Development Genes and Evolution

Speksnijder, Johanna; Jong, Kees; Wisselaar, Heleen; Linnemans, Wilbert; Dohmen, M.

doi: 10.1007/BF02438936pmid: 28305713

We have studied the organization of the cortex in fertilized eggs ofNassarius reticulatus by examining rotary-shadowed whole mounts of isolated cortices in the transmission electron microscope. The following components were distinguished: (a) the plasma membrane, with clathrin-coated areas and coated pits, (b) microfilaments and microtubules, and (c) a tubulovesicular network of endoplasmic reticulum. Microfilaments were identified by labeling with heavy meromyosin, and microtubules with a monoclonal anti-tubulin antibody, using both immunofluorescence microscopy and immunogold labeling for transmission electron microscopy. The microfilaments are organized in a network parallel to and closely associated with the plasma membrane, with typical Y- and X-shaped intersections. The endoplasmic reticulum is associated with this microfilamentous lattice. The microtubules also run parallel to the plasma membrane, but they are located at a greater distance, as can be inferred from stereo images. In the uncleaved egg, numerous microtubules are present in the egg cortex. Shortly before polar lobe formation, at the onset of mitosis, the microtubules disappear almost entirely. They reappear again at the end of first cleavage, as the polar lobe is being resorbed. The synthesis of cortical microtubules at this stage appears to depend on the presence of microtubule-organizing centers in the animal hemisphere of the egg, since microtubules do not reappear in isolated polar lobes. Clathrin-coated areas are present in both the animal and vegetal hemisphere before polar lobe formation. During mitosis, the clathrin-coated plaques and pits are found almost exclusively in the animal hemisphere. After resorption of the polar lobe, at the two-cell stage, no clathrin-coated areas were found at all.

Henry, Jonathan

doi: 10.1007/BF02438937pmid: 28305714

The light emitting photocytes ofChaetopterus variopedatus larvae are bilaterally situated within the ectoderm of the post-trochal region. Their histological appearance is similar to that of the adult photocytes. The larval photocytes contain a large quantity of membranous secretory vesicles (photosomes), which probably contain the photoluminescent protein. The two-cellChaetopterus embryo contains a small AB and a large CD blastomere. Previous studies have shown that only the “larvae” resulting from isolated CD blastomeres are able to luminesce. Consistent with these findings, morphologically distinct photocytes are only found in the CD larvae. The removal of the small polar lobe that forms during first cleavage leads to the production of a “larva” that is unable to produce light. All delobed larvae contain morphologically distinct photocytes, which are identical to those in normal larvae except they appear to contain only a small quantity of photosomes. Experimental equalization of first cleavage leads to the production of a double embryo. While photocytes are found in both of the duplicated post-trochal regions, usually only one of these is capable of emitting luminescence. Apparently, the highly localized vagetal material (determinants) responsible for functional light emission is distributed to both halves in only a few cases when first cleavage is experimentally “equalized”. These results indicate that the determinative action of the polar lobe is not required for the formation of the photocytes themselves, but rather for their ability to function as emitters of light. The determinants in the polar lobe ofChaetopterus may control some aspect of the photoluminescence reaction itself, such as the production of the photoprotein.

Casanova, Jordi

doi: 10.1007/BF02438938pmid: 28305715

spalt is a gene required for head and tail development in theDrosophila embryo. When mutated it produces homeotic transformations which are restricted in the embryonic cuticle to both ends of the embryo. However analysis of the distribution of its transcripts has shown that it is also expressed in the trunk region of the embruo. Here I show that two homeotic genesUltrabithorax (Ubx) andSex combs reduced (Scr) can be ectopically expressed in the trunk region of embryos lackingspalt gene function. These results suggest thatspalt has a general role in ensuring the correct spatial expression of other homeotic genes and hence could be another member of thePolycomb class of regulatory genes.

Litvin, Judith; King, Mary

doi: 10.1007/BF02438939pmid: 28305716

We have examined the proteins ofXenopus whole oocytes and embryos that are accessible to surface iodination. These cell surface proteins appear to undergo stagespecific modulation during development. The most pronounced changes were observed between oocytes and the 32-cell stage when the number of labelled proteins doubled, and between early and late gastrula when the complexity declined by two thirds. The simplification of the pattern during gastrulation may reflect changes in cell position as endodermal cells move inside leaving ectodermal cells at the external surface. The lectin binding patterns for NP-40 solubilized proteins extracted from oocytes and embryos also changed in a stage-dependent manner. Con A and WGA recognized a complex pattern of glycoproteins containing glucose and mannose residues. In contrast, SAB and RCA binding to galactose residues recognized far fewer glycoproteins. Many of the observed changes occurred during cleavage stages before embryonic gene transcription is initiated and may be due to post-translational modifications.

