Molluscan defence mechanisms: immunity or population biology?

In recent studies of associations between host and parasite, there has been a tendency to consider the internal defence mechanisms of the mollusc as pivotal in the relationship. However, evolution is a population-based phenomenon, and it is by considering populations rather than individuals that the success of these associations can best be appreciated. Among susceptible host species within the biocoenosis, truly resistant strains appear to be remarkably rare. Clive Shiff here examines mechanisms of defence or avoidance among molluscan hosts of tremotodes from both the internal and ecological perspective.     

What drives the positive correlation between human population density and bird species richness in Australia?

Aim To test six hypotheses that could explain or mediate the positive correlation between human population density (HPD) and bird species richness while controlling for biased sampling effort. These hypotheses were labelled as follows: productivity (net primary productivity, NPP); inherent heterogeneity (diversity of vegetation types); anthropogenic heterogeneity (diversity of land uses); conservation policy (proportion of conservation land); increased productivity (human‐induced productivity increases); and the reduced‐slope hypothesis (which predicts that humans have a negative impact on species numbers across the full range of variation in HPD). Location Australia. Methods All data were collected at a spatial resolution of 1° across mainland Australia. Bird species richness was from 2007 atlas data and random subsampling was used to account for biased sampling effort. HPD was from the 2006 census. All other data were from government produced geographic information system layers. The most important biotic or abiotic factors influencing patterns in both species richness and HPD were assessed using simultaneous autoregressive models and an information theoretic approach. Results NPP appeared to be one of the main factors driving spatial congruence between bird species richness and HPD. Inherent habitat heterogeneity was weakly related to richness and HPD, although an interaction between heterogeneity and NPP indicated that the former may be an important determinant of species richness in low‐productivity regions. There was little evidence that anthropogenic landscape heterogeneity or human‐induced changes in productivity influenced the relationship between species richness and HPD, but conservation policy appeared to act as an important mediating factor and species richness was positively related to the proportion of conservation land only in regions of high HPD. Main conclusions The spatial congruence between bird species richness and HPD occurs because both respond positively to productivity and, in certain circumstances, habitat heterogeneity. Our results suggest that conservation policy could mediate this relationship, but further research is required to determine the importance of conservation reserves in supporting species in regions densely populated by humans.     

How to lift a model for individual behaviour to the population level?

The quick answer to the title question is: by bookkeeping; introduce as p(opulation)-state a measure telling how the individuals are distributed over their common i(ndividual)-state space, and track how the various i-processes change this measure. Unfortunately, this answer leads to a mathematical theory that is technically complicated as well as immature. Alternatively, one may describe a population in terms of the history of the population birth rate together with the history of any environmental variables affecting i-state changes, reproduction and survival. Thus, a population model leads to delay equations. This delay formulation corresponds to a restriction of the p-dynamics to a forward invariant attracting set, so that no information is lost that is relevant for long-term dynamics. For such equations there exists a well-developed theory. In particular, numerical bifurcation tools work essentially the same as for ordinary differential equations. However, the available tools still need considerable adaptation before they can be practically applied to the dynamic energy budget (DEB) model. For the time being we recommend simplifying the i-dynamics before embarking on a systematic mathematical exploration of the associated p-behaviour. The long-term aim is to extend the tools, with the DEB model as a relevant goal post.     

Kernel weight per spike: what contributes to it at the individual QTL level?

Spike length (SL), spikelet number (SPN) per spike, kernel number per spike (KNPS), and thousand-kernel weight (TKW) have strong genetic associations with kernel weight per spike (KWPS) in wheat. To investigate their genetic relationships at the individual quantitative trait locus (QTL) level, both unconditional and conditional QTL mapping for KWPS with respect to SL, SPN, KNPS, and TKW were conducted. Two related F_8:9 recombinant inbred line populations, comprising 485 and 229 lines, respectively, were used. The trait phenotypic performances of each population were evaluated in four different environments. Unconditional QTL mapping analysis identified 22 putative additive QTL for KWPS, five of which were stable QTL, and only QKwps-WJ-1B.2 showed significant additive-by-environment interaction effects. In comparison with unconditional QTL mapping analysis, conditional QTL mapping analysis indicated that, at the QTL level, KNPS and TKW contributed more to KWPS than did SL and SPN. Any unconditional QTL for KWPS detected in this study was associated with at least one of its four related traits. The present study will provide assistance in the understanding of the genetic relationships between KWPS and its related traits.     

