Mosaic Formation Research Articles

The influence of different vegetation patches on the spatial distribution of nests and the epigeic activity of ants ( Lasius niger) on a spoil dump after brown coal mining (Czech Republic)

A study was carried out during 2001 on mine tailings in NW Bohemia aimed at describing the spatial patterns of nests distribution and epigeic activity of ants in relation to the vegetation mosaic. Lasius niger was the most abundant species of ant and its nest mounds were significantly more numerous in patches with sparse vegetation than inside dense Calamagrostis epigejos vegetation; this was particularly true for small and medium-sized nests. Small and medium nests also occurred more frequently near the edges of a given patch than in the center. Large and medium nests were randomly distributed in the area, whereas small nests had an aggregated distribution. Pitfall trapping reveal significantly higher activity of L. niger workers in tall and dense vegetation stands in comparison with low and sparse vegetation. This pattern was particularly pronounced during the peak of foraging activity in summer and was not so significant in spring or autumn. We expect that ant preferentially forage in shaded habitats during the summer months when bare soil may be too hot. The results indicated that nesting and foraging may differ in their microclimatic requirements and the formation of vegetation mosaics may be important to changes in the ant population during succession.

Spatial mosaic formation through frequency-dependent selection in Müllerian mimicry complexes

Although contemporary models of Müllerian mimicry have considered the movement of interfacial boundaries between two distinct mimetic forms, and even the possibility of polymorphisms in two patch systems, no model has considered how multiple forms of Müllerian mimics might evolve and be maintained over large geographical areas. A spatially explicit individual-based model for the evolution of Müllerian mimicry is presented, in which two unpalatable species are distributed over discrete cells within a regular lattice. Populations in each cell are capable of genetic drift and experience localized dispersal as well as frequency-dependent selection by predators. When each unpalatable prey species was introduced into a random cell and allowed to spread, then mimicry evolved throughout the system in the form of a spatial mosaic of phenotypes, separated by narrow “hybrid zones”. The primary mechanism generating phenotypic diversity was the occasional establishment of new mutant forms in unoccupied cells and their subsequent maintenance (and spread) through frequency-dependent selection. The mean number of discrete clusters of the same morph that formed in the lattice was higher the higher the intensity of predation, and higher the lower the dispersal rate of unpalatable prey. Under certain conditions the hybrid zones moved, in a direction dependent on the curvature of their interfacial boundaries. However, the mimetic mosaics were highly stable when the intensity of predation was high and the rate of prey dispersal was low. Overall, this model highlights how a stable mosaic of different mimetic forms can evolve from a range of starting conditions through a combination of chance effects and localized frequency-dependent selection.

Developmental improvement in the regularity and packing of mouse horizontal cells: Implications for mechanisms underlying mosaic pattern formation

The present investigation has sought to determine whether the population of retinal horizontal cells undergoes an increase in the precision of its mosaic patterning during postnatal development, and if so, whether this increase is compatible with three different mechanistic accounts of retinal mosaic formation. Horizontal cells were labeled with antibodies to neurofilaments or calbindin at different developmental stages, and then visualized in retinal wholemounts. Multiple fields were sampled from each retina to determine horizontal cell density, while the X-Y coordinates of each cell in a field were determined. An estimate of total horizontal cell number was calculated for each retina, while the Voronoi domain regularity index and the packing factor were computed for each field. Two strains of mice showing a two-fold difference in the size of their horizontal cell population in maturity were sampled, C57BL/6J and A/J. Horizontal cell number in C57BL/6J was approximately twice that observed in A/J at all postnatal stages, with neither strain showing an effect of age on horizontal cell number. In both strains, however, the Voronoi domain regularity index and the packing factor were significantly lower at P-1 relative to later developmental stages. These results show that accounts of mosaic formation proposing the selective death of irregularly positioned cells, or the periodic occurrence of fate-determining events, are insufficient to establish the final patterning achieved by horizontal cells. Rather, they support the hypothesis that tangential dispersion enhances mosaic patterning during postnatal development.

