Interactions of biotic and abiotic environmental factors in an ectomycorrhizal symbiosis, and the potential for selection mosaics

BACKGROUND

Geographic selection mosaics, in which species exert different evolutionary impacts on each other in different environments, may drive diversification in coevolving species. We studied the potential for geographic selection mosaics in plant-mycorrhizal interactions by testing whether the interaction between bishop pine (Pinus muricata D. Don) and one of its common ectomycorrhizal fungi (Rhizopogon occidentalis Zeller and Dodge) varies in outcome, when different combinations of plant and fungal genotypes are tested under a range of different abiotic and biotic conditions.

RESULTS

We used a 2 x 2 x 2 x 2 factorial experiment to test the main and interactive effects of plant lineage (two maternal seed families), fungal lineage (two spore collections), soil type (lab mix or field soil), and non-mycorrhizal microbes (with or without) on the performance of plants and fungi. Ecological outcomes, as assessed by plant and fungal performance, varied widely across experimental environments, including interactions between plant or fungal lineages and soil environmental factors.

CONCLUSION

These results show the potential for selection mosaics in plant-mycorrhizal interactions, and indicate that these interactions are likely to coevolve in different ways in different environments, even when initially the genotypes of the interacting species are the same across all environments. Hence, selection mosaics may be equally as effective as genetic differences among populations in driving divergent coevolution among populations of interacting species.

Coevolution drives temporal changes in fitness and diversity across environments in a bacteria-bacteriophage interaction

Coevolutionary interactions are thought to play a crucial role in diversification of hosts and parasitoids. Furthermore, resource availability has been shown to be a fundamental driver of species diversity. Yet, we still do not have a clear understanding of how resource availability mediates the diversity generated by coevolution between hosts and parasitoids over time. We used experiments with bacteria and bacteriophage to test how resources affect variation in the competitive ability of resistant hosts and temporal patterns of diversity in the host and parasitoid as a result of antagonistic coevolution. Bacteria and bacteriophage coevolved for over 150 bacterial generations under high and low-resource conditions. We measured relative competitive ability of the resistant hosts and phenotypic diversity of hosts and parasitoids after the initial invasion of resistant mutants and again at the end of the experiment. Variation in relative competitive ability of the hosts was both time- and environment-dependent. The diversity of resistant hosts, and the abundance of host-range mutants attacking these phenotypes, differed among environments and changed over time, but the direction of these changes differed between the host and parasitoid. Our results demonstrate that patterns of fitness and diversity resulting from coevolutionary interactions can be highly dynamic.

Enzymic pathways of hyaluronan catabolism

The enzymic degradation of hyaluronan in mammalian tissues takes place in two phases, encompassing breakdown of the polysaccharide to its monosaccharide constituents and subsequent utilization of the monosaccharide products. Degradation to the monosaccharide components is effected by the concerted action of three enzymes, hyaluronidase, beta-D-glucuronidase and beta-N-acetyl-D-hexosaminidase. The relative contributions of hyaluronidase and the two exoglycosidases to the physiological catabolism of hyaluronan are not yet known but consideration of the kinetic properties of the three enzymes clearly indicates that hyaluronidase is best suited for the initial attack on the polysaccharide, inasmuch as its Km for hyaluronan is 1000- to 10,000-fold lower than that estimated for beta-D-glucuronidase. Recent investigations in the authors' laboratories have been focused on the catabolism of hyaluronan and other complex carbohydrates in liver, since the sinusoidal endothelial cells in this organ are the main sites for degradation of circulating hyaluronan. Assay of ten lysosomal hydrolases in isolated rat liver cells showed considerably higher activities in Kupffer cells and endothelial cells than in hepatocytes for nine of the enzymes, including beta-D-glucuronidase and beta-N-acetyl-D-hexosaminidase. The activity of N-acetylglucosamine-6-phosphate deacetylase, a key enzyme in the metabolism of the N-acetylglucosamine released by the lysosomal degradation of hyaluronan and other complex carbohydrates, has also been determined. High deacetylase activities were observed in both Kupffer cells and endothelial cells but, surprisingly, virtually no activity was detected in hepatocytes. This finding implies that N-acetylglucosamine cannot be degraded in hepatocytes and must be largely reutilized in the synthesis of new macromolecules. Further studies of the enzymes involved in hyaluronan degradation and N-acetylglucosamine utilization in the liver are under way.

