bluuebunny/crossref_metadata_2025 huggingface dataset:
Xenopus Distal-less related homeobox genes are expressed in the developing forebrain and are induced by planar signals
Authors: Papalopulu Nancy , Kintner Chris
Published: April 2021

The polymerase chain reaction (PCR) was used to isolated five Xenopus homeobox clones (X-dll1 to 5) that are related to the Drosophila Distal-less (Dll) gene and we propose a subdivision of the vertebrate distal-less gene family according to sequence similarities. cDNA clones were isolated for X-dll2, 3 and 4, and their expression was studied by RNase protection and in situ hybridization. X-dll2, which belongs to a separate subfamily than X-dll3 and 4, is not expressed in the neural ectoderm. X-dll3 and X-dll4, which belong to the same subfamily, have a similar but not identical pattern of expression that is restricted to anterior ectodermal derivatives, namely the ventral forebrain, the cranial neural crest and the cement gland. X-dll3 is also expressed in the olfactory and otic placodes while X-dll4 is expressed in the developing eye. X-dll3 differs from the other Xenopus genes and the previously isolated Dll-related mouse genes, in that localized expression can be detected by in situ hybridization very early in development, in the anterior-transverse ridge of the open neural plate. Based on that early expression pattern, we suggest that X-dll3 marks the rostral-most part of the neural plate, which gives rise to the ventral forebrain. Finally, we have used these Xenopus distal-less genes to show that the anterior neural plate can be induced by signals that spread within the plane of neural ectoderm, indicating that at least the initial steps of forebrain development do not require signals from underlying mesoderm.
The role of the ovarian tumor locus in Drosophila melanogaster germ line sex determination
Authors: Pauli Daniel , Oliver Brian , Mahowald Anthony P.
Published: April 2021

The locus ovarian tumor (otu) is involved in several aspects of oogenesis in Drosophila melanogaster. The possible role of otu in the determination of the sexual identity of germ cells has not been extensively explored. Some otu alleles produce a phenotype known as ovarian tumors: ovarioles are filled with numerous poorly differentiated germ cells. We show that these mutant germ cells have a morphology similar to primary spermatocytes and that they express male germ line-specific reporter genes. This indicates that they are engaged along the male pathway of germ line differentiation. Consistent with this conclusion, we found that the splicing of Sex-lethal (Sxl) pre-mRNAs occurs in the male-specific mode in otu transformed germ cells. The position of the otu locus in the regulatory cascade of germ line sex determination has been studied by using mutations that constitutively express the feminizing activity of the Sxl gene. The sexual transformation of the germ cells observed with several combinations of otu alleles can be reversed by constitutive expression of Sxl. This shows that otu acts upstream of Sxl in the process of germ line sex determination. Other phenotypes of otu mutations were not rescued by constitutive expression of Sxl, suggesting that several functions of otu are likely to be independent of sex determination. Finally, we show that the gene dosage of otu modifies the phenotype of ovaries heterozygous for the dominant alleles of ovo, another gene involved in germ line sex determination. One dose of otu+ enhances the ovoD ovarian phenotypes, while three doses partially suppress these phenotypes. Synergistic interaction between ovoD1 and otu alleles leads to the occasional transformation of chromosomally female germ cells into early spermatocytes. These interactions are similar to those observed between ovoD and one allele of the sans fille (snf) locus. Altogether, our results imply that the otu locus acts, along with ovo, snf, and Sxl, in a pathway (or parallel pathways) required for proper sex determination of the female germ line.
Genetic analysis of hedgehog signalling in the Drosophila embryo
Authors: Forbes A. J. , Nakano Y. , Taylor A. M. , Ingham P. W.
Published: April 2021

The segment polarity genes play a fundamental role in the patterning of cells within individual body segments of the Drosophila embryo. Two of these genes wingless (wg) and hedgehog (hh) encode proteins that enter the secretory pathway and both are thought to act by instructing the fates of cells neighbouring those in which they are expressed. Genetic analysis has identified the transcriptional activation of wg as one of the targets of hh activity: here we present evidence that transduction of the hh-encoded signal is mediated by the activity of four other segment polarity genes, patched, fused, costal-2 and cubitus interruptus. The results of our genetic epistatsis analysis together with the molecular structures of the products of these genes where known, suggest a pathway of interactions leading from reception of the Aft-encoded signal at the cell membrane to transcriptional activation in the cell nucleus. We have also found that transcription of patched is regulated by the same pathway and describe the identification of cis-acting upstream elements of the ptc transcription unit that mediate this regulation.
Migration of Drosophila germ cells: analysis using enhancer trap lines
Authors: Howard Ken , Jaglarz Mariusz , Zhang Nian , Shah Jaymini , Warrior Rahul
Published: April 2021

Cell migration is a common feature of development. In order to understand more about the factors that controf these movements we have embarked on further analysis of the migration of Drosophila germ cells. This process involves passage of the germ cells across the gut pri mordium and migration toward the mesoderm where the somatic gonad forms. We are particularly interested in the early phase of this migration when the germ cells interact with the amnioproctodeal invagination, the developing gut, before entering into association with the mesoderm. We will summarize the results of our and other studies of these events before describing a number of enhancer trap lines which show expression in the amnioproctodeal invagination during the early phase of germ cell migration. These reveal more about the com plexity of this tissue and suggest this tissue is capable of guiding the early phase of germ cell migration.
Genes controlling cellular polarity in Drosophila
Authors: Gubb David
Published: April 2021

