Nobel laureates' letter to President Bush.
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Embryo Disposition↗
Biomedical subjects
E F Wieschaus
Publications and source records attributed to E F Wieschaus.
Cell and developmental biology--a shared past, an intertwined future.
Cell and developmental biology are distinct disciplines with clear differences in emphasis and domains of interest, yet they also share a common historic origin and benefit from an increasingly productive exchange of insights and influences. Our goal in this commentary is to examine the common origin of cell and developmental biology, to explore ways in which they currently interact, and to consider the connections and differences that exist between these two fields.
Animals↗
Dynein-mediated cargo transport in vivo. A switch controls travel distance.
Cytoplasmic dynein is a microtubule-based motor with diverse cellular roles. Here, we use mutations in the dynein heavy chain gene to impair the motor's function, and employ biophysical measurements to demonstrate that cytoplasmic dynein is responsible for the minus end motion of bidirectionally moving lipid droplets in early Drosophila embryos. This analysis yields an estimate for the force that a single cytoplasmic dynein exerts in vivo (1.1 pN). It also allows us to quantitate dynein-mediated cargo motion in vivo, providing a framework for investigating how dynein's activity is controlled. We identify three distinct travel states whose general features also characterize plus end motion. These states are preserved in different developmental stages. We had previously provided evidence that for each travel direction, single droplets are moved by multiple motors of the same type (Welte et al. 1998). Droplet travel distances (runs) are much shorter than expected for multiple motors based on in vitro estimates of cytoplasmic dynein processivity. Therefore, we propose the existence of a process that ends runs before the motors fall off the microtubules. We find that this process acts with a constant probability per unit distance, and is typically coupled to a switch in travel direction. A process with similar properties governs plus end motion, and its regulation controls the net direction of transport.
Animals↗
Developmental regulation of vesicle transport in Drosophila embryos: forces and kinetics.
In Drosophila embryos, microtubules oriented along apical-basal directions support saltatory vesicle movement. Vesicle traffic includes lipid droplets whose distribution shifts twice during early embryogenesis. Using microscopy, optical tweezers, and a novel squashed-mount embryo preparation, we tracked single droplets and measured the forces these generated. Droplet stalling forces change developmentally, in a roughly quantized fashion, consistent with variation in the number of active motors. We characterized a mutation, klarsicht, that affects droplet transport. Klar+ facilitates changes in force, possibly by coordinating the activity of multiple motors. Alterations in transport affected motion in both apical and basal directions, indicating tight coupling between motors of opposite polarity. Mutations in klar also affect nuclear migration during eye development, suggesting multiple roles for klar-based transport.
Animals↗
A novel basic helix-loop-helix protein is expressed in muscle attachment sites of the Drosophila epidermis.
We have found that a novel basic helix-loop-helix (bHLH) protein is expressed almost exclusively in the epidermal attachments sites for the somatic muscles of Drosophila melanogaster. A Drosophila cDNA library was screened with radioactively labeled E12 protein, which can dimerize with many HLH proteins. One clone that emerged from this screen encoded a previously unknown protein of 360 amino acids, named delilah, that contains both basic and HLH domains, similar to a group of cellular transcription factors implicated in cell type determination. Delilah protein formed heterodimers with E12 that bind to the muscle creatine kinase promoter. In situ hybridization with the delilah cDNA localized the expression of the gene to a subset of cells in the epidermis which form a distinct pattern involving both the segmental boundaries and intrasegmental clusters. This pattern was coincident with the known sites of attachment of the somatic muscles to tendon cells in the epidermis. delilah expression persists in snail mutant embryos which lack mesoderm, indicating that expression of the gene was not induced by attachment of the underlying muscles. The similarity of this gene to other bHLH genes suggests that it plays an important role in the differentiation of epidermal cells into muscle attachment sites.
Amino Acid Sequence↗
The nullo protein is a component of the actin-myosin network that mediates cellularization in Drosophila melanogaster embryos.
After the 13th nuclear division cycle of Drosophila embryogenesis, cortical microfilaments are reorganized into a hexagonal network that drives the subsequent cellularization of the syncytial embryo. Zygotic transcription of the nullo and serendipity-alpha genes is required for normal structuring of the microfilament network. When either gene is deleted, the network assumes an irregular configuration leading to the formation of multinucleate cells. To investigate the role of these genes during cellularization, we have made monoclonal antibodies to both proteins. The nullo protein is present from cycle 13 through the end of cellularization. During cycle 13, it localizes between interphase actin caps and within metaphase furrows. In cellularizing embryos, nullo co-localizes with the actin-myosin network and invaginates along with the leading edge of the plasma membrane. The serendipity-alpha (sry-alpha) protein co-localizes with nullo protein to the hexagonal network but, unlike the nullo protein, it localizes to the sides rather than the vertices of each hexagon. Mutant embryos demonstrate that neither protein translationally regulates the other, but the localization of the sry-alpha protein to the hexagonal network is dependent upon nullo.
Actin Cytoskeleton↗
Maternal effect mutations of the sponge locus affect actin cytoskeletal rearrangements in Drosophila melanogaster embryos.
