bluuebunny/crossref_metadata_2025 huggingface dataset:
Genetic control of flower shape in Antirrhinum majus
Authors: Almeida Jorge , Rocheta Margarida , Galego Lisete
Published: April 2021

The asymmetric shape of the Antirrhinum corolla is determined by genes acting differentially along the dorsoventral axis of the flower. Genes so far identified determine asymmetry by acting in dorsal regions. We describe a gene, divaricata, which in contrast to previously identified genes acts in ventral regions of the flower. We show by the analysis of mutant combinations that the divaricata gene is negatively regulated by the dorsal genes cycloidea and dichotoma. In addition, we show by the analysis of chromosomal duplications that the divaricata gene acts in a dosage-dependent manner, suggesting that the level of its product is critical for determining ventral identities.
Examination of the function of two kelch proteins generated by stop codon suppression
Authors: Robinson Douglas N. , Cooley Lynn
Published: April 2021

The Drosophila kelch gene produces a single transcript with a UGA stop codon separating two open reading frames (ORF1 and ORF2). From the transcript, 76 kDa ORF1 and 160 kDa full-length (ORF1 + ORF2) proteins are made. The expression of these two proteins is regulated in a tissue-specific manner causing the ratio of full-length to ORF1 protein to vary in different tissues. The only detected defect for kelch mutants is female sterility, and kelch protein is localized to the ovarian ring canals. kelch mutant ring canals are disorganized and have partly occluded lumens, causing a failure to transport cytoplasm. ORF1 and full-length kelch proteins co-sediment with ring canals suggesting that both proteins are found in the ring canals. Transgenetic analysis reveals that ORF1 kelch protein is sufficient to rescue ring canal morphology and fertility. In addition, we have mutated the UGA stop codon to a UAA stop codon and to three sense codons that allow constitutive readthrough. Analysis of these mutants reveals that a full-length kelch protein can partially compensate for the loss of endogenous kelch, but the residue included at the stop codon is critical for function. Finally, these studies suggest that the mechanism of stop codon suppression of kelch is by tRNA suppression.
Rescue of Drosophila engrailed mutants with a highly divergent mosquito engrailed cDNA using a homing, enhancer-trapping transposon
Authors: Whiteley Mary , Kassis Judith A.
Published: April 2021

Specific fragments of Drosophila regulatory DNA can alter the insertional specificity of transposable elements causing them to ‘home’ to their parent gene. We used this property to insert a transposon-encoded functional coding region near a defective one and rescue a null mutation. This approach differs from homologous recombination in that the endogenous defective coding region is left in place and the genomic DNA is altered by the addition of the therapeutic transposon. We constructed a P-element-based transposon in which an engrailed cDNA from Anopheles gambiae (a mosquito) is expressed from a Drosophila engrailed minimal promoter. The promoter fragment used includes 2.6 kb of regulatory DNA that causes transposons to home to the endogenous Drosophila engrailed gene at high frequencies. We inserted this transposon onto a Drosophila chromosome that produces no functional engrailed proteins. When this transposon integrated near the engrailed promoter, adult viability was restored to engrailed mutant flies showing that the highly divergent mosquito engrailed protein can replace the Drosophila engrailed protein at all stages of development. Insertion of this transposon into the adjacent invected gene, which is transcribed in a pattern similar to engrailed, led to only embryonic rescue, suggesting an important difference in the regulation of these two genes.
The roles of maternal α-catenin and plakoglobin in the early Xenopus embryo
Authors: Kofron M. , Spagnuolo A. , Klymkowsky M. , Wylie C. , Heasman J.
Published: April 2021

Catenins (α-, β- and γ- or plakoglobin) are cytoplasmic cadherin-associated proteins. Studies on cultured cells have suggested that both α-catenin and plakoglobin are important for the adhesive function of cadherins. α-catenin binds to both β-catenin and plakoglobin and may link the cadherin/catenin complex to actin filaments. Separate domains of plakoglobin bind to cadherin and α-catenin, suggesting it may act as a bridge between these molecules. However, plakoglobin may have other activities: it is expressed in both desmosomal junctions in association with desmogleins and the cytoplasm in conjunction with APC, and previous work suggests it may act in a dorsal signalling pathway when overexpressed in Xenopus embryos. Here, we have studied the roles of α-catenin and plakoglobin directly, by depleting the maternal mRNAs coding for each of them in developing Xenopus embryos. We find that depletion of maternal α-catenin causes the loss of intercellular adhesion at the blastula stage, similar to that reported previously for EP cadherin. Depletion of plakoglobin results in a partial loss of adhesion, and a loss of embryonic shape, but does not affect dorsal signalling.
Toll, a muscle cell surface molecule, locally inhibits synaptic initiation of the RP3 motoneuron growth cone in Drosophila
Authors: Rose Demian , Zhu Xiaomao , Kose Hiroyuki , Hoang Bao , Cho John , Chiba Akira
Published: April 2021

