Thomas C Day

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 nonliving 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 …

Whole-genome duplication (WGD) is widespread across eukaryotes and can promote adaptive evolution. However, given the instability of newly-formed polyploid genomes, understanding how WGDs arise in a population, persist, and underpin adaptations remains a challenge. Using our ongoing Multicellularity Long Term Evolution Experiment (MuLTEE), we show that diploid snowflake yeast (Saccharomyces cerevisiae) under selection for larger multicellular size rapidly undergo spontaneous WGD. From its origin within the first 50 days of the experiment, tetraploids persist for the next 950 days (nearly 5,000 generations, the current leading edge of our experiment) in ten replicate populations, despite being genomically unstable. Using synthetic reconstruction, biophysical modeling, and counter-selection experiments, we found that tetraploidy evolved because it confers immediate fitness benefits in this environment, by producing larger, longer cells that yield larger clusters. The same selective benefit also maintained tetraploidy over long evolutionary timescales, inhibiting the reversion to diploidy that is typically seen in laboratory evolution experiments. Once established, tetraploidy facilitated novel genetic routes for adaptation, playing a key role in the evolution of macroscopic multicellular size via the origin of evolutionarily conserved aneuploidy. These results provide unique empirical insights into the evolutionary dynamics and impacts of WGD, showing how it can initially arise due to its immediate adaptive benefits, be maintained by selection, and fuel long-term innovations by creating additional dimensions of heritable genetic variation.

The evolution of multicellular life spurred evolutionary radiations, fundamentally changing many of Earth’s ecosystems. Yet little is known about how early steps in the evolution of multicellularity affect eco-evolutionary dynamics. Through long-term experimental evolution, we observed niche partitioning and the adaptive divergence of two specialized lineages from a single multicellular ancestor. Over 715 daily transfers, snowflake yeast were subjected to selection for rapid growth, followed by selection favouring larger group size. Small and large cluster-forming lineages evolved from a monomorphic ancestor, coexisting for over ~4,300 generations, specializing on divergent aspects of a trade-off between growth rate and survival. Through modelling and experimentation, we demonstrate that coexistence is maintained by a trade-off between organismal size and competitiveness for dissolved oxygen. Taken together …

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 …

Multicellular organisms often exhibit morphologies featuring branching chains of cells. Examples include root systems, mycelial networks, cyanobacterial mats, and dense brush and bushes. Notably, filamentous, branching objects can wind around each other, thus forming tangles that are difficult to disassemble. This phenomenon, called''entanglement'', is well-known to occur in non-growing materials. However, entanglement is typically studied in non-living, and thus non-growing materials, such as polymers. Further, with few exceptions, currently studied systems are filamentous rather than branching, and generally flexible rather than rigid. Much less studied is the emergence of entanglement in the regime of growing, branching, and relatively rigid systems, despite their prevalence across the domains of life. Consequently, many open questions regarding entanglement remain unclear: how readily does …

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. Using analytical models, synthetic biology, and biologically-informed simulations, we explored the emergence of trait heritability during the evolution of multicellularity. Prior work on the evolution of multicellularity has asserted that substantial collective-level trait heritability either emerges only late in the transition or requires some evolutionary change subsequent to the formation of clonal multicellular groups. In a prior analytical model, we showed that collective-level heritability not only exists but is usually more heritable than the underlying cell-level trait upon which it is based, as soon as multicellular groups form. Here, we show that key assumptions and predictions of that model are borne out in a real engineered biological system, with important implications for the emergence of collective-level heritability.

While early multicellular lineages necessarily started out as relatively simple groups of cells, little is known about how they became Darwinian entities capable of sustained multicellular evolution, –. Here we investigate this with a multicellularity long-term evolution experiment, selecting for larger group size in the snowflake yeast (Saccharomyces cerevisiae) model system. Given the historical importance of oxygen limitation, our ongoing experiment consists of three metabolic treatments—anaerobic, obligately aerobic and mixotrophic yeast. After 600 rounds of selection, snowflake yeast in the anaerobic treatment group evolved to be macroscopic, becoming around 2 × 104 times larger (approximately mm scale) and about 104-fold more biophysically tough, while retaining a clonal multicellular life cycle. This occurred through biophysical adaptation—evolution of increasingly elongate cells that initially reduced the …

