Cross-Scale Interactions and the Distribution-Abundance Relationship

Positive interspecific relationships between local abundance and extent of regional distribution are among the most ubiquitous patterns in ecology. Although multiple hypotheses have been proposed, the mechanisms underlying distribution-abundance (d-a) relationships remain poorly understood. We examined the intra- and interspecific distribution-abundance relationships for a metacommunity of 13 amphibian species sampled for 15 consecutive years. Mean density of larvae in occupied ponds was positively related to number of ponds occupied by species; employing the fraction of ponds uniquely available to each species this same relationship sharply decelerates. The latter relationship suggested that more abundant species inhabited most available habitats annually, whereas rarer species were dispersal limited. We inferred the mechanisms responsible for this pattern based on the dynamics of one species, Pseudacris triseriata, which transitioned between a rare, narrowly distributed species to a common, widely distributed species and then back again. Both transitions were presaged by marked changes in mean local densities driven by climatic effects on habitat quality. We identified threshold densities separating these population regime shifts that differed with landscape configuration. Our data suggest that these transitions were caused by strong cross-scale interactions between local resource/niche processes and larger scale metapopulation processes. The patterns we observed have relevance for understanding the mechanisms of interspecific d-a relationships and critical thresholds associated with habitat fragmentation.     

Characterization of schistosome tegumental alkaline phosphatase (SmAP)

Schistosomes are parasitic platyhelminths that currently infect over 200 million people globally. The parasites can live for years in a putatively hostile environment - the blood of vertebrates. We have hypothesized that the unusual schistosome tegument (outer-covering) plays a role in protecting parasites in the blood; by impeding host immunological signaling pathways we suggest that tegumental molecules help create an immunologically privileged environment for schistosomes. In this work, we clone and characterize a schistosome alkaline phosphatase (SmAP), a predicted ～60 kDa glycoprotein that has high sequence conservation with members of the alkaline phosphatase protein family. The SmAP gene is most highly expressed in intravascular parasite life stages. Using immunofluorescence and immuno-electron microscopy, we confirm that SmAP is expressed at the host/parasite interface and in internal tissues. The ability of living parasites to cleave exogenous adenosine monophosphate (AMP) and generate adenosine is very largely abolished when SmAP gene expression is suppressed following RNAi treatment targeting the gene. These results lend support to the hypothesis that schistosome surface enzymes such as SmAP could dampen host immune responses against the parasites by generating immunosuppressants such as adenosine to promote their survival. This notion does not rule out other potential functions for the adenosine generated e.g. in parasite nutrition.     

Preliminary crystallographic data for NAD(P)H quinone reductase isolated from the Walker 256 rat carcinoma cell line

An NAD(P)H quinone reductase isolated from Walker rat 256 carcinoma cells has been crystallized in a form suitable for high-resolution structural analysis. The crystals belong to orthorhombic space group P2(1)2(1)2(1) with cell parameters a = 168.15 A, b = 105.09 A and c = 67.38 A and contain four monomeric or two dimeric enzyme molecules per asymmetric unit. Diffraction extends beyond 2.3 A resolution.     

Active management of labour: care of the fetus.

A prospective study of 1000 consecutive primigravidae was conducted to assess the relevance of the colour of the liquor to the welfare of the child. The results showed that clear liquor early in labour virtually ensures the birth of a healthy infant, provided the duration of labour is limited and delivery is effected without trauma; and, conversely, that meconium--or no liquor--marks the fetus who may suffer death or brain damage during normal labour. Special techniques were reserved for the second group of cases.     

Cell surface phenotype of lymphoid cells from normal mice and mice treated with monoclonal anti-IgD from birth

Mice treated from birth with mouse monoclonal anti-IgD antibodies develop low frequencies of B cells in the spleen, a small percentage of which express very low levels of sIgD on their cell surface and extremely low frequencies of B cells in their lymph nodes, lacking sIgD entirely. However, the splenic B cells are phenotypically mature in that a high percentage of these cells express Lyb-5, indicating that the expression of sIgD is not a prerequisite for the acquisition of a mature surface antigen repertoire of B cells. In contrast, a high density of sIgM on splenic B cells is expressed, which suggests a predominance of cells with the phenotype of immature B cells and/or activated B cells. Furthermore, the spleen cells from anti-IgD-treated mice lack cells that respond to in vitro stimulation by LPS with an increase in the density of their sIa.     

Metacommunity patterns in larval odonates

The growth of metacommunity ecology as a subdiscipline has increased interest in how processes at different spatial scales structure communities. However, there is still a significant knowledge gap with respect to relating the action of niche- and dispersal-assembly mechanisms to observed species distributions across gradients. Surveys of the larval dragonfly community (Odonata: Anisoptera) in 57 lakes and ponds in southeast Michigan were used to evaluate hypotheses about the processes regulating community structure in this system. We considered the roles of both niche- and dispersal-assembly processes in determining patterns of species richness and composition across a habitat gradient involving changes in the extent of habitat permanence, canopy cover, area, and top predator type. We compared observed richness patterns and species distributions in this system to patterns predicted by four general community models: species sorting related to adaptive trade-offs, a developmental constraints hypothesis, dispersal assembly, and a neutral community assemblage. Our results supported neither the developmental constraints nor the neutral-assemblage models. Observed patterns of richness and species distributions were consistent with patterns expected when adaptive tradeoffs and dispersal-assembly mechanisms affect community structure. Adaptive trade-offs appeared to be important in limiting the distributions of species which segregate across the habitat gradient. However, dispersal was important in shaping the distributions of species that utilize habitats with a broad range of hydroperiods and alternative top predator types. Our results also suggest that the relative importance of these mechanisms may change across this habitat gradient and that a metacommunity perspective which incorporates both niche- and dispersal-assembly processes is necessary to understand how communities are organized. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.