In the syncytial blastoderm stage of Drosophila embryogenesis, dome-shaped actin "caps" are observed above the interphase nuclei. During mitosis, this actin rearranges to participate in the formation of pseudocleavage furrows, transient membranous invaginations between dividing nuclei. Embryos laid by homozygous sponge mothers lack these characteristic actin structures, but retain other actin associated structures and processes. Our results indicate that the sponge product is specifically required for the formation of actin caps and metaphase furrows. The specificity of the sponge phenotype permits dissection of both the process of actin cap formation and the functions of actin caps and metaphase furrows. Our data demonstrate that the distribution of actin binding protein 13D2 is unaffected in sponge embryos and suggest that 13D2 is upstream of actin in cortical cap assembly. Although actin caps and metaphase furrows have been implicated in maintaining the fidelity of nuclear division and the positions of nuclei within the cortex, our observations indicate that these structures are dispensible during the early syncytial blastoderm cell cycles. A later requirement for actin metaphase furrows in preventing the nucleation of mitotic spindles between inappropriate centrosomes is observed. Furthermore, the formation of actin caps and metaphase furrows is not a prerequisite for the formation of the hexagonal array of actin instrumental in the conversion of the syncytial embryo into a cellular blastoderm.
Actin Depolymerizing Factors↗
short gastrulation, a mutation causing delays in stage-specific cell shape changes during gastrulation in Drosophila melanogaster.
Mutations at the short gastrulation locus affect the timing of certain early morphogenetic events occurring during gastrulation in Drosophila melanogaster. Specifically, the invagination and subsequent closing of the posterior midgut and the anterior midgut appear to be delayed in these embryos. In addition, their germbands do not extent the full distance anteriorly on the dorsal side of the embryo. The dorsal cells are abnormally thick and fall into extremely deep dorsal folds as the germband extends. sog embryos continue development, but form disorganized first instar larvae. Normal sog expression is required in the zygote, but not in the mother for normal embryonic development and viability. Analysis of adult and larval gynandromorphs indicates that sog expression is required only in the ventral and/or anterior and posterior ends of the embryo, arguing that the dorsal abnormalities caused by the mutation are secondary consequences of defects elsewhere in mutant embryos.
Alleles↗
Dominant maternal-effect mutations of Drosophila melanogaster causing the production of double-abdomen embryos.
Dominant mutations at two loci, BicaudalC (BicC) and BicaudalD (BicD), cause heterozygous females to produce double-abdomen embryos. These mutations cause the production of embryos with a range of defects extending from the anterior end of the differentiated embryo. The same array of defective embryos is caused by mutations at either locus and is similar to that produced by the original mutation at bicaudal (bic). The array of defective embryos suggests that these mutations cause the loss of positional values from the anterior end of the embryo, associated with a duplication of the posterior end if too few positional values remain. BicaudalD mutations appear to be antimorphic, gain-of-function mutations, whereas BicaudalC mutations are likely to be hypomorphic or amorphic mutations. Mutations at all these loci (bic, BicC and BicD) act as mutual enhancers of each other, and a number of other maternal-effect mutations also act to either enhance or suppress the expression of these dominant bicaudal mutations.
Abdomen↗
The localized requirements for a gene affecting segmentation in Drosophila: analysis of larvae mosaic for runt.
The runt gene is required in a developing Drosophila embryo for proper segmentation. Mutant embryos fail to hatch but secrete a larval cuticle in which pattern defects are apparent. In runt embryos, there are pattern deletions spaced at two segment intervals along the antero-posterior axis of the animal. The deleted regions are replaced by mirror-image duplications of the remaining regions. This paper investigates the localized requirements for runt+ activity by analyzing the segmentation patterns in larval genetic mosaics. This analysis is aided by the faintoid and shavenbaby mutations which affect larval cuticle morphology without affecting segmentation. These two mutations serve as markers of the regions of larval cuticle secreted by genetically runt cells. The analysis of the runt mosaic patterns indicates the effects of the gene on segmentation are primarily cell autonomous. This includes both the pattern deletions and the associated mirror-image duplications. This indicates the mirror-image duplications are not due to regeneration but result from a more direct effect of runt on patterning in the embryo. The mosaic patterns also reveal other aspects of the process of pattern formation in the larval epidermis. Based on these results a model is presented for the generation of the larval pattern.
Animals↗
Requirements for zygotic gene activity during gastrulation in Drosophila melanogaster.
Mutations at the folded gastrulation (fog) and twisted gastrulation (tsg) loci interfere with early morphogenetic movements in Drosophila melanogaster. fog embryos do not form a normal posterior midgut and although their germbands do elongate, they do not extend dorsally. As a result, when normal embryos have fully extended germbands, the germbands in mutant embryos are folded into the interior on the ventral side of the embryo. tsg embryos have abnormally deep dorsal folds during early gastrulation, associated with the failure of dorsal cells to slip laterally to make way for the expanding germband. Both fog and tsg embryos continue to develop, but form disorganized first instar larvae. fog and tsg are zygotically active genes expressed at least by 10 and 20 min after the onset of gastrulation. Both mutations are viable in homozygous germ cells and the wild-type genes need not be expressed during oogenesis for survival of heterozygous progeny. Elimination of fog+ gene product from maternal germ cells does, however, affect the extent of folding observed during gastrulation in viable heterozygotes. Analysis of fog adult and larval gynandromorphs indicates that normal folded gastrulation gene function is only required at the posterior region of the embryo, most probably in the cells giving rise to the posterior midgut or proctodeum. The relative survival of fog mosaics suggests that embryos with mosaic "lethal foci" also die during embryogenesis, although the typical fog phenotype is only produced when the entire focus is mutant. In contrast to the fog focus, no particular cell must be wild type in tsg mosaics for survival. Wild-type cells on the dorsal side of the embryo, however, are most effective in rescuing the embryo. This indicates that normal tsg gene product may be required only on the dorsal side of the embryo, potentially in the region which gives rise to the amnion serosa.
Animals↗
Bicaudal mutations of Drosophila melanogaster: alteration of blastoderm cell fate.
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Animals↗