smaug in Drosophila embryos. The RNA has a short poly(A) tail at steady state and decays gradually during the first 2-3 h of development. Smaug has recently also been implicated in mRNA deadenylation. To study the mechanism of sequence-dependent deadenylation, we have developed a cell-free system from Drosophila embryos that displays rapid deadenylation of nanos mRNA. The Smaug response elements contained in the nanos 3'-untranslated region are necessary and sufficient to induce deadenylation; thus, Smaug is likely to be involved. Unexpectedly, deadenylation requires the presence of an ATP regenerating system. The activity can be pelleted by ultracentrifugation, and both the Smaug protein and the CCR4.NOT complex, a known deadenylase, are enriched in the active fraction. The same extracts show pronounced translational repression mediated by the Smaug response elements. RNAs lacking a poly(A) tail are poorly translated in the extract; therefore, SRE-dependent deadenylation contributes to translational repression. However, repression is strong even with RNAs either bearing a poly(A) tract that cannot be removed or lacking poly(A) altogether; thus, an additional aspect of translational repression functions independently of deadenylation.

Zebrafish MiR-430 Promotes Deadenylation and Clearance of Maternal mRNAs

MicroRNAs (miRNAs) comprise 1 to 3% of all vertebrate genes, but their in vivo functions and mechanisms of action remain largely unknown. Zebrafish miR-430 is expressed at the onset of zygotic transcription and regulates morphogenesis during early development. By using a microarray approach and in vivo target validation, we find that miR-430 directly regulates several hundred target messenger RNA molecules (mRNAs). Most targets are maternally expressed mRNAs that accumulate in the absence of miR-430. We also show that miR-430 accelerates the deadenylation of target mRNAs. These results suggest that miR-430 facilitates the deadenylation and clearance of maternal mRNAs during early embryogenesis.

MicroRNA Function and Mechanism: Insights from Zebra Fish

MicroRNAs (miRNAs) are small RNAs that bind to the 3 UTR of mRNAs. We are using zebra fish as a model system to study the developmental roles of miRNAs and to determine the mechanisms by which miRNAs regulate target mRNAs. We generated zebra fish embryos that lack the miRNA-processing enzyme Dicer. Mutant embryos are devoid of mature miRNAs and have morphogenesis defects, but differentiate multiple cell types. Injection of miR-430 miRNAs, a miRNA family expressed at the onset of zygotic transcription, rescues the early morphogenesis defects in dicer mutants. miR-430 accelerates the decay of hundreds of maternal mRNAs and induces the deadenylation of target mRNAs. These studies suggest that miRNAs are not obligatory components of all fate specification or signaling pathways but facilitate developmental transitions and induce the deadenylation and decay of hundreds of target mRNAs.

31 EFFECT OF TREATMENT OF OVINE CYTOPLASTS WITH CAFFEINE ON THE TRANSLOCALIZATION OF HSP27 IN SOMATIC CELL NUCLEAR TRANSFER RECONSTRUCTED EMBRYOS

