Haploid male germ cells package their DNA into a volume that is typically 10% or less that of a somatic cell nucleus. To achieve this remarkable level of compaction, spermatozoa replace most of their ...histones with smaller, highly basic arginine and (in eutherians) cysteine rich protamines. One reason for such a high level of compaction is that it may help optimise nuclear shape and hence support the gametes' swimming ability for the long journey across the female reproductive tract to the oocyte. Super-compaction of the genome may confer additional protection from the effects of genotoxic factors. However, many species including the human retain a fraction of their chromatin in the more relaxed nucleosomal configuration that appears to run counter to the ergonomic, toroidal and repackaging of sperm DNA. Recent research suggests that the composition of this 'residual' nucleosomal compartment, a generally overlooked feature of the male gamete, is far more significant and important than previously thought. In this respect, the transport and incorporation of modified paternal histones by the spermatozoon to the zygote has been demonstrated and indicates another potential paternal effect in the epigenetic reprogramming of the zygote following fertilisation that is independent of imprinting status. In this review, the most recent research into mammalian spermatozoal chromatin composition is discussed alongside evidence for conserved, non-randomly located nucleosomal domains in spermatozoal nuclei, all supporting the hypothesis that the spermatozoon delivers a novel epigenetic signature to the egg that may be crucial for normal development. We also provide some thoughts on why this signature may be required in early embryogenesis.
Current understanding of life‐history evolution and how demographic parameters contribute to population dynamics across species is largely based on assumptions of either constant environments or ...stationary environmental variation. Meanwhile, species are faced with non‐stationary environmental conditions (changing mean, variance, or both) created by climate and landscape change. To close the gap between contemporary reality and demographic theory, we develop a set of transient life table response experiments (LTREs) for decomposing realised population growth rates into contributions from specific vital rates and components of population structure. Using transient LTREs in a theoretical framework, we reveal that established concepts in population biology will require revision because of reliance on approaches that do not address the influence of unstable population structure on population growth and mean fitness. Going forward, transient LTREs will enhance understanding of demography and improve the explanatory power of models used to understand ecological and evolutionary dynamics.
Understanding the scales at which environmental variability affects populations is critical for projecting population dynamics and species distributions in rapidly changing environments. Here we used ...a multilevel Bayesian analysis of range‐wide survey data for Adélie penguins to characterize multidecadal and annual effects of sea ice on population growth. We found that mean sea ice concentration at breeding colonies (i.e., “prevailing” environmental conditions) had robust nonlinear effects on multidecadal population trends and explained over 85% of the variance in mean population growth rates among sites. In contrast, despite considerable year‐to‐year fluctuations in abundance at most breeding colonies, annual sea ice fluctuations often explained less than 10% of the temporal variance in population growth rates. Our study provides an understanding of the spatially and temporally dynamic environmental factors that define the range limits of Adélie penguins, further establishing this iconic marine predator as a true sea ice obligate and providing a firm basis for projection under scenarios of future climate change. Yet, given the weak effects of annual sea ice relative to the large unexplained variance in year‐to‐year growth rates, the ability to generate useful short‐term forecasts of Adélie penguin breeding abundance will be extremely limited. Our approach provides a powerful framework for linking short‐ and longer term population processes to environmental conditions that can be applied to any species, facilitating a richer understanding of ecological predictability and sensitivity to global change.
Temporal scales at which sea ice most strongly predicts population growth of ice‐obligate penguins is unresolved. Using Bayesian multilevel analysis, we found that multidecadal population trends of Adélie penguins are strongly predicted by long‐term sea ice concentration at colony locations. Our results are consistent with an intermediate sea ice optimum hypothesis. However, annual fluctuations in population abundance are not predicted by annual sea ice anomalies, potentially limiting the ability to produce useful short‐term forecasts.
Summer temperature on the Cape Churchill Peninsula (Manitoba, Canada) has increased rapidly over the past 75 years, and flowering phenology of the plant community is advanced in years with warmer ...temperatures (higher cumulative growing degree days). Despite this, there has been no overall shift in flowering phenology over this period. However, climate change has also resulted in increased interannual variation in temperature; if relationships between phenology and temperature are not linear, an increase in temperature variance may interact with an increase in the mean to alter how community phenology changes over time. In our system, the relationship between phenology and temperature was log‐linear, resulting in a steeper slope at the cold end of the temperature spectrum than at the warm end. Because below‐average temperatures had a greater impact on phenology than above‐average temperatures, the long‐term advance in phenology was reduced. In addition, flowering phenology in a given year was delayed if summer temperatures were high the previous year or 2 years earlier (lag effects), further reducing the expected advance over time. Phenology of early‐flowering plants was negatively affected only by temperatures in the previous year, and that of late‐flowering plants primarily by temperatures 2 years earlier. Subarctic plants develop leaf primordia one or more years prior to flowering (preformation); these results suggest that temperature affects the development of flower primordia during this preformation period. Together, increased variance in temperature and lag effects interacted with a changing mean to reduce the expected phenological advance by 94%, a magnitude large enough to account for our inability to detect a significant advance over time. We conclude that changes in temperature variability and lag effects can alter trends in plant responses to a warming climate and that predictions for changes in plant phenology under future warming scenarios should incorporate such effects.
