Context-dependent fitness costs of reproduction despite stable body mass costs in an Arctic herbivore.

The cost of reproduction on demographic rates is often assumed to operate through changing body condition. Several studies have found that reproduction depresses body mass more if the current conditions are severe, such as high population densities or adverse weather, than under benign environmental conditions. However, few studies have investigated the association between the fitness components and body mass costs of reproduction. Using 25 years of individual-based capture-recapture data from Svalbard reindeer Rangifer tarandus platyrhynchus, we built a novel Bayesian state-space model that jointly estimated interannual change in mass, annual reproductive success and survival, while accounting for incomplete observations. The model allowed us to partition the differential effects of intrinsic and extrinsic factors on both non-reproductive mass change and the body mass cost of reproduction, and to quantify their consequences on demographic rates. Contrary to our expectation, the body mass cost of reproduction (mean = -5.8 kg) varied little between years (CV = 0.08), whereas the between-year variation in body mass changes, that were independent of the previous year's reproductive state, varied substantially (CV = 0.4) in relation to autumn temperature and the amount of rain-on-snow in winter. This body mass loss led to a cost of reproduction on the next reproduction, which was amplified by the same environmental covariates, from a 10% reduction in reproductive success in benign years, to a 50% reduction in harsh years. The reproductive mass loss also resulted in a small reduction in survival. Our results show how demographic costs of reproduction, driven by interannual fluctuations in individual body condition, result from the balance between body mass costs of reproduction and body mass changes that are independent of previous reproductive state. We illustrate how a strong context-dependent fitness cost of reproduction can occur, despite a relatively fixed body mass cost of reproduction. This suggests that female reindeer display a very conservative energy allocation strategy, either aborting their reproductive attempt at an early stage or weaning at a relatively constant cost. Such a strategy might be common in species living in a highly stochastic and food limited environment.

The neglected season: Warmer autumns counteract harsher winters and promote population growth in Arctic reindeer.

Arctic ungulates are experiencing the most rapid climate warming on Earth. While concerns have been raised that more frequent icing events may cause die-offs, and earlier springs may generate a trophic mismatch in phenology, the effects of warming autumns have been largely neglected. We used 25 years of individual-based data from a growing population of wild Svalbard reindeer, to test how warmer autumns enhance population growth. Delayed plant senescence had no effect, but a six-week delay in snow-onset (the observed data range) was estimated to increase late winter body mass by 10%. Because average late winter body mass explains 90% of the variation in population growth rates, such a delay in winter-onset would enable a population growth of r = 0.20, sufficient to counteract all but the most extreme icing events. This study provides novel mechanistic insights into the consequences of climate change for Arctic herbivores, highlighting the positive impact of warming autumns on population viability, offsetting the impacts of harsher winters. Thus, the future for Arctic herbivores facing climate change may be brighter than the prevailing view.

Continental-scale tree-ring-based projection of Douglas-fir growth: Testing the limits of space-for-time substitution.

A central challenge in global change research is the projection of the future behavior of a system based upon past observations. Tree-ring data have been used increasingly over the last decade to project tree growth and forest ecosystem vulnerability under future climate conditions. But how can the response of tree growth to past climate variation predict the future, when the future does not look like the past? Space-for-time substitution (SFTS) is one way to overcome the problem of extrapolation: the response at a given location in a warmer future is assumed to follow the response at a warmer location today. Here we evaluated an SFTS approach to projecting future growth of Douglas-fir (Pseudotsuga menziesii), a species that occupies an exceptionally large environmental space in North America. We fit a hierarchical mixed-effects model to capture ring-width variability in response to spatial and temporal variation in climate. We found opposing gradients for productivity and climate sensitivity with highest growth rates and weakest response to interannual climate variation in the mesic coastal part of Douglas-fir's range; narrower rings and stronger climate sensitivity occurred across the semi-arid interior. Ring-width response to spatial versus temporal temperature variation was opposite in sign, suggesting that spatial variation in productivity, caused by local adaptation and other slow processes, cannot be used to anticipate changes in productivity caused by rapid climate change. We thus substituted only climate sensitivities when projecting future tree growth. Growth declines were projected across much of Douglas-fir's distribution, with largest relative decreases in the semiarid U.S. Interior West and smallest in the mesic Pacific Northwest. We further highlight the strengths of mixed-effects modeling for reviving a conceptual cornerstone of dendroecology, Cook's 1987 aggregate growth model, and the great potential to use tree-ring networks and results as a calibration target for next-generation vegetation models.