We have previously reported that treatment of enucleated ovine oocytes with 10 mM caffeine for 6 h increases the activities of maturation-promoting factor (MPF) and mitogen-activated protein kinase (MAPK) (Lee and Campbell 2006 Biol. Reprod. 74, 691–698). In addition, when used as cytoplast recipients for somatic cell nuclear transfer (SCNT), the resultant blastocyst stage embryos show an increase in total cell numbers. Furthermore, we have also found that gene expression profiles observed following caffeine treatment more closely resemble those of IVF embryos than non-treated SCNT embryos. In particular, heat shock protein 27 (HSP-27, a molecular chaperon was down-regulated in non-treated SCNT embryos. In contrast, in caffeine-treated SNCT embryos, levels of transcripts were similar to those of IVF embryos (Choi et al. 2006 Reprod. Fertil. Dev. 18, 122). Several studies on heat shock protein 25 (HSP-25)/HSP-27 demonstrated that HSP-27 was underexpressed in SCNT bovine embryos (Pfister-Genskow et al. 2005 Biol. Reprod. 72, 546–555), and phosphorylated HSP-25 becomes sub-localized in nucleus following heat shock condition in mouse embryos (Kim et al. 2002 Mol. Reprod. Dev. 61, 3–13). In cultured cells, HSP-25 phosphorylation was mediated by mitogen-activated protein kinase (MAPK), suggesting that nuclear-translocated HSP-25 might function to protect nuclear structure, thereby preventing apoptosis (Geum et al. 2002 J. Biol. Chem. 277, 19 913–19 921). In this study, we investigated the effects of caffeine treatment on the localization of HSP-27 by fluorescene immunocytochemistry in SCNT and control embryos. HSP-27 was detected primarily in the cytoplasm of non-treated SCNT embryos from the 1-cell stage onwards; however, some nulear staining was observed in some embryos. In contrast, in caffeine-treated SCNT embryos, a more intense immunostaining of HSP-27 was observed in the nucleus than in the cytoplasm. This result indicated that HSP-27 was phosphorylated and translocated into nucleus in SCNT embryos treated with caffeine. In addition, we also examined the levels of transcripts of upstream genes of HSP-27 such as p38MAPKs, and MAPK-activated protein kinase 5 (Mapkapk5) by semi-quantitative RT-PCR. The levels of Mapkapk5 and p38MAPKs transcriptional expression were constant among groups. These observations suggest that the effects mediated by caffeine on HSP-27 occur before the onset of zygotic transcription, are independent of gene expression, but are maintained until the blastocyst stage. Taken together, our results suggest that the down-regulated expression of HSP-27 in SCNT embryos may be linked with the increased incidence of programmed cell death. In contrast, the elevated expression of HSP-27 and increased nuclear translocation observed in caffeine-treated SCNT embryos may contribute to the increase in total cell number we have previously reported at the blastocyst stage.

DNA damage response and Ku80 function in the vertebrate embryo

Cellular responses to DNA damage reflect the dynamic integration of cell cycle control, cell–cell interactions and tissue-specific patterns of gene regulation that occurs in vivo but is not recapitulated in cell culture models. Here we describe use of the zebrafish embryo as a model system to identify determinants of the in vivo response to ionizing radiation-induced DNA damage. To demonstrate the utility of the model we cloned and characterized the embryonic function of the XRCC5 gene, which encodes Ku80, an essential component of the nonhomologous end joining pathway of DNA repair. After the onset of zygotic transcription, Ku80 mRNA accumulates in a tissue-specific pattern, which includes proliferative zones of the retina and central nervous system. In the absence of genotoxic stress, zebrafish embryos with reduced Ku80 function develop normally. However, low dose irradiation of these embryos during gastrulation leads to marked apoptosis throughout the developing central nervous system. Apoptosis is p53 dependent, indicating that it is a downstream consequence of unrepaired DNA damage. Results suggest that nonhomologous end joining components mediate DNA repair to promote survival of irradiated cells during embryogenesis.

Xenopus flotillin1, a novel gene highly expressed in the dorsal nervous system

The two paralogues of the Xenopus flotillin1 gene (flotillin1A and flotillin1B), which encodes a putative membrane-associated protein, were cloned from egg, cleavage, and tadpole cDNA libraries. Both mRNAs are present during oogenesis and cleavage stages. After the onset of zygotic transcription, flotillin1 transcripts are first expressed throughout the embryonic ectoderm and become enhanced in the presumptive neural ectoderm as the neural plate forms. As the neural tube forms and differentiates, flotillin1 transcripts become enriched in the dorsal half, with particularly high expression in dorsal primary neurons. At early tail bud stages, there is additional expression in the paraxial mesoderm. At late tail bud stages, flotillin1A is expressed in branchial arch mesenchyme, the overlying branchial ectoderm and in dorsal somitic mesoderm, whereas flotillin1B expression is more restricted in the dorsal neural tube and head sensory structures. This report is the first comprehensive developmental description in any animal of the expression pattern of this gene, whose protein product in several systems plays important roles in signal transduction events.