Toll, a transmembrane molecule with extracellular leucine-rich repeats, is dynamically expressed by the Drosophila embryonic musculature. Growth cones of RP3 and other motoneurons normally grow past Toll-positive muscle cells and innervate more distal muscle cells, which have down- regulated their Toll expression. In this study, we show that reciprocal genetic manipulations of Toll proteins can produce reciprocal RP3 phenotypes. In Toll null mutants, the RP3 growth cone sometimes innervates incorrect muscle cells, including those that are normally Toll- positive. In contrast, heterochronic misexpression of Toll in the musculature leads to the same growth cone reaching its correct target region but delaying synaptic initiation. We propose that Toll acts locally to inhibit synaptogenesis of specific motoneuron growth cones and that both temporal and spatial control of Toll expression is crucial for its role in development.
The SAR1 gene of Arabidopsis acts downstream of the AXR1 gene in auxin response
Authors: Cernac A. , Lincoln C. , Lammer D. , Estelle M.
Published: April 2021

A screen for suppressors of the auxin resistant mutant axr1 in Arabidopsis thaliana has identified at least three second site suppressor loci called Suppressor of Auxin Resistance (SAR). In this study we focus on the SAR1 gene. Previous studies have documented the effects of the axr1 mutations on auxin-inhibition of root growth, auxin-induced gene expression, seedling morphology and aerial morphology. In this study, we show that the axr1 mutations also affect root hair development and epidermal cell length. The sar1-1 mutation suppresses at least partially, every aspect of the axr1 phenotype. Genetic experiments indicate that this sup- pression is gene specific. When crossed with the auxin- resistant mutant aux1-7, the suppressor has little affect on auxin response. However, the morphology of sar1-1 aux1-7 inflorescences is different from either of the single mutants indicating that both genes play a role in auxin mediated development of the inflorescence. The sar1-1 mutation also affects morphology in an AXR1 background. sar1-1 plants are shorter than wild-type, have altered leaf morphology, flower earlier than wild-type plants and appear to have reduced cell division in the primary root. In most respects sar1-1 axr1 and sar1 AXR1 plants are indistinguishable, indicating that sar1 both suppresses and is epistatic to axr1. Based on these results, we propose that SAR1 acts after AXR1 and that a major function of AXR1 is to relieve SAR1 mediated repression of auxin response.
A vegetally localized T-box transcription factor in Xenopus eggs specifies mesoderm and endoderm and is essential for embryonic mesoderm formation
Authors: Horb Marko E. , Thomsen Gerald H.
Published: April 2021

Pattern formation in early embryogenesis is guided by maternal, localized determinants and by inductive interactions between cells. In Xenopus eggs, localized molecules have been identified and some, such as Vg1 and Xwnt-11, can specify cell fates by functioning as inducers or patterning agents. We have used differential screening to identify new Xenopus genes that regulate mesodermal patterning, and we have isolated a new member of the T-box family of transcription factors. This gene, named Brat, is expressed maternally and its transcripts are localized to the vegetal hemisphere of the egg. During early embryonic cleavage, Brat mRNA becomes partitioned primarily within vegetal cells that are fated to form the endoderm. Zygotic expression of Brat begins at the onset of gastrulation within the presumptive mesoderm of the marginal zone. Consistent with its zygotic expression pattern, Bratinduces, in a dose-dependent manner, a full spectrum of mesodermal genes that mark tissues across the dorsalventral axis, from the blood through the Spemann organizer. Brat also induces endoderm, consistent with its vegetal localization, making Brat a good candidate for a maternal determinant of the endoderm. We tested whether endogenous Brat is required for mesoderm formation by expressing a dominant-negative, transcriptional repressor form of Brat in embryos. This treatment inhibited mesoderm formation and severely disrupted normal development, thereby establishing that Brat plays a critical role in embryonic mesoderm formation and body patterning.
The Groucho-like transcription factor UNC-37 functions with the neural specificity gene unc-4 to govern motor neuron identity in C. elegans
Authors: Pflugrad Amy , Meir James Y.-J. , Barnes Thomas M. , Miller David M.
Published: April 2021

Groucho and Tup1 are members of a conserved family of WD repeat proteins that interact with specific transcription factors to repress target genes. Here we show that mutations in WD domains of the Groucho-like protein, UNC-37, affect a motor neuron trait that also depends on UNC-4, a homeodomain protein that controls neuronal specificity in Caenorhabditis elegans. In unc-4 mutants, VA motor neurons assume the pattern of synaptic input normally reserved for their lineal sister cells, the VB motor neurons; the loss of normal input to the VAs produces a distinctive backward movement defect. Substitution of a conserved residue (H to Y) in the fifth WD repeat in unc-37(e262) phenocopies the Unc-4 movement defect. Conversely, an amino acid change (E to K) in the sixth WD repeat of UNC-37 is a strong suppressor of unc-37(e262) and of specific unc-4 missense mutations. We have previously shown that UNC-4 expression in the VA motor neurons specifies the wild-type pattern of presynaptic input. Here we demonstrate that UNC-37 is also expressed in the VAs and that unc-37 activity in these neurons is sufficient to restore normal movement to unc-37(e262) animals. We propose that UNC-37 and UNC-4 function together to prevent expression of genes that define the VB pattern of synaptic inputs and thereby generate connections specific to the VA motor neurons. In addition, we show that the WD repeat domains of UNC-37 and of the human homolog, TLE1, are functionally interchangeable in VA motor neurons which suggests that this highly conserved protein domain may also specify motor neuron identity and synaptic choice in more complex nervous systems.
Genes necessary for C. elegans cell and growth cone migrations
Authors: Forrester Wayne C. , Garriga Gian
Published: April 2021

