Biology subjects

Yunker, P. J.

Publications and source records attributed to Yunker, P. J..

At least 19 recordsLinked to original sources

bioRxiv2026

Matrix-controlled emergence of biofilm architecture shapes antimicrobial survival

Biofilms are structured microbial communities whose extracellular matrix is widely regarded as a basis of their protection against antimicrobial compounds. Yet how matrix production by individual bacteria gives rise to collective architecture and antimicrobial protection remains poorly understood. Here, we systematically varied expression of the master biofilm regulator csgD in Salmonella enterica and found that increasing matrix production reorganizes biofilms from dense, isotropic packings into sparse, nematically aligned communities by altering cell-cell interactions. By combining experimentally measured biofilm architectures with reaction-diffusion modeling, we show that these structural changes produce distinct patterns of antimicrobial killing, ranging from preferential killing near the liquid-biofilm interface to more uniform killing throughout the community. Consequently, increasing matrix production unexpectedly reduces antimicrobial survival by shifting the biofilm into different transport regimes, while strain-specific physiological differences further modulate antimicrobial depletion. Rather than acting as a passive barrier, EPS therefore shapes antimicrobial susceptibility by reorganizing biofilm architecture and its transport properties. EPS thus provides a physical link between molecular regulation, collective architecture and antimicrobial survival, providing a quantitative framework for understanding how cellular matrix production generates emergent biofilm function.

microbiology↗

bioRxiv2026

Rapid Bacterial Identification and Antibiotic Susceptibility Testing through Interferometry-based Surface Topography Measurement

Scientists have long classified organisms based on shared morphological characteristics. However, modern taxonomy, particularly for bacteria, is largely defined at the genetic level. Yet the information encoded in genetics propagates through many levels of biological organization: genes determine patterns of gene expression and protein production which, in turn, influence cellular physiology and behavior which, in turn, influence the collective morphology of growing populations. It is unclear how far up this hierarchy that the lower level taxonomic information remains preserved. Here, we show that bacterial genera can be identified from differences in the three-dimensional topographies of their growing populations. Using coherence-scanning white-light interferometry (WLI), which is capable of nanometer-resolution, we measure bacterial population topographies of 77 different clinical isolates representing four different genera. We then extract ten biophysically relevant features describing their surface structure. Using a simple machine-learning classifier, these topographic features identified bacterial genus with 97% accuracy, demonstrating that genus-level information remains present at the scale of population morphology. We hypothesize that these topographic "fingerprints" arise because differences in the underlying biophysics of bacterial growth propagate upward into measurable differences in population structure. To test this hypothesis, we performed biophysical simulations of bacterial population growth and applied our experimentally trained classifier directly to the resulting simulated topographies. By systematically modifying simulation parameters, we determined which changes were necessary to generate topographies classified as each of the experimentally observed genera. We find that a relatively small number of differences in cellular growth, cell-surface interactions, and initial conditions are sufficient to recreate the genus-associated differences observed experimentally, and we then show that using the topographic features from the simulations we can recover the underlying parameters used to simulate the topographies. Together, these results show that differences in lower-level bacterial behavior can propagate into distinct population-scale morphologies, and that taxonomic information can remain recoverable far above the molecular scale at which bacterial identity is conventionally defined.

biophysics↗

bioRxiv2025

Higher levels of antibiotic resistance are less competitive: the hidden ecological cost of no-metabolic cost resistance

