Relationship between assemblages of mycorrhizal fungi and bacteria on grass roots

Soils support an enormous microbial diversity, but the ecological drivers of this diversity are poorly understood. Interactions between the roots of individual grass species and the arbuscular mycorrhizal (AM) fungi and bacteria in their rhizoplane were studied in a grazed, unimproved upland pasture. Individual root fragments were isolated from soil cores, DNA extracted and used to identify plant species and assess rhizoplane bacterial and AM fungal assemblages, by amplifying part of the small-subunit ribosomal RNA gene, followed by terminal restriction fragment length polymorphism analysis. For the first time we showed that AM fungal and bacterial assemblages are related in situ and that this relationship occurred at the community level. Principal coordinate analyses of the data show that the AM fungi were a major factor determining the bacterial assemblage on grass roots. We also report a strong influence of the composition of the plant community on AM fungal assemblage. The bacterial assemblage was also influenced by soil pH and was spatially structured, whereas AM fungi were influenced neither by the bacteria nor by soil pH. Our study shows that linkages between plant roots and their microbial communities exist in a complex web of interactions that act at individual and at community levels, with AM fungi influencing the bacterial assemblage, but not the other way round.     

Mutations of intermediate effect are responsible for adaptation in evolving Pseudomonas fluorescens populations

The fixation of a beneficial mutation represents the first step in adaptation, and the average effect of such mutations is therefore a fundamental property of evolving populations. It is nevertheless poorly characterized because the rarity of beneficial mutations makes it difficult to obtain reliable estimates of fitness. We obtained 68 genotypes each containing a single fixed beneficial mutation from experimental populations of Pseudomonas fluorescens, evolving in medium with serine as the sole carbon source and estimated the selective advantage of each by competition with the ancestor. The distribution of selection coefficients is modal and closely resembles the Weibull distribution. The average selection coefficient (2.1) and beneficial mutation rate (3.8x10(-8)) are high relative to previous studies, possibly because the ancestral population grows poorly in serine-limited medium. Our experiment suggests that the initial stages of adaptation to stressful environments will involve the substitution of mutations with large effect on fitness.     

Wind of change: new insights on the ecology and evolution of pollination and mating in wind-pollinated plants

Background

The rich literature that characterizes the field of pollination biology has focused largely on animal-pollinated plants. At least 10 % of angiosperms are wind pollinated, and this mode of pollination has evolved on multiple occasions among unrelated lineages, and hence this discrepancy in research interest is surprising. Here, the evolution and functional ecology of pollination and mating in wind-pollinated plants are discussed, a theoretical framework for modelling the selection of wind pollination is outlined, and pollen capture and the occurrence of pollen limitation in diverse wind-pollinated herbs are investigated experimentally.

Scope and Conclusions

Wind pollination may commonly evolve to provide reproductive assurance when pollinators are scarce. Evidence is presented that pollen limitation in wind-pollinated plants may not be as common as it is in animal-pollinated species. The studies of pollen capture in wind-pollinated herbs demonstrate that pollen transfer efficiency is not substantially lower than in animal-pollinated plants as is often assumed. These findings challenge the explanation that the evolution of few ovules in wind-pollinated flowers is associated with low pollen loads. Floral and inflorescence architecture is crucial to pollination and mating because of the aerodynamics of wind pollination. Evidence is provided for the importance of plant height, floral position, and stamen and stigma characteristics in promoting effective pollen dispersal and capture. Finally, it is proposed that geitonogamous selfing may alleviate pollen limitation in many wind-pollinated plants with unisexual flowers.Key words: Wind pollination, reproductive assurance, pollen limitation, geitonogamy, sex allocation, inflorescence architecture, mating systems     

Prevalence of Burkholderia sp. nodule symbionts on four mimosoid legumes from Barro Colorado Island, Panama

