Pattern formation and traveling waves in myxobacteria: theory and modeling

Locality versus globality in bacterial signalling: can local communication stabilize bacterial communities?

BACKGROUND

Microbial consortia are a major form of life; however their stability conditions are poorly understood and are often explained in terms of species-specific defence mechanisms (secretion of extracellular matrix, antimicrobial compounds, siderophores, etc.). Here we propose a hypothesis that the primarily local nature of intercellular signalling can be a general mechanism underlying the stability of many forms of microbial communities.

PRESENTATION OF THE HYPOTHESIS

We propose that a large microbial community can be pictured as a theatre of spontaneously emerging, partially overlapping, locally recruited microcommunities whose members interact primarily among themselves, via secreted (signalling) molecules or cell-cell contacts. We hypothesize that stability in an open environment relies on a predominantly local steady state of intercellular communication which ensures that i) deleterious mutants or strains can be excluded by a localized collapse, while ii) microcommunities harbouring useful traits can persist and/or spread even in the absence of specific protection mechanisms.

TESTING THE HYPOTHESIS

Some elements of this model can be tested experimentally by analyzing the behaviour of synthetic consortia composed of strains having well-defined communication systems and devoid of specific defence mechanisms. Supporting evidence can be obtained by in silico simulations.

IMPLICATIONS OF THE HYPOTHESIS

The hypothesis provides a framework for a systematic comparison of bacterial community behavior in open and closed environments. The model predicts that local signalling may enable multispecies communities to colonize open, structured environments. On the other hand, a confined niche or a host may be more likely to be colonized by a bacterial mono-species community, and local communication here provides a control against spontaneously arising cheaters, provided that survival depends on cooperation.

Deciphering the hunting strategy of a bacterial wolfpack

Myxococcus xanthus is a common soil bacterium with an intricate multicellular lifestyle that continues to challenge the way in which we conceptualize the capabilities of prokaryotic organisms. Myxococcus xanthus is the preferred laboratory representative from the Myxobacteria, a family of organisms distinguished by their ability to form highly structured biofilms that include tentacle-like packs of surface-gliding cell groups, synchronized rippling waves of oscillating cells and massive spore-filled aggregates that protrude upwards from the substratum to form fruiting bodies. But most of the Myxobacteria are also predators that thrive on the degradation of macromolecules released through the lysis of other microbial cells. The aim of this review is to examine our understanding of the predatory life cycle of M. xanthus. We will examine the multicellular structures formed during contact with prey, and the molecular mechanisms utilized by M. xanthus to detect and destroy prey cells. We will also examine our understanding of microbial predator-prey relationships and the prospects for how bacterial predation mechanisms can be exploited to generate new antimicrobial technologies.

Predataxis behavior in Myxococcus xanthus

Spatial organization of cells is important for both multicellular development and tactic responses to a changing environment. We find that the social bacterium, Myxococcus xanthus utilizes a chemotaxis (Che)-like pathway to regulate multicellular rippling during predation of other microbial species. Tracking of GFP-labeled cells indicates directed movement of M. xanthus cells during the formation of rippling wave structures. Quantitative analysis of rippling indicates that ripple wavelength is adaptable and dependent on prey cell availability. Methylation of the receptor, FrzCD is required for this adaptation: a frzF methyltransferase mutant is unable to construct ripples, whereas a frzG methylesterase mutant forms numerous, tightly packed ripples. Both the frzF and frzG mutant strains are defective in directing cell movement through prey colonies. These data indicate that the transition to an organized multicellular state during predation in M. xanthus relies on the tactic behavior of individual cells, mediated by a Che-like signal transduction pathway.

Genetic circuitry controlling motility behaviors of Myxococcus xanthus

M. xanthus has a complex multicellular lifestyle including swarming, predation and development. These behaviors depend on the ability of the cells to achieve directed motility across solid surfaces. M. xanthus cells have evolved two motility systems including Type-IV pili that act as grappling hooks and a controversial engine involving mucus secretion and fixed focal adhesion sites. The necessity for cells to coordinate the motility systems and to respond rapidly to environmental cues is reflected by a complex genetic network involving at least three complete sets of chemosensory systems and eukaryotic-like signaling proteins. In this review, we discuss recent advances suggesting that motor synchronization results from spatial oscillations of motility proteins. We further propose that these dynamics are modulated by the action of multiple upstream complementary signaling systems. M. xanthus is thus an exciting emerging model system to study the intricate processes of directed cell migration.