The control of cellular polarity is one of the least under- stood aspects of development. Genes have been identified in Drosophila that affect the polarity of embryonic cells in all three axes, apical-basal, proximodistal and dorsoventral. Mutations that affect adult polarity are also known and mutant Hies show several types of pattern alteration, including rotations and mirror-image duplications, imaginai discs are much greater in size, however, than the embryo, and adult structures contain very large numbers of cells, many of which are not visibly differentiated with respect to their immediate neighbours. In regions where neighbouring cells are similar to each other, the imaginai polarity mutants alter the orientation of bristles and hairs, but do not change cellular fate. Other regions, such as the tarsal segments of the legs, the ommatidia of the eye and the brack’d bristle sockets on the tibia, behave as discrete fields. Within these fields, fine-scale mirror-image reversals and pattern duplications are observed, analogous to those caused by the embryonic segment polarity mutants. Thus, the polarised transmission of informa- tion can affect either orientation or fate depending on whether cells are differentiated from their immediate neighbours. C ellular polarity will be critically dependent on both the internal cytoskeletal arehitecture and the spatial organisation of signal transduction molecules within the cell membrane.
The innervation of supernumerary and replacing grafts of limbs in Eleutherodactylus martinicensis
Authors: Hughes Arthur
Published: April 2021

The innervation of grafted limbs in Eleutherodactylus martinicensis has been studied, mainly in comparison of supernumerary grafts with those which replace a host limb. If batches of each type are fixed at different times after operation, it is found that the extent to which motor nerves penetrate into the grafts decreases with time after operation with supernumerary grafts, but not with replacing grafts. The muscles of supernumerary grafts are innervated from collateral branches of axons, usually from among those which supply the normal limb. It thus seems that these collaterals which develop as far as the formation of functional nerve-muscle junctions become unstable in late stages of embryonic life. In general, grafted fore limbs become functional much more readily than do hind limbs. The degeneration of the nerves to supernumerary fore limbs may be followed in the living animal by the loss of movement in the graft. Their loss of function is more marked when grafted heterotopically than when near the orthotopic position. Histological evidence for the breakdown of the nerves to supernumerary grafts is presented, both in respect to fibres and the Schwann cells. A nerve to a graft degenerates more readily when it belongs outside the brachial or lumbar series. Degeneration of nerves and loss of function of the graft proceeds more drastically with supernumerary homografts than with the corresponding autografts. In supernumerary autografts in which motor nerves have degenerated, cutaneous branches may extend further into the graft. The breakdown of nerves is retarded in embryos kept at sub-normal temperatures.
The embryology of Dacus tryoni (Diptera). 3. Origins of imaginal rudiments other than the principal discs
Authors: Anderson D. T.
Published: April 2021

The imaginal segmental abdominal hypodermis, salivary glands, fore-, mid-and hind-gut, lateral cerebral ganglia and segmental tracheal system each arise from a group of blastoderm cells situated within the blastodermal presumptive area of the corresponding larval structure.

The cyclorrhaphan blastoderm can be zoned into presumptive areas for all major parts of the body. Each area includes presumptive larval cells and also presumptive cells of the imaginal part which replaces the larval part at metamorphosis. Except for mesodermal structures, all major imaginal parts and their larval equivalents are probably determined in the blastoderm. The theory is developed that one of the fundamental steps in cyclorrhaphan evolution has been separation of the blastodermal rudiments of the major parts of the hypodermal body wall and gut into determined larval and adult components. In addition, it is suggested, the larval components have acquired a capacity to evócate differentiation of mesodermal structures independently of the adult components.
Further experiments on the innervation and function of grafted supernumerary limbs in the embryo of Eleutherodactylus martinicensis
Authors: Hughes Arthur
Published: April 2021

Previous studies on the transplantation of limbs in the embryo of Eleuthero-dactylus martinicensis have shown that supernumerary limbs tend to be less well innervated than are corresponding replacing grafts, and that the innervation of the former, either autografts or homografts, is progressively less extensive with increase in time after operation. The degree to which grafted supernumerary forelimbs show movement in life parallels the extent of their motor innervation when studied subsequently in serial sections. The loss of innervation and of movement is particularly drastic near the time when the animal normally hatches and the tail undergoes atrophy.
The radiosensitivity of oögonia and oöcytes in the chick embryo
Authors: Hughes Gillian C.
Published: April 2021

The effect of X-irradiation on the germ cells of the developing chick ovary depends on both the dose administered, and on the age of the embryo at the time of exposure. Germ cells are particularly radiosensitive at 11 days’ incubation; they become less so up to 17 days. Peak radiosensitivity at 11 days’ incubation coincides with a high mitotic activity of oögonia, but not with its maximal incidence (9th day of incubation). Oögonia undergoing their last (pre-meiotic) mitosis may be particularly radio-sensitive. Oocytes at the leptotene, zygotene and pachytene stages of meiotic prophase apparently become increasingly radioresistant. X-irradiation induces a slight retardation in the subsequent development of the ovary.
Initial stimulus and subsequent interactions in embryonic induction
Authors: Saxén Lauri , Toivonen Sulo , Vainio Tapani
Published: April 2021