The surface-mosaic model in host-parasite relationships.

The dynamics of protein adsorption to a microbial surface could be of significance in host-parasite relationships because non-defense proteins might interfere with the binding of defense proteins. A surface mosaic of defense and non-defense proteins formed on the microbial surface could activate one of the tissue reactivity programs via a binary code (help or silence) generated by the adsorbed proteins. Understanding the mechanisms of the mosaic formation and its evolution might help to identify evasion mechanisms used by virulent microorganisms. This also provides a conceptual framework to design new strategies to control the infectious diseases they cause.

Mathematical model of retinal mosaic formation

The retina is one of the best examples of modular organisation in neural circuitry. This modular structure enables it to perform parallel processing. A mathematical model of the retina has been set up, focusing on mechanical features of retinal neurons and on the interaction and dendritic overlapping among retinal cells. The model focuses on the actions of local mechanical forces on the neuron’s cytoskeleton. The cytoskeleton is regarded as a structure in which elastic and rigid elements are combined according to the tensegrity concept. We have assumed that dendritic overlap takes place in such a way as to favour uniform retinal neurons’ distribution and that dendritic overlap is the only cause of neuron’s motion on the retinal surface. This overlap depends on the growth of the dendrites due to the cytoskeletic deformation. The results obtained are in agreement with experimental results that support the notion that local mechanical interaction and dendritic overlapping are capable to transform random cell distributions into regular mosaics.

Oxygen-dosage effect on the structure and composition of ultrathin NiO layers reactively grown on Ag(001)

NiO ultrathin films have been prepared under UHV conditions on Ag(001) substrate by metal deposition in ${\mathrm{O}}_{2}$ atmosphere. The films were prepared by deposition of the amount of metal correspondent to 1 ML (monolayer) of NiO in the presence of different oxygen-to-metal flux ratios, to investigate the oxygen dosage effect on the structure and composition of the growing layer and on the stoichiometry of Ni oxide. Thicker films (up to 20 ML) were also prepared. The structure has been monitored both in reciprocal and direct space by low-energy electron diffraction and primary-beam diffraction-modulated electron emission. Core-level x-ray photoemission spectroscopy has been used to study film chemistry and composition. At low coverage (below 2 ML) a dramatic dependence of structure and composition on the oxygen-to-nickel flux ratio has been observed. Low oxygen dosage induces a $(2\ifmmode\times\else\texttimes\fi{}1)$ reconstruction in the 1-ML films that evolves to a $(1\ifmmode\times\else\texttimes\fi{}1)$ phase as the dosage and/or the film thickness increases. In the layers prepared at low oxygen dosage a large fraction of metallic Ni coexists with the Ni oxide, but the oxidized fraction largely prevails for high oxygen dosage. The oxygen dosage during the growth also affects thicker films. The mosaic formation, which has been ascribed to misfit strain relaxation, is related to a low oxygen-to-nickel flux ratio.

A moving wave patterns the cone photoreceptor mosaic array in the zebrafish retina

In this paper, we describe the embryonic origin and patterning of the planar mosaic array of cone photoreceptor spectral subtypes in the zebrafish retina. A discussion of possible molecular mechanisms that might generate the cone mosaic array considers but discards a model that accounts for formation of neuronal mosaics in the inner retina and discusses limitations of mathematical simulations that reproduce the zebrafish cone mosaic pattern. The formation and organization of photoreceptors in the ommatidia of the compound eye of Drosophila is compared with similar features in the developing zebrafish cone mosaic, and a model is proposed that invokes spatiotemporally coordinated cell-cell interactions among cone progenitors to determine the identity and positioning of cone spectral subtypes.