Travelling-wave behaviour in a multiphase model of a population of cells in an artificial scaffold

This paper analyses travelling-wave behaviour in a recently-formulated multiphase model for the growth of biological tissue that comprises motile cells and water inside a porous scaffold. The model arises in the context of tissue engineering, and its purpose is to study how cells migrate and proliferate inside porous biomaterials. In suitable limits, tissue growth in the model is shown to occur in the form of travelling waves that can propagate either forwards or backwards, depending on the values of the parameters. In the case where the drag force between the scaffold and the cells is non-zero, the growth of the aggregate can be analysed in terms of the propagation of a constant-speed wavefront in a semi-infinite domain. A numerical (shooting) method is described for calculating the wave speed, and detailed results for how the speed varies with respect to the parameters are given. In the case where the drag force is zero, the size of the aggregate is shown either to grow or to shrink exponentially with time. These results may be of importance in determining the experimental factors that control tissue invasiveness in scaffolds thereby allowing greater control over engineered tissue growth.

Biology by numbers: mathematical modelling in developmental biology

In recent years, mathematical modelling of developmental processes has earned new respect. Not only have mathematical models been used to validate hypotheses made from experimental data, but designing and testing these models has led to testable experimental predictions. There are now impressive cases in which mathematical models have provided fresh insight into biological systems, by suggesting, for example, how connections between local interactions among system components relate to their wider biological effects. By examining three developmental processes and corresponding mathematical models, this Review addresses the potential of mathematical modelling to help understand development.

Mutations of the act promoter in Myxococcus xanthus

Mutations within the -12 and -24 elements provide evidence that the act promoter is recognized by sigma-54 RNA polymerase. Deletion of the -20 base pair, which lies between the two conserved elements of sigma-54 promoters, decreased expression by 90%. In addition, mutation of a potential enhancer sequence, around -120, led to an 80% reduction in act gene expression. actB, the second gene in the act operon, encodes a sigma-54 activator protein that is proposed to be an enhancer-binding protein for the act operon. All act genes, actA to actE, are expressed together and constitute an operon, because an in-frame deletion of actB decreased expression of actA and actE to the same extent. After an initially slow phase of act operon expression, which depends on FruA, there is a rapid phase. The rapid phase is shown to be due to the activation of the operon expression by ActB, which completes a positive feedback loop. That loop appears to be nested within a larger positive loop in which ActB is activated by the C signal via ActA, and the act operon activates transcription of the csgA gene. We propose that, as cells engage in more C signaling, positive feedback raises the number of C-signal molecules per cell and drives the process of fruiting body development forward.

A microbial genetic journey

Fortunately, I began research in 1950 when the basic concepts of microbial genetics could be explored experimentally. I began with bacteriophage lambda and tried to establish the colinearity of its linkage map with its DNA molecule. My students and I worked out the regulation of lambda repressor synthesis for the establishment and maintenance of lysogeny. We also investigated the proteins responsible for assembly of the phage head. Using cell extracts, we discovered how to package DNA inside the head in vitro. Around 1972, I began to use molecular genetics to understand the developmental biology of Myxococcus xanthus. In particular, I wanted to learn how myxococcus builds its multicellular fruiting body within which it differentiates spores. We identified two cell-to-cell signals used to coordinate development. We have elucidated, in part, the signal transduction pathway for C-signal that directs the morphogenesis of a fruiting body.

Aggregation during fruiting body formation in Myxococcus xanthus is driven by reducing cell movement

When starved, Myxococcus xanthus cells assemble themselves into aggregates of about 10(5) cells that grow into complex structures called fruiting bodies, where they later sporulate. Here we present new observations on the velocities of the cells, their orientations, and reversal rates during the early stages of fruiting body formation. Most strikingly, we find that during aggregation, cell velocities slow dramatically and cells orient themselves in parallel inside the aggregates, while later cell orientations are circumferential to the periphery. The slowing of cell velocity, rather than changes in reversal frequency, can account for the accumulation of cells into aggregates. These observations are mimicked by a continuous agent-based computational model that reproduces the early stages of fruiting body formation. We also show, both experimentally and computationally, how changes in reversal frequency controlled by the Frz system mutants affect the shape of these early fruiting bodies.