Sequences of 16S rRNA and partial 23S rRNA genes and PCR assays with genotype-specific primers indicated that bacteria in the genus Burkholderia were the predominant root nodule symbionts for four mimosoid legumes (Mimosa pigra, M. casta, M. pudica, and Abarema macradenia) on Barro Colorado Island, Panama. Among 51 isolates from these and a fifth mimosoid host (Pithecellobium hymenaeafolium), 44 were Burkholderia strains while the rest were placed in Rhizobium, Mesorhizobium, or Bradyrhizobium. The Burkholderia strains displayed four distinct rRNA sequence types, ranging from 89% to 97% similarity for 23S rRNA and 96.5-98.4% for 16S rRNA. The most common genotype comprised 53% of all isolates sampled and was associated with three legume host species. All Burkholderia genotypes formed nodules on Macroptilium atropurpureum or Mimosa pigra, and sequencing of rRNA genes in strains re-isolated from nodules verified identity with inoculant strains. Sequence analysis of the nitrogenase alpha-subunit gene (nifD) in two of the Burkholderia genotypes indicated that they were most similar to a partial sequence from the nodule-forming strain Burkholderia tuberum STM 678 from South Africa. In addition, a PCR screen with primers specific to Burkholderia nodB genes yielded the expected amplification product in most strains. Comparison of 16S rRNA and partial 23S rRNA phylogenies indicated that tree topologies were significantly incongruent. This implies that relationships across the rRNA region may have been altered by lateral gene transfer events in this Burkholderia population.     

Self-mortification and the stigma of leprosy in northern India

This article examines the biocultural dynamics of social discrimination and physical disfigurement among people with leprosy, or Hansen's disease (HD), in Banaras, northern India. Based on the narratives and observations ofpeople living in colony and street settings, I trace three destructive processes by which the social stigmata of leprosy become physically expressed. First, strategies of concealment further the progression and spread of HD through late detection and undertreatment. Second, the internalization of stigma can lead to bodily dissociation and injury through self-neglect. Finally, some people intentionally seek injuries under conditions of desperate poverty. As a result of such mortification processes, these people came to embody, quite literally, the prejudices that exacerbated their condition in the first place.     

Long-Term Persistence and Leaching of Escherichia coli in Temperate Maritime Soils

Enteropathogen contamination of groundwater, including potable water sources, is a global concern. The spreading on land of animal slurries and manures, which can contain a broad range of pathogenic microorganisms, is considered a major contributor to this contamination. Some of the pathogenic microorganisms applied to soil have been observed to leach through the soil into groundwater, which poses a risk to public health. There is a critical need, therefore, for characterization of pathogen movement through the vadose zone for assessment of the risk to groundwater quality due to agricultural activities. A lysimeter experiment was performed to investigate the effect of soil type and condition on the fate and transport of potential bacterial pathogens, using Escherichia coli as a marker, in four Irish soils (n = 9). Cattle slurry (34 tonnes per ha) was spread on intact soil monoliths (depth, 1 m; diameter, 0.6 m) in the spring and summer. No effect of treatment or the initial soil moisture on the E. coli that leached from the soil was observed. Leaching of E. coli was observed predominantly from one soil type (average, 1.11 ± 0.77 CFU ml⁻¹), a poorly drained Luvic Stagnosol, under natural rainfall conditions, and preferential flow was an important transport mechanism. E. coli was found to have persisted in control soils for more than 9 years, indicating that autochthonous E. coli populations are capable of becoming naturalized in the low-temperature environments of temperate maritime soils and that they can move through soil. This may compromise the use of E. coli as an indicator of fecal pollution of waters in these regions.The contamination of groundwater, including potable water supplies, with microbial pathogens continues to be a global concern (52, 59). Of particular importance in developed countries are the high levels of contamination associated with small-scale and very-small-scale drinking water supplies (5, 19, 57), often groundwater, which serve an estimated 10% of the total population in the European Union (13). The high numbers of these water supplies found to be contaminated with fecal bacteria and thus considered to be unfit for human consumption are worrying because the water from them is often untreated or inadequately treated prior to consumption. Microbial pathogens are known to survive for considerable periods of time in groundwater (29), which increases the health risk due to utilization of contaminated supplies. There are various sources of contamination, but evidence suggests that contamination from the spreading of animal slurries and manures on land can be a significant contributor (3, 33, 53). Spreading of agricultural slurries and manures on land is used by the agricultural sector as a means of nutrient recycling. The health risks associated with the spreading of animal and human wastes containing enteric pathogens have been recognized for a long time (10, 18). Animal manure and wastewaters may contain a broad range of pathogenic microorganisms, including Escherichia coli O157:H7, Campylobacter, Cryptosporidium, Salmonella spp., and pathogenic viruses, which are released into the environment during spreading (15, 22, 55). The levels and incidence of pathogens present in animal manures and slurries are influenced by a number of factors, including herd health, age demographics, stress factors, diet, season, and manure management and storage (37, 39).Soils (and subsoils) often act as a zone for mitigating microbial contamination of groundwater associated with the spreading of animal slurries and manures on land. Some of the pathogenic microorganisms applied to agricultural soils have, however, been observed to leach through the soil into groundwater, which can affect drinking water quality and pose a risk to public health (16, 26, 28, 42, 50), confirming that soil is not always a sufficient obstruction for protection of groundwater (16, 53). Consequently, characterization of the movement of pathogens through the unsaturated soil and subsoil zone (vadose zone) has become critical for assessment of the risk to groundwater posed by agricultural activities (8, 14, 42). The soil and subsoil type is believed to be a major factor influencing the potential transfer of pathogens through soil to groundwater (3, 34, 41, 50). The preapplication moisture status of a soil, which may be influenced by the season, also impacts pathogen survival, fate, and transport (2, 11, 43, 54).E. coli is widely used as an indicator of fecal contamination of water, and certain strains are known to be pathogenic (12). Thus, characterizing this organism''s transport through soil is important because of the health risk posed by the organism itself and with regard to its validity as an indicator of the fate of enteropathogens in the environment. E. coli strains have diverse properties and capabilities that affect their survival and transport in soils (9, 36, 56, 60). Consequently, data obtained by using total E. coli rather than individual surrogate strains can be more representative of the fate and transport of E. coli present in animal slurries. E. coli O157 die-off in soils has been reported to be the same as or quicker than total E. coli die-off, suggesting that data for total E. coli provide a conservative estimate of the survival potential (38, 56). Although many field and laboratory studies have investigated E. coli transport through soil columns (4, 6, 16, 43, 46, 47, 50, 51), most studies have investigated transport through soil to a depth of less than 30 cm. For assessment of the risk of transport to groundwater, such studies may not take into account the variation in soil physical and chemical characteristics with depth (e.g., the frequency and continuity of macropores, organic matter, and moisture contents) that affect bacterial transport. Furthermore, rainfall was often simulated in previous studies, which allows experimental conditions to be controlled but may not be representative of the risk due to variable natural rainfall events over time. In this study, we used intact soil monoliths that were 1 m deep to assess the risk of leaching of total E. coli in four representative Irish soil types under natural rainfall and environmental conditions.The objective of this study was to quantitatively investigate the impact of soil type and season (soil moisture content) on the fate and transport of E. coli spread on four different temperate maritime soil types under natural rainfall conditions. We hypothesized that there would be a greater microbial risk to underlying groundwater with better-drained soil types than with relatively poorly drained soil types following the application of animal slurry. In addition, we hypothesized that E. coli cells spread on wetter spring soils would be transported in greater numbers than E. coli cells spread on drier soils in the summer.     

