Chemical Patterns, Compartments and a Binary Epigenetic Code in Drosophila

I propose a model which accounts for the geometries and sequencein which compartmental boundary lines arise on the differentimaginal discs, and on the blastoderm of Drosophila melanogaster;and propose that successive lines are recorded by differentbinary switches, to create a binary epigenetic code word specifyingeach disc, and disc compartment. I suppose a biochemical systemundergoing reaction and diffusion acts throughout development.As an imaginal disc grows, a succession of differently shapedchemical concentration patterns form at a discrete set of discsizes. I suppose a specific concentration of one chemical isa threshold. Concentrations above or below threshold switchcells to one or another of two commitments. Then the line acrossthe imaginal disc with the threshold concentration is a predictedcompartmental boundary. The sequence and geometries of suchlines predict the compartmental boundaries seen on the wingdisc, the other discs, and on the blastoderm stage egg. Thecompartmental lines on the wing disc suggest that a terminalcompartment is specified by a combination of binary names recordinga sequence of binary commitments: anterior, not posterior; dorsal,not ventral; wing, not thorax; proximal, not distal. Each combinationcomprises a binary epigenetic code word. Recently I constructedan independent model for transdetermination in Drosophila whichproposed a similar binary epigenetic code for the differentdiscs. The clone restriction lines predicted on the blastodermby my transdetermination model, the chemical pattern model,and analogy with the wing disc, are nearly identical. Severalare already confirmed. The resultant binary code scheme correctlypredicts many relative transdetermination frequencies and accountssimply for the action of most homeotic mutants as genes whichalter a single switch state in one or more discs.     

Heterotopic transplantation in the syncytial blastoderm ofDrosophila: Evidence for anterior and posterior nuclear commitments

Summary The interaction ofDrosophila syncytial blastoderm nuclei and cortical cytoplasm in the control of somatic developmental commitments was studied by transplanting genetically marked nuclei and surrounding cytoplasm between anterior and posterior flanks. After completion of cellularization the host egg was cut. Host anterior or posterior partial embryos were cultured in adult abdomens for 8–10 days, then the larval tissue removed and injected into larval hosts for metamorphosis. Differentiated ectodermal implants were recovered from emerged adults and characterized. One hundred sixteen clearly interpretable control and experimental implants were found. Of the 73 experimental implants 15 were derived from donor nuclei.Among the 15 donor implants, 14 autonomously formed donor site anterior (head and thoracic) or posterior (abdomen and genital) structures. This donor autonomy is interpreted to mean that nuclear and cytoplasmic factors necessary for anterior and posterior somatic commitments are present and transplantable prior to the completion of cellularization. Since donor nuclei injected directly into host flanks, or premixed with host cytoplasm, would have been well exposed to any host cytoplasmic factors, donor nuclei appear to have adopted anterior or posterior somatic commitments which are stable to significant cytoplasmic alterations.In 14 implants, host nuclei exposed to donor material altered somatic fate and formed donor type structures. These conversions are interpreted to imply that cytoplasmic factors controlling anterior or posterior somatic fates are present in the syncytial balstoderm embryo.     

Auxin gradients trigger de novo formation of stem cells during somatic embryogenesis

Single or a group of somatic cells could give rise to the whole plant, which require hormones, or plant growth regulators. Although many studies have been done during past years, how hormones specify cell fate during in vitro organogenesis is still unknown. To uncover this mechanism, Arabidopsis somatic embryogenesis has been recognized as a model for studying in vitro plant organogenesis. In this paper, we showed that establishment of auxin gradients within embryonic callus is essential for inducing stem cell formation via PIN1 regulation. This study sheds new light on how hormone regulates stem cell formation during in vitro organogenesis.Key words: auxin gradients, PIN proteins, stem cell, somatic embryogenesis     

Auxin-induced WUS expression is essential for embryonic stem cell renewal during somatic embryogenesis in Arabidopsis