An Artificial environmental stimulus of short duration (10-15 min.) leads to either mesodermal or neural differentiation of competent amphibian ectoderm in certain experimental conditions (Johnen, 1956a; Yamada, 1962). Subsequent to this stimulus development into well organized structures is autonomous, independent of environmental ‘inductive’ factors. Hence, two alternatives for the control mechanism have to be considered; either the whole pathway to the complex end-result is triggered and determined during the short-term initial pulse (primary induction), or only the first step of a long chain reaction has been activated, and this is followed by secondary interactions between the tissue components. Recently Grobstein (1963) discussed these alternatives in the mechanism of embryonic induction and concluded that a single step process is most unlikely in many known examples of induction (kidney tubule induction, induction of cartilage etc.). Seen from the viewpoint of primary induction, there seems to be good reason to agree with him, and we have in previous discussions often stressed the obvious significance of a neural/mesodermal interaction subsequent to the primary inductive stimulus (Saxén & Toivonen, 1961, 1962). The following experiment was planned to gain further evidence with this view in mind.
Methylation of nucleolar RNA during development of the amphibian oöcyte
Authors: Ozban N. , Tandler C. J. , Sirlin J. L.
Published: April 2021

Transmethylation is known to occur from methionine to ribonucleic acid (RNA) derivatives (Fleissner & Borek, 1962). Most of these methylated derivatives are typical of transfer RNA (Dunn, 1959). In dipteran salivary cells it was possible to distinguish with autoradiography between the transmethylation to RNA and the incorporation of methionine into protein (Sirlin, Jacob & Tandler, 1963). The same approach was used in the present experiments to trace the formation of transfer RNA in amphibian oöcytes.
The abnormalities of the inner ear in kreisler mice
Authors: Deol M. S.
Published: April 2021

The recessive gene kreisler (symbol kr) was discovered by Hertwig (1942a), who later described its effects on behaviour (1942b) and the inner ear (1944, 1956). She found that it was possible to trace the anomalies of the ear back to 9-day embryos, when the otic vesicle can be seen to be situated in an abnormal position. The present study was aimed at discovering the cause of this abnormality, and at giving a fuller account of the later development of the ear.
The use of inherited abnormalities in studies on the embryogenesis ofDermestes maculatus(Coleoptera)
Authors: Ede D. A. , Rogers Anne M.
Published: April 2021

The possibility of using inherited abnormalities in place of conventional experimental embryological methods has been explored by several workers in Drosophila, where there is a large range of lethal and female sterile genes readily available (reviewed Waddington, 1956). These genes are usually pleiotropic, i.e. with manifold effects, and tracing these diverse effects back as far as possible to a single primary defect gives information about the corresponding developmental mechanisms in the normal embryo. Two disadvantages have attended these studies: (1) dipteran embryology is extremely specialized and difficult to relate to that of other insects, (2) it is almost entirely mosaic in character, and therefore throws no light on the important subject of tissue interactions.
The formation of the second maturation spind le in the eggs of various Limnaeidae
Authors: Raven Chr. P.
Published: April 2021

In a number of papers (Raven, 1949; Raven et al., 1958; Raven, 1959) I have reported on egg maturation, especially the formation of the second maturation spindle, in various pulmonate snails. It appeared that this early and quite fundamental process of development exhibits a surprising variation.
β4 integrin expression in myelinating Schwann cells is polarized, developmentally regulated and axonally dependent
Authors: Feltri M. Laura , Scherer Steven S. , Nemni Raffaello , Kamholz John , Vogelbacker Helen , Scott Marion Oronzi , Canal Nicola , Quaranta Vito , Wrabetz Lawrence
Published: April 2021

In developing and regenerating peripheral nerve, Schwann cells interact with axons and extracellular matrix in order to ensheath and myelinate axons. Both of these interactions are likely to be mediated by adhesion molecules, including integrins, which mediate cell-cell and cell-extracellular matrix interactions. Recently, the β4 integrin subunit was reported to be expressed by Schwann cells in peripheral nerve. We have examined the expression of β4, β1 and their common heterodimeric partner, the α6 integrin subunit, in developing and regenerating rat peripheral nerve. β4 and α6 are enriched in peripheral nerve and they co-localize at the abaxonal surface of myelinating Schwann cells, opposite the Schwann cell basal lamina, which contains possible ligands of α6β4. In contrast, β4 and α6 are expressed in a different pattern in non-myelinating Schwann cells. The level of 4, but not α6 or β1 mRNAs, increases progressively in developing nerves, reaching a peak in adult nerves well after the peak of the myelinspecific mRNAs. After axotomy, the expression of β4 mRNA and protein, but not α6 or β1 mRNAs, fall rapidly but subsequently are reinduced by regenerating axons. Similarly, in cultured Schwann cells, the expression of β4 mRNA, but not α6 mRNA, is significantly modulated by forskolin, a drug that elevates cAMP and mimics some of the effects of axonal contact. β4 integrin expression in Schwann cells, therefore, is regulated by Schwann cellaxon interactions, which are known to be critical in determining the Schwann cell phenotype. Furthermore, the polarized expression of α6β4 to the abaxonal surface of myelinating Schwann cells suggests that α6β4 may mediate in part the morphological changes required of Schwann cells in the process of myelination in the peripheral nervous system.
Expression of the HSP 70.1 gene, a landmark of early zygotic activity in the mouse embryo, is restricted to the first burst of transcription
Authors: Christians Elisabeth , Campion Evelyne , Thompson Eric M. , Renard Jean-Paul
Published: April 2021

