Colony size and not nest density drives reproductive output in the Common Tern Sterna hirundo

Density is known to be an important factor in population size regulation. Several mechanisms of density limitation have been identified in colonial birds. We studied competition in Common Terns Sterna hirundo to assess whether the factor limiting reproductive output was competition for nest‐sites, which is dependent on local nest density, or density‐dependent competition for food resources, which is dependent on overall colony size using the same foraging area. We found strong associations of both colony size and nest density with reproductive output in five colonies of Common Terns in three different habitats (one marine, two freshwater). Based on detailed long‐term datasets of six separate sub‐colonies of the Banter See colony that differed in nest density, we found that reproductive success was not related to nest density but to overall colony size, possibly a result of resource depletion and food competition. We also found carry‐over effects of colony size during rearing on post‐fledging return rate. These results have important implications for the conservation management plans aimed at recovering declining populations of Common Terns.     

Evidence of facultative daytime hypothermia in a small passerine wintering at northern latitudes

The use of hypothermia as a means to save energy is well documented in birds. This energy‐saving strategy is widely considered to occur exclusively at night in diurnally active species. However, recent studies suggest that facultative hypothermia may also occur during the day. Here, we document the use of daytime hypothermia in foraging Black‐capped Chickadees Poecile atricapillus wintering in eastern Canada. We measured the body temperature (T_b) of 126 individuals (plus 48 repeated measures) during a single winter and related values to ambient temperature (T_a) at the time of capture. We also tested whether daytime hypothermia was correlated with the size of body reserves (residuals of mass on structural size and fat score) and levels of metabolic performance (basal metabolic rate and maximum thermogenic capacity). We found that T_b of individual birds was lower when captured at low T_a, reaching values as low as 35.5 °C in actively foraging individuals. T_b was unrelated to metabolic performance or measures of body reserves. Therefore, daytime hypothermia does not result from individuals being unable to maintain T_b during cold spells or to a lack of body reserves. Our data also demonstrated a high level of individual variation in the depth of hypothermia, the causes of which remain to be explored.     

Lower foraging efficiency of offspring constrains use of optimal habitat in birds with extended parental care

After reproducing successfully, birds with extended parental care form family groups. Despite being the dominant social unit, such family groups have been reported to switch to alternative habitat earlier than adults without offspring, with potential negative carry‐over effects for the next breeding season. Here we test a proposed mechanism for this earlier habitat switch, namely a low foraging efficiency in juveniles. Such a test is best performed under controlled conditions because in the field families may occupy food patches of a different quality than singles or pairs without young. We studied this mechanism in Bewick's Swans Cygnus columbianus bewickii, which trample (or ‘treadle’) for food buried in the sediment. The gross intake rate of juveniles was as low as 60% of that of adults, depending on the burial depth of the food. Trampling effort did not differ between age classes, but differences in intake rate were related to body size, suggesting that larger or heavier birds were trampling more efficiently. Corresponding giving‐up densities in the field were calculated to be c. 60% higher for juveniles than for adults. Our findings are consistent with the hypothesis that the lower foraging efficiency of juveniles may be responsible for the segregation of family groups from adults without offspring.     

Substrate water exchange in photosystem II core complexes of the extremophilic red alga Cyanidioschyzon merolae

The binding affinity of the two substrate–water molecules to the water-oxidizing Mn₄CaO₅ catalyst in photosystem II core complexes of the extremophilic red alga Cyanidioschyzon merolae was studied in the S₂ and S₃ states by the exchange of bound ¹⁶O-substrate against ¹⁸O-labeled water. The rate of this exchange was detected via the membrane-inlet mass spectrometric analysis of flash-induced oxygen evolution. For both redox states a fast and slow phase of water-exchange was resolved at the mixed labeled m/z 34 mass peak: k_f = 52 ± 8 s^− 1 and k_s = 1.9 ± 0.3 s^− 1 in the S₂ state, and k_f = 42 ± 2 s^− 1 and k_slow = 1.2 ± 0.3 s^− 1 in S₃, respectively. Overall these exchange rates are similar to those observed previously with preparations of other organisms. The most remarkable finding is a significantly slower exchange at the fast substrate–water site in the S₂ state, which confirms beyond doubt that both substrate–water molecules are already bound in the S₂ state. This leads to a very small change of the affinity for both the fast and the slowly exchanging substrates during the S₂ → S₃ transition. Implications for recent models for water-oxidation are briefly discussed.     