Growth and surface morphology: epitaxial MgO films and the Ag(1,1,19) substrate

The morphology of Ag(1,1,19) and the growth of thin MgO layers on it, mostly at room temperature, was investigated by LEED spot profile analysis. These measurements have been supplemented by UPS, XPS and EELS data in order to clarify the electronic structure of the films. Our LEED analysis shows that only steps of monoatomic height are formed on the vicinal Ag surface with a geometric terrace width distribution. The 2 monolayer (ML) thick MgO film does not completely cover the substrate. It forms pyramidal clusters on the surface. Thicker films are closed, and the coexistence of a stepped MgO film with the formation of mosaics is observed. At 5 ML about half of the MgO film consists of mosaics and of non-tilted MgO-crystallites. Their steps are mostly one atom high, with edges that point on average both in non-polar and in polar directions. The influence of the substrate structure on the morphology of the MgO films is discussed.

Influence of cell fate mechanisms upon retinal mosaic formation: a modelling study.

Many types of retinal neurone are arranged in a spatially regular manner so that the visual scene is uniformly sampled. Several mechanisms are thought to be involved in the development of regular cellular positioning. One early-acting mechanism is the lateral inhibition of neighbouring cells from acquiring the same fate, mediated by Delta-Notch signalling. We have used computer modelling to test whether lateral inhibition might transform an initial population of undifferentiated cells into more regular populations of two types of differentiated cells. Initial undifferentiated cells were positioned randomly, subject only to a minimal distance constraint. Each undifferentiated cell then acquired either primary or secondary fate using one of several lateral inhibition mechanisms. Mosaic regularity was assessed using the regularity index and the packing factor. We found that for irregular undifferentiated mosaics, the arrangement of resulting primary (but not secondary) fate cells was more regular than in the initial undifferentiated population. However, for regular undifferentiated mosaics, no further increases in the regularity of the primary fate mosaics were observed. We have used this model to test the specific hypothesis that on- and off-centre retinal ganglion cells emerge from an initial, irregular undifferentiated population of ganglion cells. Lateral inhibition can subdivide an initially irregular population into two types of cell that are mildly regular. However, lateral inhibition alone is insufficient to produce mosaics of the same regularity as observed experimentally. Likewise, and in contrast to earlier reports, cell death alone is insufficient to match the regularity of experimental mosaics. We conclude that lateral inhibition can transform irregular distributions into regular mosaics, upon which subsequent processes (such as lateral cell movement or cell death) can further refine mosaic regularity.

Tn5041-like transposons: molecular diversity, evolutionary relationships and distribution of distinct variants in environmental bacteria.

A detailed study on the geographic distribution, molecular diversity and evolutionary relationships of 24 closely related variants of the Tn5041 transposon found among 182 mercury resistant environmental Gram-negative strains from the IMG-Hg Reference Collection is reported here. RFLP analysis, followed by the determination of partial DNA sequences, identified 14 distinct types of these transposons, which differed from each other by 1-7 single-event DNA polymorphisms. No polymorphisms were detected at the right arm of the transposons except an insertion of a new mobile DNA element carrying a mer operon (named the mer2 cassette) within the Tn5041 mer operon. According to the model presented here, the insertion occurred via homologous recombination with a circular form of the mer2 cassette. A total of 8 point mutations, 1 internal deletion, 2 end-involving deletions, 3 mosaic regions and 2 insertions were detected at the left arm of the transposons. The insertions were a transposon closely related to Tn21 but lacking the integron and a new group II intron (named INT5041C). Inspection of the geographic distribution of the Tn5041 variants suggested that at least three long-distance waves of dissemination of these variants had occurred, accompanied by homologous recombination between different Tn5041 lineages. Movements of circular DNAs by homologous recombination as a source of mosaic genes and new mer genes, and formation of unusual mosaics ending or beginning at the Tn5041 att site are discussed.