Somatic embryogenesis requires auxin and establishment of the shoot apical meristem (SAM). WUSCHEL ( WUS ) is critical for stem cell fate determination in the SAM of higher plants. However, regulation of WUS expression by auxin during somatic embryogenesis is poorly understood. Here, we show that expression of several regulatory genes important in zygotic embryogenesis were up-regulated during somatic embryogenesis of Arabidopsis. Interestingly, WUS expression was induced within the embryonic callus at a time when somatic embryos could not be identified morphologically or molecularly. Correct WUS expression, regulated by a defined critical level of exogenous auxin, is essential for somatic embryo induction. Furthermore, it was found that auxin gradients were established in specific regions that could then give rise to somatic embryos. The establishment of auxin gradients was correlated with the induced WUS expression. Moreover, the auxin gradients appear to activate PIN1 polar localization within the embryonic callus. Polarized PIN1 is probably responsible for the observed polar auxin transport and auxin accumulation in the SAM and somatic embryo. Suppression of WUS and PIN1 indicated that both genes are necessary for embryo induction through their regulation of downstream gene expression. Our results reveal that establishment of auxin gradients and PIN1-mediated polar auxin transport are essential for WUS induction and somatic embryogenesis. This study sheds new light on how auxin regulates stem cell formation during somatic embryogenesis.     

Isocitrate dehydrogenase in D. melanogaster imaginal discs: pattern development and alteration by homoeotic mutant genes

The distribution of the soluble form of NADP+-dependent isocitrate dehydrogenase (ICDH) was examined in Drosophila melanogaster imaginal discs. Development of the enzyme patterns and the specific transformations of the patterns by homoeotic mutants were studied. ICDH pattern formation was followed in eye-antennal discs and wing discs from the late 2nd instar stage through 3rd instar and 8 hours into prepupal development. The patterns formed gradually in both disc types. The most interesting pattern developed in the eye portion of the eye-antennal disc complex. ICDH distribution as well as staining intensity correlated well with differentiation of the ommatidia. The spatial distribution of ICDH within the discs was under genetic control. The patterns reflected the state of determination of the disc. When the presumptive tissue type was transformed via mutant homoeotic genes to different determinative states, the ICDH pattern likewise transformed to the pattern characteristic of the newly acquired structure.     

Pattern formation and determination in the antenna of the homoeotic mutant Antennapedia of Drosophila melanogaster

The development of the antenna in the antennal-leg homoeotic mutant Antennapedia (Antp^R) was investigated using somatic crossing-over to mark clones of cells in Antp^R antennal appendages. Antp^R antennae ranged from a nearly normal antenna to a nearly normal leg. The arrangement of clones of marked bristles and cuticle in the more antennalike antennae was similar to the wild type antenna, and that of the leglike antennae was similar to the wild-type leg. The contiguity of clones argued against extensive individual cell migration. The regions occupied by homoeotic leg varied considerably between different Antp^R antennae. Observation of Antp^R antennae in these phenotypic mosaics showed that specific leg parts replaced specific antennal parts. Even small groups of leg sensilla appeared only at precise locations in the antenna. These results suggest that homoeotic leg cells and antennal cells can both respond to the same positional information or prepattern. An analysis of clone size provided estimates for cell number in the Antp^R antenna. It was found that cell numbers in the wild-type and Antp^R antennae are about the same until the third instar, when the Antp^R cells start dividing more rapidly than wild type. Previous work had shown that clonal inheritance of a commitment for homoeotic leg also did not occur prior to the early third instar. It is suggested that determination for homoeotic leg occurs in the early third instar, and that thereafter this commitment is inherited by the progeny of the determined cells. The increase in growth rate is probably due to a faster growth rate in cells with a leg commitment than in cells with an antennal commitment. The results suggest that, once initiated, determination may be of two types—a clonally inherited determination (for example, to be homoeotic leg) and an environmental determination (for example, to be a specific part of a homoeotic leg). Clonal inheritance of determination in normal embryonic development and in sex determination in intersexes is discussed.     

A cell arrangement specific to thoracic ganglia in the central nervous system of theDrosophila embryo: Its behaviour in homoeotic mutants

Summary We describe a set of cells in the central nervous system of theDrosophila embryo which are restricted to the thoracic ganglia in the wildtype. Taking these cells as indication of thoracic identity, we find that the ventral cord of embryos homozygous mutant for different bithorax functions and for Polycomb undergoes homoeotic transformations equivalent to those observed in the larval cuticle.     

