Is there direct and delayed density dependent variation in population structure in a temperate European cyclic vole population?

Population structure, in terms of the body mass, condition, sex and reproductive status of individuals, has been found to vary in phase with population density in cyclic populations of microtine rodents. Because sustained population cycles involve delayed density dependent changes in the population growth rate, we would expect at least some life history traits also to depend on past densities. Detailed, long-term studies of changes in vole life history traits are however few, and are largely restricted to northern Europe. In view of the uncertainty as to whether the cyclic microtine populations of western Europe represent the same phenomenon as those of northern Europe, we studied temporal variation in the structure of a clearly cyclic population of the common vole Microtus arvalis Pallas, in the cereal plains of mid-western France. Our data set contains seasonal, individual-level data from long-term, large-scale trapping covering four entire population cycles. We found considerable cyclic variation in population structure in spring (April), but less so in summer (June). In spring of post-peak years, animals were of low body weight and body condition (particularly females), litter sizes were smaller and there was a reduction in the proportion of breeders. All of these could be proximal drivers of increased mortality rates, or decreased birth rates, contributing to the population declines. Few life history traits, however, showed direct density dependent variation, and none of the traits studied here showed delayed density dependence. We have shown declines in the fecundity and body condition of voles from a western European population that coincides with, and may be a proximal cause of, cyclic declines in population density. Closer attention to proximal causes, by which ecological processes drive cycles, could clarify the extent to which microtine cycles across Europe represent a single phenomenon.     

Spatial heterogeneity,population “regulation” and local extinction in simulated host-parasitoid interactions

Summary Simulation models have recently been used to suggest that spatial heterogeneity, acting on small spatial scales within local populations, may allow parasitoids and other natural enemies to regulate host or prey populations in ways that would not be detected by conventional (k-factor) analyses of life table data. However, additional study of these models suggests that local extinction may be a frequent event in the simulated interactions. The spreading of risk concept appears more applicable to the simulated populations than a classical view emphasizing tight regulation around stable equilibrium points. The spreading of risk viewpoint also appears to shed additional light on questions raised in the recent debate between Dempster (1983); Hassel (1985); Dempster and Pollard (1986), concerning the modeling of spatial heterogeneity and regulation in temperate-zone insect populations.     

Size-specific interaction patterns and size matching in a plant�Cpollinator interaction web

Background and Aims

Many recent studies show that plant–pollinator interaction webs exhibit consistent structural features such as long-tailed distributions of the degree of generalization, nestedness of interactions and asymmetric interaction dependencies. Recognition of these shared features has led to a variety of mechanistic attempts at explanation. Here it is hypothesized that beside size thresholds and species abundances, the frequency distribution of sizes (nectar depths and proboscis lengths) will play a key role in determining observed interaction patterns.

Methods

To test the influence of size distributions, a new network parameter is introduced: the degree of size matching between nectar depth and proboscis length. The observed degree of size matching in a Spanish plant–pollinator web was compared with the expected degree based on joint probability distributions, integrating size thresholds and abundance, and taking the sampling method into account.

Key Results

Nectar depths and proboscis lengths both exhibited right-skewed frequency distributions across species and individuals. Species-based size matching was equally close for plants, independent of nectar depth, but differed significantly for pollinators of dissimilar proboscis length. The observed patterns were predicted well by a model considering size distributions across species. Observed size matching was closer when relative abundances of species were included, especially for flowers with openly accessible nectar and pollinators with long proboscises, but was predicted somewhat less successfully by the model that included abundances.

Conclusions

The results suggest that in addition to size thresholds and species abundances, size distributions are important for understanding interaction patterns in plant–pollinator webs. It is likely that the understanding will be improved further by characterizing for entire communities how nectar production of flowers and energetic requirements of pollinators covary with size, and how sampling methods influence the observed interaction patterns.Key words: Plant–pollinator community, flower morphology, generalization, nectar, pollination network, body size, size matching, specialization     

Is aggregation of beta-amyloid peptides a mis-functioning of a current interaction process?

