Structure of Flower in Arabidopsis thaliana: Spatial Pattern Formation

Morphological analysis of flowers was carried out in Arabidopsis thaliana wild type plants and agamous and apetala2 mutants. No direct substitution of organs takes place in the mutants, since the number and position of organs in them do not correspond to the structure of wild type flower. In order to explain these data, a notion of spatial pattern formation in the meristem was introduced, which preceded the processes of appearance of organ primordia and formation of organs. Zones of acropetal and basipetal spatial pattern formation in the flower of wild type plants were postulated. It was shown that the acropetal spatial pattern formation alone took place in agamous mutants and basipetal spatial pattern formation alone, in apetala2 mutants. Different variants of flower structure are interpreted as a result of changes in the volume of meristem (space) and order of spatial pattern formation (time).     

Distinct roles of Shh and Fgf signaling in regulating cell proliferation during zebrafish pectoral fin development

Background  

Cell proliferation in multicellular organisms must be coordinated with pattern formation. The major signaling pathways directing pattern formation in the vertebrate limb are well characterized, and we have therefore chosen this organ to examine the interaction between proliferation and patterning. Two important signals for limb development are members of the Hedgehog (Hh) and Fibroblast Growth Factor (Fgf) families of secreted signaling proteins. Sonic hedgehog (Shh) directs pattern formation along the anterior/posterior axis of the limb, whereas several Fgfs in combination direct pattern formation along the proximal/distal axis of the limb.     

Chemically Based Mathematical Model for Development of Cerebral Cortical Folding Patterns

The mechanism for cortical folding pattern formation is not fully understood. Current models represent scenarios that describe pattern formation through local interactions, and one recent model is the intermediate progenitor model. The intermediate progenitor (IP) model describes a local chemically driven scenario, where an increase in intermediate progenitor cells in the subventricular zone correlates to gyral formation. Here we present a mathematical model that uses features of the IP model and further captures global characteristics of cortical pattern formation. A prolate spheroidal surface is used to approximate the ventricular zone. Prolate spheroidal harmonics are applied to a Turing reaction-diffusion system, providing a chemically based framework for cortical folding. Our model reveals a direct correlation between pattern formation and the size and shape of the lateral ventricle. Additionally, placement and directionality of sulci and the relationship between domain scaling and cortical pattern elaboration are explained. The significance of this model is that it elucidates the consistency of cortical patterns among individuals within a species and addresses inter-species variability based on global characteristics and provides a critical piece to the puzzle of cortical pattern formation.     

A comparative ultrastructural analysis of exine pattern development in wild-typeArabidopsis and a mutant defective in pattern formation

Summary In order to identify factors necessary for the establishment of the reticulate pollen wall pattern, we have characterized a T-DNA tagged mutant ofArabidopsis thaliana that is defective in pattern formation. This study reports the results of an ultrastructural comparison of pollen wall formation in the mutant to wall development in wild-type plants. Pollen wall development in the mutant parallels that of wild-type until the early tetrad stage. At this point in wild-type plants, the microspore plasma membrane assumes a regular pattern of ridges and valleys. Initial sporopollenin deposition occurs on the ridges marking the beginning of probacula formation. In contrast, the plasma membrane in the mutant appears irregular with flattened protuberances and rare invaginations. As a result, the wild-type regular pattern of ridges and valleys is not formed. Sporopollenin is randomly deposited on the plasma membrane and aggregates on the locule wall; it is not anchored to the membrane. Our finding that the mutation blocks the normal invagination of the plasma membrane and disrupts the proper deposition of sporopollenin during wall formation suggests that the mutation could be in a gene responsible for pattern formation. These results also provide direct evidence that the plasma membrane plays a critical role in the establishment of the pollen wall pattern.     

