Paleolimnological perspectives on the shifting geographic template of permafrost landscapes and its implications for Arctic freshwater biodiversity

Geographic context matters when trying to understand how permafrost thaw impacts northern freshwater biodiversity in a warming climate. Most risk to freshwater from thawing permafrost is associated with abrupt thaw processes known as thermokarst. Lake sediments can provide a record of thermokarst landscape development and associated biogeochemical and biodiversity trends over long timescales, providing a tool to link thermokarst geomorphology with freshwater biodiversity. We describe how paleolimnology, with its inherent emphasis on long-term perspectives, can characterize the shifting geographic template of warming thermokarst landscapes and its implications for biodiversity. We suggest aligning thermokarst lake paleolimnological research with hypothesis-testing frameworks used by permafrost hydrologists and biogeochemists and by the Freshwater Circumpolar Biodiversity Monitoring Program, and advocate for knowledge co-production with northern Indigenous communities. Lastly, we stress the importance of considering geographic context in the choice of study sites to ensure that diverse thermokarst landscapes are represented (especially those most vulnerable to warming) and that the fine-scale differences in limnological settings that influence ecosystem response to thermokarst stressors are accounted for.

Computational modelling and near-complete kinetochore tracking reveal how chromosome dynamics during cell division are co-ordinated in space and time

Mitotic chromosome segregation is a self-organising process that achieves high fidelity separation of 46 duplicated chromosomes into two daughter cells. Chromosomes must be captured by the microtubule-based spindle, aligned at the spindle equator where they undergo oscillatory motion (metaphase) and then pulled to opposite spindle poles (anaphase). These large and small-scale chromosome movements are driven by kinetochores, multi-protein machines, that link chromosomes to microtubules and generate directional forces. Through automated near-complete tracking of kinetochores at fine spatio-temporal resolution over long timescales, we produce a detailed atlas of kinetochore dynamics throughout metaphase and anaphase in human cells. We develop a hierarchical biophysical model of kinetochore dynamics and fit this model to 4D lattice light sheet experimental data using Bayesian inference. We demonstrate that location in the metaphase plate is the largest factor in the variation in kinetochore dynamics, exceeding the variation between cells, whilst within the spindle there is local spatio-temporal coordination between neighbouring kinetochores of directional switching events, kinetochore-fibre (K-fibre) polymerization/depolymerization state and the segregation of chromosomes. Thus, metaphase oscillations are robust to variation in the mechanical forces throughout the spindle, whilst the spindle environment couples kinetochore dynamics across the plate. Our methods provide a framework for detailed quantification of chromosome dynamics during mitosis in human cells.

Mechanism of life-long maintenance of neuron identity despite molecular fluctuations

Cell fate is maintained over long timescales, yet molecular fluctuations can lead to spontaneous loss of this differentiated state. Our simulations identified a possible mechanism that explains life-long maintenance of ASE neuron fate in C. elegans by the terminal selector transcription factor CHE-1. Here, fluctuations in CHE-1 level are buffered by the reservoir of CHE-1 bound at its target promoters, which ensures continued che-1 expression by preferentially binding the che-1 promoter. We provide experimental evidence for this mechanism by showing that che-1 expression was resilient to induced transient CHE-1 depletion, while both expression of CHE-1 targets and ASE function were lost. We identified a 130 bp che-1 promoter fragment responsible for this resilience, with deletion of a homeodomain binding site in this fragment causing stochastic loss of ASE identity long after its determination. Because network architectures that support this mechanism are highly conserved in cell differentiation, it may explain stable cell fate maintenance in many systems.

Timescale Analyses of Fluctuations in Coexisting Populations of a Native and Invasive Tree Squirrel

1. Competition from invasive species is an increasing threat to biodiversity. In Southern California, the western gray squirrel (Sciurus griseus, WGS) is facing increasing competition from the fox squirrel (Sciurus niger, FS), an invasive congener. 2. We used spectral methods to analyze 140 consecutive monthly censuses of WGS and FS within a 11.3 ha section of the California Botanic Garden. Variation in the numbers for both species and their synchrony was distributed across long timescales (> 15 months). 3. After filtering out annual changes, concurrent mean monthly temperatures from nearby Ontario Airport (ONT) yielded a spectrum with a large semiannual peak and significant spectral power at long timescales (> 30 months). Squirrel-temperature cospectra showed significant negative covariation at long timescales (> 35 months) for WGS and smaller significant negative peaks at 6 months for both species. 4. Simulations from a Lotka-Volterra model of two competing species indicates that the risk of extinction for the weaker competitor increases quickly as environmental noise shifts from short to long timescales. 5. We analyzed the timescales of fluctuations in detrended mean annual temperatures for the time period 1915-2014 from 1218 locations across the continental USA. In the last two decades, significant shifts from short timescales to long timescales have occurred, changing from less than 3 years to 4-6 years. 6. Our results indicate that (i) population fluctuations in co-occurring native and invasive tree squirrels are synchronous, occur over long timescales, and may be driven by fluctuations in environmental conditions; (ii) long timescale population fluctuations increase the risk of extinction in competing species, especially for the inferior competitor; and (iii) the timescales of interannual environmental fluctuations may be increasing from recent historical values. These results have broad implications for the impact of climate change on the maintenance of biodiversity.