Geographic structure in a widespread plant?mycorrhizal interaction: pines and false truffles

Mutualistic interactions are likely to exhibit a strong geographic mosaic in their coevolutionary dynamics, but the structure of geographic variation in these interactions is much more poorly characterized than in host-parasite interactions. We used a cross-inoculation experiment to characterize the scales and patterns at which geographic structure has evolved in an interaction between three pine species and one ectomycorrhizal fungus species along the west coast of North America. We found substantial and contrasting patterns of geographic interaction structure for the plants and fungi. The fungi exhibited a clinal pattern of local adaptation to their host plants across the geographic range of three coastal pines. In contrast, plant growth parameters were unaffected by fungal variation, but varied among plant populations and species. Both plant and fungal performance measures varied strongly with latitude. This set of results indicates that in such widespread species interactions, interacting species may evolve asymmetrically in a geographic mosaic because of differing evolutionary responses to clinally varying biotic and abiotic factors.

Pancreas-sparing distal duodenectomy for infrapapillary neoplasms

INTRODUCTION

For neoplasms that arise in the third and fourth parts of the duodenum (D(3), D(4)), a duodenectomy that preserves the pancreas can provide adequate tumour clearance while avoiding the additional dissection and risk of the common alternative, pancreatoduodenectomy.

PATIENTS AND METHODS

Pancreas-sparing distal duodenectomy (PSDD) was performed in 14 patients with infrapapillary duodenal neoplasms between 1991-2002, and the clinical outcome is reviewed. The operation entails careful separation of the lower pancreatic head from D(3), complete mobilisation of the ligament of Treitz and end-to-end duodenojejunal anastomosis 1-3 cm below the major duodenal papilla.

RESULTS

There were 9 men and 5 women of median age 56 years, who presented with iron-deficiency anaemia (n = 8), gastric outlet obstruction (n = 4), anaemia and gastric outlet obstruction (n = 1), epigastric pain or mass (1 each). There were 11 malignant neoplasms (adenocarcinoma 5, stromal tumour 4, recurrent seminoma 1, plasmacytoma 1), 2 benign neoplasms (villous adenoma, lipoma) and 1 patient with steroid-induced ulceration. In addition to D(3) and D(4), resection included the distal part of D(2) in 5 patients, while 4 required concomitant partial colectomy. Median operation time was 240 min and median blood loss 1197 ml, being greater for malignant than benign lesions (1500 ml versus 700 ml). There was one death from gangrenous cholecystitis, one early re-operation for anastomotic bleeding and one late re-operation for delayed gastric emptying secondary to anastomotic stricture, but no pancreatic complications. At a median follow-up of 47 months, three patients had died of recurrent disease while the other 10 were alive and well with no upper gastrointestinal symptoms.

CONCLUSIONS

Provided there is a minimum 1-cm clearance at the papilla, PSDD is a useful alternative to formal pancreatoduodenectomy in patients with unusual neoplasms arising from the third and fourth parts of the duodenum. Although a major undertaking in its own right, it avoids the extra time of a pancreatic resection and the extra risk of a pancreatic anastomosis.

Gene flow reverses an adaptive cline in a coevolving host-parasitoid interaction

Many natural populations are characterized by clinal patterns of adaptation, but it is unclear how gene flow and environmental gradients interact to drive such clines. We addressed this question by directly manipulating dispersal and productivity in an experimental landscape containing a microbial parasitoid, the bacteriophage T7, and its host, the bacterium Escherichia coli. We observed that the adaptation of parasitoids increased on hosts originating from lower-productivity communities in the absence of gene flow. However, adaptation decreased along the same productivity gradient with experimentally imposed gene flow of the host and parasitoid. This occurred despite relatively low rates of gene flow.