Antibiotic resistance is often assumed to be constrained by fitness costs that limit the spread of highly resistant strains. Yet, many resistance mechanisms - including enzymatic antibiotic degradation - can arise with little or no metabolic cost, raising an important question: why is extreme resistance not more widespread? Here, we show that community-level interactions impose a hidden ecological cost on high resistance. By performing experiments with simple communities comprised of antibiotic resistant clinical isolates and an antibiotic susceptible strain, we find that when exposed to betalactam antibiotics, strains with a higher degree of antibiotic resistance can promote the survival of cohabiting susceptible strains. Guided by mean-field modeling, we find that highly resistant bacteria accelerate detoxification of the shared environment, shortening the period during which resistance confers a competitive advantage. Experiments with an engineered strain with tunable resistance level confirm that susceptible cells grow best in the presence of highly resistant strains. Importantly, this effect does not require evolved cooperation or active enzyme secretion; experimental and modeling results show that unavoidable processes, such as cell death or passive leakage, prevent complete privatization of resistance, giving rise to "accidental co-operation". These findings suggest that resistance evolution is not only shaped by intrinsic cellular costs but also by ecological feedback that limits the benefits of incremental increases in resistance. This result may be reflected in the phenotypic responses of the clinical strains tested in this work, which fell into distinct low- and high-resistance classes with no intermediate phenotypes. Thus, this work demonstrates the important role of community dynamics in understanding the evolution of antibiotic resistance and treatment outcomes.

microbiology↗

bioRxiv2025

Phase separation and coexistence in spatial coordination games between microbes

Dense, microbial communities are shaped by local interactions between cells. Both the nature of interactions, spanning antagonistic to cooperative, and the strength of interactions vary between and across microbial species and strains. These local interactions can influence the emergence and maintenance of microbial diversity. However, it remains challenging to link features of local interactions with spatially mediated coexistence dynamics given the significant variation in the microscopic mechanisms involved in cell-to-cell feedback. Here, we explore how microbial interactions over a broad range of ecological contexts spanning antagonism to cooperation can enable coexistence as spatially explicit domains emerge. To do so, we introduce and analyze a family of stochastic coordination games, where individuals do better when playing (i.e., interacting) with individuals of the same type than when playing with individuals of a different type. Using this game-theoretic framework, we show that the population dynamics for coordination games is governed by a double-well shaped interaction potential. We find that in a spatial setting this double-well potential induces phase separation, facilitating coexistence. Moreover, we show that for microbes engaged in symmetric coordination games, phase separation takes on a universal scaling that follows Model A coarsening, consistent with prior experimental observations for Vibrio cholerae mutual killers. Finally, we derive a PDE equivalent of the spatial stochastic game, confirming both the double-well nature of spatial coordination games and the universality of phase separation. Altogether, this work extends prior findings on the link between microbial interactions and population structure and suggests generic mechanisms embedded in local interactions that can enable coexistence.

microbiology↗

bioRxiv2025

Cell-substrate friction controls biofilm development

Bacteria often live in biofilms, surface-attached communities that can form on nearly any surface, from coarse sands to smooth glass. It is often hypothesized that cell-substrate friction can impact biofilm growth and development on these disparate surfaces. However, the experimental difficulty in measuring the friction between a biofilm and its surface has limited our understanding of how friction forces impact emergent colony-level behaviors and morphologies. Here, we demonstrate that increasing cell-substrate friction increases the biofilm contact angle, which, in turn, decreases the horizontal biofilm range expansion rate. We first used individual-based simulations to isolate the impact of friction. In this simple model, we found that increasing friction increases biofilm contact angle, and, in turn, that the contact angle emerges from a simple dynamic force balance between cell-substrate friction, cell-cell steric forces, and cell-cell adhesion, reminiscent of Youngs equation for sessile liquid drops. We developed an approach to directly measure the friction between bacterial colonies and agar surfaces, and found that the friction between the substrate and the biofilm is higher on substrates made with higher agar weight percentage. Additionally, we observed that biofilm contact angles increase with friction, following a Young-equation-like balance. Finally, we found that contact angle increases with agar percentage for a wide range of bacteria, suggesting that the biophysical impact of friction may play a role for a wide array of microbes.