Pigment pattern formation in the larval salamander Ambystoma maculatum

As part of an ongoing comparative study of pigment patterns and their formation in embryos and larvae of ambystomatid salamanders, Ambystoma maculatum from two differnt populations, one in the northern (New York) and one in the central (Tennessee) United States, were investigated. Scanning electron microscopy was used to study early neural crest development. Light microscopy in combination with markers for the two pigment cell types (xanthophores and melanophores) made it possible to follow pigment cell migration before the pigment cells were fully differentiated. A bilateral pigment pattern consisting of two horizontal melanophore stripes surrounding an interstripe area populated by xanthophores formed in the larvae. In both populations, some variation was present in the form of a continuum ranging from clear horizontal stripes to extreme cases with a random pattern. Unlike the other ambystomatids that have been investigated, the neural crest cells in A. maculatum do not form aggregates and no vertical bars are formed. Instead, both the pattern and its formation are very similar to what has been reported for salamandrids. If pattern formation mechanisms can act as developmental constraints we would expect the A. maculatum pattern to be the primitive condition in the Ambystomatidae, using the Salamandridae as the outgroup. There is no strong support for this when aggregate formation is used as a character and mapped onto phylogenies for the group. The aggregate formation mechanism, and the pigment pattern that it leads to, have most likely been secondarily lost in A. maculatum. © 1993 Wiley-Liss, Inc.     

Quantitative analysis of major axis development inViburnum dilatatum andV. wrightii (Caprifoliaceae)

The architectural development ofViburnum dilatatum andV. wrightii was investigated quantitatively. In both species, the major axis is developed from terminal buds of vegetative shoots and from axillary buds on the most distal nodes of reproductive shoots. The architecture of the two species is formed mainly by four branching patterns: a monopodial pattern (M), a sympodial pattern producing a pair of opposite daughter shoots (SP), a sympodial pattern producing a single daughter shoot (SS), and a pattern terminated with a dormant or dead bud (D). In the process of the architecture formation, four successive stages are recognized: 1) height growth, where the M pattern is dominant; 2) crown formation, in addition to the M pattern, the SP pattern occurs frequently; 3) crown expansion, the M and SP patterns are also frequent; 4) over mature, the M pattern becomes dominant again. These four stages are common to the two species, butViburnum wrightii proceeds with the crown formation stage more rapidly and stays in the crown expansion stage for a longer time thanV. dilatatum. The crown ofViburnum wrightii is thus more branched than that ofV. dilatatum.     

Pattern formation during early amphibian development: Embryogenesis in inverted anuran and urodele eggs

Early embryogenesis was monitored in Xenopus, Rana (anurans), and Ambystoma (urodele) eggs which were inverted at various times between fertilization and first cleavage. The pattern of cleavage furrow formation, site of involution, and extent of organogenesis were observed. In several instances, pattern formation was dramatically altered. The small/large blastomere pattern was, for example, reversed in some inverted embryos. Developmental arrest at early organogenesis usually followed pattern reversal. By employing a series of tissue transplantations, it was possible to establish that the activity of the primary embryonic organizer of inverted embryos was diminished drastically. The developmental competence of the prospective ectoderm of inverted embryos was, however, reversed. Incomplete organogenesis in inverted embryos is therefore probably due to either abnormal mesoderm formation or defective tissue interactions.     

Floral patterning.

In recent years, flower development has emerged as a model system for studying pattern formation in plants. Homeotic mutants with an altered pattern of floral organs have been found in many species. Recently, several of the floral homeotic genes have been isolated; and the mechanisms underlying pattern formation during flower development are beginning to be elucidated.     

Turing patterns in a predator–prey model with seasonality

Many ecological systems show striking non-homogeneous population distributions. Diffusion-driven instabilities are commonly studied as mechanisms of pattern formation in many fields of biology but only rarely in ecology, in part because some of the conditions seem quite restrictive for ecological systems. Seasonal variation is ubiquitous in temperate ecosystems, yet its effect on pattern formation has not yet been explored. We formulate and analyze an impulsive reaction–diffusion system for a resource and its consumer in a two-season environment. While the resource grows throughout the ‘summer’ season, the consumer reproduces only once per year. We derive conditions for diffusion-driven instability in the system, and we show that pattern formation is possible with a Beddington–DeAngelis functional response. More importantly, we find that a low overwinter survival probability for the resource enhances the propensity for pattern formation: diffusion-driven instability occurs even when the diffusion rates of prey and predator are comparable (although not when they are equal).

     

Spatial organization of transport domains and subdomain formation in the plasma membrane of Chara corallina

Pattern formation mechanisms in developing organisms determine cellular differentiation and function. However, the components that interact during the manifestation of a spatial pattern are in general unknown. Characean algae represent a model system to study pattern formation. These algae develop alternating acid and alkaline transport domains that influence the pattern of growth. In the present study, it will be demonstrated that a diffusion mechanism is implicated in acid and alkaline domain formation and this growth pattern. Experiments on the characean growth pattern were performed that resulted in pronounced, however, unpredictable modifications in the original pattern. A major component involved in this pattern-forming mechanism emerged from the nonlinear kinetics of the H⁺-ATPase that is located in the plasma membrane of these algae. Based on these kinetics, a mathematical model was developed and numerically analyzed. As a result, the contribution of a diffusional component to the characean acid/alkaline pattern appeared most likely.This work was supported by the Deutsche Forschungsgemeinschaft (grant #571 1/1) to JF.     