Landscape structure, habitat fragmentation, and the ecology of insects

Landscape structure, habitat fragmentation, and the ecology of insects

Pattern Formation of the Cone Mosaic in the Zebrafish Retina: A Cell Rearrangement Model

In fish retinas, cone photoreceptor cells are arranged in two-dimensional regular patterns, called cone mosaics. In the zebrafish retina, four subtypes of cone cells, which are maximally sensitive to different wavelengths of light, appear in quasi-periodic patterns. The pattern formation mechanism is unknown. Here, I develop a mathematical model to examine whether cell adhesion can explain the formation of the zebrafish mosaic. I assume that the movement of differentiated cells is responsible for generating the pattern, and that the movement rate is modified by cell adhesion. The pattern is formed if the magnitudes of cell adhesion between cell types are chosen appropriately. I determine the conditions of cell adhesion for generating the pattern. I also compare this cell rearrangement model with a previously studied model in which the pattern is formed by transitions of cell fate. The condition for obtaining the focal pattern is looser in the cell rearrangement model than in the fate transition model.

The role of tangential dispersion in retinal mosaic formation.

Individual types of retinal nerve cell are spaced across the retina in an orderly manner, ensuring a uniform sampling of the visual field. This regularity in cellular spacing has been commonly attributed to fate determination mechanisms operating around the time of cell birth, an hypothesis presuming that the position of a nerve cell is fixed within the plane of the retina from the time of its determination. At odds with this view, recent results from X-inactivation mosaic mice indicate that certain classes of retinal nerve cell, those known to form orderly mosaics in the adult retina, disperse tangentially during development. Furthermore, studies defining the spatial characteristics of developing and mature retinal mosaics suggest that cell-cell interactions around the time of morphological differentiation lead to mutual repulsion. Modelling studies in turn show that nothing more than a simple minimal spacing rule between neighboring cells of the same type is sufficient for the creation of the global patterning observed in biological retinal mosaics. For some cell types, the size of this "exclusion zone" surrounding individual cells is shown to be an intrinsic characteristic of each cell type, invariant across the retina, and accounting for the variation in mosaic regularity across changes in cell density. These results show how short-distance movements driven by intercellular interactions at the local level may mediate the emergence of the global patterning characteristic of retinal mosaics during development.

Growth of NiO and MgO Films on Ag(100)

The growth morphology of NiO and MgO films deposited in the temperature range 300–350 K has been studied by Spot Profile Analysis of Low Energy Electron Diffraction (SPA-LEED) patterns. The oxide films grow layer-by-layer. They are pseudomorphic during the initial stages. Relaxation via the injection of misfit dislocations with {110} glide planes occurs for coverages larger than a critical coverage (Θ c ≈5 ML for NiO, Θ c =1–2 ML for MgO). The relaxation process leads to the formation of mosaics on the film surface. The tilt angle of the mosaics decreases with increasing oxide thickness due to minimisation of the deformation energy.

Gradual and partial loss of corner cone-occupied area in the retina of rainbow trout

Several studies have indicated that the rainbow trout ( Oncorhynchus mykiss) loses ultraviolet (UV) sensitivity and the associated UV-sensitive corner cones when the animal transforms from a small (parr) juvenile to a larger, silver-coloured, smolt. Similar changes supposedly take place when parr juveniles are treated with thyroid hormone (T 4) or retinoic acid. In contrast to previous investigations, this study shows that parr juveniles lack corner cones throughout the lower half of the ventral retina, suggesting that corner cones cease to be incorporated into the ventral retina some time after hatching. This uneven incorporation of corner cones across the retina, when combined with retinal growth, creates a progressively smaller area of lower retina occupied by corner cones. Because in previous studies, the stimulating illumination was directed primarily at the ventral retina, the reported age-dependent changes in UV or polarization sensitivities can be explained by differences in the area of corner cones that was illuminated, and not necessarily by a loss of corner cones. This study also shows: (1) that the double cones from non-ventral mosaics of parr rainbow trout may change in cross-sectional shape, altering the mosaic formation from a square to a row, (2) the existence of a ‘pure’ (non-changing) square mosaic in the ventral retina, and (3) a potential method, based on differential staining of cone nuclei, to classify paired cones into double or twin cones.