The migrations of cells and growth cones contribute to form and pattern during metazoan development. To study the mechanisms that regulate cell motility, we have screened for C. elegans mutants defective in the posteriorly directed migrations of the canal-associated neurons (CANs). Here we describe 14 genes necessary for CAN cell migration. Our characterization of the mutants has led to three conclusions. First, the mutations define three gene classes: genes necessary for cell fate specification, genes necessary for multiple cell migrations and a single gene necessary for final positioning of migrating cells. Second, cell interactions between the CAN and HSN, a neuron that migrates anteriorly to a position adjacent to the CAN, control the final destination of the HSN cell body. Third, C. elegans larval development requires the CANs. In the absence of CAN function, larvae arrest development, with excess fluid accumulating in their pseudocoeloms. This phenotype may reflect a role of the CANs in osmoregulation.
Cortical granules of the sea urchin translocate early in oocyte maturation
Authors: Berg Linnea K. , Wessel Gary M.
Published: April 2021

Cortical granules are secretory vesicles poised at the cortex of an egg that, upon stimulation by sperm contact at fertilization, secrete their contents. These contents modify the extracellular environment and block additional sperm from reaching the egg. The role of cortical granules in blocking polyspermy is conserved throughout much of phylogeny. In the sea urchin, cortical granules accumulate throughout the cytoplasm during oogenesis, but in mature eggs the cortical granules are attached to the plasma membrane, having translocated to the cortex at some earlier time. To study the process of cortical granule translocation to the cell surface we have devised a procedure for maturation of sea urchin oocytes in vitro. Using this procedure, we examined the rate of oocyte maturation by observing the movement and breakdown of the germinal vesicle, the formation of polar bodies and the formation of the egg pronucleus. We find that oocyte maturation takes approximately 9 hours in the species used here (Lytechinus variegatus), from the earliest indication of maturation (germinal vesicle movement) to formation of a distinct pronucleus. We then observed the translocation of cortical granules in these cells by immunolocalization using a monoclonal antibody to hyalin, a protein packaged specifically in cortical granules. We found that the translocation of cortical granules in in vitro-matured oocytes begins with the movement of the germinal vesicle to the oocyte cell surface, and is 50% complete 1 hour after germinal vesicle breakdown. In the in vitro-matured egg, 99% of the cortical granules are at the cortex, indistinguishable from translocation in oocytes that mature in vivo. We have also found that eggs that mature in vitro are functionally identical to eggs that mature in vivo by four criteria. (1) The matured cells undergo a selective turnover of mRNA encoding cortical granule contents. (2) The newly formed pronucleus begins transcription of histone messages. (3) Cortical granules that translocate in vitro are capable of exocytosis upon activation by the calcium ionophore, A23187. (4) The mature egg is fertilizable and undergoes normal cleavage and development. In vitro oocyte maturation enables us to examine the mechanism of cortical granule translocation and other processes that had previously only been observed in static sections of fixed ovaries.
The parthenocarpic fruit (pat) mutant of tomato (Lycopersicon esculentum Mill.) sets seedless fruits and has aberrant anther and ovule development
Authors: Mazzucato Andrea , Taddei Anna Rita , Soressi Gian Piero
Published: April 2021

Among the different sources of genetic parthenocarpy described in tomato, the mutation referred to as parthenocarpic fruit (pat) is of particular interest because of its strong expressivity and because it confers earlier ripening, higher fruit set and enhanced fruit quality. As a pleiotropic effect, pat flowers have aberrantly developing androecia and reduced male and female fertility. In this work we extend the early description of the pat phenotype by investigating the expression of parthenocarpy in three different environments and by using light and scanning electron microscopy to analyse the development of male and female floral organs. The degree of parthenocarpy was high in the three experimental environments and was characterised by a precocious initiation of ovary growth to pre-anthesis floral stages. Aberrations in anther development were evident at flower bud stages and resulted in shorter, irregular and teratoid organs. Ectopic production of carpel-like structures bearing external ovules was evident in the most severely altered androecia. Analysis of ovule development revealed that a fraction of pat ovules becomes aberrant from very early stages, having defective integument growth. Meiosis was irregular in aberrant ovules and megaspore or gamete production was severely hampered. The described pat syndrome suggests that parthenocarpy in this mutant could be a secondary effect of a gene controlling, at early stages, organ identity and development.
Transcriptional and post-transcriptional control mechanisms coordinate the onset of spermatid differentiation with meiosis I in Drosophila
Authors: White-Cooper Helen , Schäfer Mireille A. , Alphey Luke S. , Fuller Margaret T.
Published: April 2021

