Multiple roles of the gene zinc finger homeodomain-2 in the development of the Drosophila wing

The gene zfh2 and its human homolog Atbf1 encode huge molecules with several homeo- and zinc finger domains. It has been reported that they play important roles in neural differentiation and promotion of apoptosis in several tissues of both humans and flies. In the Drosophila wing imaginal disc, Zfh2 is expressed in a dynamic pattern and previous results suggest that it is involved is proximal–distal patterning. In this report we go further in the analysis of the function of this gene in wing development, performing ectopic expression experiments and studying its effects in genes involved in wing development. Our results suggest that Zfh2 plays an important role controlling the expression of several wing genes and in the specification of those cellular properties that define the differences in cell proliferation between proximal and distal domains of the wing disc.     

Metamorphosis ofHydractinia echinata Insights into pattern formation in Hydroids

Summary Hydractinia echinata is a marine, colony-forming coelenterate. Fertilized eggs develop into freely swimming planula larvae, which undergo metamorphosis to a sessile (primary) polyp. Metamorphosis can be triggered by means of certain marine bacteria and by Cs⁺. Half a day after this treatment a larva will have developed into a polyp. The induction of metamorphosis can be prevented by addition of inhibitor I, a substance partially purified from tissue ofHydra. The larvae ofH. echinata also appear to contain this substance. Inhibitor I appliedafter the onset of metamorphosis blocks its continuation as long as it remains in the culture medium. Cs⁺ applied within the same period of time also blocks the continuation of metamorphosis. However, these two agents have opposite effects on the body pattern of the resultant polyps. The experiments indicate that application of Cs⁺ triggers the generation of the pre-pattern. Inhibitor I appears to be a factor of this prepattern. A model is proposed which describes the basic features of head and foot/stolon formation not only forHydractinia but also for other related hydroids.     

The role of trans-membrane signal transduction in turing-type cellular pattern formation

The Turing mechanism (Phil. Trans. R. Soc. B 237 (1952) 37) for the production of a broken spatial symmetry in an initially homogeneous system of reacting and diffusing substances has attracted much interest as a potential model for certain aspects of morphogenesis (Models of Biological Pattern Formation, Academic Press, London, 1982; Nature 376 (1995) 765) such as pre-patterning in the embryo. The two features necessary for the formation of Turing patterns are short-range autocatalysis and long-range inhibition (Kybernetik 12 (1972) 30) which usually only occur when the diffusion rate of the inhibitor is significantly greater than that of the activator. This observation has sometimes been used to cast doubt on applicability of the Turing mechanism to cellular patterning since many messenger molecules that diffuse between cells do so at more-or-less similar rates. Here we show that Turing-type patterns will be able to robustly form under a wide variety of realistic physiological conditions though plausible mechanisms of intra-cellular chemical communication without relying on differences in diffusion rates. In the mechanism we propose, reactions occur within cells. Signal transduction leads to the production of messenger molecules, which diffuse between cells at approximately equal rates, coupling the reactions occurring in different cells. These mechanisms also suggest how this process can be controlled in a rather precise way by the genetic machinery of the cell.     

Physiologically induced color-pattern changes in butterfly wings: mechanistic and evolutionary implications

A mechanistic understanding of the butterfly wing color-pattern determination can be facilitated by experimental pattern changes. Here I review physiologically induced color-pattern changes in nymphalid butterflies and their mechanistic and evolutionary implications. A type of color-pattern change can be elicited by elemental changes in size and position throughout the wing, as suggested by the nymphalid groundplan. These changes of pattern elements are bi-directional and bi-sided dislocation toward or away from eyespot foci and in both proximal and distal sides of the foci. The peripheral elements are dislocated even in the eyespot-less compartments. Anterior spots are more severely modified, suggesting the existence of an anterior-posterior gradient. In one species, eyespots are transformed into white spots with remnant-like orange scales, and such patterns emerge even at the eyespot-less "imaginary" foci. A series of these color-pattern modifications probably reveal "snap-shots" of a dynamic morphogenic signal due to heterochronic uncoupling between the signaling and reception steps. The conventional gradient model can be revised to account for these observed color-pattern changes.     

Genetic analysis of the wing vein pattern of Drosophila

Summary Of the many mutations known to affect the wing vein pattern we have selected the most extreme in 29 genes for study. Their phenotype can be classified in two major classes: lack-of-veins and excess-of-veins, and in several internally coherent groups. The study of multiple mutant combinations, within groups and between groups, reveals several genetic operations at work in the generation of the vein pattern. The finding that some of these mutations also affect cell proliferation in characteristic ways has prompted a generative model of wing morphogenetic and pattern formation based on cell behaviour properties defined by the corresponding wild-type genes.     

