Local and systemic replicative fitness for viruses in specialist, generalist, and non-specialist interactions with salmonid hosts.

Host tissues represent diverse resources or barriers for pathogen replicative fitness. We tested whether viruses in specialist, generalist, and non-specialist interactions replicate differently in local entry tissue (fin), and systemic target tissue (kidney) using infectious hematopoietic necrosis virus (IHNV) and three salmonid fish hosts. Virus tissue replication was host specific, but one feature was shared by specialists and the generalist which was uncommon in the non-specialist interactions: high host entry and replication capacity in the local tissue after contact. Moreover, specialists showed increased replication in systemic target tissues early after host contact. By comparing ancestral and derived IHNV viruses, we also characterized replication tradeoffs associated with specialist and generalist evolution. Compared with the ancestral virus, a derived specialist gained early local replicative fitness in the new host but lost replicative fitness in the ancestral host. By contrast, a derived generalist showed small replication losses relative to the ancestral virus in the ancestral host but increased early replication in the local tissue of novel hosts. This study shows that the mechanisms of specialism and generalism are host specific and that local and systemic replication can contribute differently to overall within host replicative fitness for specialist and generalist viruses.

The influence of diet on gut microbiome and body mass dynamics in a capital-breeding migratory bird.

Gut-associated microbial communities are known to play a vital role in the health and fitness of their hosts. Though studies investigating the factors associated with among-individual variation in microbiome structure in wild animal species are increasing, knowledge of this variation at the individual level is scarce, despite the clear link between microbiome and nutritional status uncovered in humans and model organisms. Here, we combine detailed observational data on life history and foraging preference with 16S rRNA profiling of the faecal microbiome to investigate the relationship between diet, microbiome stability and rates of body mass gain in a migratory capital-breeding bird, the light-bellied Brent goose (Branta bernicla hrota). Our findings suggest that generalist feeders have microbiomes that are intermediate in diversity and composition between two foraging specialisms, and also show higher within-individual plasticity. We also suggest a link between foraging phenotype and the rates of mass gain during the spring staging of a capital breeder. This study offers rare insight into individual-level temporal dynamics of the gut microbiome of a wild host. Further work is needed to uncover the functional link between individual dietary choices, gut microbiome structure and stability, and the implications this has for the reproductive success of this capital breeder.

Teaching in Paediatrics for UK Foundation Doctors: A Cross-Sectional Study.

Background Most doctors will care for children regularly during their careers in settings such as the emergency department, general practice, surgery, or, for a minority, during paediatric specialist training. As such, exposure to topics related to child health ought to be part of the broad curriculum of learning offered to UK Foundation Programme doctors. Objective This study aimed to quantify teaching in paediatrics that is accessed by Foundation doctors. Methods A cross-sectional electronic survey of foundation year one or two (F1/F2) doctors at the end of the 2020-2021 academic year. Ethical approval was granted by the Imperial College London (ICL) Education Ethics Review Process (EERP 2021-082). Results Two-hundred and five Foundation doctors completed the survey, from 16 of the 18 Foundation schools. Respondents attended a median of two hours (interquartile range (IQR) 0-10) of paediatric teaching during the past 12 months, including a median of one hour (IQR 0-2) of core teaching and a median of one hour (IQR 0-9) of non-core teaching. Those who had worked in a paediatric post in the past 12 months, or who were interested in Paediatrics as a career, attended more median hours of teaching. Conclusions Although many doctors will care for children routinely during their later careers, the number of teaching hours in paediatrics experienced by Foundation doctors is low. The UK Foundation Programme should incorporate more teaching in paediatrics to increase exposure to child health amongst newly graduated and as-yet unspecialised doctors.

Patterns of host-parasite coinvasion promote enemy release and specialist parasite spillover.