The aly, can, mia and sa genes of Drosophila are essential in males both for the G2-meiosis I transition and for onset of spermatid differentiation. Function of all four genes is required for transcription in primary spermatocytes of a suite of spermatid differentiation genes. aly is also required for transcription of the cell cycle control genes cyclin B and twine in primary spermatocytes. In contrast can, mia and sa are required for accumulation of twine protein but not twine transcript. We propose that the can, mia and sa gene products act together or in a pathway to turn on transcription of spermatid differentiation genes, and that aly acts upstream of can, mia and sa to regulate spermatid differentiation. We also propose that control of translation or protein stability regulates entry into the first meiotic division. We suggest that a gene or genes transcribed under the control of can, mia and sa allow(s) accumulation of twine protein, thus coordinating meiotic division with onset of spermatid differentiation.
Organogenic responses in tissue culture of srd mutants of Arabidopsis thaliana
Authors: Ozawa Shoichi , Yasutani Izumi , Fukuda Hiroo , Komamine Atsushi , Sugiyama Munetaka
Published: April 2021

In Arabidopsis thaliana, shoot redifferentiation and root redifferentiation can be induced at high frequency from hypocotyl and root explants by a two-step culture method. Tissues are precultured on callus-inducing medium and then transferred onto shoot-inducing medium for shoot redifferentiation or onto root-inducing medium for root redifferentiation. In an attempt to dissect these organogenic processes genetically, we characterized the responses in tissue culture of srd1, srd2 and srd3 mutants that were originally isolated as temperature-sensitive strains with defects in shoot redifferentiation (Yasutani, I., Ozawa, S., Nishida, T., Sugiyama, M. and Komamine, A. (1994) Plant Physiol. 105, 815-822). These mutants exhibited temperature sensitivity at different steps of organogenesis, which allowed the identification of three states associated with organogenic competence: IC (incompetent); CR (competent with respect to root redifferentiation); and CSR (competent with respect to shoot and root redifferentiation). Hypocotyl explants were shown to be in the IC state at the initiation of culture and to enter the CSR state, via the CR state, during preculture on callus-inducing medium, whereas root explants seemed to be in the CR state at the initiation of culture. The transition from IC to CR and that from CR to CSR appeared to require the functions of SRD2 and SRD3, respectively. It appears that explants in the CSR state redifferentiate shoots with the aid of the products of SRD1 and SRD2 when transplanted onto shoot-inducing medium. Histological examination of the srd mutants revealed that the function of SRD2 is required not only for organogenesis but also for the reinitiation of cell proliferation in hypocotyl explants during culture on callus-inducing medium. Linkage analysis using RFLP markers indicated that SRD1, SRD2, and SRD3 are located at the lower region, the central region, and the upper region of chromosome 1, respectively.
Embryonic phenotype of Unc5h3 mutant mice suggests chemorepulsion during the formation of the rostral cerebellar boundary
Authors: Przyborski Stefan A. , Knowles Barbara B. , Ackerman Susan L.
Published: April 2021

Mutation of the Unc5h3 (formally known as rcm) gene has important consequences on neuronal migration during cerebellar development. Unc5h3 transcripts are expressed early (embryonic day 8.5) in the hindbrain region and later in the cerebellar primordia. In Unc5h3 mutant embryos, both the development and initial migration of Purkinje cell progenitors occur as in wild-type controls. The rhombic lip, from which granule cell precursors arise, also appears to form normally in mutants. However, at E13.5, an abnormal subpopulation of granule cell and Purkinje cell precursors becomes detectable in rostral areas of the Unc5h3 mutant brain stem. These ectopic cerebellar cells increase in number and continue moving in a rostral direction throughout the remainder of embryogenesis and early stages of postnatal development invading the lateral regions of the pontine area and eventually the inferior colliculus. Cell proliferation markers demonstrate the mitotic nature of these subpial ectopic granule neurons indicating the displacement of the rostral external germinal layer in mutant animals. Our data suggest that establishment of the rostral cerebellar boundary may rely on chemorepulsive signaling events that require UNC5H3 expressed by cerebellar neurons and extracellular ligands that are functionally related to the UNC5H3-binding, guidance molecule netrin1. Although the phenotype resulting from the Unc5h3 mutation is apparently limited to the formation of the cerebellum, additional sites of Unc5h3 expression are also found during development suggesting the compensatory function of other genes.
STYLOSA and FISTULATA: regulatory components of the homeotic control of Antirrhinum floral organogenesis
Authors: Motte Patrick , Saedler Heinz , Schwarz-Sommer Zsuzsanna
Published: April 2021

