Strategies of ultraviolet-B protection in microscopic algae

Different species of microalgae show a wide range of susceptibility to ultraviolet-B (UV-B) radiation. To identify factors responsible for the UV-B tolerance of some of the algae, we compared 8 species that are highly tolerant to UV-B to 8 species that are highly susceptible. The tolerant species contained substantial amounts of an acetolysis-resistant residue. The residue consists of sporopollenin, a biopolymer of variable chemical composition that occurs in the algal cell walls and absorbs UV-B radiation. The susceptible species contained little or no sporopollenin. We propose that sporopollenin provides protection to the tolerant species by screening the incident UV-B radiation. Previous studies showed that the mycosporine-like amino acids (MAA) also act as effective UV-B screens. Our data indicate that sporopollenin provides a constant protection while MAA are induced by radiation stress and occur with some delay. The tolerant species also differ from the susceptible species in their capacity to repair the reaction centers damaged by UV-B. The tolerant algae became vulnerable to UV-B when protein synthesis needed for repair was blocked by streptomycin. In the susceptible species, streptomycin had no effect during the UV-B stress. The repair deficiency in the susceptible species can be explained either by relatively less effective protein synthesis or by an inhibition of the protein synthesis by UV-B. In the tolerant species, the structures needed for protein synthesis are protected by UV-B screening of sporopollenin and MAA.     

Aphids do not avoid resistance in Australian lupin (Lupinus angustifolius, L. luteus) varieties

Laboratory bioassays and field trials were used to characterize resistance to three aphid species (Myzus persicae (Sulzer), Acyrthosiphon kondoi Shinji, Aphis craccivora (Koch) in two aphid-resistant varieties (Kalya, Tanjil) and one susceptible variety (Tallerack) of Lupinus angustifolius L., and in one resistant variety (Teo) and one susceptible variety (Wodjil) of L. luteus L. Host selection tests in the glasshouse showed that alates of all three species preferred L. luteus to L. angustifolius, but provided no evidence that alates selected susceptible varieties over resistant. These results were supported by a field trial, which showed no difference in the number of colonizing A. kondoi alates collected from the resistant and susceptible lines of each lupin species, but there were significantly more late-instar nymphs and apterous adults on the susceptible lines. In laboratory host suitability experiments, there was much greater suppression of aphid growth and survival on Teo than on Kalya and Tanjil. In field trials, the numbers of aphids were generally lower on resistant compared to susceptible lines of both lupin species with one notable exception: M. persicae numbers were not lower on the resistant variety Tanjil compared to the susceptible variety Tallerack (L. angustifolius). These results suggest that the resistance mechanisms in both lupin species do not affect the selection of hosts by colonizing aphids, but rather are affecting the growth, survival and possibly reproduction of aphids after settling.     

Diffuse coevolution: constraints on a generalist parasite favor use of a dead-end host

Many evolutionary models and empirical studies of parasite-host interactions consider single species of parasites exploiting single host species. However, many parasites are generalists in that they parasitize more than one host species (often many more) and establish associations with other hosts that cannot be described as true parasitism. We identify such an association, explain how constraints may maintain it, and indicate why such diffuse interactions are deserving of attention. We describe the use of two closely related Sympetrum dragonfly species by larvae of the water mite Arrenurus planus Marshall. Adults of one dragonfly species are resistant whereas adults of the other species are almost wholly susceptible to A. planus . However, A. planus attaches as often to the resistant host as it does to the susceptible host species when relative abundance and seasonal timing of adult emergence of both species is considered. We present evidence that mites track the susceptible host and are most active early in the season, when early-emerging unsuitable hosts are also present. Thus, use of resistant hosts appears an unavoidable outcome of constraints promoting discovery and use of susceptible hosts. Such findings have implications for studies of local adaptation and host switching.     

The Susceptibility of Monocotyledons to Agrobacterium tumefaciens

The susceptibility of 257 monocotyledon species belonging to 139 genera and 27 families, has been tested and the literature on this subject reviewed. In contrast with dicotyledons and gymnosperms, monocotyledons are much less susceptible to Agrobacterium tumefaciens: only 3 % of the species of monocotyledons tested were host plants, whereas 60 % of the dicotyledons and gymnosperms were susceptible. Only the closely related monocotyledon orders Liliales (6 families) and Arales (1 family) contained species that were susceptible. These results are consistent with a proposed taxonomic relationship between the Liliales and the dicotyledons. A correlation between host biochemical capabilities and susceptibility to crown gall is suggested.     