Molecular Cloning and Expression of Two Closely Related GTP-binding Proteins from Zebrafish

Developmentally regulated GTP-binding proteins (DRGs) are a subclass of GTP-binding proteins that have been discovered recently. Here we report two zebrafish DRG cDNA clones closely related to human and mouse DRG genes. The two DRG sequences showed a high degree of similarity (55% identity, 72% similarity) at the amino acids level. Whole mount in situ hybridization revealed expression of zebrafish DRGs maternally, following the onset of zygotic transcription at the mid-blastula transition (MBT) and throughout embryonic. The expression of these two genes in different tissues follows a similar pattern, suggesting that they may serve a similar function.

Molecular characterization of genomic activities at the onset of zygotic transcription in mammals.

In rabbit embryos, zygotic transcripts are required for the development of the embryo only from the 8- to 16-cell stage onward, more than 44 h after fertilization (i.e., zygotic gene activation; ZGA). In order to characterize the first zygotic transcripts expressed in this species we used a suppression subtractive hybridization approach to isolate RNA that was present after the major transcriptional activation (morula stage), but absent at the 1-cell stage as maternal transcripts. One hundred fourteen differentially expressed inserts were selected and sequenced. A statistical analysis of expression patterns throughout the preimplantation period of development shows that genes transcribed from ZGA onward follow different patterns of expression. Considering their early post-ZGA behavior, we describe at least two main patterns: a gradual increase from ZGA onward, and a sharp increase in expression at ZGA followed by a marked decrease at the morula stage. Our data show that both ZGA and some early post-ZGA events are involved in the establishment of specific patterns of embryonic gene expression.

Wnt Signaling Through Different Transcription Factors

The Wnt signaling pathway regulates various processes during development. In Xenopus , Wnt signaling components encoded by maternal mRNAs establish the dorsal embryonic axis very early in development. However, after the onset of zygotic transcription, Xwnt-8 promotes ventral and lateral mesoderm development and restricts dorsal mesoderm development. Roël et al. demonstrated that the mechanism whereby Wnt signaling elicits distinct responses at different embryonic stages depends on differential expression of two transcription factors, XTcf-3 and XLef-1, downstream of the Wnt pathway. By injecting dominant-negative mutant forms of XTcf-3 and XLef-1 that could not bind β-catenin into various blastomeres in Xenopus embryos, the authors confirmed that Xtcf-3 was necessary for dorsal axis formation. They also showed that XLef-1 was required for zygotic, ventrolateral mesoderm-promoting development and for expression of tissue-specific genes regulated by Xwnt-8. The dominant-negative XLef-1 mutant rescued embryos from ectopic Xwnt-8 signaling but not from ectopic maternal Wnt signaling, whereas the Xtcf-3 mutant had the reverse effect. Antisense morpholino oligonucleotides against XLef-1 injected into blastomeres inhibited development and interfered with expression of the marker gene XmyoD similarly to the constitutively repressing XLef-1 mutant. Co-expressing human Lef-1, which is not inhibited by the antisense morpholino, rescued XmyoD expression, confirming that this phenotype results from loss of the XLef-1 gene product. These results suggest that the same β-catenin-dependent Wnt signaling pathway mediates both dorsal and ventrolateral development and that differential expression of the two transcription factors leads to tissue-specific responses to Wnt signaling. G. Roël, F. S. Hamilton, Y. Gent, A. A. Bain, O. Destree, S. Hoppler, Lef-1 and Tcf-3 transcription factors mediate tissue-specific Wnt signaling during Xenopus development. Curr. Biol. 12 , 1941-1945 (2002). [Online Journal]

Survivin mRNA is down-regulated during early Xenopus laevis embryogenesis.