The identity and developmental pattern of the four organ types constituting the flower is governed by three developmental functions, A, B and C, which are defined by homeotic genes and established in two adjacent whorls. In this report we morphologically and genetically characterise mutants of two genes, STYLOSA (STY) and FISTULATA (FIS) which control floral homeotic meristem- and organ-identity genes and developmental events in all floral whorls. The morphology of the reproductive organs in the first and second whorls of sty fis double mutant flowers indicate that the two genes are part of the mechanism to prevent ectopic expression of the C-function in the perianth of wild-type flowers. This is verified by the detection of the expansion of the expression domain of the class C gene PLENA (PLE) towards the perianth. Interestingly, in the second whorl of sty and fis mutants, spatial differences in stamenoid features and in the pattern of ectopic expression of the PLE gene were observed. This suggests that, with respect to the negative control of PLE, petals are composed of two regions, a lateral and a central one. Mutation in ple is epistatic to most of the sty/fis-related homeotic defects. PLE, however, is not the primary target of STY/FIS control, because dramatic reduction of expression of FIMBRIATA, meristem identity genes (FLORICAULA and SQUAMOSA) and of class B organ identity genes (GLOBOSA) occur before changes in the PLE expression pattern. We propose that STY/FIS are hierarchically high-ranking genes that control cadastral component(s) of the A-function. SQUAMOSA as a potential target of this control is discussed. Retarded growth of second whorl organs, subdivision of third whorl primordia and the failure to initiate them in sty/fis mutants may be mediated by the FIMBRIATA gene.
Continuing organizer function during chick tail development
Authors: Knezevic Vladimir , Santo Ronald De , Mackem Susan
Published: April 2021

Development of the posterior body (lumbosacral region and tail) in vertebrates is delayed relative to gastrulation. In amniotes, it proceeds with the replacement of the regressed node and primitive streak by a caudal blastemalike mass of mesenchyme known as the tail bud. Despite apparent morphological dissimilarities, recent results suggest that tail development in amniotes is in essence a continuation of gastrulation, as is the case in Xenopus. However, this has been inferred primarily from the outcome of fate mapping studies demonstrating discrete, regionalized cell populations in the tail bud, like those present at gastrulation. Our analysis of the tail bud distribution of several molecular markers that are expressed in specific spatial domains during chick gastrulation confirms these results. Furthermore, we present evidence that gastrulation-like ingression movements from the surface continue in the early chick tail bud and that the established tail bud retains organizer activity. This ‘tail organizer’ has the expected properties of being able to recruit uncommitted host cells into a new embryonic axis and induce host neural tissue with posteriorly regionalized gene expression when grafted to competent host cells that are otherwise destined to form only extra-embryonic tissue. Together, these results indicate that chick tail development is mechanistically continuous with gastrulation and that the developing tail in chick may serve as a useful experimental adjunct to investigate the molecular basis of inductive interactions operating during gastrulation, considering that residual tail organizing activity is still present at a surprisingly late stage.
Bundle sheath cells and cell-specific plastid development in Arabidopsis leaves
Authors: Kinsman E. A. , Pyke K. A.
Published: April 2021

Bundle sheath cells form a sheath around the entire vascular tissue in Arabidopsis leaves and constitute a distinct leaf cell type, as defined by their elongate morphology, their position adjacent to the vein and by differences in their chloroplast development compared to mesophyll cells. They constitute about 15% of chloroplast-containing cells in the leaf. In order to identify genes which play a role in the differential development of bundle sheath and mesophyll cell chloroplasts, a screen of reticulate leaf mutants of Arabidopsis was used to identify a new class of mutants termed dov (differential development of vascular-associated cells). The dov1 mutant clearly demonstrates a cell-specific difference in chloroplast development. Mutant leaves are highly reticulate with a green vascular pattern. The underlying bundle sheath cells always contain normal chloroplasts, whereas chloroplasts in mesophyll cells are abnormal, reduced in number per cell and seriously perturbed in morphology at the ultrastructural level. This demonstrates that differential chloroplast development occurs between the bundle sheath and mesophyll cells in the Arabidopsis leaf.
Genetic interactions and cell behaviour in blistered mutants during proliferation and differentiation of the Drosophila wing
Authors: Roch Fernando , Baonza Antonio , Martín-Blanco Enrique , García-Bellido Antonio
Published: April 2021

In this work, we analyse the blistered function in wing vein development by studying genetic mosaics of mutant cells, genetic interactions with other genes affecting vein development and blistered expression in several mutant backgrounds. blistered encodes for a nuclear protein homologous to the mammalian Serum Response Factor and is expressed in presumptive intervein cells of third larval instar and pupal wing discs. Clones of blistered mutant cells proliferate normally but tend to grow along veins and always differentiate as vein tissue. These observations indicate that vein-determined wing cells show a particular behaviour that is responsible for their allocation to vein regions. We observe strong genetic interactions between blistered, veinlet and genes of the Ras signaling cascade. During disc proliferation, blistered expression is under the control of the Ras signal transduction pathway, but its expression is independent of veinlet. During the pupal period, blistered and veinlet expression become interdependent and mutually exclusive. These results link the activity of the Ras pathway to the process of early determination of intervein cells, by the transcriptional control of the blistered nuclear factor.
HASTY: a gene that regulates the timing of shoot maturation in Arabidopsis thaliana
Authors: Telfer Abby , Poethig R. Scott
Published: April 2021