Species invasion and redistribution, driven by climate change and other anthropogenic influences, alter global biodiversity patterns and disrupt ecosystems. As host species move, they can bring their associated parasites with them, potentially infecting resident species, or leave their parasites behind, enhancing their competitive ability in their new ranges. General rules to predict why invading hosts will retain some parasites but not others are relatively unexplored, and the potential predictors are numerous, ranging from parasite life history to host community composition. In this study, we focus on the parasite retention process during host invasion. We used the Global Mammal Parasite Database to identify terrestrial mammal hosts sampled for parasites in both native and non-native ranges. We then selected predictors likely to play a role in parasite retention, such as parasite type, parasite specialism, species composition of the invaded community, and the invading host's phylogenetic or trait-based similarity to the new community. We modelled parasite retention using boosted regression trees, with a suite of 25 predictors describing parasite and host community traits. We further tested the generality of our predictions by cross-validating models on data for other hosts and invasion locations. Our results show that parasite retention is nonrandom and predictable across hosts and invasions. It is broadly shaped by parasite type and parasite specialism, with more specialist parasites that infect many closely related hosts more likely to be retained. This trend is pronounced across parasite types; helminths, however, show a more uniform likelihood of retention regardless of specificity. Overall, we see that most parasites are not retained (11% retained), meaning many invasive species may benefit from enemy release. However, species redistribution does have the potential to spread parasites, and this also has great relevance to understanding conservation implications of species invasions. We see that specialist parasites are most likely to coinvade with their hosts, which suggests that species closely related to the invasive hosts are most likely to be affected by parasite spillover.

Variation in within-host replication kinetics among virus genotypes provides evidence of specialist and generalist infection strategies across three salmonid host species.

Theory of the evolution of pathogen specialization suggests that a specialist pathogen gains high fitness in one host, but this comes with fitness loss in other hosts. By contrast, a generalist pathogen does not achieve high fitness in any host, but gains ecological fitness by exploiting different hosts, and has higher fitness than specialists in nonspecialized hosts. As a result, specialist pathogens are predicted to have greater variation in fitness across hosts, and generalists would have lower fitness variation across hosts. We test these hypotheses by measuring pathogen replicative fitness as within-host viral loads from the onset of infection to the beginning of virus clearance, using the rhabdovirus infectious hematopoietic necrosis virus (IHNV) in salmonid fish. Based on field prevalence and virulence studies, the IHNV subgroups UP, MD, and L are specialists, causing infection and mortality in sockeye salmon, steelhead, and Chinook salmon juveniles, respectively. The UC subgroup evolved naturally from a UP ancestor and is a generalist infecting all three host species but without causing severe disease. We show that the specialist subgroups had the highest peak and mean viral loads in the hosts in which they are specialized, and they had low viral loads in nonspecialized hosts, resulting in large variation in viral load across hosts. Viral kinetics show that the mechanisms of specialization involve the ability to both maximize early virus replication and avoid clearance at later times, with different mechanisms of specialization evident in different host-virus combinations. Additional nuances in the data included different fitness levels for nonspecialist interactions, reflecting different trade-offs for specialist viruses in other hosts. The generalist UC subgroup reached intermediate viral loads in all hosts and showed the smallest variation in fitness across hosts. The evolution of the UC generalist from an ancestral UP sockeye specialist was associated with fitness increases in steelhead and Chinook salmon, but only slight decreases in fitness in sockeye salmon, consistent with low- or no-cost generalism. Our results support major elements of the specialist-generalist theory, providing evidence of a specialist-generalist continuum in a vertebrate pathogen. These results also quantify within-host replicative fitness trade-offs resulting from the natural evolution of specialist and generalist virus lineages in multi-host ecosystems.

"Jack-of-all-trades" is parthenogenetic.