One of the hallmarks of early development is the rapid proliferation of cells immediately after fertilization. Many of the rules that govern cell division in normal somatic cells, such as contact inhibition and apoptosis, seem temporarily suspended in the early embryo. A similar suspension of mechanisms normally regulating cell division occurs in the development of cancer. Survivin, an inhibitor of apoptosis and a positive regulator of progression through the cell cycle, localizes to the mitotic spindle and interacts with several proapoptotic caspases. Survivin protein expression has been studied during the development of the salivary gland in mouse. However, the regulation of survivin during the critical transitions defining oocyte maturation and the early restriction of developmental potential are not easily examined in the mouse. We therefore studied survivin mRNA expression during oogenesis and early embryogenesis in Xenopus laevis. We found that survivin mRNA is present in the earliest stages of Xenopus oocytes and that it accumulates during oogenesis. Progesterone-induced maturation of Xenopus oocytes leads to polyadenylation of the survivin transcript. Survivin mRNA is also present in early Xenopus embryos. After the onset of zygotic transcription, however, the amount of survivin mRNA declines rapidly to undetectable levels. This decrease in survivin mRNA correlates temporally with both the slowing of the cell cycle and the onset of endogenous embryonic apoptosis. With the exception of the ovary, survivin mRNA was undetectable in all adult Xenopus tissues examined.

Lef-1 and Tcf-3 Transcription Factors Mediate Tissue-Specific Wnt Signaling during Xenopus Development

Wnt signaling functions repeatedly during embryonic development to induce different but specific responses. What molecular mechanisms ensure that Wnt signaling triggers the correct tissue-specific response in different tissues? Early Xenopus development is an ideal model for addressing this fundamental question, since there is a dramatic change in the response to Wnt signaling at the onset of zygotic gene transcription: Wnt signaling components encoded by maternal mRNA establish the dorsal embryonic axis [1]; zygotically expressed Xwnt-8 causes almost the opposite, by promoting ventral and lateral and restricting dorsal mesodermal development [2–4]. Although Wnt signaling can function through different signal transduction cascades, the same β-catenin-dependent, canonical Wnt signal transduction pathway mediates Wnt signaling at both stages of Xenopus development [5, 6]. Here we show that, while the function of the transcription factor XTcf-3 is required for early Wnt signaling to establish the dorsal embryonic axis, closely related XLef-1 is required for Wnt signaling to pattern the mesoderm after the onset of zygotic transcription. Our results show for the first time that different transcription factors of the Lef/Tcf family function in different tissues to bring about tissue-specific responses downstream of canonical Wnt signaling.

Evolution of developmental functions by the Eomesodermin, T-brain-1, Tbx21 subfamily of T-box genes: insights from amphioxus.

We have identified an amphioxus T-box gene that is orthologous to the Eomesodermin, T-brain-1, and Tbx21 genes of vertebrates, and we have characterized its expression pattern during embryonic and larval development. AmphiEomes/Tbr1/Tbx21 is maternally expressed in oocytes and cleavage stage embryos. After the onset of zygotic transcription at the blastula stage, it is expressed in invaginating mesendoderm cells during gastrulation, but it is downregulated in presumptive ectoderm and neurectoderm. Expression is seen in both axial and paraxial mesendoderm in neurulae and early larvae, but it is not detected in differentiated endoderm, somites, or notochord. Expression persists in mesendoderm cells of the tail bud in early larvae, but it disappears between 1 to 1.5 days post fertilization. Unlike orthologous genes in basal deuterostomes or vertebrates, no anterior neural expression domain is detected at any stage of development. Integrating phylogenetic and developmental data, we have reconstructed the evolutionary history of the Eomesodermin/Tbr1/Tbx21 subfamily of T-box genes from a single ancestral locus that originated very early in metazoan evolution, before the evolution of triploblasts from their diploblast ancestor.

Cyclin-dependent kinase 8 is expressed both maternally and zygotically during zebrafish embryo development

Cyclin-dependent kinase 8 (cdk8) regulates transcription by phosphorylating RNA polymerase II and TFIIH. The mechanism of zygotic transcription activation during vertebrate embryonic development is poorly understood. Here we describe the cloning and developmental expression pattern of zebrafish cdk8 mRNA. It is highly conserved, sharing 79% DNA and 95% amino acid sequence identity with human cdk8, thereby indicating an important role for the protein. Northern blotting and whole mount in situ hybridisation revealed expression of zebrafish cdk8 maternally, following the onset of zygotic transcription at the mid-blastula transition (MBT) and throughout embryonic development.