In a previous study, Hughes et al. [Proc. Natl. Acad. Sci. USA 93 (1996) 2065-2070] demonstrated that the amyloid peptide is able to interact with itself in a two-hybrid system and that interaction is specific. They further supported that the method could be used to define the sequences that might be important in nucleation-dependent aggregation. The sequence of the amyloid peptide can be split into four clusters, two hydrophilic (1-16 and 22-28) and two hydrophobic (17-21 and 29-42). We designed by molecular modeling and tested by the two-hybrid approach, series of mutations spread all over the sequence and changing the distribution of hydrophobicity and/or the spatial hindrance. In the two-hybrid assay, interaction of native Abeta is reproduced. Screening of mutations demonstrates that the C-domain (residues 29-40 (42)), the median domain (residues 17-22) and the N-domain (1-16) are all crucial for interaction. This demonstrates that almost all fragments of the amyloid peptide but a loop (residues 23-28) and the C-term amino acid are important for the native interaction. We support that the folded three-dimensional (3D) structure is the Abeta-Abeta interacting species, that the whole sequence is involved in that 3D fold which has a low secondary structure propensity and a high susceptibility to mutations and thus should have a low stability. The native fold of Abeta could be stabilized in Abeta-Abeta complexes which could in other circumstances facilitate the nucleation event of aggregation that leads to the formation of stable senile plaques.     

Density‐dependent parasitism in biological control of soil‐borne insects,nematodes, fungi and bacteria

Temporal density‐dependent parasitism appears to explain the development of some pest‐ and pathogen‐suppressive soils. In many examples, an enrichment of hosts (soil‐borne pests or pathogens) precedes and supports a build up of beneficial parasites in perennial crops or in monocultures, and the population dynamics of hosts and parasites may be described in epidemiological terms. Although the examples support the concept of a balance of nature based on density‐dependent regulation, we lack detailed understanding of the processes that drive these beneficial epidemics. In addition, the epidemics develop slowly and often do not provide the control expected by farmers. Current research, therefore, emphasizes inundative release of beneficial agents. Nevertheless, we should study epidemics in soil in order to learn how they might be enhanced; to learn how better to utilize inoculum that is inundatively added to soil; and to contribute to the general discussion of population biology and regulation.     

Is parasitism of metazoa “a one-way ticket”?

Various phenomena interpreted as possible cases of metazoans returning from obligate parasitism to the free-living state (a violation of the ecological interpretation of Dollo’s law) are critically considered. It is shown that among temporary and phase parasites, reversal to the free-living state is a very rare but not exceptional phenomenon. Such cases occur only among the temporary ectoparasites whose ancestors were predators. The only case of violation of Dollo’s law among permanent parasites, although an incompletely proved one, was detected in acariform mites of the family Pyroglyphidae (Acariformes: Psoroptoidea).     

Does risk of egg parasitism affect choice of oviposition sites by a moth? A field and laboratory study

In many insects the eggs are highly vulnerable and egg parasitism can represent a major cause of mortality. It is thus important for the ovipositing female to find a protected site for her eggs. We have examined how egg parasitation can affect host plant choice by Spodoptera littoralis (Boisduval) (Lepidoptera: Noctuidae). Ovipositing moths exhibited significantly higher preference for alfalfa compared to cotton, both in the laboratory and the field. However, larval performance, in terms of development rate and the attained larval and pupal weight, was significantly better on cotton than on alfalfa. In laboratory tests no difference in larval survival on the two host plants was observed. To test the hypothesis that seeking enemy-free space could be a factor behind the preference for alfalfa, the parasitation levels on the two plants were investigated. In field tests, the egg parasitoid Chelonus inanitus (Linnaeus) (Hymenoptera: Braconidae) parasitized a significantly higher number of egg batches on cotton than on alfalfa. Furthermore, the parasitoid was significantly faster in finding and parasitizing S. littoralis eggs on cotton than on alfalfa in a cage experiment. These findings support the assumption that the preference for the inferior larval food plant, alfalfa, reflects a search for a host plant species on which attacks by natural enemies are less likely. Possible factors accounting for the preference of S. littoralis for alfalfa and the higher parasitation rates on cotton are discussed.     

Do laboratory studies predict parasitoid interaction outcomes in the field?

