The ascidian larval brain and palps (a putative rudimentary placode) are specified by two transcription factor genes, ZicL and FoxC, respectively. FGF9/16/20 induces ZicL expression soon after the bipotential ancestral cells divide into the brain and palp precursors at the early gastrula stage. FGF9/16/20 begins to be expressed at the 16-cell stage, and induces several target genes, including Otx, before the gastrula stage. Here, we show that ZicL expression in the brain lineage is transcriptionally repressed by Hes-a and two Blimp-1-like zinc finger proteins, BZ1 and BZ2, in the bi-potential ancestral cells. ZicL is precociously expressed in the bi-potential cells in embryos in which these repressors are knocked down. This precocious ZicL expression produces extra brain cells at the expense of palp cells. The expression of BZ1 and BZ2 is turned off by a negative autofeedback loop. This auto-repression acts as a delay circuit that prevents ZicL from being expressed precociously before the brain and palp fates split, thereby making room within the neural plate for the palps to be specified. Addition of the BZ1/2 delay timer circuit to the gene regulatory network responsible for brain formation might represent a key event in the acquisition of the primitive palps/placodes in an ancestral animal.

Highlights d Single pool of heterogenous stem cells supports homeostasis in mouse spermatogenesis d Stem cells move reversibly between renewal-biased and differentiation-primed states d Stem cell dynamics depends on distinct rates of state transition and cell division d Such multistate dynamics reduces mitotic load, while keeping stem cell density high

Maternally provided mRNAs and proteins direct early development and activate the zygotic genome. Using microarrays, we examined the dynamics of transcriptomes during the early development of a basal chordate, Ciona intestinalis. Microarray analysis of unfertilized eggs, as well as 8-, and 16- and 32-cell embryos revealed that nearly half of the genes encoded in the genome were expressed maternally, and that approximately only one-fourth of these genes were expressed at similar levels among eggs obtained from different individuals. Genes encoding proteins involved in protein phosphorylation were enriched in this latter group. More than 90% of maternal RNAs were not reduced before the 16-cell stage when the zygotic developmental program begins. Additionally we obtained gene expression profiles of individual blastomeres from the 8- and 16-cell embryos. On the basis of these profiles, we concluded that the posterior-most localization, which has been reported for over 20 different transcripts, is the only major localization pattern of maternal transcripts. Our data also showed that maternal factors establish only nine distinct patterns of zygotic gene expression at the 16-cell stage. Therefore, one of the main developmental functions of maternally supplied information is to establish these nine distinct expression patterns in the 16-cell embryo. The dynamics of transcriptomes in early-stage embryos provides a foundation for studying how maternal information starts the zygotic program.

In embryos of an invertebrate chordate, Ciona intestinalis, two transcription factors, Foxa.a and Zic-r.b, are required for specification of the brain and the notochord, which are derived from distinct cell lineages. In the brain lineage, Foxa.a and Zic-r.b are expressed with no temporal overlap. In the notochord lineage, Foxa.a and Zic-r.b are expressed simultaneously. In the present study, we found that the temporally non-overlapping expression of Foxa.a and Zic-r.b in the brain lineage was regulated by three repressors: Prdm1-r.a (formerly called BZ1), Prdm1-r.b (BZ2) and Hes.a. In morphant embryos of these three repressor genes, Foxa.a expression was not terminated at the normal time, and Zic-r.b was precociously expressed. Consequently, Foxa.a and Zic-r.b were expressed simultaneously, which led to ectopic activation of Brachyury and its downstream pathways for notochord differentiation. Thus, temporal controls by transcriptional repressors are essential for specification of the two distinct fates of brain and notochord by Foxa.a and Zic-r.b. Such a mechanism might enable the repeated use of a limited repertoire of transcription factors in developmental gene regulatory networks.

Pancreatic islets play an essential role in regulating blood glucose level. Although the molecular pathways underlying islet cell differentiation are beginning to be resolved, the cellular basis of islet morphogenesis and fate allocation remain unclear. By combining unbiased and targeted lineage tracing, we address the events leading to islet formation in the mouse. From the statistical analysis of clones induced at multiple embryonic timepoints, here we show that, during the secondary transition, islet formation involves the aggregation of multiple equipotent endocrine progenitors that transition from a phase of stochastic amplification by cell division into a phase of sublineage restriction and limited islet fission. Together, these results explain quantitatively the heterogeneous size distribution and degree of polyclonality of maturing islets, as well as dispersion of progenitors within and between islets. Further, our results show that, during the secondary transition, α- and β-cells are generated in a contemporary manner. Together, these findings provide insight into the cellular basis of islet development.