The maintenance of extreme amino acid diversity at the disease resistance gene, RPP13, in Arabidopsis thaliana

We have used the naturally occurring plant-parasite system of Arabidopsis thaliana and its common parasite Peronospora parasitica (downy mildew) to study the evolution of resistance specificity in the host population. DNA sequence of the resistance gene, RPP13, from 24 accessions, including 20 from the United Kingdom, revealed amino acid sequence diversity higher than that of any protein coding gene reported so far in A. thaliana. A significant excess of amino acid polymorphism segregating within this species is localized within the leucine-rich repeat (LRR) domain of RPP13. These results indicate that single alleles of the gene have not swept through the population, but instead, a diverse collection of alleles have been maintained. Transgenic complementation experiments demonstrate functional differences among alleles in their resistance to various pathogen isolates, suggesting that the extreme amino acid polymorphism in RPP13 is maintained through continual reciprocal selection between host and pathogen.     

Extreme climatic events change the dynamics and invasibility of semi-arid annual plant communities

Extreme climatic events represent disturbances that change the availability of resources. We studied their effects on annual plant assemblages in a semi-arid ecosystem in north-central Chile. We analysed 130 years of precipitation data using generalised extreme-value distribution to determine extreme events, and multivariate techniques to analyse 20 years of plant cover data of 34 native and 11 exotic species. Extreme drought resets the dynamics of the system and renders it susceptible to invasion. On the other hand, by favouring native annuals, moderately wet events change species composition and allow the community to be resilient to extreme drought. The probability of extreme drought has doubled over the last 50 years. Therefore, investigations on the interaction of climate change and biological invasions are relevant to determine the potential for future effects on the dynamics of semi-arid annual plant communities.