Rippling is a predatory behavior in Myxococcus xanthus

Cells of Myxococcus xanthus will, at times, organize their movement such that macroscopic traveling waves, termed ripples, are formed as groups of cells glide together on a solid surface. The reason for this behavior has long been a mystery, but we demonstrate here that rippling is a feeding behavior which occurs when M. xanthus cells make direct contact with either prey or large macromolecules. Rippling has been observed during two fundamentally distinct environmental conditions: (i) starvation-induced fruiting body development and (ii) predation of other organisms. Our results indicate that case (i) does not occur in all wild-type strains and is dependent on the intrinsic level of autolysis. Analysis of predatory rippling indicates that rippling behavior is inducible during predation on proteobacteria, gram-positive bacteria, yeast (such as Saccharomyces cerevisiae), and phage. Predatory efficiency decreases under genetic and physiological conditions in which rippling is inhibited. Rippling will also occur in the presence of purified macromolecules such as peptidoglycan, protein, and nucleic acid but does not occur in the presence of the respective monomeric components and also does not occur when the macromolecules are physically separated from M. xanthus cells. We conclude that rippling behavior is a mechanism utilized to efficiently consume nondiffusing growth substrates and that developmental rippling is a result of scavenging lysed cell debris.

A generalized discrete model linking rippling pattern formation and individual cell reversal statistics in colonies of myxobacteria

Self-organization processes in multicellular aggregates of bacteria and amoebae offer fascinating insights into the evolution of cooperation and differentiation of cells. During myxobacterial development a variety of spatio-temporal patterns emerges such as counterpropagating waves of cell density that are known as rippling. Recently, several models have been introduced that qualitatively reproduce these patterns. All models include active motion and a collision-triggered reversal of individual bacteria. Here, we present a systematic study of a generalized discrete model that is based on similar assumptions as the continuous model by Igoshin et al (2001 Proc. Natl Acad. Sci. USA 98 14913). We find counterpropagating as well as unidirectional rippling waves in extended regions of the parameter space. If the interaction strength and the degree of cooperativity are large enough, rippling patterns appear even in the absence of a refractory period. We show for the first time that the experimentally observed double peak in the reversal statistics of bacteria in rippling colonies (Welch and Kaiser 2001 Proc. Natl Acad. Sci. USA 98 14907) can be reproduced in simulations of counterpropagating rippling waves which are dominant in experiments. In addition, the reversal statistics in the pre-rippling phase is correctly reproduced.

From biochemistry to morphogenesis in myxobacteria

Many aspects of metazoan morphogenesis find parallels in the communal behavior of microorganisms. The cellular slime mold D. discoideum has long provided a metaphor for multicellular embryogenesis. However, the spatial patterns in D.d. colonies are generated by an intercellular communication system based on diffusible morphogens, whereas the interactions between embryonic cells are more often mediated by direct cell contact. For this reason, the myxobacteria have emerged as a contending system in which to study spatial pattern formation, for their colony strutures rival those of D.d. in complexity, yet communication between cells in a colony is carried out by direct cell contacts. Here I sketch some of the progress my laboratory has made in modeling the life cycle of these organisms.

Two continuum models for the spreading of myxobacteria swarms

We analyze the phenomenon of spreading of a Myxococcus xanthus bacterial colony on plates coated with nutrient. The bacteria spread by gliding on the surface. In the first few hours, cell growth is irrelevant to colony spread. In this case, bacteria spread through peninsular protrusions from the edge of the initial colony. We analyze the diffusion through the narrowing reticulum of cells on the surface mathematically and derive formulae for the spreading rates. On the time scale of tens of hours, effective diffusion of the bacteria, combined with cell division and growth, causes a constant linear increase in the colony's radius. Mathematical analysis and numerical solution of reaction-diffusion equations describing the bacterial and nutrient dynamics demonstrate that, in this regime, the spreading rate is proportional to the square root of both the effective diffusion coefficient and the nutrient concentration. The model predictions agree with the data on spreading rate dependence on the type of gliding motility.

