The determination of spatial pattern inDictyostelium discoideum

Free-living amoebae of the cellular slime mouldDictyostelium discoideum aggregate when starved and give rise to a long and thin multicellular structure, the slug. The slug resembles a metazoan embryo, and as with other embryos it is possible to specify a fate map. In the case ofDictyostelium discoideum the map is especially simple: cells in the anterior fifth of the slug die and form a stalk while the majority of those in the posterior differentiate into spores. The genesis of this anterior-posterior distinction is the subject of our review. In particular, we ask: what are the relative roles of individual pre-aggregative predispositions and post-aggregative position in determining cell fate? We review the literature on the subject and conclude that both factors are important. Variations in nutritional status, or in cell cycle phase at starvation, can bias the probability that an amoeba differentiates into a stalk cell or a spore. On the other hand, isolates, or slug fragments, consisting of only prestalk cells or only prespore cells can regulate so as to result in a normal range of both cell types. We identify three levels of control, each being responsible for guiding patterning in normal development: (i) ‘coin tossing’, whereby a cell autonomously exhibits a preference for developing along either the stalk or the spore pathway with relative probabilities that can be influenced by the environment; (ii) ‘chemical kinetics’, whereby prestalk and prespore cells originate from undifferentiated amoebae on a probabilistic basis but, having originated, interact (e.g. via positive and negative feedbacks), and the interaction influences the possibility of conversion of one cell type into the other; and (iii) ‘positional information’, in which the spatial distribution of morphogens in the slug influences the pathway of differentiation. In the case of possibilities (i) and (ii), sorting out of like cell types leads to the final spatial pattern. In the case of possibility (iii), the pattern arisesin situ     

The reinforcement-extinction process of selector gene activity: a positive feed-back loop and cell-cell interactions in Ultrabithorax patterning

Summary Most viable alleles of homeotic genes cause partial transformations within given lineages in a topographically specific fashion. We study this phenomenon as a way to understand the normal mechanisms involved in the spatial regulation of homeotic gene expression. To this end we have investigated the distribution of Ultrabithorax (Ubx) proteins in imaginai discs mutant for hypomorphic and neomorphic alleles of Ubx and alleles of trans-acting genes. We find that the morphological discontinuities observed in the adult transformations are caused by corresponding new patterns of the Ubx proteins in the imaginai anlagen. These novel patterns of Ubx proteins are understood as a consequence of a process of reinforcement-extinction of Ubx expression. The evidence suggesting that this process results from a positive feed-back loop and cell-cell interactions is discussed.     

Tracking down the "head blob": comparative analysis of wingless expression in the developing insect procephalon reveals progressive reduction of embryonic visual system patterning in higher insects

The evolution of larval head morphology in holometabolous insects is characterized by reduction of antennal appendages and the visual system components. Little insight has been gained into molecular developmental changes underlying this morphological diversification. Here we compare the expression of the segment polarity gene wingless (wg) in the pregnathal head of fruit fly, flour beetle and grasshopper embryos. We provide evidence that wg activity contributes to segment border formation, and, subsequently, the separation of the visual system and protocerebrum anlagen in the anterior procephalon. In directly developing insects like grasshopper, seven expression domains are formed during this process. The activation of four of these, which correspond to polar expression pairs in the optic lobe anlagen and the protocerebral ectoderm, has shifted to postembryonic stages in flour beetle and Drosophila. The remaining three domains map to the protocerebral neuroectoderm, and form by disintegration of a large precursor domain in flour beetle and grasshopper. In Drosophila, the precursor domain remains intact, constituting the previously described “head blob”. These data document major changes in the expression of an early patterning gene correlated with the dramatic evolution of embryonic visual system development in the Holometabola.     