1. Bumblebees inhabit spatially heterogeneous landscapes that are likely characterized by population sources and sinks. To date, most studies of bumblebee habitat requirements have examined static ...relationships between worker abundance and habitat characteristics. However, if habitat types are linked by dispersal, source-sink dynamics could alter overall population sensitivity to habitat loss, changing conclusions from static approaches. 2. Here, we used empirically derived spatial matrix models to study source-sink population dynamics of bumblebees in heterogeneous environments. We used these models to assess population sensitivity to habitat loss and to examine the population dynamics that could occur under rapid habitat change. 3. Loss of natural habitat reduced long-term population growth rates, and more natural habitat was required to sustain bumblebee species with long-distance dispersal than species with short-distance dispersal. The long-term relative density of colonies in each habitat type depended on landscape structure and bumblebee dispersal ability. Under most conditions, high-quality habitat contained higher densities of colonies than low-quality habitats. However, low-quality habitat could contain higher densities than high-quality habitats if populations were in long-term decline. 4. Rapid loss or restoration of natural habitat produced transient population dynamics that differed from long-term dynamics. After large landscape perturbations that affected population structure, transient dynamics persisted for 4-8 years and were longest for species with short dispersal in highly aggregated landscapes. While transient dynamics were short in duration, they caused large effects on long-term population density. 5. Synthesis and applications. When habitats are linked by dispersal or populations have recently experienced perturbations, patterns of abundance on the landscape may not reflect variation in habitat quality. Spatially structured matrix population models are practical tools than can be used to account for nonequilibrium and source-sink dynamics. For bumblebees, population sensitivity to habitat loss and the duration of transient dynamics depend on dispersal ability and landscape configuration. Demographic approaches such as ours can help to disentangle population patterns from processes and will therefore be valuable for guiding conservation and management.
Time series of vital rates are often used to construct “environment-blind” stochastic population projections and calculate the elasticity of population growth to increased temporal variance in vital ...rates. Here, we show that the utility of this widely used demographic tool is greatly limited by shifts in vital rate correlations that occur as environmental drivers become increasingly variable. The direction and magnitude of these shifts are unpredictable without environmentally explicit models. Shifting vital rate correlations had the largest fitness effects on life histories with short to medium generation times, potentially hampering comparative analyses based on elasticities to vital rate variance for a wide range of species. Shifts in vital rate correlations are likely ubiquitous in increasingly variable environments, and further research should empirically evaluate the life histories for which detailed mechanistic relationships between vital rates and environmental drivers are required for making reliable predictions versus those for which summarized demographic data are sufficient.
The Paris Agreement is a multinational initiative to combat climate change by keeping a global temperature increase in this century to 2°C above preindustrial levels while pursuing efforts to limit ...the increase to 1.5°C. Until recently, ensembles of coupled climate simulations producing temporal dynamics of climate en route to stable global mean temperature at 1.5 and 2°C above preindustrial levels were not available. Hence, the few studies that have assessed the ecological impact of the Paris Agreement used ad‐hoc approaches. The development of new specific mitigation climate simulations now provides an unprecedented opportunity to inform ecological impact assessments. Here we project the dynamics of all known emperor penguin (Aptenodytes forsteri) colonies under new climate change scenarios meeting the Paris Agreement objectives using a climate‐dependent‐metapopulation model. Our model includes various dispersal behaviors so that penguins could modulate climate effects through movement and habitat selection. Under business‐as‐usual greenhouse gas emissions, we show that 80% of the colonies are projected to be quasiextinct by 2100, thus the total abundance of emperor penguins is projected to decline by at least 81% relative to its initial size, regardless of dispersal abilities. In contrast, if the Paris Agreement objectives are met, viable emperor penguin refuges will exist in Antarctica, and only 19% and 31% colonies are projected to be quasiextinct by 2100 under the Paris 1.5 and 2 climate scenarios respectively. As a result, the global population is projected to decline by at least by 31% under Paris 1.5 and 44% under Paris 2. However, population growth rates stabilize in 2060 such that the global population will be only declining at 0.07% under Paris 1.5 and 0.34% under Paris 2, thereby halting the global population decline. Hence, global climate policy has a larger capacity to safeguard the future of emperor penguins than their intrinsic dispersal abilities.