Bud formation inHydra: Inhibition by an endogenous morphogen

Summary From crude extracts ofHydra tissue a substance has been purified which prevents or retards the asexual reproduction by budding. The molecular weight is in the range of 300 to 1000 daltons. Inhibition of bud formation can be observed with concentrations equivalent to the extract from one hydra per 4 ml, that is, to a more than 10,000-fold dilution of the initial crude extract of a hydra. The purified inhibitor is active at a concentration of less than 10^–8 M.Most of the inhibitor present inHydra is bound to cells. Within the cells the substance is mainly bound to particulate structures which sediment at 10,000 g. Its concentration is highest in the hypostomal region and decreases in the direction of the tentacles and peduncle. A second, lower, peak has been found in the basal disc. Treatment of the animals with a toxic agent (nitrogen mustard) which depletes the animal of interstitial cells, nematocytes and nematoblasts excludes the possibility that the inhibitor is present to any great extent in these cells. In conjunction with cell separation experiments by centrifugation of fixed cells in suspension, these results indicate that nerve cells are the most likely sites of storage of the inhibiting substance, although epithelial cells are not excluded as sources for the inhibitor.     

Modeling the bicoid gradient: diffusion and reversible nuclear trapping of a stable protein

The Bicoid gradient in the Drosophila embryo provided the first example of a morphogen gradient studied at the molecular level. The exponential shape of the Bicoid gradient had always been interpreted within the framework of the localized production, diffusion, and degradation model. We propose an alternative mechanism, which assumes no Bicoid degradation. The medium where the Bicoid gradient is formed and interpreted is very dynamic. Most notably, the number of nuclei changes over three orders of magnitude from fertilization, when Bicoid synthesis is initiated, to nuclear cycle 14 when most of the measurements were taken. We demonstrate that a model based on Bicoid diffusion and nucleocytoplasmic shuttling in the presence of the growing number of nuclei can account for most of the properties of the Bicoid concentration profile. Consistent with experimental observations, the Bicoid gradient in our model is established before nuclei migrate to the periphery of the embryo and remains stable during subsequent nuclear divisions.     

Electron Microscopy and Spectroscopical Studies on the Coloured Patches on the Wing of a Butterfly,Graphium sarpedon (Lepidoptera: Papillionidae) With Reference to Their Photobiological and Electrical Properties

The surface ultra-structural features of the coloured patches on the wing of a butterfly Graphium sarpedon have been studied with the help of scanning electron microscopy. Comparisons have been made between the dark brown area and the light green patches of the wing. A diffraction grating pattern with 15 lines per μm² with a uniform spacing of about 1 μm is present in the light green patches. A slightly coarser grating is present on the dark brown area, which constitutes the major portion of the wing. Sensilla chaetica was found on both the light green and dark brown area. A special type of sensilla trichodea with a big socket and some elongated projections were localized only on the light green patches. This region of the wing also contains some spherical structures with a diameter of 0.5 to 0.6 μm. The infra-red spectroscopy has revealed some differences in the nature and position of the peaks in the low-energy region in the dark brown area and the light green patches. The atomic absorption spectroscopy also shows qualitative as well as quantitative differences in the inorganic set up of the two regions. The electron spin resonance spectroscopy reveals the presence of a peak in the dark brown region only, indicating the presence of free radical in it. The differences observed in the ultra-structural and spectroscopical features, and also in the inorganic components of the two regions, are discussed in relation to their physical and physiological properties.     

Genetic variation of density responses in relation to wing polymorphism in the oriental chinch bug,Cavelerius saccharivorus Okajima (Heteroptera: Lygaeidae)

Responses to nymphal density in the determination of wing form were compared between the offspring from brachypter x brachypter crosses and those from macropter x macropter crosses of the oriental chinch bug, Cavelerius saccharivorus. In the offspring from crosses between macropters, there was a strong tendency for macropters to increase to a rather high level with increasing nymphal density in both sexes. In contrast to this, in the offspring from crosses between brachypters, the appearance rate of macropters attained a maximum value in moderately crowded conditions and conversely decreased to a lower level in more crowded conditions in both sexes. Thus density response patterns concerning the determination of wing form were quite different between the offspring from different crosses in the wing form, indicating that there is a genetic basis underlying wing polymorphism in this species. As for the body size of emerged adults, macropters tended to be larger than brachypters in the same crowded condition. Moreover, the rate of decrease of body size with nymphal density was lower in the offspring from crosses between macropters than in the offspring from crosses between brachypters. This indicated that the former offspring are more tolerant of nymphal crowding than the latter. The difference in such a tolerance against nymphal crowding between the offspring from different crosses was considered to be related to the difference in the appearance of macropters in the crowded conditions between them.     