Density dependence,spatial scale and patterning in sessile biota

Sessile biota can compete with or facilitate each other, and the interaction of facilitation and competition at different spatial scales is key to developing spatial patchiness and patterning. We examined density and scale dependence in a patterned, soft sediment mussel bed. We followed mussel growth and density at two spatial scales separated by four orders of magnitude. In summer, competition was important at both scales. In winter, there was net facilitation at the small scale with no evidence of density dependence at the large scale. The mechanism for facilitation is probably density dependent protection from wave dislodgement. Intraspecific interactions in soft sediment mussel beds thus vary both temporally and spatially. Our data support the idea that pattern formation in ecological systems arises from competition at large scales and facilitation at smaller scales, so far only shown in vegetation systems. The data, and a simple, heuristic model, also suggest that facilitative interactions in sessile biota are mediated by physical stress, and that interactions change in strength and sign along a spatial or temporal gradient of physical stress.     

Spatial patterning of the distribution of Ca2+ in Dictyostelium discoideum as assayed in fine glass capillaries

We have shown previously that the Ca²⁺-specific fluorescent dyes chlortetracycline (CTC) and indo-1/AM can be used to distinguish between prestalk and prespore cells inDictyostelium discoideum at a very early stage. In the present study, pre-and post-aggregative amoebae ofDictyostelium discoideum were labelled with CTC or indo-1 and their fluorescence monitored after being drawn into a fine glass capillary. The cells rapidly form two zones of Ca²⁺-CTC or Ca²⁺-indo-1 fluorescence. Anterior (air side) cells display a high level of fluorescence; the level drops in the middle portion of the capillary and rises again to a lesser extent in the posteriormost cells (oil side). When bounded by air on both sides, the cells display high fluorescence at both ends. When oil is present at both ends of the capillary, there is little fluorescence except for small regions at the ends. These outcomes are evident within a couple of minutes of the start of the experiment and the fluorescence pattern intensifies over the course of time. By using the indicator neutral red, as well as with CTC and indo-1, we show that a band displaying strong fluorescence moves away from the anterior end before stabilizing at the anterior-posterior boundary. We discuss our findings in relation to the role of Ca²⁺ in cell-type differentiation inDictyostelium discoideum.     

Components of positional information in the developing wing margin of the Lyra mutant of Drosophila

Summary A number of parameters characteristic of the wing margin precursor in imaginal discs of Drosophila are known: the zone of non-proliferating cells or ZNC (O'Brochta and Bryant 1985), aldehyde oxidase (AO) and other enzyme staining patterns (Sprey et al. 1982), E1C antigen localization in a narrow band along the margin (Piovant and Lena 1988). To test our hypothesis that such parameters, and others, act in concert to determine margin identity and the positional information that specifies the bristles and hairs appropriate to the anterior, posterior and distal margins, we have examined these parameters in the dominant mutant Lyra, in which much of the anterior and posterior margins is missing. After establishing that Lyra phenotype is already evident in the early pupal wing, we tested the known imaginal disc parameters and found that only Mab E1C (Piovant and Lena 1988) distribution differs from wild type, suggesting that E1C antigen may be a component of positional information. Sibatani's (1983) model for specification of positional information (PI) applied to wing discs predicts the Lyra adult wing shape as well as the reduced distribution of E1C antigen in Lyra wing discs. The model is based on the assumption that specification of positional information depends on interactions of multiple, independent factors. Clonal analysis with shaggy (Simpson et al. 1988 and Ripoll et al. 1988) indicates that factors in addition to E1C antigen contribute to margin PI in Lyra wings and should allow us to test the multi-component hypothesis further.     

Local and spatial dynamics of a host–parasitoid system in a field experiment

Local extinction and colonisation rates are key factors in host–parasitoid metapopulation theory, but experimental evidence from the field is scarce. We studied the host–parasitoid system consisting of the aphid Metopeurum fuscoviride and its specialist parasitoid Lysiphlebus hirticornis. This system is characterised by a patchy distribution of the host plants (Tanacetum vulgare) and by frequent extinctions of local aphid populations. In a first field experiment, we found that the presence of the parasitoid increases the likelihood of extinction of local host populations (=all aphids living on one plant). In a second field experiment, we manipulated the distance between local host populations. Parasitoid colonisation rate strongly decreased with increasing distance between local host populations. Thus, our results show the importance of parasitoids for local host populations extinction and of distance between local host populations for parasitoid colonisation rate, suggesting the importance of spatial processes for host–parasitoid systems in the field.     

