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Abstract: The protein β-catenin plays a critically important role in establishing axial polarity during early animal development. In many organisms, β-catenin is degraded preferentially on one side of the cleavage stage embryo. On the opposite side of the embryo, β-catenin is stabilized and accumulates in the nucleus, where it functions in concert with members of the LEF/TCF family to activate the transcription of diverse target genes. Genes that are activated by β-catenin play an essential role in the specification of endomesoderm and in the establishment of key signaling centers in the early embryo. In several organisms, the asymmetric distribution of maternal components of the canonical Wnt pathway has been shown to be responsible for the polarized stabilization of β-catenin. In this study, we identified all Wnt and Wnt receptor mRNAs that are present in unfertilized sea urchin eggs and early embryos and analyzed their distributions along the primary (AV) axis. Our findings indicate that the asymmetric distribution of a maternal Wnt or Wnt receptor mRNA is unlikely to be a primary determinant of the polarized stabilization of β-catenin along the AV axis. This contrasts sharply with findings in other organisms and points to remarkable evolutionary flexibility in the molecular mechanisms that underlie this otherwise very highly conserved patterning process. [Copyright &y& Elsevier]

Abstract: The purpose of this study was to investigate the effect of photoperiod advanced spawning on egg quality and egg transcriptome. Experimentation was performed using 2 groups of rainbow trout (Oncorhynchus mykiss) from an autumn-spawning strain. In 2003, a first group of females was exposed, after their first natural spawning, to a long–short photoperiod regime beginning in January (photoperiod manipulated group, PM group, n =17) resulting in a second reproduction in June–July. A second group of females achieved their first reproduction in the fall (2003) and was used as a control (n =25). For each female, egg quality was assessed by monitoring survival at eyeing (E) and yolk–sac resorption (YSR) as well as the occurrence of malformations at YSR. We observed a significant decrease of egg quality in PM group. Survival at eyeing was 49±18% (means±95% CI) in PM group while a 93±3% survival was observed in control group. At YSR, the frequencies of alive alevins exhibiting no noticeable malformation were 37±16% in PM group and 84±5% in the control group. This increase of mortality was associated with a higher individual variability between females and an increase of malformations observed on alive alevins. These malformations were mostly characterized by defects of yolk–sac resorption (48% of malformations observed in PM group). Transcriptome analysis was performed using nylon microarrays displaying 9152 rainbow trout cDNAs spotted. Differential analysis allowed the identification of 6 genes significantly less abundant in PM eggs than in control eggs. These results suggest that photoperiod manipulation of spawning date can induce significant egg quality defects. Moreover, the transcriptome of unfertilized oocyte appears to be influenced by environmental perturbations, such as photoperiod regime, and seems to be reflective of egg developmental capacities after fertilization. [Copyright &y& Elsevier]

Circadian (∼24 h) clocks are endogenous time-keeping systems that drive the daily biological rhythms observed in most living organisms. The oscillation is generated by a transcriptional/translational autoregulatory feedback loop that is reset by external time cues such as the light/dark cycle and which in turn controls rhythms in physiology and behavior through downstream clock-controlled genes (Nature 417 (2002) 329). Genetic and biochemical analysis of Drosophila and mammalian clock genes has provided a comprehensive model for the molecular oscillator that generates these rhythms, but the ontogeny of this oscillator remains poorly understood. A circadian oscillator involving the clock genes Per3 and Rev-erbα was identified during early development in zebrafish (Science 289 (2000) 297). Here, we report the isolation of zebrafish Per2 and show the presence of a Per2 maternal mRNA in early embryos as for Per3. However, Per2 rhythmic expression occurs late during embryogenesis as compared to that of Per3. Furthermore, our data indicate that Per2 is not required during embryogenesis for the rhythmicity of physiological outputs such as melatonin synthesis. In addition, Per2 but not Per3 is constitutively expressed in the developing olfactory bulb and pituitary. This differential spatio-temporal expression patterns suggest specific roles for Per2 and Per3 in the establishment of the embryonic circadian system. [Copyright &y& Elsevier]

The mouse oocyte-specific linker histone H1oo (1) constitutes a novel mammalian homologue of the oocyte-specific linker histone B4 of the frog and of the cs-H1 linker histone of the sea urchin; (2) is expressed as early as the germinal vesicle (PI) stage oocyte, persisting into the MII stage oocyte, the oocytic polar bodies, and the 2-cell embryo, extinction becoming apparent at the 4–8 cell embryonic stage; and (3) may play a key role in the control of gene expression during oogenesis and early embryogenesis, presumably through the perturbation of chromatin structure. [Copyright &y& Elsevier]

The posterior-vegetal cytoplasm (PVC) of fertilized ascidian eggs plays important roles in embryo development. It has been reported that some maternal RNAs are localized to the PVC. We identified four novel type I postplasmic mRNAs that are localized to the PVC through the use of data from a cDNA project of maternal mRNAs in the eggs of Halocynthia roretzi (MAGEST database). The mRNAs are HrGLUT, HrPEN-1, and HrPEM-3, which show similarity to a glucose transporter, a g1-related protein, and Ciona pem-3, respectively; and HrPEN-2, with no similarity. Maternal mRNAs of all four genes were identically localized to the PVC after ooplasmic segregation. During cleavage, they were concentrated in the centrosome-attracting body (CAB) and were then segregated into the small blastomeres located at the posterior pole. This localization pattern is common to all known type I postplasmic mRNAs found so far. HrGLUT, HrPEN-1, and HrPEM-3 were expressed zygotically in various tissues later in embryogenesis: HrGLUT and HrPEM-3 in the mesenchyme and nervous system, and HrPEN-1 in the ectodermal cells. [Copyright &y& Elsevier]

Summary Animal germ cells possess a specific post-transcriptional regulatory context allowing the storage of maternal transcripts in the oocyte until their translation at a specific point in early development. As key regulators of gene expression, miRNAs repress translation mainly through mRNA destabilization. Thus, germline miRNAs likely use distinct ways to regulate their targets. Here, we use C. elegans to compare miRNA function within germline and somatic tissues. We show that the same miRNA displays tissue-specific gene regulatory mechanisms. While translational repression occurs in both tissues, targeted mRNAs are instead stabilized in the germline. Comparative analyses of miRNA silencing complexes (miRISC) demonstrate that their composition differs from germline to soma. We show that germline miRNA targets preferentially localize to perinuclear regions adjacent to P granules, and their repression is dependent on the core P granule component GLH-1. Together, our findings reveal the existence of different miRISC in animals that affect targeted mRNAs distinctively. Graphical Abstract Highlights • Germline and somatic miRNA silencing complexes differ in composition • Germline miRNAs protect from degradation rather than destabilize targeted transcripts • Germline miRNA targeting leads to mRNA perinuclear localization adjacent to P granules • Repression of germline miRNA targets is dependent on the P granule component GLH-1 Dallaire et al. show that in different animal tissues one microRNA uses different mechanisms to regulate gene expression. Although both somatic and germline microRNAs abrogate protein expression, somatic microRNAs direct target degradation whereas germline-expressed microRNAs protect targeted mRNA from degradation through specific localization mediated by the VASA homolog GLH-1. [ABSTRACT FROM AUTHOR]