biophysics↗

bioRxiv2025

Biofilm Vertical Growth Dynamics Are Captured by An Active Fluid Framework

Bacterial biofilms, surface-attached microbial communities, grow horizontally across surfaces and vertically above them. Although a simple heuristic model for vertical growth was experimentally shown to accurately describe the behavior of diverse microbial species, the biophysical implications and theoretical basis for this empirical model were unclear. Here, we demonstrate that this heuristic model emerges naturally from fundamental principles of active fluid dynamics. By analytically deriving exact solutions for an active fluid model of vertical biofilm growth, we show that the governing equations reduce to the same form as the empirical model in both early- and late-stage growth regimes. Our analysis reveals that cell death and decay rates likely play key roles in determining the characteristic parameters of vertical growth. The active fluid model produces a single, simple equation governing growth at heights above and below the diffusion limit; surprisingly, this "full" expression is simpler than the heuristic. With this theoretical basis, we explain why the vertical growth rate reaches a maximum at a height greater than the previously identified characteristic length scale. These results provide a theoretical foundation for a simple mathematical model of vertical growth, enabling deeper understanding of how biological and biophysical factors interact during biofilm development.

biophysics↗

bioRxiv2024

Metabolically-driven flows enable exponential growth in macroscopic multicellular yeast

The ecological and evolutionary success of multicellular lineages is due in no small part to their increased size relative to unicellular ancestors. However, large size also poses biophysical challenges, especially regarding the transport of nutrients to all cells; these constraints are typically overcome through multicellular innovations (e.g., a circulatory system). Here we show that an emergent biophysical mechanism -- spontaneous fluid flows arising from metabolically-generated density gradients -- can alleviate constraints on nutrient transport, enabling exponential growth in nascent multicellular clusters of yeast lacking any multicellular adaptations for nutrient transport or fluid flow. Surprisingly, beyond a threshold size, the metabolic activity of experimentally-evolved snowflake yeast clusters drives large-scale fluid flows that transport nutrients throughout the cluster at speeds comparable to those generated by the cilia of extant multicellular organisms. These flows support exponential growth at macroscopic sizes that theory predicts should be diffusion limited. This work demonstrates how simple physical mechanisms can act as a biophysical scaffold to support the evolution of multicellularity by opening up phenotypic possibilities prior to genetically-encoded innovations. More broadly, our findings highlight how cooption of conserved physical processes is a crucial but underappreciated facet of evolutionary innovation across scales.

evolutionary biology↗

bioRxiv2024

Fluctuations and freezing of biofilm-air interfaces

The study of interfacial roughening is common in physics, from epitaxial growth in the lab to pio-neering mathematical descriptions of universality in models of growth processes. These studies led to the identification of a series of general principles. Typically, stochastic growth produces an interface that becomes rougher as the deposit grows larger; this roughening can only be counteracted by mechanisms that act on the top of deposit, such as surface tension or surface diffusion. However, even when relaxation mechanisms are present, interfaces that continue to grow stochastically continue to change; new peaks and troughs emerge and disappear as stochastic growth produces a constantly changing, dynamic interface. These universal phenomena have been observed for bacterial colonies in a variety of contexts. However, previous studies have not characterized the interfacial phenomena at the top surface of a colony, i.e., the colony-air interface, when activity is only present at the bottom surface, i.e., the colony-solid interface, where nutrients are available, over long times. As traditional interfacial roughening models primarily focus on activity occurring at the top surface it is unclear what phenomena to expect over long times. Here, we use white light interferometry to study the roughening of bacterial biofilms, from many different species. We find that these colonies are remarkably smooth, suggesting that a mechanism of interfacial relaxation is at play. However, colonies remain remarkably smooth even after growing large. We discover that topographic fluctuations "freeze" in place, despite the fact that growth continues for hundreds of microns more. With simple simulations, we show that this emergent freezing is due to the dampening of fluctuations from cell growth by the cells between the growing zone and the surface. We find that the displacement field caused by a single perturbation decays exponentially, with a decay length of{delta} L. In line with that observation we also show that the topography ceases to change when perturbations are a distance{delta} L away from the surface. Thus, over-damped systems in which activity occurs at the bottom surface represent a distinct class of interfacial growth phenomena, capable of producing frozen topographies and remarkably smooth surfaces from spatially and temporally stochastic growth.