The cytoskeleton of spreadingDictyostelium amoebae

Summary The cytoarchitectural elements ofDictyostelium discoideum amoeba have been visualized by light and electron microscopy in cells prepared with mixtures of glutaraldehyde and Triton-X-100. After negative staining, the peripheral regions of spreading amoebae show a complex meshwork of actin filaments, the majority of which were less than 0.25 microns in length. Multiple branch points, end to side abutments and cross-overs were characteristic features of the actin meshworks. Filopodia extending from the cell periphery consisted of bundles of actin filaments that penetrated into and merged with the actin meshworks in the spreading lamellae. Microtubules emanating from the nucleus associated body penetrated to differing extents into the actin meshworks, sometimes extending close to the cell periphery.Dictyostelium cytoskeletons preparted as described here should prove useful for further studies on the locomotory mechanism.     

Positional information in imaginal discs transformed by homoeotic mutations

Summary Mutations of the bithorax complex result in segmental transformations in the thorax and abdomen ofDrosophila. The haltere discs from larvae homozygous forbx ³ orpbx are transformed so that the discs contain cells that will produce wing cuticle as well as cells that produce haltere cuticle. The pattern regulation behavior of these discs has been examined. The fate maps of the two discs were established, and then the regulative behavior of a number of fragments from both types of mutant discs was established by culturing the fragments in vivo prior to metamorphosis. The most important conclusion from this work is that the cells producing, haltere cuticle and wing cuticle within the same disc share the same positional information and that they communicate during pattern regulation.     

The Salmonella typhimurium glycine cleavage enzyme system

Summary A glycine cleavage enzyme system, inducible by glycine, has been demonstrated in Salmonella typhimurium. The induced enzyme levels, however, are only about 20% of the induced levels found in Escherichia coli. Starting with a serine auxotroph, mutants were isolated that grow with a serine supplement, but not with a glycine supplement. Three independently isolated mutants have reduced or nondetectable glycine cleavage enzyme levels. The new mutations, designated gcv, were mapped between the serA and lys genes at 62.5 min on the S. typhimurium chromosome.Abbreviations C1 one-carbon - GCV glycine cleavage - GM glucose minimal - L agar Luria agar - LB Luria broth - Tc tetracycline     

Hirano bodies in health and disease

Recent findings shed light on the physiological function of enigmatic structures called Hirano bodies, which were first described more than 30 years ago.     

Origin and function of the stalk-cell vacuole in Dictyostelium

Large vacuoles are characteristic of plant and fungal cells, and their origin has long attracted interest. The cellular slime mould provides a unique opportunity to study the de novo formation of vacuoles because, in its life cycle, a subset of the highly motile animal-like cells (prestalk cells) rapidly develops a single large vacuole and cellulosic cell wall to become plant-like cells (stalk cells). Here we describe the origin and process of vacuole formation using live-imaging of Dictyostelium cells expressing GFP-tagged ammonium transporter A (AmtA-GFP), which was found to reside on the membrane of stalk-cell vacuoles. We show that stalk-cell vacuoles originate from acidic vesicles and autophagosomes, which fuse to form autolysosomes. Their repeated fusion and expansion accompanied by concomitant cell wall formation enable the stalk cells to rapidly develop turgor pressure necessary to make the rigid stalk to hold the spores aloft. Contractile vacuoles, which are rich in H⁺-ATPase as in plant vacuoles, remained separate from these vacuoles. We further argue that AmtA may play an important role in the control of stalk-cell differentiation by modulating the pH of autolysosomes.     