An experimental test of the independent action hypothesis in virus–insect pathosystems

The ‘independent action hypothesis’ (IAH) states that each pathogen individual has a non-zero probability of causing host death and that pathogen individuals act independently. IAH has not been rigorously tested. In this paper, we (i) develop a probabilistic framework for testing IAH and (ii) demonstrate that, in two out of the six virus–insect pathosystems tested, IAH is supported by the data. We first show that IAH inextricably links host survivorship to the number of infecting pathogen individuals, and develop a model to predict the frequency of single- and dual-genotype infections when a host is challenged with a mixture of two genotypes. Model predictions were tested using genetically marked, near-identical baculovirus genotypes, and insect larvae from three host species differing in susceptibility. Observations in early-instar larvae of two susceptible host species support IAH, but observations in late-instar larvae of susceptible host species and larvae of a less susceptible host species were not in agreement with IAH. Hence the model is experimentally supported only in pathosystems in which the host is highly susceptible. We provide, to our knowledge, the first qualitative experimental evidence that, in such pathosystems, the action of a single virion is sufficient to cause disease.     

Susceptibility of species A, B, and C of Anopheles culicifacies complex to Plasmodium yoelii yoelii and Plasmodium vinckei petteri infections

The comparative susceptibilities of colonized species A, B, and C of Anopheles culicifacies complex and Anopheles stephensi were determined for 2 rodent malaria parasites Plasmodium vinckei petteri and Plasmodium yoelii yoelii. All the 3 members of the complex were found to support complete sporogony with varying success. Controls, A. stephensi, become readily infected, with >70% developing oocysts. Of the test groups, species A had the highest percentage of mosquitoes with oocysts (>25%) and sporozoites (>15%). Anopheles culicifacies species B were least susceptible; less than 10% had oocysts and sporozoites in the salivary glands. The results demonstrate that A. culicifacies species A is most susceptible and species B is least susceptible to infections with both the parasites.     

Interspecific proteomic comparisons reveal ash phloem genes potentially involved in constitutive resistance to the emerald ash borer

The emerald ash borer (Agrilus planipennis) is an invasive wood-boring beetle that has killed millions of ash trees since its accidental introduction to North America. All North American ash species (Fraxinus spp.) that emerald ash borer has encountered so far are susceptible, while an Asian species, Manchurian ash (F. mandshurica), which shares an evolutionary history with emerald ash borer, is resistant. Phylogenetic evidence places North American black ash (F. nigra) and Manchurian ash in the same clade and section, yet black ash is highly susceptible to the emerald ash borer. This contrast provides an opportunity to compare the genetic traits of the two species and identify those with a potential role in defense/resistance. We used Difference Gel Electrophoresis (DIGE) to compare the phloem proteomes of resistant Manchurian to susceptible black, green, and white ash. Differentially expressed proteins associated with the resistant Manchurian ash when compared to the susceptible ash species were identified using nano-LC-MS/MS and putative identities assigned. Proteomic differences were strongly associated with the phylogenetic relationships among the four species. Proteins identified in Manchurian ash potentially associated with its resistance to emerald ash borer include a PR-10 protein, an aspartic protease, a phenylcoumaran benzylic ether reductase (PCBER), and a thylakoid-bound ascorbate peroxidase. Discovery of resistance-related proteins in Asian species will inform approaches in which resistance genes can be introgressed into North American ash species. The generation of resistant North American ash genotypes can be used in forest ecosystem restoration and urban plantings following the wake of the emerald ash borer invasion.     

Host species composition influences infection severity among amphibians in the absence of spillover transmission

