Interleukin 7 receptor alpha chain (IL7R) shows allelic and functional association with multiple sclerosis

Multiple sclerosis is a demyelinating neurodegenerative disease with a strong genetic component. Previous genetic risk studies have failed to identify consistently linked regions or genes outside of the major histocompatibility complex on chromosome 6p. We describe allelic association of a polymorphism in the gene encoding the interleukin 7 receptor alpha chain (IL7R) as a significant risk factor for multiple sclerosis in four independent family-based or case-control data sets (overall P = 2.9 x 10(-7)). Further, the likely causal SNP, rs6897932, located within the alternatively spliced exon 6 of IL7R, has a functional effect on gene expression. The SNP influences the amount of soluble and membrane-bound isoforms of the protein by putatively disrupting an exonic splicing silencer.     

Nutritional constraints in terrestrial and freshwater food webs

Biological and environmental contrasts between aquatic and terrestrial systems have hindered analyses of community and ecosystem structure across Earth's diverse habitats. Ecological stoichiometry provides an integrative approach for such analyses, as all organisms are composed of the same major elements (C, N, P) whose balance affects production, nutrient cycling, and food-web dynamics. Here we show both similarities and differences in the C:N:P ratios of primary producers (autotrophs) and invertebrate primary consumers (herbivores) across habitats. Terrestrial food webs are built on an extremely nutrient-poor autotroph base with C:P and C:N ratios higher than in lake particulate matter, although the N:P ratios are nearly identical. Terrestrial herbivores (insects) and their freshwater counterparts (zooplankton) are nutrient-rich and indistinguishable in C:N:P stoichiometry. In both lakes and terrestrial systems, herbivores should have low growth efficiencies (10-30%) when consuming autotrophs with typical carbon-to-nutrient ratios. These stoichiometric constraints on herbivore growth appear to be qualitatively similar and widespread in both environments.     

Small-amplitude cycles emerge from stage-structured interactions in Daphnia-algal systems

A long-standing issue in ecology is reconciling the apparent stability of many populations with robust predictions of large-amplitude population cycles from general theory on consumer-resource interactions. Even when consumers are decoupled from dynamic resources, large-amplitude cycles can theoretically emerge from delayed feedback processes found in many consumers. Here we show that resource-dependent mortality and a dynamic developmental delay in consumers produces a new type of small-amplitude cycle that coexists with large-amplitude fluctuations in coupled consumer-resource systems. A distinctive characteristic of the small-amplitude cycles is slow juvenile development for consumers, leading to a developmental delay that is longer than the cycle period. By contrast, the period exceeds the delay in large-amplitude cycles. These theoretical predictions may explain previous empirical results on coexisting attractors found in Daphnia-algal systems. To test this, we used bioassay experiments that measure the growth rates of individuals in populations exhibiting each type of cycle. The results were consistent with predictions. Together, the new theory and experiments establish that two very general features of consumers--a resource-dependent juvenile stage duration and resource-dependent mortality--combine to produce small-amplitude resource-consumer cycles. This phenomenon may contribute to the prevalence of small-amplitude fluctuations in many other consumer-resource populations.     

Habitat structure and population persistence in an experimental community

Understanding spatial population dynamics is fundamental for many questions in ecology and conservation. Many theoretical mechanisms have been proposed whereby spatial structure can promote population persistence, in particular for exploiter-victim systems (host-parasite/pathogen, predator-prey) whose interactions are inherently oscillatory and therefore prone to extinction of local populations. Experiments have confirmed that spatial structure can extend persistence, but it has rarely been possible to identify the specific mechanisms involved. Here we use a model-based approach to identify the effects of spatial population processes in experimental systems of bean plants (Phaseolus lunatus), herbivorous mites (Tetranychus urticae) and predatory mites (Phytoseiulus persimilis). On isolated plants, and in a spatially undivided experimental system of 90 plants, prey and predator populations collapsed; however, introducing habitat structure allowed long-term persistence. Using mechanistic models, we determine that spatial population structure did not contribute to persistence, and spatially explicit models are not needed. Rather, habitat structure reduced the success of predators at locating prey outbreaks, allowing between-plant asynchrony of local population cycles due to random colonization events.     

Single-species models for many-species food webs

Most species live in species-rich food webs; yet, for a century, most mathematical models for population dynamics have included only one or two species. We ask whether such models are relevant to the real world. Two-species population models of an interacting consumer and resource collapse to one-species dynamics when recruitment to the resource population is unrelated to resource abundance, thereby weakening the coupling between consumer and resource. We predict that, in nature, generalist consumers that feed on many species should similarly show one-species dynamics. We test this prediction using cyclic populations, in which it is easier to infer underlying mechanisms, and which are widespread in nature. Here we show that one-species cycles can be distinguished from consumer resource cycles by their periods. We then analyse a large number of time series from cyclic populations in nature and show that almost all cycling, generalist consumers examined have periods that are consistent with one-species dynamics. Thus generalist consumers indeed behave as if they were one-species populations, and a one-species model is a valid representation for generalist population dynamics in many-species food webs.     

Stage-structured cycles promote genetic diversity in a predator-prey system of Daphnia and algae

Competition theory predicts that population fluctuations can promote genetic diversity when combined with density-dependent selection. However, this stabilizing mechanism has rarely been tested, and was recently rejected as an explanation for maintaining diversity in natural populations of the freshwater herbivore Daphnia pulex. The primary limitation of competition theory is its failure to account for the alternative types of population cycles that are caused by size- or stage-dependent population vital rates--even though such structure both explains the fluctuating dynamics of many species and may alter the outcome of competition. Here we provide the first experimental test of whether alternative types of cycles affect natural selection in predator-prey systems. Using competing Daphnia genotypes, we show that internally generated, stage-structured cycles substantially reduce the magnitude of selection (thereby contributing to the maintenance of genetic diversity), whereas externally forced cycles show rapid competitive exclusion. The change in selection is ecologically significant, spanning the observed range in natural populations. We argue that structured cycles reduce selection through a combination of stalled juvenile development and stage-specific mortality. This potentially general fitness-equalizing mechanism may reduce the need for strong stabilizing mechanisms to explain the maintenance of genetic diversity in natural systems.     

Interferon action--sequence specificity of the ppp(A2'p)nA-dependent ribonuclease

The oligonucleotides pppA2'p5'A2'p5'A and related oligomers (2-5A) are synthesized by an enzyme that is widely distributed in a variety of cells, the activity of which varies with interferon treatment, growth and hormone status. Because significant amounts of 2-5A have recently been detected in interferon-treated cells, it has been suggested that the oligonucleotides may be involved in interferon action and in the control of cell metabolism. In both intact cells and cell-free systems 2-5A has been shown to activate a ribonuclease. We report here investigations of the sequence specificities of the 2-5A-dependent ribonucleases in extracts of rabbit reticulocytes, mouse ascites tumour cells and human lymphoblastoid cells in conditions of partial digestion using terminally labelled RNA substrates. The enzymes cleaved on the 3'-side of UN sequences to yield UpNp terminated products. Cleavage was observed predominantly at UA and UU sequences.