In Arabidopsis thaliana, leaves produced at different stages of shoot development can be distinguished by the distribution of trichomes on the abaxial and adaxial surfaces. Leaves produced early in the development of the rosette (juvenile leaves) have trichomes on their adaxial, but not their abaxial surface, whereas leaves produced later in rosette development (adult leaves) have trichomes on both surfaces. In order to identify genes that regulate the transition between these developmental phases we screened for mutations that accelerate the production of leaves with abaxial trichomes. 9 alleles of the HASTY gene were recovered in this screen. In addition to accelerating the appearance of adult leaves these mutations also accelerate the loss of adaxial trichomes (a trait typical of bracts), reduce the total number of leaves produced by the shoot, and have a number of other effects on shoot morphology. The basis for this phenotype was examined by testing the interaction between hasty and genes that affect flowering time (35S::LEAFY, 35S::APETALA1, terminal flower1), gibberellin production (ga1-3) or perception (gai), and floral morphogenesis (leafy, apetala1, agamous). We found that hasty increased the reproductive competence of the shoot, and that its does not require gibberellin or a gibberellin response for its effect on vegetative or reproductive development. The phenotype of hasty is not suppressed by leafy, apetala1 and agamous, demonstrating that this phenotype does not result from the inappropriate expression of these genes. We suggest that HASTY promotes a juvenile pattern of vegetative development and inhibits flowering by reducing the competence of the shoot to respond to LEAFY and APETALA1.
Expression of the murine Hoxa4 gene requires both autoregulation and a conserved retinoic acid response element
Authors: Packer Alan I. , Crotty David A. , Elwell Vivian A. , Wolgemuth Debra J.
Published: April 2021

Analysis of the regulatory regions of the Hox genes has revealed a complex array of positive and negative cis-acting elements that control the spatial and temporal pattern of expression of these genes during embryogenesis. In this study we show that normal expression of the murine Hoxa4 gene during development requires both autoregulatory and retinoic acid-dependent modes of regulation. When introduced into a Hoxa4 null background, expression of a lacZ reporter gene driven by the Hoxa4 regulatory region (Hoxa4/lacZ) is either abolished or significantly reduced in all tissues at E10.5-E12.5. Thus, the observed autoregulation of the Drosophila Deformed gene is conserved in a mouse homolog in vivo, and is reflected in a widespread requirement for positive feedback to maintain Hoxa4 expression. We also identify three potential retinoic acid response elements in the Hoxa4 5′ flanking region, one of which is identical to a well-characterized element flanking the Hoxd4 gene. Administration of retinoic acid to Hoxa4/lacZ transgenic embryos resulted in stage-dependent ectopic expression of the reporter gene in the neural tube and hindbrain. When administered to Hoxa4 null embryos, however, persistent ectopic expression was not observed, suggesting that autoregulation is required for maintenance of the retinoic acid-induced expression. Finally, mutation of the consensus retinoic acid response element eliminated the response of the reporter gene to exogenous retinoic acid, and abolished all embryonic expression in untreated embryos, with the exception of the neural tube and prevertebrae. These data add to the evidence that Hox gene expression is regulated, in part, by endogenous retinoids and autoregulatory loops.
Drosophila EcR-B ecdysone receptor isoforms are required for larval molting and for neuron remodeling during metamorphosis
Authors: Schubiger Margrit , Wade Andrew A. , Carney Ginger E. , Truman James W. , Bender Michael
Published: April 2021

During the metamorphic reorganization of the insect central nervous system, the steroid hormone 20-hydroxyecdysone induces a wide spectrum of cellular responses including neuronal proliferation, maturation, cell death and the remodeling of larval neurons into their adult forms. In Drosophila, expression of specific ecdysone receptor (EcR) isoforms has been correlated with particular responses, suggesting that different EcR isoforms may govern distinct steroid-induced responses in these cells. We have used imprecise excision of a P element to create EcR deletion mutants that remove the EcR-B promoter and therefore should lack EcR-B1 and EcR-B2 expression but retain EcR-A expression. Most of these EcR-B mutant animals show defects in larval molting, arresting at the boundaries between the three larval stages, while a smaller percentage of EcR-B mutants survive into the early stages of metamorphosis.

Remodeling of larval neurons at metamorphosis begins with the pruning back of larval-specific dendrites and occurs as these cells are expressing high levels of EcR-B1 and little EcR-A. This pruning response is blocked in the EcR-B mutants despite the fact that adult-specific neurons, which normally express only EcR-A, can progress in their development. These observations support the hypothesis that different EcR isoforms control cell-type-specific responses during remodeling of the nervous system at metamorphosis.
sma-1 encodes a βH-spectrin homolog required for Caenorhabditis elegans morphogenesis
Authors: McKeown Caroline , Praitis Vida , Austin Judith
Published: April 2021

Morphogenesis transforms the C. elegans embryo from a ball of cells into a vermiform larva. During this transformation, the embryo increases fourfold in length; present data indicates this elongation results from contraction of the epidermal actin cytoskeleton. In sma-1 mutants, the extent of embryonic elongation is decreased and the resulting sma-1 larvae, although viable, are shorter than normal. We find that sma-1 mutants elongate for the same length of time as wild-type embryos, but at a decreased rate. The sma-1 mutants we have isolated vary in phenotypic severity, with the most severe alleles showing the greatest decrease in elongation rate. The sma-1 gene encodes a homolog of βH-spectrin, a novel β-spectrin isoform first identified in Drosophila. sma-1 RNA is expressed in epithelial tissues in the C. elegans embryo: in the embryonic epidermis at the start of morphogenesis and subsequently in the developing pharynx, intestine and excretory cell. In Drosophila, βH-spectrin associates with the apical plasma membrane of epithelial cells; βspectrin is found at the lateral membrane. We propose that SMA-1 is a component of an apical membrane skeleton in the C. elegans embryonic epidermis that determines the rate of elongation during morphogenesis.
Spatial and temporal targeting of gene expression in Drosophila by means of a tetracycline-dependent transactivator system
Authors: Bello Bruno , Resendez-Perez Diana , Gehring Walter J.
Published: April 2021