Wildlife epidemiological outcomes can depend strongly on the composition of an ecological community, particularly when multiple host species are affected by the same pathogen. However, the relationship between host species richness and disease risk can vary with community context and with the degree of spillover transmission that occurs among co‐occurring host species. We examined the degree to which host species composition influences infection by Batrachochytrium dendrobatidis (Bd), a widespread fungal pathogen associated with amphibian population declines around the world, and whether transmission occurs from one highly susceptible host species to other co‐occurring host species. By manipulating larval assemblages of three sympatric amphibian species in the laboratory, we characterized the relationship between host species richness and infection severity, whether infection mediates growth and survivorship differently across various combinations of host species, and whether Bd is transmitted from experimentally inoculated tadpoles to uninfected tadpoles. We found evidence of a dilution effect where Bd infection severity was dramatically reduced in the most susceptible of the three host species (Anaxyrus boreas). Infection also mediated survival and growth of all three host species such that the presence of multiple host species had both positive (e.g., infection reduction) and negative (e.g., mortality) effects on focal species. However, we found no evidence that Bd infection is transmitted by this species. While these results demonstrate that host species richness as well as species identity underpin infection dynamics in this system, dilution is not the product of reduced transmission via fewer infectious individuals of a susceptible host species. We discuss various mechanisms, including encounter reduction and antagonistic interactions such as competition and opportunistic cannibalism that may act in concert to mediate patterns of infection severity, growth, and mortality observed in multihost communities.     

Metabolic Variations of Proteus in the Memphis Area and Other Geographical Areas

The number of strains of Proteus studied was 413, and these were obtained from all clinical materials with the exception of fecal specimens. Lactose was fermented by 37 strains (P. inconstans, 29%; P. rettgeri, 16%; P. mirabilis, 4.2%; P. morganii, 3.6%; and P. vulgaris, 0%) of which 33 were from the genitourinary system. These 33 strains constituted 12.7% of the 260 strains isolated from this source. Biochemically, P. mirabilis was the least variable, and P. rettgeri was the most variable of the five species of Proteus tested. P. inconstans and P. rettgeri resembled each other more closely than any of the other species of Proteus. Comparison of results obtained in the Memphis area with those found in other locations showed that biochemical characteristics varied most with the substances citrate, salicin, xylose, trehalose, and mannitol. In contrast to earlier reports from Israel and England, none of the strains of P. inconstans in the present study was able to attack urea. All five species of Proteus tested (by the disc method) were highly susceptible to methenamine mandelate. P. mirabilis, P. morganii, and P. vulgaris were also highly susceptible to nitrofurantoin. All strains of P. mirabilis were susceptible to ampicillin. P. inconstans was the most resistant species of Proteus. Of the other 356 urease-positive strains tested, 79% were susceptible to chloramphenicol, whereas only 3.8% of the 56 urease-negative strains (P. inconstans) were susceptible. When tested with streptomycin, 61% of urease-positive strains were susceptible and 1.8% of the urease-negative strains were susceptible. Of 36 lactose-positive strains, 33.8% were susceptible to chloramphenicol, whereas 72.8% of all lactose-negative strains were susceptible. Again, of the lactose-positive strains, 17% were susceptible to streptomycin, whereas 56.3% of all lactose-negative strains were susceptible.     

Potential structural and biochemical mechanisms of compositae wild species resistance to <Emphasis Type="Italic">Alternaria tenuissima</Emphasis>

Disease assessment to measure severity of alternaria leaf spot (Alternaria tenuissima (Fr.) Wiltsh.) was performed in 32 wild species of Compositae family by seedling inoculation. It was found that two species were resistant, four were moderately resistant, and others were susceptible to various degrees. Some leaf morphological traits of two resistant and two highly susceptible wild species were studied. Trichome density and height and wax content were found to be associated with plant passive resistance, while stomata density was not associated. After A. tenuissima inoculation, enzymes phenylalanine ammonia-lyase, β-1,3-glucanase, and chitinase were also assayed. Their higher activities in the selected resistant wild species and lower activities in the highly susceptible ones were found. This suggests the possibility that these enzymes are not only constituents of resistance but also could be used as biochemical markers for screening Compositae wild species for alternaria leaf spot resistance.     

Porcine prion protein amyloid

ABSTRACT

Mammalian prions are composed of misfolded aggregated prion protein (PrP) with amyloid-like features. Prions are zoonotic disease agents that infect a wide variety of mammalian species including humans. Mammals and by-products thereof which are frequently encountered in daily life are most important for human health. It is established that bovine prions (BSE) can infect humans while there is no such evidence for any other prion susceptible species in the human food chain (sheep, goat, elk, deer) and largely prion resistant species (pig) or susceptible and resistant pets (cat and dogs, respectively). PrPs from these species have been characterized using biochemistry, biophysics and neurobiology. Recently we studied PrPs from several mammals in vitro and found evidence for generic amyloidogenicity as well as cross-seeding fibril formation activity of all PrPs on the human PrP sequence regardless if the original species was resistant or susceptible to prion disease. Porcine PrP amyloidogenicity was among the studied. Experimentally inoculated pigs as well as transgenic mouse lines overexpressing porcine PrP have, in the past, been used to investigate the possibility of prion transmission in pigs. The pig is a species with extraordinarily wide use within human daily life with over a billion pigs harvested for human consumption each year. Here we discuss the possibility that the largely prion disease resistant pig can be a clinically silent carrier of replicating prions.     