Fu, Yuchang; Hosokawa, Keiichi; Shiokawa, Koichiro

doi: 10.1007/BF02438940pmid: 28305717

Circular and linearized plasmids containing bacterial chloramphenicol acetyltransferase (CAT) genes connected or not connected to viral promoters were injected into fertilized eggs, unfertilized eggs and oocyte nuclei ofXenopus laevis, and CAT enzyme expression was studied under different conditions. Circular DNAs injected into fertilized eggs did not change their molecular form greatly, but CAT enzyme activity was expressed by the blastula or gastrula stage depending on the strength of the enhancer contained within the promoter. Linearized plasmid DNAs injected into fertilized eggs were concatemerized and replicated extensively by the blastula stage, and were expressed also actively irrespective of whether DNAs contained the promoter or not. The CAT enzyme activity was roughly comparable to the level of CAT mRNA in injected embryos. Similar results were obtained for both circular and linearized DNAs in unfertilized eggs, although the level of CAT enzyme expressed here was quite low. In oocyte nuclei, by contrast, only circular DNAs were expressed, and the expression was independent of whether or not the DNAs contained the promoter. The large concatemers formed in embryos comigrated with host DNA, but the majority of them disappeared later, at the tadpole stage, suggesting the extrachromosomal nature of these DNAs. The pronounced decrease in the amount of comigrating DNAs was accompanied by the disappearance of both CAT mRNA and enzyme activity. Therefore, we conclude that active CAT enzyme expression induced by injection of linearized DNAs in embryos and early stage larvae is due mainly to transcription from the transiently existing extra-chromosomal concatermers rather than from long-lasting, probably genome-integrated DNAs.

Cohen, Stephen; Jürgens, Gerd

doi: 10.1007/BF02438941pmid: 28305718

The development of all of the adult limbs inDrosophila depends upon the activity of theDistal-less gene. We report here the phenotypic characterization of a number of hypomorphicDistal-less alleles which indicates that there is a graded requirement forDistal-less activity in the developing limbs. Previous analysis of genetically mosaic animals indicated that cells in the early primordia of the limb imaginal dises possess a graded proximal-distal positional information which depends on the presence of theDistal-less gene for its expression. Taken together these data suggest thatDistal-less may directly encode the graded positional information that is required to organise the proximal-distal axis of the developing limbs.

Pratt, Hester; George, Martin

doi: 10.1007/BF02438942pmid: 28305719

An attempt was made to understand the ways in which ‘newly inserted’ membrane was organised in relation to existing membrane during early cleavage of the mouse embryo by (i) monitoring the redistribution of a variety of surface-binding ligands (applied to the embryo during the previous cell cycle) and (ii) analysing the localisation of newly synthesised lipid at defined stages during the second cell cycle. The membrane dynamics of the embryo appear similar to those of somatic cells during cytokinesis and/or motility, and are consistent with previous suggestions (Pratt 1985) that the main cytocortical domains of the polarised 8-cell blastomere may start to diverge during early cleavage as a result of localised assembly and reorganisation of the embryo cytocortex.

Yazaki, Ikuko; Uemura, Isao

doi: 10.1007/BF02438943pmid: 28305720

The blastomeres of sea urchin embryos have two surface regions with different properties. Numerous microvilli are present in the apical surface region, while the baso-lateral surface region, either on adjoining adjacent cells or facing the blastocoel, is smooth. When blastomeres are isolated from embryos and stained with fluorescein-isothiocyanate-labelled anti-(egg surface) antibody (anti-ES) prepared against membranes isolated from fertilized eggs, the apical microvillous region fluoresces while the smooth region does not [Yazaki I (1984) Acta Embryol Morphol Exp 5∶3–22]. In order to study quantitatively the ‘bindability’ of the membrane in the two regions to anti-ES, immunoelectron microscopy was used. Blastomeres isolated from embryos ofHemicentrotus pulcherrimus at the eight-cell stage were treated with rabbit anti-ES serum or pre-immune serum and then with ferritin-conjugated goat anti-(rabbit IgG) for 10 min at 0°C, mainly before fixation. About 10 times (maximally 45 times) more ferritin particles were counted per contour length in the microvillous surface region than in the smooth surface region.