Herd size in large herbivores: encoded in the individual or emergent?

In large mammalian herbivores, the increase of group size with habitat openness was first assumed to be an adaptive response, encoded in the individual. However, it could, alternatively, be an emergent property: if groups were nonpermanent units, often fusing and splitting up, then any increase of the distance at which animals perceive one another could increase the rate of group fusion and thus mean group size. Dynamical models and empirical data support this second hypothesis. This is not to say that adaptive modifications of mean herd size cannot occur. However, this changes the way in which we can envisage the history of gregariousness in large herbivores during the Tertiary.     

Diet divergence in two sympatric congeneric butterflies: Community or species level phenomenon?

Summary Two species ofEuphydryas butterflies were studied in California, USA, and showed considerable diet overlap at the species level. They utilize many of the same plant genera for oviposition. However,E. editha is less likely to use woody perennials than isE. chalcedona.Both butterfly species are known to specialize on different host plants in different populations, so species level divergence may not be a good predictor of community level divergence. Within five communities,E. editha andE. chalcedona showed no dietary overlap. A major component of the niche ofE. editha in one community was occupied byE. chalcedona in a second community, even though both butterfly species occupied both communities. These resource use patterns indicate that community level interactions may affect diet divergence. The degree to which divergence within communities is greater (or less) than expected from a species level comparison may be used to provide a measure of community organization. Equations are given in the Appendix for calculating overlap probabilities from presence/absence types of data; in this study, presence is oviposition on a particular plant species, absence is no oviposition on that plant species. Given the various assumptions of the model,E. editha andE. chalcedona show significant community level components of their dietary divergence in the areas studied. However, in some other communitiesE. editha andE. chalcedona do share host plant species. Therefore, we could not demonstrate community level divergence conclusively, nor has this been demonstrated for any other pair of insect herbivore species. We do not know whether this is because the phenomenon is truly rare or just very hard to demonstrate.     

Is high species richness at intermediate disturbance level valuable for phylogenetic conservation? A test on bird species in South‐East Botswana

The intermediate disturbance hypothesis (IDH) is one of the most debated theories in ecology. However, even when evidence is provided to support the hypothesis, its relevance for phylogenetic conservation has rarely been tested. Here, we investigated this question on birds in the South‐East district of Botswana along a disturbance gradient across three types of landscapes. We first reconstructed the phylogeny for all species recorded. Next, we assessed the relationship between dissimilarity measures and habitat types using the permutational MANOVA. Finally, we tested the IDH by fitting a generalized linear mixed effect model to account for errors due to spatial pseudo‐replications of our collection design. We found that, although species richness and phylogenetic diversity (PD) follow the prediction of the IDH, the evolutionary component of PD (i.e. PSV, phylogenetic species variability) contributes little to the prediction, suggesting that the correlation between PD and disturbance level is driven by the richness component of PD (i.e. PSR, phylogenetic species richness). However, the increased richness at the intermediate disturbance level does not result in phylogenetically diverse bird communities, indicating that the IDH contributes little to phylogenetic diversity. Our study adds to the body of literature questioning the relevance of IDH in ecology.     

Allostery in the lac operon: population selection or induced dissociation?

Allostery, the modulation of function of a protein at one site by the binding of a ligand at a different site, is a property of many proteins. Two kinetically distinct models have been proposed: i) The induced fit model in which the ligand binds to the protein and then induces the conformational change. ii) The population selection model, in which the protein spontaneously undergoes a conformational change, which is then ‘captured’ by the ligand. Using measured kinetic constants for the lac repressor the contribution of population selection vs. induced dissociation is quantified by simulating the kinetics of allostery. At very low inducer concentration, both mechanisms contribute significantly. Total induction, though, is small under these conditions. At increasing levels of induction the induced dissociation mechanism soon dominates, first due to binding of one inducer, and then from two inducers binding.     