Programmed cell death at the periphery of the pupal wing of the butterfly,Pieris rapae

The outline of the adult wing of lepidopteran insects (butterflies and moths) emerges as a result of disappearance of a group of cells at the periphery of the pupal wing. Histological observation of the pupal wing of Pieris rapae showed that, just after apolysis of the wing epithelium from the pupal cuticle, there occurs a rapid and localized decrease of the number of cells at the periphery of the wing. This decrease occurs through cell death, which lasts 1–1.5 days at 20°C. Dying cells lose contact with the neighbouring cells and show condensation of chromatin and cytoplasm. They then appear to be phagocytosed by neighbouring epithelial cells or discharged through the basal surface of the epithelium into the lumen within the wing and taken up by phagocytes. Fragmentation of DNA in the nuclei was detected in the dead cells or their debris. These results indicate that programmed cell death in the lepidopteran wing proceeds through a mechanism closely similar to that of apoptosis in the vertebrate.     

Dpp and Gbb exhibit different effective ranges in the establishment of the BMP activity gradient critical for Drosophila wing patterning

Morphogen gradients ensure the specification of different cell fates by dividing initially unpatterned cellular fields into distinct domains of gene expression. It is becoming clear that such gradients are not always simple concentration gradients of a single morphogen; however, the underlying mechanism of generating an activity gradient is poorly understood. Our data indicate that the relative contributions of two BMP ligands, Gbb and Dpp, to patterning the wing imaginal disc along its A/P axis, change as a function of distance from the ligand source. Gbb acts over a long distance to establish BMP target gene boundaries and a variety of cell fates throughout the wing disc, while Dpp functions at a shorter range. On its own, Dpp is not sufficient to mediate the low-threshold responses at the end points of the activity gradient, a function that Gbb fulfills. Given that both ligands signal through the Tkv type I receptor to activate the same downstream effector, Mad, the difference in their effective ranges must reflect an inherent difference in the ligands themselves, influencing how they interact with other molecules. The existence of related ligands with different functional ranges may represent a conserved mechanism used in different species to generate robust long range activity gradients.     

Bicoid mRNA diffusion as a mechanism of morphogenesis in Drosophila early development

We show that mRNA diffusion is the main morphogenesis mechanism that consistently explains the establishment of Bicoid protein gradients in the embryo of Drosophila, contradicting the current view of protein diffusion. Moreover, we show that if diffusion for both bicoid mRNA and Bicoid protein were assumed, a steady distribution of Bicoid protein with a constant concentration along the embryo would result, contradicting observations.     

Rapid responses to high temperature and desiccation but not to low temperature in the freeze tolerant sub-Antarctic caterpillar Pringleophaga marioni (Lepidoptera,Tineidae)

A broad definition of rapid cold hardening (RCH) is that it is the process whereby insects increase their survival of a sub-zero temperature after a brief (h) pre-exposure to a less severe low temperature. The effects of various pre-treatments on survival of two h at -7.9 degrees C were investigated in the freeze tolerant sub-Antarctic caterpillar Pringleophaga marioni (Lepidoptera: Tineidae), the first time RCH has been investigated in a freeze tolerant arthropod. All caterpillars froze when exposed to -7.9 degrees C, and none of the low temperature pre-treatments (-5, 0, 5 and 15 degrees C, as well as -5 degrees C and 0 degrees C with a delay before freezing) nor slow cooling (0.1 degrees C/min) elicited any improvement in survival of -7.9 degrees C as compared to controls. However, high temperature treatments (25, 30 and 35 degrees C), desiccation and acclimation for 5 days at 0 degrees C did result in significant increases in survival of the test temperature, possibly as a result of heat shock protein production. Haemolymph osmolality was elevated only by the 35 degrees C pre-treatment. It is suggested that the unpredictable environment of Marion Island means that P. marioni must always be physiologically prepared to survive cold snaps, and that this year-round cold hardiness therefore supersedes a rapid cold hardening response.     

Formation and deformation of patterns through diffusion

Simulating various patterns exhibited on biological forms with mathematical models has become an important supplement to theoretical biology. Models based on a certain mechanism are intended to provide explanations to the formation of a basic pattern. However, in real phenomena, among a basic pattern there always exist some difference between any two individuals. Such differences are consequences of environmental factors posed during the developmental processes. These factors, such as temperature, affect the diffusion rates of corresponding morphogenes which, in turn, alter a basic pattern to certain extent. We provide, in this paper, a quantitative characterization of this effect for a class of reaction-diffusion models.Mathematically, we study the emergence of stationary patterns and their dependence on diffusion rates for this class of models (RD-equations) with no-flux boundary conditions. The results are generalized to systems with homogeneous Dirichlet boundary conditions when the kinetic terms are odd functions. Through an analysis of the phase dynamics, we show that the deformation of stationary patterns, as the diffusion rates change, is governed by the variation of certain plane curves in the phase space. A constructive proof is given which shows explicitly how to obtain such curves.Applications of this study are illustrated with three model examples. We use these models to explain the biological implications of the mathematical features we investigated. Results from computer simulations are presented and compared with physical patterns.