Accordion waves in Myxococcus xanthus

Myxococcus xanthus are Gram-negative bacteria that glide on solid surfaces, periodically reversing their direction of movement. When starved, M. xanthus cells organize their movements into waves of cell density that sweep over the colony surface. These waves are unique: Although they appear to interpenetrate, they actually reflect off one another when they collide, so that each wave crest oscillates back and forth with no net displacement. Because the waves reflect the coordinated back and forth oscillations of the individual bacteria, we call them "accordion" waves. The spatial oscillations of individuals are a manifestation of an internal biochemical oscillator, probably involving the Frz chemosensory system. These internal "clocks," each of which is quite variable, are synchronized by collisions between individual cells using a contact-mediated signal-transduction system. The result of collision signaling is that the collective spatial behavior is much less variable than the individual oscillators. In this work, we present experimental observations in which individual cells marked with GFP can be followed in groups of unlabeled cells in monolayer cultures. These data, together with an agent-based computational model demonstrate that the only properties required to explain the ripple patterns are an asymmetric biochemical limit cycle that controls direction reversals and asymmetric contact-induced signaling between cells: Head-to-head signaling is stronger than head-to-tail signaling. Together, the experimental and computational data provide new insights into how populations of interacting oscillators can synchronize and organize spatially to produce morphogenetic patterns that may have parallels in higher organisms.

Reversing cell polarity: evidence and hypothesis

The long, rod-shaped cells of myxobacteria are polarized by their gliding engines. At the rear, A-engines push while pili pull the front end forward. An hypothesis is developed whereby both engines are partially dis-assembled, then re-assembled at the opposite pole when cells reverse their movement direction. Reversals are induced by an Mgl G-protein switch that controls engine polarity. The switch is driven by an oscillatory circuit of Frizzy proteins. In growing cells, the circuit gives rise to an occasional reversal that makes swarming possible. Then, as myxobacteria begin fruiting body development, a rising level of C-signal input drives the oscillator and changes the reversal pattern. Cells reverse regularly every eight minutes in traveling waves, the reversal period is then prolonged enabling cells to form streams that enlarge tiny random aggregates into fruiting bodies.

A three-dimensional model of myxobacterial aggregation by contact-mediated interactions

Myxobacteria provide one of the simplest models of cell-cell interaction and organized cell movement leading to cellular differentiation. When starved, tens of thousands of cells change their movement pattern from outward spreading to inward concentration; they form aggregates that become fruiting bodies. Cells inside fruiting bodies differentiate into round, nonmotile, environmentally resistant spores. Traditionally, cell aggregation has been considered to imply chemotaxis; a long-range cell interaction. However, myxobacterial aggregation is the consequence of direct cell-contact interactions, not chemotaxis. We present here a 3D stochastic lattice-gas cellular automata model of cell aggregation based on local cell-cell contact, and no chemotaxis. We demonstrate that a 3D discrete stochastic model can simulate two stages of cell aggregation. First, a "traffic jam" forms embedded in a field of motile cells. The jam then becomes an aggregation center that accumulates more cells. We show that, at high cell density, cells stream around the traffic jam, generating a 3D hemispherical mound. Later, when the nuclear traffic jam dissolves, the aggregation center becomes a 3D ring of streaming cells.

An individual based model of rippling movement in a myxobacteria population

Migrating cells of Myxococcus xanthus (MX) in the early stages of starvation-induced development exhibit elaborate patterns of propagating waves. These so-called rippling patterns are formed by two sets of waves travelling in opposite directions. It has been experimentally shown that formation of these waves is mediated by cell-cell contact signalling (C-signalling). Here, we develop an individual-based model to study the formation of rippling patterns in MX populations. Following the work of Igoshin et al. (Proc. Natl. Acad. Sci. 98 (2001) 14913) we consider each moving cell to have an internal clock which controls its turning behaviour and sensitivity to C-signal. Specifically, we examine the effects of changing: C-signal strength, sensitivity/refractoriness, cell density, and noise upon the formation and structure of the rippling patterns. We also consider three modified models that have no explicit refractory period and examine their ability to produce rippling patterns.