Activation of the mouse embryonic genome at the 2-cell stage is characterized by the synthesis of several α-amanitin-sensitive polypeptides, some of which belong to the multigenic hsp 70 family. In the present work we show that a member of this family, the HSP 70.1 gene, is highly transcribed at the onset of zygotic genome activation. Transcription of this gene began as early as the 1-cell stage. Expression of the gene continued through the early 2-cell stage but was repressed before the completion of the second round of DNA replication. During this period we observed that the level of transcription was modulated by in vitro culture conditions. The coincidence of repression of HSP70.1 transcription with the second round of DNA replication was not found for other transcription-dependent polypeptides synthesized at the 2-cell stage.
Development of hematopoietic cells lacking transcription factor GATA-1
Authors: Pevny Larysa , Lin Chyuan-Sheng , D’Agati Vivette , Simon M. Celeste , Orkin Stuart H. , Costantini Frank
Published: April 2021

GATA-1 is a zinc-finger transcription factor believed to play an important role in gene regulation during the development of erythroid cells, megakaryocytes and mast cells. Other members of the GATA family, which can bind to the same DNA sequence motif, are co-expressed in several of these hemopoietic lineages, raising the possibility of overlap in function. To examine the specific roles of GATA-1 in hematopoietic cell differentiation, we have tested the ability of embryonic stem cells, carrying a targeted mutation in the X-linked GATA-1 gene, to contribute to various blood cell types when used to produce chimeric embryos or mice. Previously, we reported that GATA-1− mutant cells failed to contribute to the mature red blood cell population, indicating a requirement for this factor at some point in the erythroid lineage (L. Pevny et al., (1991)Nature 349, 257-260). In this study, we have used in vitro colony assays to identify the stage at which mutant erythroid cells are affected, and to examine the requirement for GATA-1 in other lineages. We found that the development of erythroid progenitors in embryonic yolk sacs was unaffected by the mutation, but that the cells failed to mature beyond the proerythroblast stage, an early point in terminal differentiation. GATA-1− colonies contained phenotypically normal macrophages, neutrophils and megakaryocytes, indicating that GATA-1 is not required for the in vitro differentiation of cells in these lineages. GATA-1− megakaryocytes were abnormally abundant in chimeric fetal livers, suggesting an alteration in the kinetics of their formation or turnover. The lack of a block in terminal megakaryocyte differentiation was shown by the in vivo production of platelets expressing the ES cell-derived GPI-1C isozyme. The role of GATA-1 in mast cell differentiation was examined by the isolation of clonal mast cell cultures from chimeric fetal livers. Mutant and wild-type mast cells displayed similar growth and histochemical staining properties after culture under conditions that promote the differentiation of cells resembling mucosal or serosal mast cells. Thus, the mast and megakaryocyte lineages, in which GATA-1 and GATA-2 are co-expressed, can complete their maturation in the absence of GATA-1, while erythroid cells, in which GATA-1 is the predominant GATA factor, are blocked at a relatively early stage of maturation.
Mutations affecting the radial organisation of the Arabidopsis root display specific defects throughout the embryonic axis
Authors: Scheres Ben , Laurenzio Laura Di , Willemsen Viola , Hauser Marie-Therès , Janmaat Kees , Weisbeek Peter , Benfey Philip N.
Published: April 2021

The primary root of Arabidopsis thaliana has a remarkably uniform cellular organisation. The fixed radial pattern of cell types in the mature root arises from proliferative divisions within the root meristem. The root meristem, in turn, is laid down during embryogenesis. We have analysed six mutations causing alterations in the radial organisation of the root. Embryonic phenotypes resulting from wooden leg, gollum, pinocchio, scarecrow, shortroot and fass mutations are described. While mutations in the fass gene affect morphogenesis of all cells, the five other mutations cause alterations in specific layers. Wooden leg and gollum mutations interfere with the proper organisation of the vascular tissue. Shortroot, scarecrow and pinocchio affect the endodermis and cortex. The layer-specific phenotypes caused by all five mutations are also apparent in the hypocotyl. All these phenotypes originate from defects in the radial organisation of the embryonic axis. Secondary roots, which are formed post-embryonically, also display layer-specific phenotypes.
The prospero transcription factor is asymmetrically localized to the cell cortex during neuroblast mitosis in Drosophila
Authors: Spana Eric P. , Doe Chris Q.
Published: April 2021