Axis specification in animal development

Axis specification is the first step in defining specific regions of the developing embryo. Embryos exploit asymmetries, either pre-existing in the egg or triggered by external cues, to establish embryonic axes. The axial information is then used to generate regional differences within the embryo. In this review, we discuss experiments in animals which address three questions: whether the unfertilized egg is constructed with pre-determined axes, what cues are used to specify the embryonic axes, and how these cues are interpreted to generate the initial regional differences within the embryo. Based on mapping the data onto an animal phylogeny, we then propose a scenario for how this primary developmental decision occurred in ancestral metazoans.     

Blastomere Explants to Test for Cell Fate Commitment During Embryonic Development

Fate maps, constructed from lineage tracing all of the cells of an embryo, reveal which tissues descend from each cell of the embryo. Although fate maps are very useful for identifying the precursors of an organ and for elucidating the developmental path by which the descendant cells populate that organ in the normal embryo, they do not illustrate the full developmental potential of a precursor cell or identify the mechanisms by which its fate is determined. To test for cell fate commitment, one compares a cell''s normal repertoire of descendants in the intact embryo (the fate map) with those expressed after an experimental manipulation. Is the cell''s fate fixed (committed) regardless of the surrounding cellular environment, or is it influenced by external factors provided by its neighbors? Using the comprehensive fate maps of the Xenopus embryo, we describe how to identify, isolate and culture single cleavage stage precursors, called blastomeres. This approach allows one to assess whether these early cells are committed to the fate they acquire in their normal environment in the intact embryo, require interactions with their neighboring cells, or can be influenced to express alternate fates if exposed to other types of signals.     

A gradient of JAK pathway activity patterns the anterior-posterior axis of the follicular epithelium

The Drosophila egg develops through closely coordinated activities of associated germline and somatic cells. An essential aspect of egg development is the differentiation of the somatic follicle cells into several distinct subpopulations with specific functions. Here we demonstrate that the graded activity of the Janus kinase (JAK) pathway, stimulated by the Unpaired ligand, patterns the anterior-posterior axis of the follicular epithelium. Different levels of JAK activity instruct adoption of distinct anterior cell fates. Further, the coordinated activities of the JAK/STAT and epidermal growth factor receptor (EGFR) pathways are required to specify the posterior terminal cell fate. We propose that Upd secreted from the polar cells may act as a morphogen to stimulate A/P-derived follicular fates through JAK pathway activation.     

Transgenic manipulation of plant embryo sacs tracked through cell-type-specific fluorescent markers: cell labeling, cell ablation, and adventitious embryos

Expression datasets relating to the Arabidopsis female gametophyte have enabled the creation of a tool set which allows simultaneous visual tracking of each specific cell type (egg, synergids, central cell, and antipodals). This cell-specific, fluorescent labeling tool-set functions from gametophyte cellularization through fertilization and early embryo development. Using this system, cell fates were tracked within Arabidopsis ovules following molecular manipulations, such as the ablation of the egg and/or synergids. Upon egg cell ablation, it was observed that a synergid can switch its developmental fate to become egg/embryo-like upon loss of the native egg. Also, manipulated was the fate of the somatic ovular cells, which can become egg- and embryo-like, reminiscent of adventitious embryony. These advances represent initial steps toward engineering synthetic apomixis resulting in seed derived wholly from the maternal plant. The end goal of applied apomixis research, fixing important agronomic traits such as hybrid vigor, would be a key benefit to agricultural productivity.     

Signalling in plant embryos during the establishment of the polar axis.

In brown algae fertilization takes place free from surrounding tissue layers. The cytoskeleton and transmembrane links to the cell wall are involved in establishing and stabilizing the polar axis and in determining the fate of cells in the early embryo. In seed plants, the egg cell and zygote exhibit apical basal polarity. Mutant studies suggest that axes of polarity of the early embryo depend on signalling between the apical and basal compartments, possibly involving auxin. Development of somatic cells into plant embryos involves extracellular matrix-derived arabinogalactan proteins. This suggests a role for the cell wall in plant embryogenesis.     