Clocks and patterns in myxobacteria: a remembrance of Art Winfree

At the beginning of their aggregation phase waves of cell density sweep across the surface of myxobacteria colonies. These waves are unlike any other in biology. Waves can be linear, concentric or spiral and when they collide, instead of annihilating one another they appear to pass through each other unchanged. Moreover, the wavelength determines the spacing and pattern of fruiting bodies that will rise up presaging sporulation. The explanation for these waves was suggested by the work of Art Winfree on cellular clocks, and confirmed by a mathematical model that explains all of the observed wave behavior. The story of how this model evolved illustrates the roles of chance and scientific networking in the search for the explanation of a new phenomenon.

A biochemical oscillator explains several aspects of Myxococcus xanthus behavior during development

During development, Myxococcus xanthus cells produce a series of spatial patterns by coordinating their motion through a contact-dependent signal, the C-signal. C-signaling modulates the frequency at which cells reverse their gliding direction. It does this by interacting with the Frz system (a homolog of the Escherichia coli chemosensory system) via a cascade of covalent modifications. Here we show that introducing a negative feedback into this cascade results in oscillatory behavior of the signaling circuit. The model explains several aspects of M. xanthus behavior during development, including the nonrandom distribution of reversal times, and the differences in response of the reversal frequency to both moderate and high levels of C-signaling at different developmental stages. We also propose experiments to test the model.

Role of streams in myxobacteria aggregate formation

Cell contact, movement and directionality are important factors in biological development (morphogenesis), and myxobacteria are a model system for studying cell-cell interaction and cell organization preceding differentiation. When starved, thousands of myxobacteria cells align, stream and form aggregates which later develop into round, non-motile spores. Canonically, cell aggregation has been attributed to attractive chemotaxis, a long range interaction, but there is growing evidence that myxobacteria organization depends on contact-mediated cell-cell communication. We present a discrete stochastic model based on contact-mediated signaling that suggests an explanation for the initialization of early aggregates, aggregation dynamics and final aggregate distribution. Our model qualitatively reproduces the unique structures of myxobacteria aggregates and detailed stages which occur during myxobacteria aggregation: first, aggregates initialize in random positions and cells join aggregates by random walk; second, cells redistribute by moving within transient streams connecting aggregates. Streams play a critical role in final aggregate size distribution by redistributing cells among fewer, larger aggregates. The mechanism by which streams redistribute cells depends on aggregate sizes and is enhanced by noise. Our model predicts that with increased internal noise, more streams would form and streams would last longer. Simulation results suggest a series of new experiments.

Developmental waves in myxobacteria: A distinctive pattern formation mechanism

In early stages of their development, starving myxobacteria organize their motion to produce a periodic pattern of traveling cell density waves. These waves arise from coordination of individual cell reversals by contact signaling when they collide. Unlike waves generated by reaction-diffusion instabilities, which annihilate on collision, myxobacteria waves appear to pass through one another unaffected. Here we analyze a mathematical model of these waves developed earlier [Proc. Natl. Acad. Sci. USA 98, 14 913 (2001)]]. The mechanisms which generate and maintain the density waves are clearly revealed by tracing the reversal loci of individual cells. An evolution equation of reversal point density is derived in the weak-signaling limit. Linear stability analysis determines parameters favorable for the development of the waves. Numerical solutions demonstrate the stability of the fully developed nonlinear waves.

Signaling in myxobacteria

Myxobacteria use soluble and cell-contact signals during their starvation-induced formation of fruiting bodies. These signals coordinate developmental gene expression with the cell movements that build fruiting bodies. Early in development, the quorum-sensing A-signal in Myxococcus xanthus helps to assess starvation and induce the first stage of aggregation. Later, the morphogenetic C-signal helps to pattern cell movement and shape the fruiting body. C-signal is a 17-kDa cell surface protein that signals by contact between the ends of two cells. The number of C-signal molecules per cell rises 100-fold from the beginning of fruiting body development to the end, when spores are formed. Traveling waves, streams, and sporulation have increasing thresholds for C-signal activity, and this progression ensures that spores form inside fruiting bodies.