Sex is evolutionarily more costly than parthenogenesis, evolutionary ecologists therefore wonder why sex is much more frequent than parthenogenesis in the majority of animal lineages. Intriguingly, parthenogenetic individuals and species are as common as or even more common than sexuals in some major and putative ancient animal lineages such as oribatid mites and rotifers. Here, we analyzed oribatid mites (Acari: Oribatida) as a model group because these mites are ancient (early Paleozoic), widely distributed around the globe, and include a high number of parthenogenetic species, which often co-exist with sexual oribatid mite species. There is evidence that the reproductive mode is phylogenetically conserved in oribatid mites, which makes them an ideal model to test hypotheses on the relationship between reproductive mode and species' ecological strategies. We used oribatid mites to test the frozen niche variation hypothesis; we hypothesized that parthenogenetic oribatid mites occupy narrow specialized ecological niches. We used the geographic range of species as a proxy for specialization as specialized species typically do have narrower geographic ranges than generalistic species. After correcting for phylogenetic signal in reproductive mode and demonstrating that geographic range size has no phylogenetic signal, we found that parthenogenetic lineages have a higher probability to have broader geographic ranges than sexual species arguing against the frozen niche variation hypothesis. Rather, the results suggest that parthenogenetic oribatid mite species are more generalistic than sexual species supporting the general-purpose genotype hypothesis. The reason why parthenogenetic oribatid mite species are generalists with wide geographic range sizes might be that they are of ancient origin reflecting that they adapted to varying environmental conditions during evolutionary history. Overall, our findings indicate that parthenogenetic oribatid mite species possess a widely adapted general-purpose genotype and therefore might be viewed as "Jack-of-all-trades."

Large-herbivore nemabiomes: patterns of parasite diversity and sharing.

Amidst global shifts in the distribution and abundance of wildlife and livestock, we have only a rudimentary understanding of ungulate parasite communities and parasite-sharing patterns. We used qPCR and DNA metabarcoding of fecal samples to characterize gastrointestinal nematode (Strongylida) community composition and sharing among 17 sympatric species of wild and domestic large mammalian herbivore in central Kenya. We tested a suite of hypothesis-driven predictions about the role of host traits and phylogenetic relatedness in describing parasite infections. Host species identity explained 27-53% of individual variation in parasite prevalence, richness, community composition and phylogenetic diversity. Host and parasite phylogenies were congruent, host gut morphology predicted parasite community composition and prevalence, and hosts with low evolutionary distinctiveness were centrally positioned in the parasite-sharing network. We found no evidence that host body size, social-group size or feeding height were correlated with parasite composition. Our results highlight the interwoven evolutionary and ecological histories of large herbivores and their gastrointestinal nematodes and suggest that host identity, phylogeny and gut architecture-a phylogenetically conserved trait related to parasite habitat-are the overriding influences on parasite communities. These findings have implications for wildlife management and conservation as wild herbivores are increasingly replaced by livestock.

Job Satisfaction and Intentions to Leave of Foreign-National Urologists Working at German Hospitals: Results of the Cross-Sectional EUTAKD Study.

BACKGROUND: Studies exist that identify factors associated with higher professional satisfaction among clinicians. However, there are no reliable findings for clinicians with a migration background as to whether there is a correlation between particular dimensions of professional satisfaction and the desire to leave their current specialty or country of employment. For the first time, these data were collected within a questionnaire-based study from urological clinicians with a migration background (UCMBs) working in Germany. MATERIAL AND METHODS: A SurveyMonkey® with 101 items relating to characterizing features of the study participants and questions about job satisfaction (n = 39) was opened for UCMBs between August and October 2020. The influence of different dimensions of job satisfaction on the desire to quit the urological specialty/leave Germany was analyzed (group A: neither want to leave urology nor Germany; group B: can at least imagine leaving the urological profession and/or Germany). RESULTS: Eighty-one UCMBs were distributed almost equally in groups A (50.6%) and B (49.4%). Occupational satisfaction was higher in several respects in group A. Three dimensions that differed significantly with regard to occupational satisfaction were used to create an aggregate score ranging from 3 to 15 points as follows: (1) relationship to superiors (p = 0.014), (2) career opportunities in the clinic (p < 0.001), and (3) opportunities for the further development of surgical skills (p = 0.006). For each point value of this aggregate score, the UCMB's desire to quit urology or leave Germany (or at least uncertainty about this question) decreased by a relative value of 34.6% (odds ratio: 0.654, 95% confidence interval: 0.496-0.861, p = 0.002). CONCLUSIONS: Various dimensions of job satisfaction have been identified, the improvement of which could contribute to the long-term retention of UCMBs at German urological clinics.