Both intrinsic and extrinsic factors are known to regulate sibling cell fate. Here we describe a novel mechanism for the asymmetric localization of a transcription factor to one daughter cell at mitosis. The Drosophila CNS develops from asymmetrically dividing neuroblasts, which give rise to a large neuroblast and a smaller ganglion mother cell (GMC). The prospero gene encodes a transcription factor necessary for proper GMC gene expression. We show that the prospero protein is synthesized in the neuroblast where it is localized to the F-actin cell cortex. At mitosis, prospero is asymmetrically localized to the budding GMC and excluded from the neuroblast. After cytokinesis, prospero is translocated from the GMC cortex into the nucleus. Asymmetric cortical localization of prospero in neuroblasts requires entry into mitosis; it does not depend on numb function. prospero is also observed in cortical crescents in dividing precursors of the peripheral nervous system and adult midgut. The asymmetric cortical localization of prospero at mitosis is a mechanism for rapidly establishing distinct sibling cell fates in the CNS and possibly other tissues.
The metanephric blastema differentiates into collecting system and nephron epithelia in vitro
Authors: Qiao J. , Cohen D. , Herzlinger D.
Published: April 2021

The kidney forms from two tissue populations derived from intermediate mesoderm, the ureteric bud and metanephric mesenchyme. It is currently accepted that metanephric mesenchyme is committed to differentiating into nephrons while the ureteric bud is restricted to forming the renal collecting system. To test this hypothesis, we transferred lacZ into pure metanephric mes-enchyme isolated from gestation day 13 rat embryos. The fate of tagged mesenchymal cells and their progeny was characterized after co-culture with isolated ureteric buds. When induced to differentiate by the native inducer of kidney morphogenesis, lineage-tagged mesenchymal cells exhibit the potential to differentiate into collecting system epithelia, in addition to nephrons. The fate of cells deriving from isolated ureteric buds was also examined and results of these lacZ gene transfer experiments indicate that the majority of ureteric bud cells differentiate into the renal collecting system. These cell fate studies combined with in situ morphological observations raise the possibility that collecting system morphogenesis in vivo occurs by growth of the ureteric bud and recruitment of mesenchymal cells from the metanephric blastema. Thus, metanephric mes-enchyme may be a pluripotent renal stem population.
Micromeres are required for normal vegetal plate specification in sea urchin embryos
Authors: Ransick Andrew , Davidson Eric H.
Published: April 2021

Vegetal plate specification was assessed in S. purpuratus embryos after micromere deletions at the 4th, 5th and 6th cleavages, by assaying expression of the early vegetal plate marker Endo16, using whole-mount in situ hybridization. After 4th cleavage micromere deletions, the embryos typically displayed weak Endo16 expression in relatively few cells of the lineages that normally constitute the vegetal plate, while after 5th and 6th cleavage micromere deletions the embryos exhibited strong Endo16 expression in larger fractions of cells belonging to those lineages. When all four micromeres were deleted, the embryos were severely delayed in initiating gastrulation and sometimes failed to complete gastrulation. However, if only one micromere was allowed to remain in situ throughout development, the embryos exhibited strong Endo16 expression and gastrulation occurred normally, on schedule with controls. Additional measurements showed that these microsurgical manipulations do not alter cleavage rates or generally disrupt embryo organization. These results constitute direct evidence that the micromeres provide signals required by the macromere lineages for initiation of vegetal plate specification. The specification of the vegetal plate is completed in a normal manner only if micromere signaling is allowed to continue at least to the 6th cleavage stage.
Expression and transgenic studies of the mouse agouti gene provide insight into the mechanisms by which mammalian coat color patterns are generated
Authors: Millar Sarah E. , Miller Miles W. , Stevens Mary E. , Barsh Gregory S.
Published: April 2021

Expression of the agouti gene from two different promoters, one active at the midpoint of the hair cycle and the other specific for the ventrum, is responsible for generating a range of mammalian pigmentation patterns. We demonstrate that in postnatal mice transcripts from both promoters are confined to the dermal papilla of hair follicles, as predicted by classical transplantation experiments. Transcripts from the hair cycle promoter are detected in the embryonic whisker plate but not in other regions of the body before birth, whereas ventral-specific transcripts are detected in the ventral trunk of the embryo as well as ventral whisker plate. To investigate further the embryonic origins of adult pigmentation patterns, we carried out a detailed analysis of agouti expression in the embryo. The ventral-specific agouti isoform is first expressed at E10.5 in neural crest-derived ventral cells of the second branchial arch, in anterior regions of the forelimb buds and in a narrow stripe of ventral mesenchyme. By E14.5 a continuous layer of expression is observed in the upper cells of the dermis, including cells of the developing dermal papillae, and covering the entire ventral surface of the head and trunk and dorsal surfaces of the distal forelimb and hindlimb. This expression pattern reflects the domain of yellow coloration evident in adult animals and suggests that the agouti gene is regulated in part by factors responsible for establishing differences between the dorsal and ventral surfaces of the body during embryogenesis. To test the hypothesis that agouti is a paracrine signaling molecule that can influence pigment production by hair follicle melanocytes when expressed by either dermis or epidermis, as suggested by recombination and transplantation experiments, we created transgenic animals in which agouti is expressed in basal cells of the epidermis. These animals display stripes of yellow hairs corresponding to regions of epidermal agouti expression, confirming that agouti signals melanocytes to synthesize yellow pigment and providing direct evidence that it functions in a paracrine manner with a restricted radius of action.
The role of the cell cycle and cytokinesis in regulating neuroblast sublineage gene expression in the Drosophila CNS
Authors: Cui Xuan , Doe Chris Q.
Published: April 2021

