Session M01: Living timekeepers: Precision measurements, emergent simplicities and physics theory

Live

Fireflies offer a unique and rare glimpse into animal communication. Their signal comprises a species-specific on/off light pattern repeated periodically, used by individual fireflies to advertise themselves to potential mates. Detecting individuals becomes increasingly challenging at high densities of fireflies. In this talk, I will explore how fireflies approach this problem while using physics and information-theory concepts, e.g., energetic cost and compression (minimization of bits representing information) and detectability (high signal-to-noise-ratio). The first approach involves signal amplification via synchronization within swarms containing tens of thousands of individuals. Our recent quantitative measurements of the three-dimensional spatiotemporal flashing pattern of synchronous firefly swarms allow us to validate a set of mathematical models that account for short-range spatial correlations and the signal's emergent periodicity. The second approach involves the evolutionary design of light patterns with increased detectability at other individuals' expense. Using a computational model, we observe an emergent periodicity in the resulting optimal sequences and demonstrate a method of reconstructing potential cost functions from the phylogenetic relationships of extant species alongside their characteristic flash patterns.   

Live

Microtubules are an essential biophysical element of eukaryotic cells. Their localization and nucleation kinetics are heavily controlled to build intricate structures inside the cell. The vast majority of microtubules in many circumstances are nucleated as branches off of existing microtubule filaments. I will discuss recent work on the nucleation of microtubules in space and time. Using a combination of single molecule imaging and force microscopy, my colleagues and I have unraveled details of the transition state for branch nucleation and a role for dewetting via the Raylegh-Plateau instability in governing where microtubules branches are formed.   

Live

In this talk I will address how life shapes time in a simple bacterial cell, and examine the interplay between homeostasis and adaptation in this context. First, I will first establish that intergenerational bacterial cell size homeostasis is maintained under appropriate growth conditions, using our high-precision data on growth and division of individual C. Crescentus cells. Next, the emergent simplicities revealed by these data. Following this, a fitting-free theoretical framework, consistent with observed scaling laws and phenomenology. This naturally leads to a proposal for the underlying mechanistic model. Finally, the extension to time-varying growth conditions, and new emergent simplicities.   

On Demand

Our rhythmic world has been a driving force for organisms to evolve a clock to
anticipate such rhythms. Similarly, our own circadian biology leads to body rhythms that
parasites experience. Malaria major symptom is fever. Malarial rhythmic fevers are a
consequence of the synchronous bursting of host’s red blood cells (RBCs) on
completion of the malaria parasite asexual cell cycle. How is this bursting synchronous
across the parasite population? Are parasites following host cues or do they also have a
clock to anticipate host daily rhythms? Through a combination of infection challenges
where we manipulate the environment or rhythms of the host by infections of circadian
mutant hosts and assess the rhythms of the parasites. We found that without any
external cue to the parasites, their transcriptome remains rhythmic with ~60% of
parasite transcripts being expressed once a day. Thus, we propose malaria parasites to
have intrinsic clocks. Parasite rhythms are aligned to the host daily rhythms but are
generated by the parasite, possibly to anticipate its circadian environment.   

Live

Self-sustaining oscillations on the timescale of a day, known as circadian rhythms, are commonly found in living systems, but the underlying selective pressures that favor a dynamical system with stable oscillations remain unclear. Cyanobacteria have the most ancient circadian clock known and present a powerful model system to investigate questions of mechanism and fitness, and previous work has shown that this clock is coupled intimately to the metabolism of the cell. Using numerical simulations, we argue that a stable oscillator is a biochemically viable solution to the problem of inferring the metabolic state hours in the future based on past metabolic information. One essential process that may require future knowledge is DNA replication where completion of the chromosome occurs hours after the decision to initiate a replication cycle. We show that, consistent with this idea, initiation of replication is suppressed by the clock as nightfall approaches. Using live cell imaging approaches we show that the stability of replication forks in the dark depends strongly on the clock state, suggesting that clock control is also needed to allow open replication forks to complete successfully in the night. Consistent with this prediction, we find that "jet-lagged" cells accumulate incomplete chromosomes in the night. We argue that protection of genome integrity from the threat of fluctuating resource availability may have been an ancient driver leading to the creation of circadian rhythms.