Timescale Analyses of Fluctuations in Coexisting Populations of a Native and Invasive Tree Squirrel

1. Competition from invasive species is an increasing threat to biodiversity. In Southern California, the western gray squirrel (Sciurus griseus, WGS) is facing increasing competition from the fox squirrel (Sciurus niger, FS), an invasive congener. 2. We used spectral methods to analyze 140 consecutive monthly censuses of WGS and FS within a 11.3 ha section of the California Botanic Garden. Variation in the numbers for both species and their synchrony was distributed across long timescales (> 15 months). 3. After filtering out annual changes, concurrent mean monthly temperatures from nearby Ontario Airport (ONT) yielded a spectrum with a large semiannual peak and significant spectral power at long timescales (> 30 months). Squirrel-temperature cospectra showed significant negative covariation at long timescales (> 35 months) for WGS and smaller significant negative peaks at 6 months for both species. 4. Simulations from a Lotka-Volterra model of two competing species indicates that the risk of extinction for the weaker competitor increases quickly as environmental noise shifts from short to long timescales. 5. We analyzed the timescales of fluctuations in detrended mean annual temperatures for the time period 1915-2014 from 1218 locations across the continental USA. In the last two decades, significant shifts from short timescales to long timescales have occurred, changing from less than 3 years to 4-6 years. 6. Our results indicate that (i) population fluctuations in co-occurring native and invasive tree squirrels are synchronous, occur over long timescales, and may be driven by fluctuations in environmental conditions; (ii) long timescale population fluctuations increase the risk of extinction in competing species, especially for the inferior competitor; and (iii) the timescales of interannual environmental fluctuations may be increasing from recent historical values. These results have broad implications for the impact of climate change on the maintenance of biodiversity.

Multi-timescale control of Southern Ocean diapycnal mixing over Atlantic tracer budgets

Oceanic cross-density (diapycnal) mixing helps sustain the ocean den- sity stratification and its Meridional Overturning Circulation (MOC) and is key to global tracer distributions. The Southern Ocean (SO) is a key region where different overturning cells connect, allowing nutri- ent and carbon rich Indian and Pacific deep waters, and oxygen rich Atlantic deep waters to resurface. The SO is also rife with localized intense diapycnal mixing due to breaking of internal waves induced by the interaction of energetic eddies and currents with rough topogra- phy. SO diapycnal mixing is believed to be of secondary importance for the MOC. Here we show that changes to SO mixing can cause sig- nificant alterations to Atlantic biogeochemical tracer distributions over short and long timescales in an idealized model of the MOC. While such alterations are dominated by the direct impact of changes in diapycnal mixing on tracer fluxes on annual to decadal timescales, on centennial timescales they are dominated by the mixing-induced variations in the advective transport of the tracers by the Atlantic MOC. This work sug- gests that an accurate representation of spatio-temporally variable local and non-local mixing processes in the SO is essential for climate mod- els’ ability to i) simulate the biogeochemical cycles and air sea carbon fluxes on decadal timescales, ii) represent the indirect impact of mixing- induced changes to MOC on biogeochemical cycles on longer timescales.

Rational regulation of water-seeking effort in rodents

In the laboratory, animals’ motivation to work tends to be positively correlated with reward magnitude. But in nature, rewards earned by work are essential to survival (e.g., working to find water), and the payoff of that work can vary on long timescales (e.g., seasonally). Under these constraints, the strategy of working less when rewards are small could be fatal. We found that instead, rats in a closed economy did more work for water rewards when the rewards were stably smaller, a phenomenon also observed in human labor supply curves. Like human consumers, rats showed elasticity of demand, consuming far more water per day when its price in effort was lower. The neural mechanisms underlying such “rational” market behaviors remain largely unexplored. We propose a dynamic utility maximization model that can account for the dependence of rat labor supply (trials/day) on the wage rate (milliliter/trial) and also predict the temporal dynamics of when rats work. Based on data from mice, we hypothesize that glutamatergic neurons in the subfornical organ in lamina terminalis continuously compute the instantaneous marginal utility of voluntary work for water reward and causally determine the amount and timing of work.

Stellar Gravitational Lens Engineering for Interstellar Communication and Artifact SETI

Several recent works have proposed a “stellar relay” transmission system in which a spacecraft at the focus of a star’s gravitational lens achieves dramatic boosts in the gain of an outgoing or incoming interstellar transmission. We examine some of the engineering requirements of a stellar relay system, evaluate the long-term sustainability of a gravitational relay, and describe the perturbations and drifts that must be actively countered to maintain a relay-star-target alignment. The major perturbations on a relay-Sun-target alignment are the inwards gravity of the Sun and the reflex motion of the Sun imparted by the planets. These ∼m s−1 yr−1 accelerations can be countered with modern propulsion systems over century-long timescales. This examination is also relevant for telescope designs aiming to use the Sun as a focusing element. We additionally examine prospects for an artifact SETI search to observe stellar relays placed around the Sun by an extraterrestrial intelligence and suggest certain nearby stars that are relatively unperturbed by planetary systems as favorable nodes for a stellar relay communications system.

Punctuated evolution in the learned songs of African sunbirds

Learned traits are thought to be subject to different evolutionary dynamics than other phenotypes, but their evolutionary tempo and mode has received little attention. Learned bird song has been thought to be subject to rapid and constant evolution. However, we know little about the evolutionary modes of learned song divergence over long timescales. Here, we provide evidence that aspects of the territorial songs of Eastern Afromontane sky island sunbirds Cinnyris evolve in a punctuated fashion, with periods of stasis of the order of hundreds of thousands of years or more, broken up by evolutionary pulses. Stasis in learned songs is inconsistent with learned traits being subject to constant or frequent change, as would be expected if selection does not constrain song phenotypes over evolutionary timescales. Learned song may instead follow a process resembling peak shifts on adaptive landscapes. While much research has focused on the potential for rapid evolution in bird song, our results suggest that selection can tightly constrain the evolution of learned songs over long timescales. More broadly, these results demonstrate that some aspects of highly variable, plastic traits can exhibit punctuated evolution, with stasis over long time periods.