biophysics↗

bioRxiv2023

Examining the role of oxygen-binding proteins on the early evolution of multicellularity

Oxygen availability is a key factor in the evolution of multicellularity, as larger and more sophisticated organisms often require mechanisms allowing efficient oxygen delivery to their tissues. One such mechanism is the presence of oxygen-binding proteins, such as globins and hemerythrins, which arose in the ancestor of bilaterian animals. Despite their importance, the precise mechanisms by which oxygen-binding proteins influenced the early stages of multicellular evolution under varying environmental oxygen levels are not yet clear. We addressed this knowledge gap by heterologously expressing the oxygen binding proteins myoglobin and myohemerythrin in snowflake yeast, a model system of simple, undifferentiated multicellularity. These proteins increased the depth and rate of oxygen diffusion, increasing the fitness of snowflake yeast growing aerobically. Experiments show that, paradoxically, oxygen-binding proteins confer a greater fitness benefit for larger organisms under high, not low, O2 conditions. We show via biophysical modeling that this is because facilitated diffusion is more efficient when oxygen is abundant, transporting a greater quantity of O2 which can be used for metabolism. By alleviating anatomical diffusion limitations to oxygen consumption, the evolution of O2-binding proteins in the oxygen-rich Neoproterozoic may have been a key breakthrough enabling the evolution of increasingly large, complex multicellular metazoan lineages.

evolutionary biology↗

bioRxiv2023

The biophysical basis of bacterial colony growth

Bacteria often attach to surfaces and grow densely-packed communities called biofilms. As biofilms grow, they expand across the surface, increasing their surface area and access to nutrients. Thus, the overall growth rate of a biofilm is directly dependent on its "range expansion" rate. One factor that limits the range expansion rate is vertical growth; at the biofilm edge there is a direct trade-off between horizontal and vertical growth--the more a biofilm grows up, the less it can grow out. Thus, the balance of horizontal and vertical growth impacts the range expansion rate and, crucially, the overall biofilm growth rate. However, the biophysical connection between horizontal and vertical growth remains poorly understood, due in large part to difficulty in resolving biofilm shape with sufficient spatial and temporal resolution from small length scales to macroscopic sizes. Here, we experimentally show that the horizontal expansion rate of bacterial colonies is controlled by the contact angle at the biofilm edge. Using white light interferometry, we measure the three-dimensional surface morphology of growing colonies, and find that small colonies are surprisingly well-described as spherical caps. At later times, nutrient diffusion and uptake prevent the tall colony center from growing exponentially. However, the colony edge always has a region short enough to grow exponentially; the size and shape of this region, characterized by its contact angle, along with cellular doubling time, determines the range expansion rate. We found that the geometry of the exponentially growing biofilm edge is well-described as a spherical-cap-napkin-ring, i.e., a spherical cap with a cylindrical hole in its center (where the biofilm is too tall to grow exponentially). We derive an exact expression for the spherical-cap-napkin-ring-based range expansion rate; further, to first order, the expansion rate only depends on the colony contact angle, the thickness of the exponentially growing region, and the cellular doubling time. We experimentally validate both of these expressions. In line with our theoretical predictions, we find that biofilms with long cellular doubling times and small contact angles do in fact grow faster than biofilms with short cellular doubling times and large contact angles. Accordingly, sensitivity analysis shows that biofilm growth rates are more sensitive to their contact angles than to their cellular growth rates. Thus, to understand the fitness of a growing biofilm, one must account for its shape, not just its cellular doubling time.