A genome-wide association study identifies Arabidopsis thaliana genes that contribute to differences in the outcome of infection with two Turnip mosaic potyvirus strains that differ in their evolutionary history and degree of host specialization.

Viruses lie in a continuum between generalism and specialism depending on their ability to infect more or less hosts. While generalists are able to successfully infect a wide variety of hosts, specialists are limited to one or a few. Even though generalists seem to gain an advantage due to their wide host range, they usually pay a pleiotropic fitness cost within each host. On the contrary, a specialist has maximal fitness within its own host. A relevant yet poorly explored question is whether viruses differ in the way they interact with their hosts' gene expression depending on their degree of specialization. Using a genome-wide association study approach, we have identified host genes whose expression depends on whether hosts were infected with more or less specialized viral strains. Four hundred fifty natural accessions of Arabidopsis thaliana were inoculated with Turnip mosaic potyvirus strains with different past evolutionary histories and that shown different degrees of specialization. Three disease-related traits were measured and associated with different sets of host genes for each strain. The genetic architectures of these traits differed among viral strains and, in the case of the more specialized virus, also varied along the duration of infection. While most of the mapped loci were strain specific, one shared locus was mapped for both strains, a disease-resistance TIR-NBS-LRR class protein. Likewise, only putative cysteine-rich receptor-like protein kinases were involved in all three traits. The impact on disease progress of 10 selected genes was validated by studying the infection phenotypes of loss-of-function mutant plants. Nine of these mutants have altered the disease progress and/or symptoms intensity between both strains. Compared to wild-type plants six had an effect on both viral strains, three had an effect only on the more specialized, and two were significant during infection with the less specialized.

Multiple dimensions of dietary diversity in large mammalian herbivores.

Theory predicts that trophic specialization (i.e. low dietary diversity) should make consumer populations sensitive to environmental disturbances. Yet diagnosing specialization is complicated both by the difficulty of precisely quantifying diet composition and by definitional ambiguity: what makes a diet 'diverse'? We sought to characterize the relationship between taxonomic dietary diversity (TDD) and phylogenetic dietary diversity (PDD) in a species-rich community of large mammalian herbivores in a semi-arid East African savanna. We hypothesized that TDD and PDD would be positively correlated within and among species, because taxonomically diverse diets are likely to include plants from many lineages. By using DNA metabarcoding to analyse 1,281 faecal samples collected across multiple seasons, we compiled high-resolution diet profiles for 25 sympatric large-herbivore species. For each of these populations, we calculated TDD and PDD with reference to a DNA reference library for local plants. Contrary to our hypothesis, measures of TDD and PDD were either uncorrelated or negatively correlated with each other. Thus, these metrics reflect distinct dimensions of dietary specialization both within and among species. In general, grazers and ruminants exhibited greater TDD, but lower PDD, than did browsers and non-ruminants. We found significant seasonal variation in TDD and/or PDD for all but four species (Grevy's zebra, buffalo, elephant, Grant's gazelle); however, the relationship between TDD and PDD was consistent across seasons for all but one of the 12 best-sampled species (plains zebra). Our results show that taxonomic generalists can be phylogenetic specialists, and vice versa. These two dimensions of dietary diversity suggest contrasting implications for efforts to predict how consumers will respond to climate change and other environmental perturbations. For example, populations with low TDD may be sensitive to phylogenetically 'random' losses of food species, whereas populations with low PDD may be comparatively more sensitive to environmental changes that disadvantage entire plant lineages-and populations with low dietary diversity in both taxonomic and phylogenetic dimensions may be most vulnerable of all.

Generalism in Nature The Great Misnomer: Aphids and Wasp Parasitoids as Examples.