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 …

The rhino population has dropped over 50% in the last 50 years due to poaching for their strong and durable horns. But where does this strength and durability come from? Unlike other animal horns, rhino horns do not have a bony core; instead, they are made completely out of keratin, the same material comprising nails, feathers and hair. The material properties of the horn thus arise solely from the arrangement of its keratin fibers. In this experimental study, we analyze the structure, orientation, and function the fibers play in this strength. We hypothesize that the structural integrity is built through entangled and intertwined fibers. Entangled and intertwined structures increase material strength evident by a nonlinear stress-strain relationship. We characterize the effects of the keratin's structural impact by performing mechanical, histochemical, and microscopy tests on a preserved Black rhinoceros (Diceros bicornis …

The evolution of large organismal size is fundamentally important for multicellularity, creating new ecological niches and opportunities for the evolution of increased biological complexity. Yet little is known about how large size evolves, particularly in nascent multicellular organisms that lack genetically-regulated multicellular development. Here we examine how novel biophysical drivers of macroscopic multicellularity arise, including both the minimal requirements which drive novel biophysical adaptation and the dynamics of their emergence, using a combination of experiments and simulations. In a long-term evolutionary experiment, we observed multiple independent lines of multicellular snowflake yeast evolve macroscopic size rapidly, becoming 20,000 times larger and 10,000 times more physically tough over a span of 600 days. They accomplished this through sustained biophysical adaptation, evolving a …

The diversity of multicellular organisms is, in large part, due to the fact that multicellularity has independently evolved many times. Nonetheless, multicellular organisms all share a universal biophysical trait: cells are attached to each other. All mechanisms of cellular attachment belong to one of two broad classes; intercellular bonds are either reformable or they are not. Both classes of multicellular assembly are common in nature, having independently evolved dozens of times. In this review, we detail these varied mechanisms as they exist in multicellular organisms. We also discuss the evolutionary implications of different intercellular attachment mechanisms on nascent multicellular organisms. The type of intercellular bond present during early steps in the transition to multicellularity constrains future evolutionary and biophysical dynamics for the lineage, affecting the origin of multicellular life cycles, cell–cell …

A distinctive fishtail feature in multifractal spectra derived from heart rate variability (HRV) data closely resembles similar features in plots of the Gibbs free energy versus reduced pressure for a van der Waals fluid below the critical temperature, and, in analogy to the nonideal fluid, signals a phase transition from one multifractal state to another. This fishtail feature, often overlooked or dismissed in previous studies, is observed most prevalently in unhealthy patients and suggests a realignment between multifractal variables and their thermodynamic analogs. The van der Waals analogy and subsequent alternate interpretation of multifractal variables and functions lead one to construct a three-dimensional heart health phase diagram reminiscent of a diagram for the van der Waals fluid. The diagram which makes use of the Maxwell width as a measure of heart health produces an orderly progression from …

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.

Multicellular organisms are pervasive and broadly successful. Their success is due in part to their complex structures and functions, which self-assemble through a genetically regulated multicellular development plan. However, early multicellular groups likely lacked the developmental genes necessary to ensure that group traits were heritable. Without developmental genes, cells divide stochastically, meaning that the final cellular configuration emerges at the whim of random events, making it unclear how nascent multicellular groups achieve heritable size and shape. Here, we propose that stochasticity supplied by random cell division can directly lead to highly heritably group traits. We find that experimentally evolved multicellular groups formed with fixed bonds via stochastic cell division achieve robust, heritable multicellular size and reproduction due to emergent statistics from entropic cellular packing. Our …

R23. 00007: Early Multicellular Organisms Co-opt Cell-Level Characteristics into Group-Level Properties via the Principle of Maximum Entropy

The evolution of multicellular organisms on earth is one of the most transformative events in the history of life. Despite its importance, we know little about the process by which nascent microscopic multicellular organisms overcome substantial mechanical constraints and dramatically increase their size. We experimentally study this process with the snowflake yeast model system: baker's yeast (S. cerevisiae) with a single mutation in ACE2 gene allows mother and daughter cells to remain attached via uncut chitin bonds. These yeast clusters are composed of a few hundred cells and grow to a maximum diameter of 200 microns. After a year of artificial selection for larger multicellular size, five populations of snowflake yeast surprisingly evolved to grow to a diameter of 1 mm. In this work we show how small changes at the cell level trait lead to emergent properties in the microscopic level and helped to overcome …