biophysics↗

bioRxiv2023

Spatial constraints subvert microbial arms race

Biofilms, surface attached communities of microbes, grow in a wide variety of environments. Often, the size of these microbial community is constrained by their physical surroundings. However, little is known about how size constraints of a colony impact the outcome of microbial competitions. Here, we use individual-based models to simulate contact killing between two bacterial strains with different killing rates in a wide range of community sizes. We found that community size has a substantial impact on outcomes; in fact, in some competitions the identity of the most fit strain differs in large and small environments. Specifically, when at a numerical disadvantage, the strain with the slow killing rate is more successful in smaller environments than in large environments. The improved performance in small spaces comes from finite size effects; stochastic fluctuations in the initial relative abundance of each strain in small environments lead to dramatically different outcomes. However, when the slow killing strain has a numerical advantage, it performs better in large spaces than in small spaces, where stochastic fluctuations now aid the fast killing strain in small communities. Finally, we experimentally validate these results by confining contact killing strains of Vibrio cholerae in transmission electron microscopy grids. The outcomes of these experiments are consistent with our simulations. When rare, the slow killing strain does better in small environments; when common, the slow killing strain does better in large environments. Together, this work demonstrates that finite size effects can substantially modify antagonistic competitions, suggesting that colony size may, at least in part, subvert the microbial arms race. Author summaryBiofilms are often crowded with many bacteria in direct contact. As a result, the competition for space and resources often turns deadly. Bacteria have evolved many mechanisms with which to kill each other; this bacterial warfare is often studied in large communities on agar plates or in flow cells [1]. However, in nature these colonies are often smaller, due to spatial constraints or shear forces. It is unclear how bacterial warfare proceeds in small systems. We performed individual based model simulations of bacterial warfare comprising two strains, each capable of killing the other on direct contact. We found that the community size played a substantial role in determining the outcome. When at a numerical disadvantage, the slow killing strain survived at much higher rates in small communities. In fact, there were many conditions in which the slow killing strain survives in small spaces but is completely eliminated in large ones. Conversely, when the slow killing strain is more common, it performs better in large spaces. Together, these observations demonstrate that finite size effects aid the strain that is at a disadvantage, and in some conditions, can even flip which strain increases its abundance. Finally, we experimentally tested the results of these simulations. Two mutual killing strains of V. cholerae were grown unconfined on agar plates (i.e., in large spaces) or confined within square holes with sides 7.5m long (i.e., in small spaces). In these experiments we found that the slow killing strain survived at significantly higher rates in confinement, validating simulation results.

biophysics↗

bioRxiv2023

Entanglement in living systems

Many organisms exhibit branching morphologies that twist around each other and become entangled. Entanglement occurs when different objects interlock with each other, creating complex and often irreversible configurations. This physical phenomenon is well-studied in non-living materials, such as granular matter, polymers, and wires, where it has been shown that entanglement is highly sensitive to the geometry of the component parts. However, entanglement is not yet well understood in living systems, despite its presence in many organisms. In fact, recent work has shown that entanglement can evolve rapidly, and play a crucial role in the evolution of tough, macroscopic multicellular groups. Here, through a combination of experiments, simulations, and numerical analyses, we show that growth generically facilitates entanglement for a broad range of geometries. We find that experimentally grown entangled branches can be difficult or even impossible to disassemble through translation and rotation of rigid components, suggesting that there are many configurations of branches that growth can access that agitation cannot. We use simulations to show that branching trees readily grow into entangled configurations. In contrast to non-growing entangled materials, these trees entangle for a broad range of branch geometries. We thus propose that entanglement via growth is largely insensitive to the geometry of branched-trees, but instead will depend sensitively on time scales, ultimately achieving an entangled state once sufficient growth has occurred. We test this hypothesis in experiments with snowflake yeast, a model system of undifferentiated, branched multicellularity, showing that lengthening the time of growth leads to entanglement, and that entanglement via growth can occur for a wide range of geometries. Taken together, our work demonstrates that entanglement is more readily achieved in living systems than in their non-living counterparts, providing a widely-accessible and powerful mechanism for the evolution of novel biological material properties.