The precise temporal control of gene expression is critical for specifying neuronal identity in the Drosophila central nervous system (CNS). A particularly interesting class of genes are those expressed at stereotyped times during the cell lineage of identified neural precursors (neuroblasts): these are termed ‘sublineage’ genes. Although sublineage gene function is vital for CNS development, the temporal regulation of this class of genes has not been studied. Here we show that four genes (ming, even-skipped, unplugged and achaete) are expressed in specific neuroblast sublin- eages. We show that these neuroblasts can be identified in embryos lacking both neuroblast cytokinesis and cell cycle progression (string mutants) and in embryos lacking only neuroblast cytokinesis (pebble mutants). We find that the unplugged and achaete genes are expressed normally in string and pebble mutant embryos, indicating that temporal control is independent of neuroblast cytokinesis or counting cell cycles. In contrast, neuroblasts require cytokinesis to activate sublineage ming expression, while a single, identified neuroblast requires cell cycle progression to activate even-skipped expression. These results suggest that neuroblasts have an intrinsic gene regulatory hierarchy controlling unplugged and achaete expression, but that cell cycle- or cytokinesis-dependent mechanisms are required for ming and eve CNS expression.
The msl-2 dosage compensation gene of Drosophila encodes a putative DNA-binding protein whose expression is sex specifically regulated by Sex-lethal
Authors: Bashaw Greg J. , Baker Bruce S.
Published: April 2021

In Drosophila dosage compensation increases the rate of transcription of the male’s X chromosome and depends on four autosomal male-specific lethal genes. We have cloned the msl-2 gene and shown that MSL-2 protein is colocalized with the other three MSL proteins at hundreds of sites along the male polytene X chromosome and that this binding requires the other three MSL proteins. msl-2 encodes a protein with a putative DNA-binding domain: the RING finger. MSL-2 protein is not produced in females and sequences in both the 5′ and 3′ UTRs are important for this sex-specific regulation. Furthermore, msl-2 pre-mRNA is alternatively spliced in a Sex-lethal-dependent fashion in its 5′ UTR.
Midline signalling is required for Pax gene regulation and patterning of the eyes
Authors: Macdonald Rachel , Barth K. Anukampa , Xu Qiling , Holder Nigel , Mikkola Ingvild , Wilson Stephen W.
Published: April 2021

Pax6 and Pax2 are members of the Pax family of transcrip-tion factors that are both expressed in the developing visual system of zebrafish embryos. Pax6 protein is present in all cells that form the neural retina and pigment epithelium, whereas Pax2 is located primarily in cells that will give rise to the optic stalk. In this study, we have addressed the role of midline signalling in the regulation of Pax2 and Pax6 dis-tributions and in the subsequent morphogenesis of the eyes. Midline signalling is severely perturbed in cyclops mutant embryos resulting in an absence of ventral midline CNS tissue and fusion of the eyes. Mutant embryos ectopically express Pax6 in a bridge of tissue around the anterior pole of the neural keel in the position normally occupied by cells that form the optic stalks. In contrast, Pax2 protein is almost completely absent from this region in mutant embryos. Con-commitant with the changes in Pax protein distribution, cells in the position of the optic stalks differentiate as retina.

These results suggest that a signal emanating from the midline, which is absent in cyclops mutant embryos, may be required to promote Pax2 and inhibit Pax6 expression in cells destined to form the optic stalks. Sonic hedgehog (Shh also known as Vhh-1 and Hhg-1) is a midline sig-nalling molecule that is absent from the neuroepithelium of cyclops mutant embryos at early developmental stages. To test the possibility that Shh might be able to regulate the spatial expression of Pax6 and Pax2 in the optic primordia, it was overexpressed in the developing CNS. The number of cells containing Pax2 was increased following shh over-expression and embryos developed hypertrophied optic stalk-like structures. Complimentary to the changes in Pax2 distribution, there were fewer Pax6-containing cells and pigment epithelium and neural retina were reduced.

Our results suggest that Shh or a closely related sig-nalling molecule emanating from midline tissue in the ventral forebrain either directly or indirectly induces the expression of Pax2 and inhibits the expression of Pax6 and thus may regulate the partitioning of the optic primordia into optic stalks and retinal tissue.
Developing inner ear sensory neurons require TrkB and TrkC receptors for innervation of their peripheral targets
Authors: Schimmang Thomas , Minichiello Liliana , Vazquez Esther , Jose Isabel San , Giraldez Fernando , Klein Rüdiger , Represa Juan
Published: April 2021

The trkB and trkC genes are expressed during the formation of the vestibular and auditory system. To elucidate the function of trkB and trkC during this process, we have analysed mice carrying a germline mutation in the tyrosine kinase catalytic domain of these genes. Neu- roanatomical analysis of homozygous mutant mice revealed neuronal deficiencies in the vestibular and cochlear ganglia. In trkB (−/−) animals vestibular neurons and a subset of cochlear neurons responsible for the inner- vation of outer hair cells were drastically reduced. The peripheral targets of the respective neurons showed severe innervation defects. A comparative analysis of ganglia from trkC (−/−) mutants revealed a moderate reduction of vestibular neurons and a specific loss of cochlear neurons innervating inner hair cells. No nerve fibres were detected in the sensory epithelium containing inner hair cells.