Retinal development of West Australian dhufish, Glaucosoma hebraicum

An investigation of retinal specializations was carried out in larval and juvenile dhufish, Glaucosoma hebraicum (Glaucosomidae, Teleostei). The development of photoreceptors and formation of the retinal mosaic was followed by light and electron microscopy. At hatching the eye was undifferentiated. Cone photoreceptors were present by day 3 posthatch (dph), when exogenous feeding began. Single and multiple cones were present in a row arrangement from 3 dph to 20 dph, when the first rod nuclei were observed. Between 20 dph and approximately 3 months posthatch (mph), the row arrangement was replaced by a square mosaic of four double cones surrounding a single cone, and the cones increased in size, with the outer segments reaching up to 30 microm in length. During the period of spatial rearrangement, triple cones were often observed. From their first appearance, rod photoreceptors were added rapidly. Investigation of ganglion cell topography in 3-mph fish that had attained the adult-like square photoreceptor mosaic was carried out using retinal wholemounts. The highest densities of neurones in the ganglion cell layer were in temporal retina but no well-defined area centralis was observed. Microspectrophotometric measurements of the visual pigments within the outer segments of the photoreceptors of 3-mph fish revealed double cones with identical absorption spectra in each member of the outer segment, and the wavelength of maximum absorption (lambda(max)) located at 522 nm. Single cones were found to possess a visual pigment with lambda(max) at 460 nm and rods with a lambda(max) of 498 nm. The results imply that the larvae and juveniles are adapted for survival in coastal waters and may be active in relatively low light levels from early stages of development.

Cell‐type specific dendritic contacts between retinal ganglion cells during development

The extent of a neuron's dendritic field defines the region within which information is processed. The dendritic fields of functionally distinct ON and OFF center retinal ganglion cells (RGCs) form separate mosaics across the retina. Within each mosaic, neighboring dendritic fields overlap by a constant amount, sampling the visual field with the appropriate coverage. Contact-mediated lateral inhibition between neighboring RGCs has long been thought to regulate both the extent and overlap of dendritic fields during development. Here we show that dendro-dendritic contact exists between developing RGCs and occurs in a manner that would regulate the formation of ON and OFF mosaics separately. Dye-filled neighboring ON and OFF ferret alpha RGCs were reconstructed using multiphoton microscopy. At all neonatal ages examined, we observed dendro-dendritic contacts between RGCs of the same sign (ON/ON; OFF/OFF), but never between cells of opposite signs (ON/OFF). Terminal dendrites of one cell often touched a dendrite of its neighbor as they intersected. In some instances, the distal dendrite of one cell formed a fascicle with the proximal process of its neighbor. Alpha cells did not form contacts with neighboring beta cells of the same sign. Together, these observations suggest that dendro-dendritic contact between RGCs is cell-type specific. Dendritic contacts were observed even before the alpha cell arbors were completely stratified, suggesting that cell-cell recognition may take place early in their development. For each cell type, the relative overlap of dendritic fields was constant with age, despite a two-fold increase in field area. We suggest that dendro-dendritic contacts may be sites of intercellular signaling that could regulate local extension of dendrites to maintain the relative overlap of RGCs within a mosaic during development.

Factor analysis and countermeasure for decrease of anthocyanin mosaic formation in bentgrass green

Factor analysis and countermeasure for decrease of anthocyanin mosaic formation in bentgrass green

Factor analysis and countermeasure for decrease of anthocyanin mosaic formation in bentgrass green

Factor analysis and countermeasure for decrease of anthocyanin mosaic formation in bentgrass green

Factor analysis and countermeasure for decrease of anthocyanin mosaic formation in bentgrass green

Factor analysis and countermeasure for decrease of anthocyanin mosaic formation in bentgrass green