Biochemical and physiological mechanisms mediated by allelochemicals

Allelochemistry, the production and release of toxic chemicals produced by one species that affect a receiving susceptible species, has been the subject of diverse degrees of scientific enquiry. Recent advances in plant biology have permitted the revamp of allelochemistry as a biologically and ecologically sound explanation for plant invasion and plant-plant communication in the rhizosphere. Recent progress has been made in understanding the biochemical and molecular changes that are induced by allelochemicals in susceptible plant species, and the complex mechanisms that are used by allelochemical-resistant plants to defend against this toxic insult.     

Susceptibility of wild mammals to infection with Naegleria fowleri

Animals of 4 families of small wild mammals were live-trapped and inoculated intranasally with Naegleria fowleri to determine patterns of susceptibility. Of the 7 species of animals examined, only rodents were susceptible to N. fowleri. Susceptible animals were eastern gray squirrel, hispid cotton rat, muskrat, and house mouse. Mammals that were not susceptible at a dose of 10(6) were opossum, raccoon, and eastern cottontail rabbit. Perhaps rodents and humans share a common anatomical or physiological determinant that makes them susceptible to infection with N. fowleri.     

Three scleractinian coral diseases in Dominica, West Indies: distribution, infection patterns and contribution to coral tissue mortality

Coral diseases have been documented in many areas of the Caribbean, but studies in the eastern Caribbean region have been lacking. The prevalence, distribution patterns and contribution to the mortality of coral tissue by black band discase (BBD), white plague (WP) and dark spots disease (DSD) were examined at five reef sites along the west coast of Dominica. 185 of the 325 diseased colonies recorded between March and August 2000, in a survey area of 5884 m2, were WP. This disease contributed to 89% of the total 4.08 m2 of tissue mortality caused by diseases during the survey period. WP also affected the largest average tissue surface area (relative to colony size) per colony and exhibited the largest average tissue loss per infection when compared to BBD and DSD. The species most susceptible to WP and BBD in Dominica differed from most other described Caribbean locations with Siderastrea siderea being most susceptible. S. siderea was also the only species noted to be susceptible to DSD. Measurements of colony size revealed that each disease affected the larger colonies of some coral species. Comparisons between disease prevalence at each site and various physical parameters, including temperature, wave height, depth, and current patterns, did not exhibit significant correlations. The lack of a direct correlation between temperature and disease prevalence indicates that there are other seasonal factors contributing to the higher prevalence of diseases recorded during the summer months in Dominica. WP prevalence at each site was positively correlated to the relative species abundances of the species most susceptible to WP. This was the dominant factor in determining site-specific disease densities of this disease and may therefore be a valuable predictive and management tool. There were no correlations between BBD or DSD and the relative abundances of susceptible species. The spatial distribution patterns of WP, BBD and DSD were clustered, which is a distribution pattern that suggests an infectious disease.     

Seasonal abundance of Tuberolachnus salignus and its effect on flowering of host willows of varying susceptibility

The giant willow aphid Tuberolachnus salignus Gmelin is a large phloem-feeding insect which colonizes the stems of willow trees. This aphid is a new invasive species in New Zealand and there is limited knowledge of its inter-annual population patterns and the damage it can cause to willow hosts. Our study investigated the T. salignus population dynamics and its effects on the flowering parameters of fifteen willow species and hybrids in a field trial. The aphid population levels were found to vary among the willow species and hybrids. Based on the aphid population levels, the willow species and hybrids were classified as resistant, moderately resistant, susceptible or highly susceptible. T. salignus infestation had no effect on the flowering of resistant and moderately resistant willows, but significantly delayed the flowering time, extended the flowering duration, and decreased the catkin length in susceptible species and hybrids. Interestingly, aphid infestation was found to increase the catkin number and total floral output of some willow species and hybrids. It can be concluded that aphid populations and their effects on flowering are host-specific, with large differences between resistant and susceptible host plants.     