In order to evaluate the efficiency of the tetracycline-regulated gene expression system in Drosophila, we have generated transgenic lines expressing a tetracycline-controlled transactivator protein (tTA), with specific expression patterns during embryonic and larval development. These lines were used to direct expression of a tTA-responsive promoter fused to the coding region of either the β-galactosidase or the homeotic protein Antennapedia (ANTP), under various conditions of tetracycline treatment. We found that expression of β-galactosidase can be efficiently inhibited in embryos and larvae with tetracycline provided in the food, and that a simple removal of the larvae from tetracycline exposure results in the induction of the enzyme in a time- and concentration-dependent manner. Similar treatments can be used to prevent the lethality associated with the ectopic expression of ANTP in embryos and, subsequently, to control the timing of expression of the homeoprotein ANTP specifically in the antennal imaginal disc.

Our results show that the expression of a gene placed under the control of a tetracycline-responsive promoter can be tightly controlled, both spatially by the regulatory sequences driving the expression of tTA and temporally by tetracycline. This provides the basis of a versatile binary system for controlling gene expression in Drosophila, with an additional level of regulation as compared to the general method using the yeast transcription factor GAL4.
In vivo clonal analyses reveal the properties of endogenous neural stem cell proliferation in the adult mammalian forebrain
Authors: Morshead Cindi M. , Craig Constance G. , Kooy Derek van der
Published: April 2021

The adult mammalian forebrain contains a population of multipotential neural stem cells in the subependyma of the lateral ventricles whose progeny are the constitutively proliferating cells, which divide actively throughout life. The adult mammalian brain is ideal for examining the kinetics of the stem cells due to their strict spatial localization and the limited and discrete type of progeny generated (constitutively proliferating cells). Clonal lineage analyses 6 days after retrovirus infection revealed that under baseline conditions 60% of the constitutively proliferating cells undergo cell death, 25% migrate to the olfactory bulb and 15% remain confined to the lateral ventricle subependyma (where they reside for approximately 15 days). Analysis of single cell clones 31 days after retroviral infection revealed that the stem cell divides asymmetrically to self-renew and give rise to constitutively proliferating cells. Following repopulation of the depleted subependyma the average clone size is 2.8 times larger than control, yet the absolute number of cells migrating to the olfactory bulb is maintained and the stem cell retains its asymmetric mode of division. The number of neural stem cells in the adult forebrain 33 days after repopulation of the subependyma was estimated using bromodeoxyuridine labeling of subepenydmal cells. There were calculated to be 1200-1300 cells between the rostral corpus callosum and rostral anterior commissure; these data support a lineage model similar to those based on stem cell behavior in other tissue types.
ZLocal induction of patterning and programmed cell death in the developing Drosophila retina
Authors: Miller David T. , Cagan Ross L.
Published: April 2021

Local cell signaling can pattern the nervous system by directing cell fates, including programmed cell death. In the developing Drosophila retina, programmed cell death is used to remove excess cells between ommatidia. Cell ablation revealed the source and position of signals required for regulating the pattern of programmed cell death among these interommatidial cells. Two types of signals regulate this patterning event. Notch-mediated signals between interommatidial precursors result in removal of unneeded cells. Cone cells and primary pigment cells oppose this signal by supplying a ‘life’-promoting activity; evidence is provided that this signal occurs through localized activation of the EGF Receptor/Ras pathway. Together, these signals refine the highly regular pattern observed in the adult retina.
Overlapping functions of the myogenic bHLH genes MRF4 and MyoD revealed in double mutant mice
Authors: Rawls Alan , Valdez M. Renee , Zhang Wei , Richardson James , Klein William H. , Olson Eric N.
Published: April 2021