Coevolutionary alternation in antagonistic interactions

Coevolution between parasites and hosts or predators and prey often involves multiple species with similar kinds of defenses and counter-defenses. Classic examples include the interactions between phytophagous insects and their host plants, thick-shelled invertebrates and their shell-crushing predators, and ungulates and their predators. There are three major hypotheses for the nonequilibrium coevolutionary dynamics of these multispecific trophic interactions: escalation in traits, cycles in traits leading to fluctuating polymorphisms, and coevolutionary alternation. The conditions under which cycles and escalation are likely to occur have been well developed theoretically. In contrast, the conditions favoring coevolutionary alternation-evolutionary fluctuations in predator or prey preference driven by evolutionary shifts in relative levels of prey defense and vice versa-have yet to be identified. Using a set of quantitative coevolutionary models, we demonstrate that coevolutionary alternation can occur across a wide range of biologically plausible conditions. The result is often repeated, and potentially rapid, evolutionary shifts in patterns of specialization within networks of interacting species.

Coevolution: the geographic mosaic of coevolutionary arms races

Coevolutionary arms races between species can favor exaggeration of traits for attack and defense, but relentless escalation of these arms races does not necessarily occur in all populations.

Asymmetries in specialization in ant-plant mutualistic networks

Mutualistic networks involving plants and their pollinators or frugivores have been shown recently to exhibit a particular asymmetrical organization of interactions among species called nestedness: a core of reciprocal generalists accompanied by specialist species that interact almost exclusively with generalists. This structure contrasts with compartmentalized assemblage structures that have been verified in antagonistic food webs. Here we evaluated whether nestedness is a property of another type of mutualism-the interactions between ants and extrafloral nectary-bearing plants--and whether species richness may lead to differences in degree of nestedness among biological communities. We investigated network structure in four communities in Mexico. Nested patterns in ant-plant networks were very similar to those previously reported for pollination and frugivore systems, indicating that this form of asymmetry in specialization is a common feature of mutualisms between free-living species, but not always present in species-poor systems. Other ecological factors also appeared to contribute to the nested asymmetry in specialization, because some assemblages showed more extreme asymmetry than others even when species richness was held constant. Our results support a promising approach for the development of multispecies coevolutionary theory, leading to the idea that specialization may coevolve in different but simple ways in antagonistic and mutualistic assemblages.

Temporal dynamics of antagonism and mutualism in a geographically variable plant-insect interaction

Variation among sites and years in the local ecological outcome of interspecific interactions can generate a geographic mosaic of coevolution, as indicated by recent mathematical models. We evaluated whether local temporal dynamics of ecological outcome in the interaction between the moth Greya politella (Prodoxidae) and its host plant Lithophragma parviflorum (Saxifragaceae) are likely to mitigate or magnify geographic differences in ecological outcome found in earlier studies. The moths are highly host-specific pollinating floral parasites, and the mutualism can be swamped in some populations by the presence of effective co-pollinators. Hence, differing community contexts can shift the outcome of the interaction from mutualism to commensalism or antagonism. During each of four years, we evaluated the effect of Greya oviposition on seed development through a paired design that controlled for plant genotype and microenvironment. At Turnbull National Wildlife Refuge in Washington State, the interaction was significantly mutualistic in all four years. Mutualism in this population was indicated by a higher probability of development of capsules visited by ovipositing Greya than capsules not visited by Greya on the same plant. At Rapid River, Idaho, the interaction was commensalistic in three years and antagonistic in one year. Antagonism in this population was indicated by selective withering of capsules containing Greya eggs. Overall, the results suggest stable geographic differences in the range of ecological outcomes in this plant-insect interaction under different community contexts.