Using newly developed mitigation ensembles of fully coupled climate simulations consistent with meeting the Paris Agreement objectives, we derive robust projections of future population dynamics and species persistence for the emperor penguin. We show that global climate policy has the capacity to halt the future projected declines of emperor penguins in ways that their intrinsic biological properties (i.e., dispersal abilities) do not.
During the haploid phase of mammalian spermatogenesis, nucleosomal chromatin is ultimately repackaged by small, highly basic protamines to generate an extremely compact, toroidal chromatin ...architecture that is critical to normal spermatozoal function. In common with several species, however, the human spermatozoon retains a small proportion of its chromatin packaged in nucleosomes. As nucleosomal chromatin in spermatozoa is structurally more open than protamine-packaged chromatin, we considered it likely to be more accessible to exogenously applied endonucleases. Accordingly, we have used this premise to identify a population of endonuclease-sensitive DNA sequences in human and murine spermatozoa. Our results show unequivocally that, in contrast to the endonuclease-resistant sperm chromatin packaged by protamines, regions of increased endonuclease sensitivity are closely associated with gene regulatory regions, including many promoter sequences and sequences recognized by CCCTC-binding factor (CTCF). Similar differential packaging of promoters is observed in the spermatozoal chromatin of both mouse and man. These observations imply the existence of epigenetic marks that distinguish gene regulatory regions in male germ cells and prevent their repackaging by protamines during spermiogenesis. The ontology of genes under the control of endonuclease-sensitive regulatory regions implies a role for this phenomenon in subsequent embryonic development.
Upon arriving in a novel environment, invading populations are likely to be small and far from a stable stage structure. This unstable stage structure can cause transient (short‐term) population ...dynamics to differ greatly from asymptotic (long‐term) dynamics. Because the persistence of small populations depends heavily on population growth rate, short‐term dynamics may strongly influence the viability of invading populations. We used published matrix population models to study the dynamics of small ‘invading’ populations for 105 plant species spanning a range of life histories, including species classified as both invasive and non‐invasive. We simulated the matrix population models to estimate the effect of transient dynamics on population viability (i.e. potential invasiveness) after a hypothetical seed dispersal event into a novel environment. We then evaluated the predictive power of transient and long‐term population growth rates to explain variation in population viability and identified the life‐history correlates of population dynamics that best explained establishment success. Transient and long‐term population growth rates were positively but independently correlated with population viability across species. Minimum transient density (minimum population density attained en route to a stable stage structure) was the best transient predictor of population viability. This suggests that avoidance of severe short‐term population declines is more important during establishment than either the rate of decline or transient ability to increase in density following a decline. Despite a negative correlation between transient density and fecundity, species with high fecundity had disproportionately favourable transient dynamics and higher long‐term population growth rates, resulting in higher population viability. Together, these results suggest that highly fecund species are better equipped to overcome the early effects of demographic stochasticity in the establishment phase than less fecund species and help explain the common empirical finding that species invasiveness is correlated with fecundity. Synthesis. Transient and long‐term population dynamics are independent predictors of demographic performance that influence the viability of invading (i.e. small, unstable) populations subjected to strong effects of demographic stochasticity. Greater long‐term population growth rates and disproportionately favourable transient dynamics may account for the commonly observed invasiveness of highly fecund species. Given the strong dependence of population viability on population growth and the wide range of transient responses among species, transient analysis may provide critical insights into the demographic correlates of biological invasion potential.
Modulation of calcium-sensitive potassium (BK) channels by oxygen is important in several mammalian tissues, and in the carotid body it is crucial to respiratory control. However, the identity of the ...oxygen sensor remains unknown. We demonstrate that hemoxygenase-2 (HO-2) is part of the BK channel complex and enhances channel activity in normoxia. Knockdown of HO-2 expression reduced channel activity, and carbon monoxide, a product of HO-2 activity, rescued this loss of function. Inhibition of BK channels by hypoxia was dependent on HO-2 expression and was augmented by HO-2 stimulation. Furthermore, carotid body cells demonstrated HO-2-dependent hypoxic BK channel inhibition, which indicates that HO-2 is an oxygen sensor that controls channel activity during oxygen deprivation.
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