Sticholysin I–membrane interaction: An interplay between the presence of sphingomyelin and membrane fluidity

Sticholysin I (St I) is a pore-forming toxin (PFT) produced by the Caribbean Sea anemone Stichodactyla helianthus belonging to the actinoporin protein family, a unique class of eukaryotic PFT exclusively found in sea anemones. As for actinoporins, it has been proposed that the presence of sphingomyelin (SM) and the coexistence of lipid phases increase binding to the target membrane. However, little is known about the role of membrane structure and dynamics (phase state, fluidity, presence of lipid domains) on actinoporins' activity or which regions of the membrane are the most favorable platforms for protein insertion. To gain insight into the role of SM on the interaction of St I to lipid membranes we studied their binding to monolayers of phosphatidylcholine (PC) and SM in different proportions. Additionally, the effect of acyl chain length and unsaturation, two features related to membrane fluidity, was evaluated on St I binding to monolayers. This study revealed that St I binds and penetrates preferentially and with a faster kinetic to liquid-expanded films with high lateral mobility and moderately enriched in SM. A high content of SM induces a lower lateral diffusion and/or liquid-condensed phases, which hinder St I binding and penetration to the lipid monolayer. Furthermore, the presence of lipid domain borders does not appear as an important factor for St I binding to the lipid monolayer.     

A model for the coordinated stepping of cytoplasmic dynein

Cytoplasmic dynein play an important role in transporting various intracellular cargos by coupling their ATP hydrolysis cycle with their conformational changes. Recent experimental results showed that the cytoplasmic dynein had a highly variable stepping pattern including “hand-over-hand”, “inchworm” and “nonalternating-inchworm”. Here, we developed a model to describe the coordinated stepping patterns of cytoplasmic dynein, based on its working cycle, construction and the interaction between its leading head and tailing head. The kinetic model showed how change in the distance between the two heads influences the rate of cytoplasmic dynein under different stepping patterns. Numerical simulations of the distribution of step size and striding rate are in good quantitative agreement with experimental observations. Hence, our coordinated stepping model for cytoplasmic dynein successfully explained its diverse stepping patterns as a molecular motor. The cooperative mechanism carried out by the two heads of cytoplasmic dynein shed light on the strategies adopted by the cytoplasmic dynein in executing various functions.     

RATES OF FITNESS DECLINE AND REBOUND SUGGEST PERVASIVE EPISTASIS

Unraveling the factors that determine the rate of adaptation is a major question in evolutionary biology. One key parameter is the effect of a new mutation on fitness, which invariably depends on the environment and genetic background. The fate of a mutation also depends on population size, which determines the amount of drift it will experience. Here, we manipulate both population size and genotype composition and follow adaptation of 23 distinct Escherichia coli genotypes. These have previously accumulated mutations under intense genetic drift and encompass a substantial fitness variation. A simple rule is uncovered: the net fitness change is negatively correlated with the fitness of the genotype in which new mutations appear—a signature of epistasis. We find that Fisher's geometrical model can account for the observed patterns of fitness change and infer the parameters of this model that best fit the data, using Approximate Bayesian Computation. We estimate a genomic mutation rate of 0.01 per generation for fitness altering mutations, albeit with a large confidence interval, a mean fitness effect of mutations of ?0.01, and an effective number of traits nine in mutS^? E. coli. This framework can be extended to confront a broader range of models with data and test different classes of fitness landscape models.