biophysics↗

bioRxiv2023

Emergence and maintenance of stable coexistence during a long-term multicellular evolution experiment

The evolution of multicellular life spurred evolutionary radiations, fundamentally changing many of Earths ecosystems. Yet little is known about how early steps in the evolution of multicellularity transform eco-evolutionary dynamics, e.g., via niche expansion processes that may facilitate coexistence. Using long-term experimental evolution in the snowflake yeast model system, we show that the evolution of multicellularity drove niche partitioning and the adaptive divergence of two distinct, specialized lineages from a single multicellular ancestor. Over 715 daily transfers, snowflake yeast were subject to selection for rapid growth in rich media, followed by selection favoring larger group size. Both small and large cluster-forming lineages evolved from a monomorphic ancestor, coexisting for over ~4,300 generations. These small and large sized snowflake yeast lineages specialized on divergent aspects of a trade-off between growth rate and survival, mirroring predictions from ecological theory. Through modeling and experimentation, we demonstrate that coexistence is maintained by a trade-off between organismal size and competitiveness for dissolved oxygen. Taken together, this work shows how the evolution of a new level of biological individuality can rapidly drive adaptive diversification and the expansion of a nascent multicellular niche, one of the most historically-impactful emergent properties of this evolutionary transition.

evolutionary biology↗

bioRxiv2022

Vertical growth dynamics of biofilms

During the biofilm life cycle, bacteria attach to a surface then reproduce, forming crowded, growing communities. Many theoretical models of biofilm growth dynamics have been proposed; however, difficulties in measuring biofilm height accurately across relevant time and length scales have prevented testing these models or their biophysical underpinnings empirically. Using white light interferometry, we measure the heights of microbial colonies with nanometer precision from inoculation to their final equilibrium height, producing a novel and detailed empirical characterization of vertical growth dynamics. We show that models relying on logistic growth or nutrient depletion fail to capture biofilm height dynamics on short and long time scales. Our empirical results support a simple model inspired by the fact that biofilms only interact with the environment through their interfaces. This interface model captures the growth dynamics from short to long time scales (10 minutes to 14 days) of diverse microorganisms, including prokaryotes like gram-negative and gram-positive bacteria and eukaryotes like aerobic and anaerobic yeast. This model provides heuristic value, highlighting the biophysical constraints that limit vertical growth as well as establishing a quantitative model for biofilm development.

biophysics↗

bioRxiv2022

Evolution of a cis-acting SNP that controls Type VI Secretion in Vibrio cholerae

Mutations in regulatory mechanisms that control gene expression contribute to phenotypic diversity and thus facilitate the adaptation of microbes and other organisms to new niches. Comparative genomics can be used to infer rewiring of regulatory architecture based on large effect mutations like loss or acquisition of transcription factors but may be insufficient to identify small changes in non-coding, intergenic DNA sequence of regulatory elements that drive phenotypic divergence. In human-derived Vibrio cholerae, the response to distinct chemical cues triggers production of multiple transcription factors that can regulate the Type VI Secretion System (T6), a broadly distributed weapon for interbacterial competition. However, to date, the signaling network remains poorly understood because no regulatory element has been identified for the major T6 locus. Here we identify a conserved cis-acting single nucleotide polymorphism (SNP) controlling T6 transcription and activity. Sequence alignment of the T6 regulatory region from diverse V. cholerae strains revealed conservation of the SNP that we rewired to interconvert V. cholerae T6 activity between chitin-inducible and constitutive states. This study supports a model of pathogen evolution through a non-coding cis-regulatory mutation and preexisting, active transcription factors that confers a different fitness advantage to tightly regulated strains inside a human host and unfettered strains adapted to environmental niches. ImportanceOrganisms sense external cues with regulatory circuits that trigger the production of transcription factors, which bind specific DNA sequences at promoters ("cis" regulatory elements) to activate target genes. Mutations of transcription factors or their regulatory elements create phenotypic diversity, allowing exploitation of new niches. Waterborne pathogen Vibrio cholerae encodes the Type VI Secretion System "nanoweapon" to kill competitor cells when activated. Despite identification of several transcription factors, no regulatory element has been identified in the promoter of the major Type VI locus, to date. Combining phenotypic, genetic, and genomic analysis of diverse V. cholerae strains, we discovered a single nucleotide polymorphism in the Type VI promoter that switches its killing activity between a constitutive state beneficial outside hosts and an inducible state for constraint in a host. Our results support a role for non-coding DNA in adaptation of this pathogen.

