Spatially restricted patterns of expression of the homeobox-containing gene Hox 2.1. during mouse embryogenesis

The mouse Hox 2.1 gene contains a homeobox sequence and is therefore a candidate for a vertebrate gene involved in the control of embryonic patterning or positional specification. To investigate this possibility, we have used in situ hybridization to determine the pattern of Hox 2.1 expression during mouse embryogenesis. At 8.5 days post coitum, Hox 2.1 is expressed at a low level in the posterior neuroectoderm and mesoderm, and in the neuroectoderm of the presumptive hindbrain. At 12.5 days p.c., Hox 2.1 is expressed in an anteroposterior restricted domain extending from the hindbrain throughout the length of the spinal cord, predominantly in the dorsal region. Between 12.5 and 13.5 days p.c. the domain becomes localized to the occipital and cervical regions. We also detect Hox 2.1 RNA in the embryonic lung, stomach, mesonephros and metanephros, as well as in myenteric plexus, dorsal root ganglia and the nodose ganglion, and in mature granulocytes. The embryonic expression of Hox 2.1 in neural tissue is compared with that of Hox 3.1, which also shows anteroposterior restricted domains of gene expression. These patterns of expression are not clearly consistent with Hox 2.1 or Hox 3.1 having roles in segmental patterning. However, the data are consistent with these genes having regulatory roles in anteroposterior positional specification in the neuroectoderm and mesoderm, and suggest that Hox 2.1 may also have functions during organogenesis.     

Rbf1-independent termination of E2f1-target gene expression during early Drosophila embryogenesis

The initiation and maintenance of G1 cell cycle arrest is a key feature of animal development. In the Drosophila ectoderm, G1 arrest first appears during the seventeenth embryonic cell cycle. The initiation of G1(17) arrest requires the developmentally-induced expression of Dacapo, a p27-like Cyclin E-Cdk2 inhibitor. The maintenance of G1(17) arrest requires Rbf1-dependent repression of E2f1-regulated replication factor genes, which are expressed continuously during cycles 1-16 when S phase immediately follows mitosis. The mechanisms that trigger Rbf1 repressor function and mediate G1(17) maintenance are unknown. Here we show that the initial downregulation of expression of the E2f1-target gene RnrS, which occurs during cycles 15 and 16 prior to entry into G1(17), does not require Rbf1 or p27(Dap). This suggests a mechanism for Rbf1-independent control of E2f1 during early development. We show that E2f1 protein is destroyed in a cell cycle-dependent manner during S phase of cycles 15 and 16. E2f1 is destroyed during early S phase, and requires ongoing DNA replication. E2f1 protein reaccumulates in epidermal cells arrested in G1(17), and in these cells the induction of p27(Dap) activates Rbf1 to repress E2f1-target genes to maintain a stable G1 arrest.     

Cuticle differentiation during Drosophila embryogenesis

The constitutive criterion for the evolutionary successful clade of ecdysozoans is a protective exoskeleton. In insects the exoskeleton, the so-called cuticle consists of three functional layers, the waterproof envelope, the proteinaceous epicuticle and the chitinous procuticle that are produced as an extracellular matrix by the underlying epidermal cells. Here, we present our electron-microscopic study of cuticle differentiation during embryogenesis in the fruit fly Drosophila melanogaster. We conclude that cuticle differentiation in the Drosophila embryo occurs in three phases. In the first phase, the layers are established. Interestingly, we find that establishment of the layers occurs partially simultaneously rather than in a strict sequential manner as previously proposed. In the second phase the cuticle thickens. Finally, in the third phase, when secretion of cuticle material has ceased, the chitin laminae acquire their typical orientation, and the epicuticle of the denticles and the head skeleton darken. Our work will help to understand the phenotypes of embryos mutant for genes encoding essential cuticle factors, in turn revealing mechanisms of cuticle differentiation.     

A study of Drosophila spinster expression and its functions during embryogenesis

During the development of the Drosophila nervous system, the programmed cell death (PCD) regulates the cell number. The spinster (spin) encodes a multiple transmembrane protein and females showed a strong rejection response against the courting males. Mutation in spin interferes with the PCD of neurons, which subsequently induce the degeneration of adult neural cells. However, this spin functions has not been investigated yet during embryogenesis. In this study we first examined spin expression in detail and its function during embryonic development. Spin was primarily expressed in surface glial cells, including subperineurial glial cells and exit glia, but not in neural cells. In spin loss-of-function mutant embryos, Glial cells increased in number, and neural overgrowth occurred. In spin gain-of-function mutant embryos, PNS was predominantly degenerated at late embryonic stages. These results indicate that spin is involved in neurogenesis via cell death during embryogenesis.     

Autonomous requirements for the segment polarity gene armadillo during Drosophila embryogenesis

Embryos hemizygous for armadillo produce a "segment polarity" phenotype in which the naked posterior two-thirds of each segment is replaced by denticles with reversed polarity. Small patches of homozygous arm cells induced by mitotic recombination also form such denticles, indicating that the changes in cellular fate observed in homozygous arm embryos are autonomous at the level of single cells. Clonally derived arm patches do not, however, show the characteristic arm polarity reversals, arguing that this feature of the phenotype depends on cell interactions in fully mutant embryos. Few, if any, clones were found in the posterior-most regions of the naked cuticle, and none were found in the posterior compartments of the thorax.     

Heat shock gene expression and function during zebrafish embryogenesis

Recent work in the zebrafish, Danio rerio, indicates that heat shock genes are expressed in unique spatial patterns under non-stress conditions. In particular, hsp90alpha is expressed during the normal differentiation of striated muscle fibres, and hsp70-4 is expressed during normal lens development in the eye. Furthermore, disruption of the activity of either of these genes or their protein products gives rise to unique embryonic phenotypes that result from failures in proper somitic muscle development and lens development, respectively. Embryonic hsp70-4 expression is also activated in a cell-specific manner following heavy metal exposure. This has allowed for the development of a hsp70-4/eGFP reporter gene system in stable transgenic zebrafish that serves as a reliable yet extremely quick indicator of cell-specific toxicity in the context of the multicellular, living embryo.     

Zygotically active genes that affect the spatial expression of the fushi tarazu segmentation gene during early Drosophila embryogenesis

The establishment of the segmental body pattern of Drosophila requires the coordinated functions of three classes of zygotically active genes early in development. We have examined the effects of mutations in these genes on the spatial expression of the fushi tarazu (ftz) pair-rule segmentation gene. Mutations in four gap loci and in three pair-rule loci dramatically affect the initial pattern of transverse stripes of ftz-containing nuclei. Five other pair-rule genes and several other loci that affect the larval cuticular pattern do not detectably affect ftz expression. No simple regulatory relationships can be deduced. Rather, expression of the ftz gene depends upon the interactions among the different segmentation genes active at each position along the anterior-posterior axis of the early embryo.