Interspecific variation in conspecific negative density dependence can make species less likely to coexist

Conspecific negative density dependence (CNDD) is thought to promote plant species diversity. Theoretical studies showing the importance of CNDD often assumed that all species are equally susceptible to CNDD; however, recent empirical studies have shown species can differ greatly in their susceptibility to CNDD. Using a theoretical model, we show that interspecific variation in CNDD can dramatically alter its impact on diversity. First, if the most common species are the least regulated by CNDD, then the stabilising benefit of CNDD is reduced. Second, when seed dispersal is limited, seedlings that are susceptible to CNDD are at a competitive disadvantage. When parameterised with estimates of CNDD from a tropical tree community in Panama, our model suggests that the competitive inequalities caused by interspecific variation in CNDD may undermine many species’ ability to persist. Thus, our model suggests that variable CNDD may make communities less stable, rather than more stable.     

The effects of Phoma macrostoma on nontarget plant and target weed species

Phoma macrostoma 94-44B was evaluated against 94 plant species in 34 botanical families, of economically important agricultural, horticultural and ornamental species, as well as target and nontarget weeds. Fifty-seven species from 28 families were found to be resistant to P. macrostoma, while 38 species from 12 families, six of which also contained resistant species, were found to be susceptible. Those families comprising both susceptible and resistant species included the Asteraceae, Brassicaceae, Fabaceae, Lamiaceae, Plantaginaceae and Rosaceae with the foremost three containing the largest numbers of susceptible species. P. macrostoma was pathogenic to many dicotyledonous plant species, but nonpathogenic to monocots. Commercial applications for weed management in turfgrass, agriculture, horticulture and forestry seem probable, while domestically management of weeds in lawns, transplanted ornamental and annual flowering species may provide alternative markets.     

Investigation of intestinal nematode responses to naphthalophos and pyrantel using a larval development assay.

Responses of several nematode species to naphthalophos and pyrantel/levamisole were examined using a larval development assay in order to determine the potential of this assay for detection of resistance. Haemonchus contortus and Ostertagia circumcincta showed concentration-dependent responses to naphthalophos, however, the assay was unsuitable for Trichostrongylus colubriformis due to the low toxicity of the drug to the larval stages of this nematode. Measurement of concentration-dependent response to pyrantel in susceptible T. colubriformis was limited by a reduced toxicity against larvae at high drug concentrations, resulting in a parabolic response with a development-inhibition maxima of less than 100%. This limits the usefulness of the assay to detect pyrantel resistance in this species as the presence of a small resistant fraction in a field isolate may be indistinguishable from the parabolic susceptible response. On the other hand, responses of susceptible T. colubriformis to levamisole, and susceptible H. contortus to pyrantel and levamisole showed 100% development inhibition over a range of drug concentrations, indicating that the appearance of a resistant fraction in a field population would be readily discernible from the susceptible response, allowing resistance detection for these drug/parasite combinations. This study has highlighted the varied suitability of the larval development assay technique for resistance detection with different combinations of drugs and parasite species.     

Host Specificity of Pathogenic Pythium Species: Implications for Tree Species Diversity

In the Janzen–Connell hypothesis, host-specific natural enemies enhance species diversity and influence the structure of plant communities. This study tests the explicit assumption of host specificity for soil pathogens of the genus Pythium that cause damping-off disease of germinating seeds and seedlings. We isolated Pythium spp. from soil of a tropical forest in Panama. Then, in an inoculation experiment, we determined the pathogenicity of 75 tropical isolates of unknown pathogenicity and seven pathogenic temperate isolates of Pythium on seeds and/or seedlings of eight tropical tree species. Only three tropical isolates, one identified as P. ultimum and two as P. aphanidermatum , were pathogenic. Tropical pathogenic isolates were pathogenic on 4–6 of eight tree species. Temperate isolates were pathogenic on 0–4 of eight species, indicating that some tropical tree species are susceptible to novel isolates of Pythium . No tree species was susceptible to all isolates and two species were not susceptible to any isolate. Collectively, these results indicate that these Pythium isolates vary widely in their pathogenicity, causing differential mortality of potential host species; likewise, the tree species vary in their susceptibility to a given Pythium isolate. These differences in pathogenicity and susceptibility indicate some support for the Janzen–Connell assumption of host specificity. While they are not restricted to a single species, their intermediate level of specificity suggests that Pythium spp. have the potential to have some effect on forest community structure and diversity.