Co-option of an anteroposterior head axis patterning system for proximodistal patterning of appendages in early bilaterian evolution

The enormous diversity of extant animal forms is a testament to the power of evolution, and much of this diversity has been achieved through the emergence of novel morphological traits. The origin of novel morphological traits is an extremely important issue in biology, and a frequent source of this novelty is co-option of pre-existing genetic systems for new purposes (Carroll et al., 2008). Appendages, such as limbs, fins and antennae, are structures common to many animal body plans which must have arisen at least once, and probably multiple times, in lineages which lacked appendages. We provide evidence that appendage proximodistal patterning genes are expressed in similar registers in the anterior embryonic neurectoderm of Drosophila melanogaster and Saccoglossus kowalevskii (a hemichordate). These results, in concert with existing expression data from a variety of other animals suggest that a pre-existing genetic system for anteroposterior head patterning was co-opted for patterning of the proximodistal axis of appendages of bilaterian animals.     

A 1,4-β-D-glucan-synthase system fromDictyostelium discoideum

Particulate membrane preparations have been isolated from culminatingDictyostelium discoideum cells. The preparations incorporated glucose from uridine 5′-diphosphate-glucose into a glucose polymer or polymers. These have been shown to be homopolymers ofβ-linked glucose. A high percentage (78% by methylation analysis) of the linkages formed are 1,4-linkages and a lower percentage (12%) are 1,3-linkages. The glucan-synthase complex present in the particulate membrane preparation has an apparent K_m of 0.28 mM and a V_max of 1.59 nmol·min^?1·(mg protein)^?1. The enzyme system is dependent upon Mg²⁺ and cellobiose for maximal activity, but is inhibited by millimolar levels of Ca²⁺. Particulate membrane preparations were made from cells at various times during a synchronous developmental time course and demonstrated that the glucan-synthase activity appeared at the tight-aggregate stage of development.     

Patterning in the cellular slime moulds

The aim of the present article is to derive and illustrate in a simple form some of the important concepts in developmental biology. The development of the cellular slime mouldDictyostelium discoideum is an ideal model system for this purpose. I will outline the development of this organism at its multicellular stages and review some relevant studies focusing on the control of cell differentiation and pattern formation while deriving some key concepts in the current thinking about the control of development.     

Effect of light on the organogenic ability of garlic roots using a one-step in vitro system

A simple and efficient garlic in vitro shoot regeneration protocol has been developed. This system uses axenic root tips cultivated from the beginning in the presence of light and does not require any change or refreshing of the original medium during the entire process. The application of light from the beginning of the culture process did not affect the callus formation rate but did significantly improve the explant regeneration ability. In a 2-month period it was possible to obtain up to 250 shoots per gram of callus.Abbreviations BAP: 6-Benzylaminopurine - 2,4-D: 2,4-Dichlorophenoxyacetic acid - NAA: -Naphthaleneacetic acid Communicated by L. Peña     

The spatial pattern and canopy-understory association of trees in a cool temperate,mixed forest in western Japan

The spatial distribution pattern of trees and the association between canopy and understory individuals were examined with reference to the distribution of tree crowns in a cool temperate, mixed forest in Ohdaigahara, western Japan. Line transect and contact sampling methods were used to examine the pattern over various spatial scales. These methods are useful to detect patterns over a large study area. The dominance ofChamaecyparis obtusa on steep slopes forming large patches suggested that the distribution of this species is a consequence of landslides. UnderstoryFagus crenata showed a clumped distribution, and the relative coverage of this species was larger in canopy gaps than under a closed canopy. Understory individuals ofAbies homolepis showed a positive association with canopy trees ofF. crenata but a negative association with conspecific canopy individuals. These patterns suggested thatF. crenata regenerates in canopy gaps and is replaced byA. homolepis. The dynamics of these two species are consistent with the process of gap dynamics. The effects of both small- and large-scale disturbance must be evaluated to understand the mechanisms of patch formation and the coexistence of forest tree species.