A developmental study of trkB (−/−) and trkC (−/−) mice showed that some vestibular and cochlear fibres initially reached their peripheral targets but failed to maintain innervation and degenerated. TrkB and TrkC receptors are therefore required for the survival of specific neuronal populations and the maintenance of target innervation in the peripheral sensory system of the inner ear.
ventral veinless, the gene encoding the Cf1a transcription factor, links positional information and cell differentiation during embryonic and imaginal development in Drosophila melanogaster
Authors: Celis Jose F. de , Llimargas Marta , Casanova Jordi
Published: April 2021

The ventral veinless gene (vvl) encodes the previously identified Cf1a protein, a transcription factor containing a POU-domain. During embryonic development vvl function is required for the formation of the tracheal tree and in the patterning of the ventral ectoderm. During imaginal development vvl is required for cell proliferation and the differentiation of the wing veins. vvl expression is restricted to the regions where its function is required, and is dependent on the coordinate activities of signalling molecules such as decapentaplegic, wingless and hedgehog. vvl interacts with other genes involved in vein differentiation, including veinlet, thick veins, torpedo, decapentaplegic and Notch suggesting that vvl function may affect several cell-to-cell communication pathways. We propose that the gene vvl integrates information from different signalling molecules and regulates the expression of specific cell differentiation genes during tracheal development and vein differentiation.
The function of engrailed and the specification of Drosophila wing pattern
Authors: Guillén Isabel , Mullor Jose L. , Capdevila Javier , Sánchez-Herrero Ernesto , Morata Ginés , Guerrero Isabel
Published: April 2021

The adult Drosophila wing (as the other appendages) is subdivided into anterior and posterior compartments that exhibit characteristic patterns. The engrailed (en) gene has been proposed to be paramount in the specification of the posterior compartment identity. Here, we explore the adult en function by targeting its expression in different regions of the wing disc. In the anterior compartment, ectopic en expression gives rise to the substitution of anterior structures by posterior ones, thus demonstrating its role in specification of posterior patterns. The en-expressing cells in the anterior compartment also induce high levels of the hedgehog (hh) and decapentaplegic (dpp) gene products, which results in local duplications of anterior patterns.

Besides, hh is able to activate en and the engrailed-related gene invected (inv) in this compartment. In the posterior compartment we find that elevated levels of en product result in partial inactivation of the endogenous en and inv genes, indicating the existence of a negative autoregulatory mechanism. We propose that en has a dual role: a general one for patterning of the appendage, achieved through the activation of secreted proteins like hh and dpp, and a more specific one, determining posterior identity, in which the inv gene may be implicated.
Asymmetric localization of numb autonomously determines sibling neuron identity in the Drosophila CNS
Authors: Spana Eric P. , Kopczynski Casey , Goodman Corey S. , Doe Chris Q.
Published: April 2021

The central nervous system (CNS) represents an excellent model system for examining how a multitude of unique cell fates are specified. We find that asymmetric localization of the numb protein autonomously controls a binary cell fate decision in the Drosophila CNS. The simplest lineage in the Drosophila CNS is that of the MP2 precursor: it divides unequally to generate the dMP2 and vMP2 neurons. Both are interneurons but project in different directions: dMP2 projects its axon posteriorly while vMP2 projects anteriorly. During MP2 mitosis, numb is localized into dMP2 and excluded from vMP2. Loss of numb transforms dMP2 into vMP2, whereas ectopic numb produces the opposite transformation of vMP2 into dMP2. Thus, numb is asymmetrically localized in the dividing MP2 and is necessary and sufficient to autonomously specify dMP2 neuronal identity.
Lack of β-catenin affects mouse development at gastrulation
Authors: Haegel Hélène , Larue Lionel , Ohsugi Mami , Fedorov Lev , Herrenknecht Kurt , Kemler Rolf
Published: April 2021

Molecular analysis of the cadherin-catenin complex elucidated the central role of β-catenin in this adhesion complex, as it binds to the cytoplasmic domain of E-cadherin and to α-catenin. β-catenin may also function in signalling pathways, given its homology to the gene product of the Drosophila segment polarity gene armadillo, which is known to be involved in the wingless signalling cascade. To study the function of β-catenin during mouse development, gene knock-out experiments were performed in embryonic stem cells and transgenic mice were generated. β-catenin null-mutant embryos formed blastocysts, implanted and developed into egg-cylinder-stage embryos. At day 7 post coitum, the development of the embryonic ectoderm was affected in mutant embryos. Cells detached from the ectodermal cell layer and were dispersed into the proamniotic cavity. No mesoderm formation was observed in mutant embryos. The development of extraembryonic structures appeared less dramatically or not at all affected. Our results demonstrate that, although β-catenin is expressed rather ubiquitously, it is specifically required in the ectodermal cell layer.
FGF-8 isoforms activate receptor splice forms that are expressed in mesenchymal regions of mouse development
Authors: MacArthur Craig A. , Lawshé Avril , Xu Jingsong , Santos-Ocampo Sylvia , Heikinheimo Markku , Chellaiah Arasu T. , Ornitz David M.
Published: April 2021