Pigment pattern formation in larval ambystomatid salamanders: Ambystoma talpoideum,Ambystoma barbouri,and Ambystoma annulatum

The pigment pattern formation in embryos and larvae of three ambystomatid salamanders was investigated in an evolutionary context. Early neural crest development was studied with scanning electron microscopy. Pigment cell migration and pattern formation were investigated at the light microscopy level with markers that labelled the two pigment cell types specifically before they were fully differentiated. In all three species, the pigment pattern formation started when xanthophores that had first formed aggregates in the crest migrated ventrally. As previously observed in other species, vertical bars always form by a mechanism involving earlier onset of migration in melanophores than in xanthophores and aggregate formation in the crest. In Ambystoma talpoideum and A. annulatum, a pattern of vertical chromatophore bars formed, which was superimposed on a pattern of horizontal stripes. In Ambystoma barbouri, the tendency to form this pattern was obscured by the high density of melanophores. It is suggested that variation among species may be due to differences in the chromatophore density and in the melanophore/xanthophore ratio. Mapping of the evolution of vertical bars onto existing phylogenies for the group was confounded by controversies about how to interpret the phylogenetic data. On the phylogeny that takes all the available evidence into account, there are two equally parsimonious mappings. Vertical bars have either evolved only once and been lost twice, or evolved twice and been lost once. This rather conservative pattern can be explained both as an effect of stabilizing selection and as a result of developmental constraints. © 1994 Wiley-Liss, Inc.     

Ultraviolet irradiation initiates ectopic foot formation in regenerating hydra and promotes budding

We have studied the effects of ultraviolet-C (UVC) and Ultraviolet-B (UVB) on growth and pattern formation inPelmatohydra oligactis. UVC brings about a significant increase in budding in intact hydra while UVB does not exhibit such an effect. Excessive budding could be a response for survival at wavelengths that damage biological tissues. If the head or base piece of a bisected hydra is irradiated and recombined with the unirradiated missing part, regeneration proceeds normally indicating that exposure of a body part with either an intact head or foot to UVC does not influence pattern formation. Most significantly, in the middle piece, but not in the head or the base piece of a trisected hydra, UVC leads to initiation of ectopic feet formation in almost one third of the cases. Thus, UV irradiation interferes with pattern formation in regenerating hydra, possibly by changing positional values, and promotes budding in intact hydra. This is the first report on induction of ectopic feet formation by UV in regenerating hydra and opens up the possibility of using UV irradiation as a tool to understand pattern formation in the enigmatic hydra     

Circadian Rhythm of Pattern Formation in Populations of a Free-Swimming Organism,Tetrahymena*

SYNOPSIS. In a preliminary search for a phototactic response in Tetrahymena, we discovered instead an autotactic phenomenon of pattern formation produced by motile free-swimming cells in a dense culture. This pattern of cell aggregation is remarkably similar to the Bénard-cell patterns of classical physical chemistry, and reminiscent of the streaming patterns of motile microorganisms. In a study of light effects on the rate of pattern formation, we found that it has an endogenous circadian component, and is strongly dependent on cell concentration.     

Type specification and spatial pattern formation of floral organs: A dynamic development model

A mathematical model simulating spatial pattern formation (positioning) of floral organs is proposed. Computer experiment with this model demonstrated the following sequence of spatial pattern formation in a typical cruciferous flower: medial sepals, carpels, lateral sepals, long stamens, petals, and short stamens. The positioning was acropetal for the perianth organs and basipetal for the stamens and carpels. Organ type specification and positioning proceed non-simultaneously in different floral parts and organ type specification goes ahead of organ spatial pattern formation. Computer simulation of flower development in several mutants demonstrated that the AG and AP2 genes determine both organ type specification and formation of the zones for future organ development. The function of the AG gene is to determine the basipetal patterning zones for the development of the reproductive organs, while the AP2 gene maintains proliferative activity of the meristem establishing the acropetal patterning zone for the development of the perianth organs.     