Single-cell microbiology: tools, technologies, and applications

The field of microbiology has traditionally been concerned with and focused on studies at the population level. Information on how cells respond to their environment, interact with each other, or undergo complex processes such as cellular differentiation or gene expression has been obtained mostly by inference from population-level data. Individual microorganisms, even those in supposedly "clonal" populations, may differ widely from each other in terms of their genetic composition, physiology, biochemistry, or behavior. This genetic and phenotypic heterogeneity has important practical consequences for a number of human interests, including antibiotic or biocide resistance, the productivity and stability of industrial fermentations, the efficacy of food preservatives, and the potential of pathogens to cause disease. New appreciation of the importance of cellular heterogeneity, coupled with recent advances in technology, has driven the development of new tools and techniques for the study of individual microbial cells. Because observations made at the single-cell level are not subject to the "averaging" effects characteristic of bulk-phase, population-level methods, they offer the unique capacity to observe discrete microbiological phenomena unavailable using traditional approaches. As a result, scientists have been able to characterize microorganisms, their activities, and their interactions at unprecedented levels of detail.

Two-stage aggregate formation via streams in myxobacteria

In response to adverse conditions, myxobacteria form aggregates that develop into fruiting bodies. We model myxobacteria aggregation with a lattice cell model based entirely on short-range (nonchemotactic) cell-cell interactions. Local rules result in a two-stage process of aggregation mediated by transient streams. Aggregates resemble those observed in experiment and are stable against even very large perturbations. Noise in individual cell behavior increases the effects of streams and results in larger, more stable aggregates.

Rippling of myxobacteria

Myxobacteria colonies during their aggregation phase propagate complex waves over their surface. These waves are fundamentally different from the analogous phenomenon in diffusion-reaction systems or in populations of Dictyostelium discoideum where colliding waves annhilate. Myxobacterial waves appear to pass through one another, analogous to solitons. Moreover, individual bacteria oscillate back and forth, exhibiting no net mass transfer. A mathematical model can explain virtually all of the experimentally observed properties of these waves and draw several conclusions about the properties of the intercelular signaling system.

Waves and aggregation patterns in myxobacteria

Under starvation conditions, a population of myxobacteria aggregates to build a fruiting body whose shape is species-specific and within which the cells sporulate. Early in this process, cells often pass through a "ripple phase" characterized by traveling linear, concentric, and spiral waves. These waves are different from the waves observed during slime mold aggregation that depend on diffusible morphogens, because myxobacteria communicate by direct contact. The difference is most dramatic when waves collide: rather than annihilating one another, myxobacterial waves appear to pass through one another unchanged. Under certain conditions, the spacing and location of the nascent fruiting bodies is determined by the wavelength and pattern of the waves. Later in fruiting body development, waves are replaced by streams of cells that circulate around small initial aggregates enlarging and rounding them. Still later, pairs of motile aggregates coalesce to form larger aggregates that develop into fruiting bodies. Here we present a mathematical model that quantitatively explains these wave and aggregation phenomena.

Dynamics of fruiting body morphogenesis

Myxobacteria build their species-specific fruiting bodies by cell movement and then differentiate spores in specific places within that multicellular structure. New steps in the developmental aggregation of Myxococcus xanthus were discovered through a frame-by-frame analysis of a motion picture. The formation and fate of 18 aggregates were captured in the time-lapse movie. Still photographs of 600 other aggregates were also analyzed. M. xanthus has two engines that propel the gliding of its rod-shaped cells: slime-secreting jets at the rear and retractile pili at the front. The earliest aggregates are stationary masses of cells that look like three-dimensional traffic jams. We propose a model in which both engines stall as the cells' forward progress is blocked by other cells in the traffic jam. We also propose that these blockades are eventually circumvented by the cell's capacity to turn, which is facilitated by the push of slime secretion at the rear of each cell and by the flexibility of the myxobacterial cell wall. Turning by many cells would transform a traffic jam into an elliptical mound, in which the cells are streaming in closed orbits. Pairs of adjacent mounds are observed to coalesce into single larger mounds, probably reflecting the fusion of orbits in the adjacent mounds. Although fruiting bodies are relatively large structures that contain 10(5) cells, no long-range interactions between cells were evident. For aggregation, M. xanthus appears to use local interactions between its cells.