Studying competitive interactions among natural enemies is important to elucidate the success and non-target impact of candidate biological control agents. Increased regulation of new introductions requires that studies on non-target species be carried out in confined conditions. Hypotheses about potential impacts of biological control agents in the field are based on data from Petri dish or small cage experiments conducted in the laboratory. This study compared the performance of the diamondback moth, Plutella xylostella (L.) (Lepidoptera: Plutellidae), parasitoids Diadegma insulare (Cresson) (Hymenoptera: Ichneumonidae) and Microplitis plutellae (Muesebeck) (Hymenoptera: Braconidae) in experiments conducted in small cages in the laboratory and in large cages in the field. Results showed no significant differences between laboratory and field outcomes for D. insulare alone and when D. insulare and M. plutellae were combined. For M. plutellae alone, parasitism in the laboratory cages was significantly less than in the field cages. These results demonstrate that laboratory studies may be useful to develop hypotheses on competitive interactions of candidate parasitoid biological control agents.     

Metapopulation dynamics of a persisting predator–prey system in the laboratory: time series analysis

The scarcity of experimental evidence for the persistence of predator–prey systems at the metapopulation level inspired us to develop a simple predator–prey experiment that could be used for testing several theoretical predictions concerning persistence and its causes. The experimental system used consisted of one or several islands with small bean plants, the phytophagous mite Tetranychus urticae and the predatory mite Phytoseiulus persimilis. In the first experiment, one large system was used consisting of 90 small bean plants, prey and predators. The system persisted for only 120 days. Second, a system was used consisting of eight islands with ten plants each where the islands were connected by bridges. Two replicate experiments showed persistence for at least 393 days. The difference between the first and the second experiments suggests that the longer persistence is caused by a limited migration between the eight islands. Despite efforts to start both replicates of the second experiment with similar initial conditions, the dynamics of both replicates varied substantially. In one replicate the prey and predator numbers showed a trend through time, whereas the numbers fluctuated around a fixed value in the other replicate. A time series analysis of the data of the prey and predators showed the presence of periodicity with a lag of 8.5 weeks in one replicate, whereas such cyclic behaviour was not found in the other replicate. The differences between the two replicates suggest that it is difficult to perform experiments where one replicate is perturbed and the other serves as an undisturbed control. We suggest using a longer time series, where a system is disturbed only during the second half of the experiment. The data from the first and second halves can subsequently be used to estimate the effect of the perturbation. The advantages and disadvantages of this method are discussed. This revised version was published online in November 2006 with corrections to the Cover Date.     

The interaction of β2-glycoprotein I with lysophosphatidic acid in platelet aggregation and blood clotting

β₂-Glycoprotein I (β₂-GPI) is a plasma protein that binds to oxidized low-density lipoprotein (LDL) and negatively charged substances, and inhibits platelet activation and blood coagulation. In this study, we investigated the interaction of β₂-GPI with a negatively charged lysophosphatidic acid (LPA) in platelet aggregation and blood clotting. Two negatively charged lysophospholipids, LPA and lysophosphatidylserine, specifically inhibited the binding of β₂-GPI to oxidized LDL in a concentration-dependent manner. Intrinsic tryptophan fluorescence studies demonstrated that emission intensity of β₂-GPI decreases in an LPA-concentration-dependent manner without a shift in wavelength maxima. LPA specifically induced the aggregation of β₂-GPI in phosphate-buffered saline, and in incubated plasma and serum, both of which are known to accumulate LPA by the action of lecithin-cholesterol acyltransferase and lysophospholipase D/autotaxin. β₂-GPI aggregated by LPA did not inhibit activated von Willebrand factor-induced aggregation, and did not prolong the activated partial thromboplastin time in blood plasma, in contrast to non-aggregated β₂-GPI. These results suggest that β₂-GPI aggregated by the binding to LPA fails to inhibit platelet aggregation and blood clotting in contrast to non-aggregated β₂-GPI.     

Corticosterone levels in host and parasite nestlings: Is brood parasitism a hormonal stressor?

Parasite chicks from non-evictor species usually try to monopolize host parental care, thereby increasing considerably the level of food competition in the nest. Here, we propose that brood parasitism is an important stressor for host and parasite nestlings and explore this hypothesis in the non-evictor great spotted cuckoo (Clamator glandarius) and its main hosts, the same-sized black-billed magpie (Pica pica) and the larger carrion crow (Corvus corone). We experimentally created 3-nestling broods of different brood compositions (only cuckoo chicks, only host chicks, or cuckoo and host chicks together) and measured baseline corticosterone levels of nestlings along their developmental period (early, middle and late). We found that brood parasitism increased corticosterone levels in magpie nestlings in the mid and late nestling period compared to those raised in unparasitized nests. Interestingly, carrion crow nestlings from parasitized nests only increased their corticosterone levels in the mid nestling period, when the competition for food with the cuckoo nestling was highest. Our results suggest that brood parasitism could be a potential physiological stressor for host nestlings, especially during the developmental stages where food requirements are highest. Conversely, cuckoo nestlings could be physiologically adapted to high competition levels since they did not show significant differences in corticosterone levels in relation to brood composition.     