Compatible invasion of a phylogenetically distant host embryo by a hymenopteran parasitoid embryo

Embryonic invasion into the tissue of genetically different organisms has been known only in mother-embryo interactions of viviparous organisms. Hence, embryonic invasions have been thought to occur only within the same or closely related species. For endoparasitic Hymenoptera, which are oviposited in their host egg but complete their development in the later stages, entry into the host embryo is essential. To date, the entry of these parasitoids is known to be accomplished by either egg deposition directly into the embryo or by the newly hatched larva boring into the embryo. However, Copidosoma floridanum is a polyembryonic parasitoid whose development is characterized by a prolonged embryonic stage, and which lacks a larval form during its host embryogenesis. We have analyzed the behavior and fate of C. floridanum embryos co-cultured with their host embryo in vitro. Here, we show that the morula-stage embryo of C. floridanum actively invades the host embryo. Histological analyses have demonstrated that C. floridanum embryonic invasion is associated with adherent junction to host cells rather than causing an obvious wound on the host cells. These findings provide a novel case of embryonic invasion into a phylogenetically distant host embryo, ensuring cellular compatibility with host tissues.     

Variation in mesoderm specification across Drosophilids is compensated by different rates of myoblast fusion during body wall musculature development

Background

It has been shown that species separated by relatively short evolutionary distances may have extreme variations in egg size and shape. Those variations are expected to modify the polarized morphogenetic gradients that pattern the dorso-ventral axis of embryos. Currently, little is known about the effects of scaling over the embryonic architecture of organisms. We began examining this problem by asking if changes in embryo size in closely related species of Drosophila modify all three dorso-ventral germ layers or only particular layers, and whether or not tissue patterning would be affected at later stages.

Principal Findings

Here we report that changes in scale affect predominantly the mesodermal layer at early stages, while the neuroectoderm remains constant across the species studied. Next, we examined the fate of somatic myoblast precursor cells that derive from the mesoderm to test whether the assembly of the larval body wall musculature would be affected by the variation in mesoderm specification. Our results show that in all four species analyzed, the stereotyped organization of the body wall musculature is not disrupted and remains the same as in D. melanogaster. Instead, the excess or shortage of myoblast precursors is compensated by the formation of individual muscle fibers containing more or less fused myoblasts.

Conclusions

Our data suggest that changes in embryonic scaling often lead to expansions or retractions of the mesodermal domain across Drosophila species. At later stages, two compensatory cellular mechanisms assure the formation of a highly stereotyped larval somatic musculature: an invariable selection of 30 muscle founder cells per hemisegment, which seed the formation of a complete array of muscle fibers, and a variable rate in myoblast fusion that modifies the number of myoblasts that fuse to individual muscle fibers.     

Plant stem cells: divergent pathways and common themes in shoots and roots

Stem cells in plant shoot and root meristems are maintained throughout the life of the plant and produce somatic daughter cells that make up the body of the plant. Plant stem cells can also be derived from somatic cells in vivo and in vitro. Recent findings are refining our knowledge of signaling pathways that define stem cell fate and specify either shoot or root stem cell function. New evidence also highlights a role for epigenetic mechanisms in controlling stem cell fate.     

Homoeosis in Drosophila. II. a Genetic Analysis of Polycomb

Three dominant mutant alleles of the Polycomb locus of Drosophila melanogaster are associated with homoeotic transformations of meso- and metathoracic to prothoracic legs, a homoeotic transformation of antennae to legs, and abnormalities of wings and some thoracic bristles. Puro and Nygrén (1975) localized Polycomb in the proximal left arm of chromosome 3 within salivary gland chromosome interval 77E,F-80. In the present study, the location and dosage relationships of this locus were examined, using translocation-generated segmental aneuploidy. The results indicate that Polycomb lies within interval 78C,D-79D, and that the locus is haplo-insufficient. Males hypoploid for this interval show meso- and metathoracic leg transformations, and both males and females show wing abnormalities. In addition, the legs of hypoploids of both sexes are shorter than those of wild-type flies, and show aberrancies of segmentation, chaetal number and distribution, and other morphological characteristics. Hypoploid flies do not express a homoeotic antennal-leg transformation, but the deficiency is associated with a Minute phenotype that is known to suppress this transformation in Polycomb flies; thus it cannot be ascertained whether the antennal-leg transformation is a haplo-insufficient phenotype. It is suggested that the expression of non-homoeotic pleiotropic effects provides a criterion for identifying homoeotic mutations that do not function directly in the establishment of determined states, but rather cause homoeosis indirectly. Polycomb is interpreted in this fashion, and it is suggested that the mutant syndrome may result from localized cell death.