microbiology↗

bioRxiv2021

De novo evolution of macroscopic multicellularity

While early multicellular lineages necessarily started out as relatively simple groups of cells, little is known about how they became Darwinian entities capable of open-ended multicellular adaptation1,2. To explore this, we initiated the Multicellularity Long Term Evolution Experiment (MuLTEE), selecting for larger group size in the snowflake yeast (Saccharomyces cerevisiae) model system. Given the historical importance of oxygen limitation3, our ongoing experiment consists of three metabolic treatments4: anaerobic, obligately aerobic, and mixotrophic yeast. After 600 rounds of selection, snowflake yeast in the anaerobic treatment evolved to be macroscopic, becoming ~2{middle dot}104 times larger (~mm scale) and ~104-fold more biophysically tough, while retaining a clonal multicellular life cycle. They accomplished this through sustained biophysical adaptation, evolving increasingly elongate cells that initially reduced the strain of cellular packing, then facilitated branch entanglements that enabled groups of cells to stay together even after many cellular bonds fracture. In contrast, snowflake yeast competing for low oxygen remained microscopic, evolving to be just ~6-fold larger, underscoring the critical role of oxygen levels in the evolution of multicellular size. Taken together, this work provides unique insight into an ongoing evolutionary transition in individuality, showing how simple groups of cells overcome fundamental biophysical limitations via gradual, yet sustained, multicellular adaptation.

evolutionary biology↗

bioRxiv2021

Cellular organization in lab-evolved and extant multicellular species obeys a maximum entropy law

The prevalence of multicellular organisms is due in part to their ability to form complex structures. How cells pack in these structures is a fundamental biophysical issue, underlying their functional properties. However, much remains unknown about how cell packing geometries arise, and how they are affected by random noise during growth - especially absent developmental programs. Here, we quantify the statistics of cellular neighborhoods of two different multicellular eukaryotes: lab-evolved "snowflake" yeast and the green alga Volvox carteri. We find that despite large differences in cellular organization, the free space associated with individual cells in both organisms closely fits a modified gamma distribution, consistent with maximum entropy predictions originally developed for granular materials. This entropic cellular packing ensures a degree of predictability despite noise, facilitating parent-offspring fidelity even in the absence of developmental regulation. Together with simulations of diverse growth morphologies, these results suggest that gamma-distributed cell neighborhood sizes are a general feature of multicellularity, arising from conserved statistics of cellular packing.

biophysics↗

bioRxiv2021

Spontaneous emergence of multicellular heritability

The Major Transitions in evolution include events and processes that result in the emergence of new levels of biological individuality. For collectives to undergo Darwinian evolution, their traits must be heritable, but the emergence of higher-level heritability is poorly understood and has long been considered a stumbling block for nascent evolutionary transitions. A change in the means by which genetic information is utilized and transmitted has been presumed necessary. Using analytical models, synthetic biology, and biologicallyinformed simulations, we explored the emergence of trait heritability during the evolution of multicellularity. Contrary to existing theory, we show that no additional layer of genetic regulation is necessary for traits of nascent multicellular organisms to become heritable; rather, heritability and the capacity to respond to natural selection on multicellular-level traits can arise "for free." In fact, we find that a key emergent multicellular trait, organism size at reproduction, is usually more heritable than the underlying cell-level trait upon which it is based, given reasonable assumptions.

evolutionary biology↗