Can caterpillar density or host-plant quality explain host-plant-related parasitism of a generalist forest caterpillar assemblage?

Herbivore–carnivore interactions are influenced by the plants on which herbivores feed. Accordingly, dietary generalist herbivores have been shown to experience differential risk of mortality from carnivores on different host-plant species. Here, we investigate whether caterpillar density and host-plant quality play a role in driving variation in generalist forest caterpillar mortality from insect parasitoids using a large-scale, multi-year observational study. A total of 4,500 polyphagous caterpillars were collected from eight host-tree species in Connecticut deciduous forests over 5 years, and frequencies of mortality from insect parasitoids (flies and wasps) were compared across the eight host-plant species for the entire generalist caterpillar assemblage (76 species). Separate comparisons were made using seven numerically dominant generalist species, allowing us to account for variation in caterpillar species-specific parasitism risk. We find significant variation in parasitism frequencies of generalist caterpillars across the eight host-plant species when accounting for variation in caterpillar density. We find no support for an influence of caterpillar density on parasitism and no clear evidence for an effect of host-plant quality on parasitism. Therefore, the results of this study discount the hypotheses that variation in caterpillar density and host-plant quality are responsible for variation in parasitism frequencies across host-plant species. Instead, our findings point to other plant-related characteristics, such as plant-derived parasitoid attractants, which may have robust, community-wide effects.     

Production and evolution pattern of “fruity smell” aggregation pheromones in genus Drosophila

Insects produce pheromones to serve a range of ecological functions throughout their lifetime. The chemical composition, production pattern, and interspecies specificity provide information for carrying out their function and biological significance. Several species of Drosophila produce a class of volatile esters considered as “fruity smells”; however, the production pattern and ecological functions of these “fruity smell” volatiles in genus Drosophila are poorly understood. Here, using the headspace solid-phase microextraction (SPME) method, we tested the production pattern of volatile pheromones in Drosophila immigrans and factors that possibly affected pheromone production, including mating, feeding conditions, age of adult flies, and geographical distribution. We also explored the evolution and production pattern of volatile pheromones in 14 species of genus Drosophila. Our result showed that male D. immigrans adult flies produce three male-specific volatile ester pheromones, which are also considered as “fruity smell” chemicals, in a relatively stable pattern. In addition, a series of “fruity smell” ester pheromones with similar structure and chemical properties were found to appear in the species of D. virilis and D. immigrans species group, but not in the species of D. melanogaster species group. The ester volatile pheromone production of male flies has a correspondence with the female's demand for host plants. Integrating the production and evolution pattern of these volatile chemicals, we inferred the interaction between insects and host plants reflected in the Drosophila “fruity smell” pheromones.     

PLCβ isoforms differ in their subcellular location and their CT-domain dependent interaction with Gαq

Phospholipase C (PLC) β isoforms are implicated in various physiological processes and pathologies. However, mechanistic insight into the localization and activation of each of the isoforms is limited. Therefore, it is crucial to gain more in-depth knowledge as to the regulation of the different isoforms. Here we describe the subcellular location of full-length PLCβ isozymes and their C-terminal (CT) domains. Strikingly, we found isoforms PLCβ1 and PLCβ4 to be enriched at the plasma membrane, contrary to isoforms PLCβ2 and PLCβ3. We determined that the CT domain is an inhibitor of Gq-mediated increases in intracellular calcium, the potency of its effect being dependent upon the CT domain isoform used. Furthermore, ratiometric fluorescence resonance energy transfer (FRET) imaging was used to study the kinetics of the Gαq–CTβx interactions. By the use of recently developed tools, which enable the on-demand activation of Gαq, we could show that the interaction between constitutively active Gαq and PLCβ3 prolongs the residence time of PLCβ3 at the plasma membrane. These findings suggest that under physiological circumstances, PLCβ3 and Gαq interact in a kiss-and-run fashion, likely due to the GTPase-activating activity of PLCβ towards Gαq.     