Genetic analysis of axon pattern formation in the embryonic CNS ofDrosophila

The major axon tracts in the embryonic CNS ofDrosophila are organised in a simple, ladder-like pattern. Each neuromere contains two commissures which connect the contra-lateral sides and two longitudinal connectives which connect the different neuromeres along the anterior-posterior axis. The commissures form in close association with only few cells located at the CNS midline. The formation of longitudinal connectives depends in part on the presence of specific lateral glial cells. To unravel the genes underlying the formation of the embryonic CNS axon pattern, we conducted a saturating F2 EMS mutagenesis, screening for mutations, which disrupt this process. The analyses of the identified mutations lead to a simple sequential model on axon pattern formation in embryonic CNS.     

Pattern Formation by Competition: A Biological Example

We present a simple model based on a reaction-diffusion equation to explain pattern formation in a multicellular bacterium (Streptomyces). We assume competition for resources as the basic mechanism that leads to pattern formation; in particular we are able to reproduce the spatial pattern formed by bacterial aerial mycelium in the case of growth in minimal (low resources) and maximal (large resources) culture media.     

Cell division and morphogenesis in angiosperm embryogenesis

Angiosperm embryogenesis generates the basic body organization of flowering plants. The underlying processes of pattern formation, which establishes the diversity of position-dependent cell fates, and morphogenesis, which brings about the shape of the embryo, may not only involve intercellular communication and controlled cell expansion but also non-random cell divisions. Genetic analysis ofArabidopsisembryogenesis which displays a large invariant pattern of cell divisions suggests that unequal cell divisions segregate cell fates and are thus involved in pattern formation whereas other oriented cell divisions and differential mitotic rates reflect patterning and rather play a role in morphogenesis.     

Patterns of adventitious root formation in English ivy

Adventitious root formation by debladed petiole cuttings of English ivy (Hedera helix L.) proceeds via a direct rooting pattern for the easy-to-root juvenile phase, while the difficult-to-root mature phase roots through an indirect rooting pattern. Juvenile petiole cuttings treated with α-naphthaleneacetic acid (NAA, 100 μM) plus the polyamine biosynthesis inhibitor, difluoromethylarginine (DFMA, 1 mM), formed an increased number of roots per cutting initiated by the indirect rooting pattern. The increased root formation and change in rooting pattern were reversed by the addition of putrescine (1 mM). Delaying auxin application to petiole cuttings for 15 days also induced juvenile petioles to root by the indirect pattern. This could be reversed by rewounding the base of the cutting prior to auxin application after day 15. The data support the use of the terms “competent root-forming cells” and “induced competent root-forming cells” to describe the target cells for the initial events of root formation for the direct and indirect rooting patterns, respectively.     

ULTRASTRUCTURE AND INITIATION OF WALL PATTERN IN PEDIASTRUM BORYANUM

The reticulate pattern in the wall of Pediastrum boryanum emerges rapidly during wall formation following aggregation of the swarming zoospores to form the coenobium. Electron micrographs during colony formation show that microtubules, present during the motile phase and aggregation, are gone prior to wall formation and probably do not participate in wall pattern regulation. A single dictyosome lies adjacent to the nucleus and from blebs of the nuclear membrane receives vesicles at its forming face. Vesicles formed at the maturing face have not been observed to contribute to the cell wall. Electron-lucent patches occur in the plasma membrane prior to wall formation. The first indication of a reticulate pattern in wall development is the deposition on the plasma membrane of interconnected plaques of outer wall material at the corners of hexagons. The sites of the plaques may correspond to clusters of ribosomes on endoplasmic reticulum underlying the plasmalemma. Following completion of the outer wall the thicker inner wall layer is deposited and within it the reticulate pattern of ridges is soon evident in tangential sections as strips of greater electron density. It is suggested that the pattern of the wall is templated by the plasma membrane.     

Surface topology assisted alignment of Min protein waves

Self-organization of proteins into large-scale structures is of pivotal importance for the organization of cells. The Min protein system of the bacterium Escherichia coli is a prime example of how pattern formation occurs via reaction–diffusion. We have previously demonstrated how Min protein patterns are influenced by compartment geometry. Here we probe the influence of membrane surface topology, as an additional regulatory element. Using microstructured membrane-clad soft polymer substrates, Min protein patterns can be aligned. We demonstrate that Min pattern alignment starts early during pattern formation and show that macroscopic millimeter-sized areas of protein patterns of well-defined orientation can be generated.