The appearances of oesophageal carcinoma demonstrated on high-resolution, T2-weighted MRI, with histopathological correlation

This paper describes the spectrum of imaging features of oesophageal adenocarcinoma seen using high-resolution T2-weighted (T2W) magnetic resonance imaging (MRI). Thirty-nine patients with biopsy-proven oesophageal adenocarcinoma were scanned using an external surface coil. A sagittal T2W sequence was used to localise the tumour and to plan axial images perpendicular to the tumour. Fast spin-echo (FSE) T2W axial sequence parameters were: TR/TE, 3,300-5,000 ms/120-80 ms; field of view (FOV) 225 mm, matrix 176x512(reconstructed) mm to 256x224 mm, giving an in-plane resolution of between 1.28x0.44 mm and 0.88x1.00 mm, with 3-mm slice thickness. Thirty-three patients underwent resection and the MR images were compared with the histological whole-mount sections. There were four T1, 12 T2, and 17 T3 tumours. The T2W high-resolution MRI sequences produced detailed images of the oesophageal wall and surrounding structures. Analysis of the imaging appearances for different tumour T stages enabled the development of imaging criteria for local staging of oesophageal cancer using high-resolution MRI. Our study illustrates the spectrum of appearances of oesophageal cancer on T2W high-resolution MRI, and using the criteria established in this study, demonstrates the potential of this technique as an alternative non-invasive method for local staging for oesophageal cancer.

Cryptic species within the cosmopolitan desiccation-tolerant moss Grimmia laevigata

The common cushion moss Grimmia laevigata (Bridel) Bridel grows on bare rock in a broad range of environments on every continent except Antarctica. As such, it must harbor adaptations to a remarkably broad set of environmental stresses, the extremes of which can include very high temperatures, prolonged nearly complete desiccation, and high ultraviolet B (UVB) exposure. Yet, like many mosses, G. laevigata shows very little morphological variability across its cosmopolitan range. This presents an evolutionary puzzle, the solution to which lies in understanding the phylogeographic structure of this morphologically simple organism. Here we report the results of an analysis of amplified fragment length polymorphisms (AFLPs) in G. laevigata, focusing on individuals from the California Floristic Province. We found evidence that populations within California constitute two distinct geographically overlapping cryptic species. Each clade harbors multiple private alleles, indicating they have been genetically isolated for some time. We suggest that the existence of cryptic species within G. laevigata, in combination with its life history, growth habits, and extreme desiccation tolerance, makes this moss an ideal research tool and a candidate for a biological indicator of climate change and pollution.

Adaptation varies through space and time in a coevolving host-parasitoid interaction

One of the central challenges of evolutionary biology is to understand how coevolution organizes biodiversity over complex geographic landscapes. Most species are collections of genetically differentiated populations, and these populations have the potential to become adapted to their local environments in different ways. The geographic mosaic theory of coevolution incorporates this idea by proposing that spatial variation in natural selection and gene flow across a landscape can shape local coevolutionary dynamics. These effects may be particularly strong when populations differ across productivity gradients, where gene flow will often be asymmetric among populations. Conclusive empirical tests of this theory have been particularly difficult to perform because they require knowledge of patterns of gene flow, historical population relationships and local selection pressures. We have tested these predictions empirically using a model community of bacteria and bacteriophage (viral parasitoids of bacteria). We show that gene flow across a spatially structured landscape alters coevolution of parasitoids and their hosts and that the resulting patterns of adaptation can fluctuate in both space and time.

Colorectal surgery: the risk and burden of adhesion-related complications

OBJECTIVES

Adhesions are associated with serious medical complications. This study examines the real-time burden of adhesion-related readmissions following colorectal surgery and assesses the impact of previous surgery on adhesion-related outcomes.

PATIENTS AND METHODS

The study used data from the Scottish National Health Service Medical Record Linkage Database to identify three cohorts of patients who had undergone open colorectal surgery during the financial years 1996-97, 1997-98 and 1998-99. Each cohort was followed up for at least 2 years and the number and category of adhesion-related readmissions was recorded. The influence of any previous operations on adhesion-related readmissions was also determined by performing a subanalysis within the 1996-97 cohort of patients who had no record of abdominal surgery within either the previous 5 or 15 years. The relative risk of adhesion-related readmissions was also assessed.