Community-wide patterns of parasitism of a host “generalist” brood-parasitic cowbird

The brown-headed cowbird (Molothrusater) is a generalist obligate brood parasite. Despite intensive study and growing concern over the negative impact of cowbird parasitism on populations of many hosts, very little is known about the factors influencing community-wide patterns of cowbird parasitism. Using systematic nest searches, nest parasitism was studied over two breeding seasons at a study site in northeastern Illinois encompassing grassland, forest-edge, and forest habitat, supporting a diverse avian community. Parasitism was observed for 18 out of 34 altricial bird species found nesting at the study site. A total of 299 cowbird eggs and nestlings were found in 191 of a total of 593 nests. Analyses revealed several ecological and behavioral factors associated with frequency of parasitism and the resulting distribution of cowbird eggs. Much higher frequencies of parasitism were found in edge and forest habitats than in grassland. Within the edge habitat, open nests were parasitized significantly more often than cavity nests. Among open nests in the edge habitat, the two largest species were never parasitized. Host behavior, particularly egg-ejection behavior, was associated with a reduced observed frequency of parasitism, but at least three species known to eject cowbird eggs were sometimes parasitized. For six common hosts capable of rearing cowbirds, we found no correlation between level of parasitism and host nest-survivorship, suggesting that fine-grained assessments of host quality by female cowbirds do not influence patterns of parasitism among acceptable host species, or that differences in host quality are not great and/or predictable enough for such fine-grained assessments. Our results suggest that when a variety of possible nests are available, the level of parasitism on a particular species is a balance between a␣cowbird's preference for a particular species and the effectiveness of host species' defenses. A conceptual model was developed that incorporates the observed correlation of cowbird eggs or nestlings with habitat, nest-type, host species' body mass, and host behavioral defenses. Additional community-wide studies of cowbird parasitism will test if this model is applicable to other avian communities. Received: 20 December 1996 / Accepted: 17 May 1997     

Survival and anti-parasite defense in a host metapopulation under heavy brood parasitism: a source–sink dynamic model

The obligate brood parasite common cuckoo Cuculus canorus, widespread in Eurasia, occasionally reaches a high parasitism rate (over 20%), which usually exists only for a short period of time and in cases of new parasitism. Recent results from Hungary proved that a remarkably high parasitism rate (50–66%) can also be maintained constantly for several decades. In this paradoxical situation the reproductive success of the strongly exploited host population is lower than would be necessary for self-reproduction. We developed a model for a hypothetical host–brood parasite system that demonstrated that immigration of naive individuals from a highly reproductive (source) host population might explain the survival of the highly parasitized (sink) population. Our results also showed the possibility of maintaining the high parasitism rate and the imperfection of the hosts counter-adaptation against the brood parasite over a longer period. Gene flow was necessary to maintain both the acceptor genes and the non-mimetic cuckoo eggs in heavy parasitism. When the immigration rate was low (1–2%), an early expansion of the mimetic cuckoos was followed by a spread of anti-parasite defense, and consequently, the parasitism rate stabilized at a lower, but still relatively high level of about 45–60%.     

The “telomereless” erythrocytes and telomere-length dependent erythropoiesis

Approximately 25 trillion erythrocytes (red blood cells) circulate in the bloodstream of an adult human, surpassing the number of circulating leukocytes (white blood cells) by a factor of about 1000. Moreover, the erythrocyte turnover rate accounts for approximately 76% of the turnover rate of all circulating blood cells. This simple math shows that the hematopoietic system principally spends its telomere length-dependent replicative capacity on building and maintaining the erythrocyte blood pool. Erythropoiesis (red blood cell production) is thus the principal cause of telomere shortening with age in hematopoietic cells (HCs), a conclusion that holds significant implications for linking telomere length dynamics in HCs to health and lifespan of modern humans.     

α-Synuclein aggregation and transmission in Parkinson’s disease: a link to mitochondria and lysosome

Science China Life Sciences - The presence of intraneuronal Lewy bodies (LBs) and Lewy neurites (LNs) in the substantia nigra (SN) composed of aggregated α-synuclein (α-syn) has been...