The Fgf8 gene is expressed in developing limb and craniofacial structures, regions known to be important for growth and patterning of the mouse embryo. Although Fgf8 is alternatively spliced to generate at least 7 secreted isoforms that differ only at their mature amino terminus, the biological significance of these multiple isoforms is not known. In this report, we demonstrate that multiple FGF-8 isoforms are present at sites of Fgf8 expression during mouse development. To address the possibility that the FGF-8 isoforms might interact with different fibroblast growth factor receptors, we prepared recombinant FGF-8 protein isoforms. We examined the ability of these proteins to activate alternatively spliced forms of fibroblast growth factor receptors 1–3, and fibroblast growth factor receptor 4. Recombinant FGF-8b and FGF-8c activate the ‘c’ splice form of FGFR3, and FGFR4, while FGF-8b also efficiently activates ‘c’ splice form of FGFR2. No activity could be detected for recombinant or cell expressed FGF-8a. Furthermore, none of the isoforms tested interact efficiently with ‘b’ splice forms of FGFR1-3, or the ‘c’ splice form of FGFR1. These results indicate that the FGF-8b and FGF8c isoforms, produced by ectodermally derived epithelial cells, interact with mesenchymally expressed fibroblast growth factor receptors. FGF-8b and FGF-8c may therefore provide a mitogenic signal to the underlying mesenchyme during limb and craniofacial development.
extradenticle determines segmental identities throughout Drosophila development
Authors: Rauskolb Cordelia , Smith Katherine M. , Peifer Mark , Wieschaus Eric
Published: April 2021

extradenticle (exd) and the homeotic selector proteins together establish segmental identities by coordinately regulating the expression of downstream target genes. The inappropriate expression of these targets in exd mutant embryos results in homeotic transformations and aberrant morphogenesis. Here we examine the role of exd in adult development by using genetic mosaics and a hypomorphic exd allele caused by a point mutation in the homeodomain. exd continues to be essential for the specification of segmental identities, consistent with a continuing requirement for exd as a cofactor of the homeotic selector proteins. Loss of exd results in the homeotic transformation of abdominal segments to an A5 or A6 segmental identity, the antenna and arista to leg, and the head capsule to dorsal thorax or notum. Proximal leg structures are particularly sensitive to the loss of exd, although exd does not affect the allocation of proximal positional values of the leg imaginal disc. Using heat-shocks to induce expression of a hsp70-exd fusion gene, we show that, in contrast to the homeotic selector genes, ubiquitously high levels of exd expression do not cause pattern abnormalities or segmental transformations.
Myogenesis in paraxial mesoderm: preferential induction by dorsal neural tube and by cells expressing Wnt-1
Authors: Stern Howard M. , Brown Anthony M. C. , Hauschka Stephen D.
Published: April 2021

Previous studies have demonstrated that the neural tube/notochord complex is required for skeletal muscle development within somites. In order to explore the localization of myogenic inducing signals within the neural tube, dorsal or ventral neural tube halves were cultured in contact with single somites or pieces of segmental plate mesoderm. Somites and segmental plates cultured with the dorsal half of the neural tube exhibited 70% and 85% myogenic response rates, as determined by immunostaining for myosin heavy chain. This response was slightly lower than the 100% response to whole neural tube/notochord, but was much greater than the 30% and 10% myogenic response to ventral neural tube with and without notochord. These results demonstrate that the dorsal neural tube emits a potent myogenic inducing signal which accounts for most of the inductive activity of whole neural tube/notochord. However, a role for ventral neural tube/notochord in somite myogenic induction was clearly evident from the larger number of myogenic cells induced when both dorsal neural tube and ventral neural tube/notochord were present. To address the role of a specific dorsal neural tube factor in somite myogenic induction, we tested the ability of Wnt-1-expressing fibroblasts to promote paraxial mesoderm myogenesis in vitro. We found that cells expressing Wnt-1 induced a small number of somite and segmental plate cells to undergo myogenesis. This finding is consistent with the localized dorsal neural tube inductive activity described above, but since the ventral neural tube/notochord also possesses myogenic inductive capacity yet does not express Wnt-1, additional inductive factors are likely involved.
Cell fate specification by even-skipped expression in the Drosophila nervous system is coupled to cell cycle progression
Authors: Weigmann Katrin , Lehner Christian F.
Published: April 2021

The correct specification of defined neurons in the Drosophila central nervous system is dependent on evenskipped. During CNS development, even-skipped expression starts in the ganglion mother cell resulting from the first asymmetric division of neuroblast NB 1-1. This first division of NB 1-1 (and of the other early neuroblasts as well) is temporally controlled by the transcriptional regulation of string expression, which we have manipulated experimentally. even-skipped expression still occurs if the first neuroblast division is delayed, but not if the division is prohibited. Moreover, even-skipped expression is also dependent on progression through S phase which follows immediately after the first division. However, cytokinesis during the first NB division is not required for even-skipped expression as revealed by observations in pebble mutant embryos. Our results demonstrate therefore that evenskipped expression is coupled to cell cycle progression, presumably in order to prevent a premature activation of expression by a positive regulator which is produced already in the neuroblast during G2 and segregated asymmetrically into the ganglion mother cell during mitosis.