The myogenic basic helix-loop-helix (bHLH) genes – MyoD, Myf5, myogenin and MRF4 – exhibit distinct, but overlapping expression patterns during development of the skeletal muscle lineage and loss-of-function mutations in these genes result in different effects on muscle development. MyoD and Myf5 have been shown to act early in the myogenic lineage to establish myoblast identity, whereas myogenin acts later to control myoblast differentiation. In mice lacking myogenin, there is a severe deficiency of skeletal muscle, but some residual muscle fibers are present in mutant mice at birth. Mice lacking MRF4 are viable and have skeletal muscle, but they upregulate myogenin expression, which could potentially compensate for the absence of MRF4. Previous studies in which Myf5 and MRF4 null mutations were combined suggested that these genes do not share overlapping myogenic functions in vivo. To determine whether the functions of MRF4 might overlap with those of myogenin or MyoD, we generated double mutant mice lacking MRF4 and either myogenin or MyoD. MRF4/myogenin double mutant mice contained a comparable number of residual muscle fibers to mice lacking myogenin alone and myoblasts from those double mutant mice formed differentiated multinucleated myotubes in vitro as efficiently as wild-type myoblasts, indicating that neither myogenin nor MRF4 is absolutely essential for myoblast differentiation. Whereas mice lacking either MRF4 or MyoD were viable and did not show defects in muscle development, MRF4/MyoD double mutants displayed a severe muscle deficiency similar to that in myogenin mutants. Myogenin was expressed in MRF4/MyoD double mutants, indicating that myogenin is insufficient to support normal myogenesis in vivo. These results reveal unanticipated compensatory roles for MRF4 and MyoD in the muscle differentiation pathway and suggest that a threshold level of myogenic bHLH factors is required to activate muscle structural genes, with this level normally being achieved by combinations of multiple myogenic bHLH factors.
Fgf8 is mutated in zebrafish acerebellar (ace) mutants and is required for maintenance of midbrain-hindbrain boundary development and somitogenesisy
Authors: Reifers Frank , Böhli Heike , Walsh Emily C. , Crossley Phillip H. , Stainier Didier Y. R. , Brand Michael
Published: April 2021

We describe the isolation of zebrafish Fgf8 and its expression during gastrulation, somitogenesis, fin bud and early brain development. By demonstrating genetic linkage and by analysing the structure of the Fgf8 gene, we show that acerebellar is a zebrafish Fgf8 mutation that may inactivate Fgf8 function. Homozygous acerebellar embryos lack a cerebellum and the midbrain-hindbrain boundary organizer. Fgf8 function is required to maintain, but not initiate, expression of Pax2.1 and other marker genes in this area. We show that Fgf8 and Pax2.1 are activated in adjacent domains that only later become overlapping, and activation of Fgf8 occurs normally in no isthmus embryos that are mutant for Pax2.1. These findings suggest that multiple signaling pathways are independently activated in the midbrain-hindbrain boundary primordium during gastrulation, and that Fgf8 functions later during somitogenesis to polarize the midbrain. Fgf8 is also expressed in a dorsoventral gradient during gastrulation and ectopically expressed Fgf8 can dorsalize embryos. Nevertheless, acerebellar mutants show only mild dorsoventral patterning defects. Also, in spite of the prominent role suggested for Fgf8 in limb development, the pectoral fins are largely unaffected in the mutants. Fgf8 is therefore required in development of several important signaling centers in the zebrafish embryo, but may be redundant or dispensable for others.
MES-2, a maternal protein essential for viability of the germline in Caenorhabditis elegans, is homologous to a Drosophila Polycomb group protein
Authors: Holdeman Richard , Nehrt Sarah , Strome Susan
Published: April 2021

A unique and essential feature of germ cells is their immortality. In Caenorhabditis elegans, germline immortality requires the maternal contribution from four genes, mes-2, mes-3, mes-4 and mes-6. We report here that mes-2 encodes a protein similar to the Drosophila Polycomb group protein, Enhancer of zeste, and in the accompanying paper that mes-6 encodes another Polycomb group protein. The Polycomb group is responsible for maintaining proper patterns of expression of the homeotic and other genes in Drosophila. It is thought that Polycomb group proteins form heteromeric complexes and control gene expression by altering chromatin conformation of target genes. As predicted from its similarity to a Polycomb group protein, MES-2 localizes to nuclei. MES-2 is found in germline nuclei in larval and adult worms and in all nuclei in early embryos. By the end of embryogenesis, MES-2 is detected primarily in the two primordial germ cells. The correct distribution of MES-2 requires the wild-type functions of mes-3 and mes-6. We hypothesize that mes-2 encodes a maternal regulator of gene expression in the early germline; its function is essential for normal early development and viability of germ cells.
Pattern formation and growth during floral organogenesis: HUELLENLOS and AINTEGUMENTA are required for the formation of the proximal region of the ovule primordium in Arabidopsis thaliana
Authors: Schneitz Kay , Baker Shawn C. , Gasser Charles S. , Redweik Antje
Published: April 2021

Our understanding of the molecular mechanisms that regulate and integrate the temporal and spatial control of cell proliferation during organ ontogenesis, particularly of floral organs, continues to be primitive. The ovule, the progenitor of the seed, of Arabidopsis thaliana has been used to develop an effective model system for the analysis of plant organogenesis. A typical feature of a generalized ovule is the linear arrangement of at least three distinct elements, the funiculus, chalaza and nucellus, along a proximal-distal axis. This pattern is supposed to be established during the early proliferative phase of ovule development. We provide genetic evidence that the young ovule primordium indeed is a composite structure. Two genes, HUELLENLOS and AINTEGUMENTA have overlapping functions in the ovule and differentially control the formation of the central and proximal elements of the primordium. The results indicate that proximal-distal pattern formation in the Arabidopsis ovule takes place in a sequential fashion, starting from the distal end. Furthermore, we show that HUELLENLOS also regulates the initiation and/or maintenance of integument and embryo sac ontogenesis and interestingly prevents inappropriate cell death in the young ovule.