Are the costs of employer-sponsored health insurance passed on to workers at the individual level?

Because employer-sponsored health insurance (ESI) is experience rated, employers have an incentive to try to offset its cost by paying lower wages to employees who have greater medical expenditures. The existing evidence on this topic, however, illustrates only that ESI is associated with lower wages for groups of workers who are costlier to cover. In contrast, I use the variation provided by the Affordable Care Act's employer mandate to examine if differences in medical expenditures are passed on to workers at the individual level. My estimates rely on Medical Expenditure Panel Survey data in a dose response difference-in-difference framework that examines how wages change for workers with varying medical expenditures when they must soon be offered ESI. I find that each $1 difference in medical expenditures is associated with a $0.35 to $0.51 wage offset after the employer mandate's announcement wherever ESI must soon be offered to workers. Placebo analyses, focusing on workers whose employers are not affected by the mandate, provide support for a causal interpretation. I also show that my findings are not sensitive to sample selection or data reliability issues and that they cannot be explained by the effects of the Great Recession, demographic characteristics that correlate with medical expenditures, or location- or industry-specific idiosyncratic shocks.     

Invasion-resistance in experimental grassland communities: species richness or species identity?

The question as to why some communities are more invasible than others has pro-found implications for conservation biology and land management. The theoretical issues involved go right to the heart of our understanding of species coexistence and community assembly. The experiment reported here indicates that for productive, small-scale grassland plots, species identity matters more than species richness in determining both the number of invading species and the total biomass of invasives.     

Mitosis in plants: how far we have come at the molecular level?

The basic mechanism of mitosis is universally conserved in all eucaryotes, but specific solutions to achieve this process have been adapted by different organisms during evolution. Although cytological studies of plant cells have contributed to our understanding of chromatin dynamics during mitosis, many of the molecular mechanisms that control mitosis have been identified in yeast and animal cells. Nevertheless, recent advances have begun to fill the gaps in our understanding of how mitosis is regulated in plants, and raise intriguing questions to be answered in the future.     

How much do we know about the breeding biology of bird species in the world?

Knowledge on species’ breeding biology is the building blocks of avian life history theory. A review for the current status of the knowledge at a global scale is needed to highlight the priority for future research. We collected all available information on three critical nesting parameters (clutch size, incubation period and nestling period) for the close to 10 000 bird species in the world and identified taxonomic, geographic and habitat gaps in the distribution of knowledge on avian breeding biology. The results show that only one third of all extant species are well known regarding the three nesting parameters analyzed, while the rest are partly or poorly known. Most data deficient taxonomic groups are tropical forest nesters, particularly from the Amazon basin, southeast Asia, Equatorial Africa and Madagascar – the places that harbor the world's highest bird diversity. These knowledge gaps could be hampering our understanding of avian life histories. Ornithologists are encouraged to pay more efforts to explore the breeding biology of those poorly‐known species.     

Cytokines in chronic heart failure: possible interaction in the neurohormonal and the cytokine system at the cAMP level?

In recent years, the pathophysiological concept of chronic heart failure (CHF) has changed from an isolated hemodynamic view to a more complex concept involving neuroendocrine and inflammatory pathways. New therapeutic strategies, such as beta-blocker therapy, are based on these new concepts and provide clinical evidence for a clinical benefit in patients with CHF. The survival benefit of beta-blocker therapy in CHF has been related to neurohumoral regulation. Thus, evidence evolved showing that following beta-blocker therapy cytokine levels in CHF patients are altered. We have shown that the levels of soluble TNF receptor type 2 correlated well with cAMP in leukocytes. Data from clinical studies in adult and infant CHF patients have demonstrated that beta-blocker therapy is accompanied by altered cytokine, cytokine antagonist, and/or soluble cytokine receptor levels. These alterations may result from a dysregulated interaction of beta-adrenergic pathways and the cytokine system, and are possibly related to cAMP-dependent regulation of the release or shedding of these mediators.     

Mutation processes at the protein level: is Lamarck back?