RESULTS

In the 1996-97 cohort, 9.0% of patients were readmitted within a year after surgery; 2.1% had complications directly related to adhesions and 6.9% had complications that were possibly related. After 4 years, 19.0% of patients were readmitted for reasons directly or possibly related to adhesions. Many patients were readmitted on more than one occasion and the relative risk of adhesion-related complications was 29.7 per 100 initial procedures over 4 years. In the subgroups that had no record of abdominal surgery within the previous 5 or 15 years, the relative risks of adhesion-related complications were 24.8% and 23.5%, respectively. There was no change in the rate of adhesion-related readmissions following colorectal surgery between 1996 and 1999.

CONCLUSION

Colorectal surgery is associated with a considerable rate of adhesion-related readmissions. Preventative measures should be considered to reduce this risk.

Premeiotic Clusters of Mutation and the Cost of Natural Selection

Haldane stated that there is a cost of natural selection for new beneficial alleles to be substituted over time. Most of this cost, which leads to "genetic deaths," is in the early generations of the substitution process when the new allele is low in frequency. It depends on the initial frequency and dominance value, but not the selection coefficient, of the advantageous allele. There have been numerous suggestions on how to reduce the cost for preexisting genetic variation that goes from disadvantageous, or neutral, to advantageous with a change in the environment. However, the cost of natural selection for new alleles that arise by mutation is assumed to be high, based on the assumption that new mutant alleles arise in natural populations as single events [1/(2N) of the total alleles]. However, not all mutant alleles arise as single events. Premeiotic mutations occur frequently in individuals (germinal mosaics), giving rise to multiple copies of identical mutant alleles called a "cluster" (C) with an initial allele frequency of C/(2N) instead of 1/(2N). These clusters of new mutant alleles reduce the cost of natural selection in direct proportion to the relative size of the cluster. Hence new advantageous alleles that arise by mutation have the greatest chance of going to fixation if they occur in large clusters in small populations.

Coevolution in variable mutualisms

Many mutualistic interactions are probably not mutualistic across all populations and years. This article explores consequences of this observation with a series of genetic models that consider how variable mutualisms coevolve. The first models, previously introduced in a general coevolutionary context, consider two coevolving species whose fitness interactions change between beneficial and antagonistic in response to independent spatial or temporal variation in the abiotic or biotic environment. The results demonstrate that both temporal and spatial variability in fitness interactions can cause partner species with tightly matched traits favored by unconditional mutualisms to be vulnerable to evolutionary invasion by alternative types. A new model presented here shows that an additional mutualistic species can have a similar effect and can even cause fitness interactions between the other two species to evolve. Under some conditions, the pairwise interactions can change unidirectionally from mutualistic to antagonistic, with virtually no evolutionary change in either partner species. In other cases, fitness interactions between the species pair can oscillate between mutualism and antagonism as a result of coevolution in the third species. Taken as a whole, these theoretical results suggest that many features of mutualistic coevolution can best be understood by considering spatial, temporal, and community-dependent patterns of fitness interactions.

Coevolution between hosts and parasites with partially overlapping geographic ranges

Many host species interact with a specific parasite within only a fraction of their geographical range. Where host and parasite overlap geographically, selection may be reciprocal constituting a coevolutionary hot spot. Host evolution, however, may be driven primarily by selection imposed by alternative biotic or abiotic factors that occur outside such hot spots. To evaluate the importance of coevolutionary hot spots for host and parasite evolution, we analyse a spatially explicit genetic model for a host that overlaps with a parasite in only part of its geographical range. Our results show that there is a critical amount of overlap beyond which reciprocal selection leads to a coevolutionary response in the host. This critical amount of overlap depends upon the explicit spatial configuration of hot spots. When the amount of overlap exceeds this first critical level, host-parasite coevolution commonly generates stable allele frequency clines rather than oscillations. It is within this region that one of the primary predictions of the geographic mosaic theory is realized, and local maladaptation is prevalent in both species. Past a further threshold of overlap between the species oscillations do evolve, but allele frequencies in both species are spatially synchronous and local maladaptation is absent in both species. A consequence of such transitions between coevolutionary dynamics is that parasite adaptation is inversely proportional to the fraction of its host's range that it occupies. Hence, as the geographical range of a parasite increases, it becomes increasingly maladapted to the host. This suggests a novel mechanism through which the geographical range of parasites may be limited.