Regulation of the mRNAs Encoding Proteins of the BMP Signaling Pathway during the Maternal Stages of Xenopus Development

Activation of the Xenopus bone morphogenetic protein (BMP) pathway is coincident with the onset of zygotic transcription but requires maternal signaling proteins. The mechanisms controlling the translation of mRNAs that encode proteins of the BMP pathway were investigated by using polysome association as an assay for translational activity. Our results indicate that five different mRNAs encoding proteins of the BMP pathway were translationally regulated during Xenopus development. These mRNAs were either not associated or inefficiently associated with polysomes in oocytes, and each was recruited to polysomes at a different developmental stage. The Smad1 and ALK-2 mRNAs were recruited to polysomes during oocyte maturation, whereas the BMP-7 and XSTK9 mRNAs were recruited during the early stages of embryogenesis. The ALK-3 mRNA was not efficiently associated with polysomes during any maternal stage of development and was efficiently recruited to polysomes only after the onset of zygotic transcription. In general, for all stages except oocytes, polysome recruitment was associated with the presence of a 3′ poly(A) tail. However, there was not an obvious correlation between the absolute length of poly(A) and the efficiency of polysome recruitment, indicating that the relationship between poly(A) tail length and translation during early frog embryogenesis is complex. We further focused on the BMP-7 mRNA and demonstrated that sequence elements within the 3′UTR were necessary for recruitment of the BMP-7 mRNA to polysomes and sufficient to direct the addition of poly(A) and activate translation of a reporter during embryogenesis. Interestingly, the BMP-7 mRNA lacks the previously defined eCPE sequences proposed to direct poly(A) addition and translational activation during embryogenesis. The implications of our findings for translational regulation of maternal mRNAs during embryogenesis and for the activation of the BMP pathway are discussed.

Expression of genes involved in mammalian meiosis during the transition from egg to embryo.

The ooplasm of higher eukaryotes provides substances necessary for completing the last stages of meiosis and initiating the first mitotic division. These processes are firmly attuned to other events in the egg and newly formed embryo, such as switching from the use of maternal transcripts to the onset of zygotic transcription. In mammals little is known about the molecular mechanisms guiding this transition, largely due to the lack of information about genes expressed in the egg and early embryos. Studies of yeast mitosis have contributed much of what is known about the vertebrate cell cycle, and recent reports indicate that homologs of yeast DNA repair genes also function during mammalian gametogenesis. To examine whether this conservation can be expanded to include genes operative in oocyte meiosis, we performed a computer-based search for homologs of yeast genes that are induced during sporulation in C. elegans, Drosophila, and mammals. Results from this study suggest that yeast and higher eukaryotes share genes that coordinate the overall process of meiosis. However intriguing differences exist, reflecting the distinctive mechanisms governing the progression of meiosis in each organism. ESTs representing more than half of the mammalian homologs are present in mouse cDNA libraries that contains genes controlling the meiosis/mitosis transition. About 50% of these genes contain potential cis-elements for cytoplasmic polyadenylation in their 3'-UTR, suggesting the importance of controlled translation in the egg and zygote.

Onset of zygotic transcription and maternal transcript legacy in the rabbit embryo

Onset of zygotic transcription is progressive from the one-cell stage onward in the rabbit embryo. Maternal transcripts remain fairly stable until the 8–16 cell stage when major transcriptional activation of the zygotic genome takes place. To understand the mechanisms of the maternal-to-zygotic transition in the genetic information governing development, we asked whether a progressive synthesis of zygotic transcripts takes over the maternal molecules, or whether the synthesis of zygotic transcripts is very abrupt and independent of the persistence of the maternal counterparts. To answer this question, we set up mRNA differential display experiments comparing the mRNA content of rabbit embryos at different stages during the preimplantation period. We isolated eight zygotic transcripts whose synthesis is abruptly turned on at the 8–16 cell stage. These transcripts are involved in general cellular metabolism and their maternal counterparts are still present up to the four-cell and even the 8–16 cell stage. This identification of early zygotic transcripts suggests that global long range modifications of chromatin structure result in a rapid increase in transcription rates during the major transcriptional activation of the zygotic genome. Mol. Reprod. Dev. 58:127–136, 2001. © 2001 Wiley-Liss, Inc.