The experimental evidence accumulated for the last half of the century clearly suggests that inherited variation is not restricted to the changes in genomic sequences. The prion model, originally based on unusual transmission of certain neurodegenerative diseases in mammals, provides a molecular mechanism for the template-like reproduction of alternative protein conformations. Recent data extend this model to protein-based genetic elements in yeast and other fungi. Reproduction and transmission of yeast protein-based genetic elements is controlled by the "prion replication" machinery of the cell, composed of the protein helpers responsible for the processes of assembly and disassembly of protein structures and multiprotein complexes. Among these, the stress-related chaperones of Hsp100 and Hsp70 groups play an important role. Alterations of levels or activity of these proteins result in "mutator" or "antimutator" affects in regard to protein-based genetic elements. "Protein mutagens" have also been identified that affect formation and/or propagation of the alternative protein conformations. Prion-forming abilities appear to be conserved in evolution, despite the divergence of the corresponding amino acid sequences. Moreover, a wide variety of proteins of different origins appear to possess the ability to form amyloid-like aggregates, that in certain conditions might potentially result in prion-like switches. This suggests a possible mechanism for the inheritance of acquired traits, postulated in the Lamarckian theory of evolution. The prion model also puts in doubt the notion that cloned animals are genetically identical to their genome donors, and suggests that genome sequence would not provide a complete information about the genetic makeup of an organism.     

Estimating bird species richness: How should repeat surveys be organized in time?

Abstract Estimates of species richness for a given area require that repeat surveys be taken, so that the statistical robustness of the estimate can be assessed. But how should these repeat surveys be organized in time? Here we present a case study of Australian woodland birds, surveyed using the ‘active timed area search’ method, which has become the standard unit for the Australian Bird Atlas, a continental‐scale bird survey. To date, there has been no assessment of how estimates of species richness derived from this method are affected by the temporal organization of the repeat surveys. For instance, can conducting the repeat surveys in sequence on the same day effectively capture richness, or will additional species be obtained by repeating the surveys on different days within a season? If so, does the spacing of the repeat visits throughout the season have an effect? To answer these questions, we surveyed woodland birds in the Mount Lofty Ranges, South Australia, during late spring–summer 1999–2000, and compared the performance of two different temporal configurations of repeat visits to sites: (i) six repeat surveys performed on the same day; and (ii) three repeat surveys on different days. For both, we calculated the average number of species actually sighted and also estimated total species richness. The data supported our hypothesis that the same‐day surveys would yield fewer species and underestimate total species richness. The different‐day repeats captured significantly more species per unit of survey effort, and yielded a higher richness estimate. However, the timespan over which different‐day surveys were conducted within a season did not have a significant influence on species richness estimates, evincing a qualitative advantage to surveying on different days, regardless of the spacing of repeat visits. These results may be of assistance to conservation managers when planning cost‐efficient monitoring regimes.     

Do environmental changes or juvenile competition act as mechanisms of species displacement in crayfishes?

The Big Creek Crayfish, Orconectes peruncus, is native to the St. Francis River drainage in Missouri, USA and is often absent where the introduced Woodland Crayfish, Orconectes hylas, has established. We performed a field experiment to determine whether effects of current abiotic conditions and interspecific competition with O. hylas were responsible for displacement of O. peruncus from parts of their former range. We examined growth and survival of juvenile male O. peruncus exposed to juvenile male O. hylas in enclosures at two sites in the former range of O. peruncus. Enclosures contained 8 (low density) or 16 individuals (high density) and had O. peruncus only (control) or both species (interspecific treatment). Juvenile O. peruncus were able to survive and grow in portions of their former range, implicating biotic versus abiotic factors in the displacement of O. peruncus. Survival rates of O. peruncus did not differ among treatments at either site. Orconectes peruncus showed significant growth in all treatments and interspecific effects were not greater than intraspecific effects on O. peruncus growth rates. High-density treatments showed significantly reduced O. peruncus growth rates compared to low-density treatments, except in Carver Creek interspecific treatments. When considered in the context of previous studies examining the effects of O. hylas on O. peruncus, results suggest that neither direct competition between juvenile males of the two species or abiotic change are responsible for the decreased range of O. peruncus. Additional research is required to determine the mechanism(s) driving the displacement of O. peruncus.