Coupling cell movement to multicellular development in myxobacteria

The myxobacteria are Gram-negative organisms that are capable of multicellular, social behaviour. In the presence of nutrients, swarms of myxobacteria feed cooperatively by sharing extracellular digestive enzymes, and can prey on other bacteria. When the food supply runs low, they initiate a complex developmental programme that culminates in the production of a fruiting body. Myxobacteria move by gliding and have two, polarly positioned engines to control their motility. The two engines undergo coordinated reversals, and changes in the reversal frequency and speed are responsible for the different patterns of movement that are seen during development. The myxobacteria communicate with each other and coordinate their movements through a cell-contact-dependent signal. Here, the cell movements that culminate in the development of the multicellular fruiting body are reviewed.

The role of mechanical forces on the patterning of the avian feather-bearing skin: A biomechanical analysis of the integumentary musculature in birds

The integumentary musculature of birds consists of three distinct components. The smooth musculature comprises feather and apterial muscles, which form a continuous musculo-elastic layer within the dermis. The feather muscles, which consistently include at least erectors and depressors, interconnect contour feathers within pterylae (i.e., feather tracts) along gridlines that are oriented diagonally to the longitudinal and transverse axes of the body. The apterial muscles interconnect pterylae by attaching to the contour feathers along their peripheries. The striated musculature is composed of individual subcutaneous muscles, most of which attach to contour feathers along the caudal periphery of pterylae A new integrative functional analysis of the integumentary musculature proposes how apterial muscles stabilize the pterylae and modulate the tension of the musculo-elastic layer, and how subcutaneous muscles provide the initial stimulus for erector muscles being able to ruffle the contour feathers within pterylae. It also shows how the arrangement of the contour feathers and integumentary muscles reflects the stresses and strains that act on the avian skin. These mechanical forces are in effect not only in the adult, especially during flight, but may also be active during feather morphogenesis. The avian integument with its complex structural organization may, therefore, represent an excellent model for analyzing the nature of interactions between the environment and genetic material. The predictions of our model are testable, and our study demonstrates the relevance of integrated analyses of complex organs as mechanically coherent systems for evolutionary and developmental biology.

Rippling patterns in aggregates of myxobacteria arise from cell-cell collisions

Experiments with myxobacterial aggregates reveal standing waves called rippling patterns. Here these structures are modeled with a simple discrete model based on the interplay between migration and collisions of cells. Head-to-head collisions of cells result in cell reversals. To correctly reproduce the rippling patterns, a refractory phase after each cell reversal has to be assumed, during which further reversal is prohibited. The duration of this phase determines the wavelength and period of the ripple patterns as well as the reversal frequency of single cells.

Cell behavior in traveling wave patterns of myxobacteria

Cells in the early stages of starvation-induced fruiting body development migrate in a highly organized periodic pattern of equispaced accumulations that move as traveling waves. Two sets of waves are observed moving in opposite directions with the same wavelength and speed. To learn how the behavior of individual cells contributes to the wave pattern, fluorescent cells were tracked within a rippling population. These cells exhibit at least three types of organized behavior. First, most cell movement occurs along the same axis as the rippling movement. Second, there is a high degree of cell alignment parallel to the direction of rippling, as indicated by the biased movement. Third, by controlling the reversal frequency, cell movement becomes periodic in a rippling field. The periodicity of individual cells matches the period of macroscopic rippling. This last behavior is unique to a rippling population and, on the basis of Myxococcus xanthus genetic data, we conclude that this periodicity is linked to the C signal, a nondiffusible cell contact-mediated signaling molecule. When two cells moving in opposite directions meet end to end, they transmit the C signal to each other and in response reverse their gliding direction. This model of traveling waves represents a new mode of biological pattern formation that depends on cell-contact interactions rather than reaction diffusion.