Mycobacterium tuberculosis Strains H37ra and H37rv have equivalent minimum inhibitory concentrations to most antituberculosis drugs.

Background: Mycobacterium tuberculosis (Mtb) strains H37Ra and H37Rv are commonly used to study new and re-evaluate old antituberculous agents with respect to their pharmacodynamic effects in vitro. The differences in membrane proteins and, in particular, differences in carrier proteins between Mtb H37Ra and Mtb H37Rv may have an impact on antibiotic potency. The question of whether H37Ra can be used as a reliable surrogate for H37Rv and clinical strains has not been addressed sufficiently. The purpose of this study is to provide a full comparison of susceptibility data of the most common antituberculosis (TB) agents against both Mtb strains. Methods: In addition to a literature review, in vitro checkerboard susceptibility study was conducted comparing the in vitro minimum inhibitory concentration (MIC) of 16 common antituberculous drugs against H37Ra and H37Rv. Heifets-Sanchez TB agar drug susceptibility plates were utilized. Results: Half of the antibiotics demonstrated similar growth inhibition against both strains, while slightly differing MIC values were found for 7 of 16 drugs. With the exception of rifampicin, no marked difference in MIC against H37Ra and H37Rv was observed. Conclusion: While neither the attenuated (H37Ra) nor the virulent strain (H37Rv) is a clinical strain, both strains predicted MICs of clinical isolates equally well, when comparing the current in vitro results to clinical susceptibility data in the literature. H37Ra comes with the benefits of lower experimental costs and less administrative barriers including the requirement of a biosafety Level III environment.

Crystal structure of cone arrestin at 2.3A: evolution of receptor specificity.

Arrestins play a fundamental role in the regulation and signal transduction of G protein-coupled receptors. Here we describe the crystal structure of cone arrestin at 2.3A resolution. The overall structure of cone visual arrestin is similar to the crystal structures of rod visual and the non-visual arrestin-2, consisting of two domains, each containing ten beta-sheets. However, at the tertiary structure level, there are two major differences, in particular on the concave surfaces of the two domains implicated in receptor binding and in the loop between beta-strands I and II. Functional analysis shows that cone arrestin, in sharp contrast to its rod counterpart, bound cone pigments and non-visual receptors. Conversely, non-visual arrestin-2 bound cone pigments, suggesting that it may also regulate phototransduction and/or photopigment trafficking in cone photoreceptors. These findings indicate that cone arrestin displays structural and functional features intermediate between the specialized rod arrestin and the non-visual arrestins, which have broad receptor specificity. A unique functional feature of cone arrestin was the low affinity for its cognate receptor, resulting in an unusually rapid dissociation of the complex. Transient arrestin binding to the photopigment in cones may be responsible for the extremely rapid regeneration and reuse of the photopigment that is essential for cone function at high levels of illumination.

The atomic structure of human methemalbumin at 1.9 A.

The high resolution structure of hemalbumin was determined by single crystal X-ray diffraction to a resolution of 1.9 A. The structure revealed the protoporphyrin IX bound to a single site within a hydrophobic cavity in subdomain IB, one of the principal binding sites for long chain fatty acid. The iron is penta coordinated with the fifth ligand comprised of the hydroxyl oxygen of Tyr-161 (phenolic oxygen to heme plane distance: 2.73 A) in an otherwise completely hydrophobic pocket. The heme propionic acid residues form salt bridges with His-142 and Lys-190, which together with a series of hydrophobic interactions, enclose and secure the heme within the IB helical motif. A detailed discussion of the structure together with its implications for the development of potential blood substitutes is presented.