In the present article we discuss why, in our view, the term 'generalism' to define the dietary breadth of a species is a misnomer and should be revised by entomologists/ecologists with the more exact title relating to the animal in question's level of phagy-mono-, oligo, or polyphagy. We discard generalism as a concept because of the indisputable fact that all living organisms fill a unique ecological niche, and that entry and exit from such niches are the acknowledged routes and mechanisms driving ecological divergence and ultimately speciation. The term specialist is probably still useful and we support its continuing usage simply because all species and lower levels of evolutionary diverge are indeed specialists to a large degree. Using aphids and parasitoid wasps as examples, we provide evidence from the literature that even some apparently highly polyphagous agricultural aphid pest species and their wasp parasitoids are probably not as polyphagous as formerly assumed. We suggest that the shifting of plant hosts by herbivorous insects like aphids, whilst having positive benefits in reducing competition, and reducing antagonists by moving the target organism into 'enemy free space', produces trade-offs in survival, involving relaxed selection in the case of the manicured agro-ecosystem.

Fructose-rich niches traced the evolution of lactic acid bacteria toward fructophilic species.

Fructophilic lactic acid bacteria (FLAB) are found in fructose-rich habitats associated with flowers, fruits, fermented foods, and the gastrointestinal tract of several insects having a fructose-based diet. FLAB are heterofermentative lactobacilli that prefer fructose instead of glucose as carbon source, although additional electron acceptor substrates (e.g. oxygen) remarkably enhance their growth on glucose. As a newly discovered bacterial group, FLAB are gaining increasing interest. In this review, the ecological context in which these bacteria exist and evolve was resumed. The wide frequency of isolation of FLAB from fructose feeding insects has been deepened to reveal their ecological significance. Genomic, metabolic data, reductive evolution, and niche specialization of the main FLAB species have been discussed. Findings to date acquired are consistent with a metabolic model in which FLAB display a reliance on environmental niches and the degree of host specificity. In light of FLAB proximity to lactic acid bacteria generally considered to be safe, and due to their peculiar metabolic traits, FLAB may be successfully exploited in food and pharmaceutical applications.

Characterizing the phylogenetic specialism-generalism spectrum of mammal parasites.

The distribution of parasites across mammalian hosts is complex and represents a differential ability or opportunity to infect different host species. Here, we take a macroecological approach to investigate factors influencing why some parasites show a tendency to infect species widely distributed in the host phylogeny (phylogenetic generalism) while others infect only closely related hosts. Using a database on over 1400 parasite species that have been documented to infect up to 69 terrestrial mammal host species, we characterize the phylogenetic generalism of parasites using standard effect sizes for three metrics: mean pairwise phylogenetic distance (PD), maximum PD and phylogenetic aggregation. We identify a trend towards phylogenetic specialism, though statistically host relatedness is most often equivalent to that expected from a random sample of host species. Bacteria and arthropod parasites are typically the most generalist, viruses and helminths exhibit intermediate generalism, and protozoa are on average the most specialist. While viruses and helminths have similar mean pairwise PD on average, the viruses exhibit higher variation as a group. Close-contact transmission is the transmission mode most associated with specialism. Most parasites exhibiting phylogenetic aggregation (associating with discrete groups of species dispersed across the host phylogeny) are helminths and viruses.

Aphid specialism as an example of ecological-evolutionary divergence.

Debate still continues around the definition of generalism and specialism in nature. To some, generalism is equated solely with polyphagy, but this cannot be readily divorced from other essential biological factors, such as morphology, behaviour, genetics, biochemistry, chemistry and ecology, including chemical ecology. Viewed in this light, and accepting that when living organisms evolve to fill new ecological-evolutionary niches, this is the primal act of specialisation, then perhaps all living organisms are specialist in the broadest sense. To illustrate the levels of specialisation that may be found in a group of animals, we here provide an overview of those displayed by a subfamily of hemipteran insects, the Aphididae, which comprises some 1600 species/subspecies in Europe alone and whose members are specialised in a variety of lifestyle traits. These include life cycle, host adaptation, dispersal and migration, associations with bacterial symbionts (in turn related to host adaptation and resistance to hymenopterous wasp parasitoids), mutualisms with ants, and resistance to insecticides. As with polyphagy, these traits cannot easily be separated from one another, but rather, are interconnected, often highly so, which makes the Aphididae a fascinating animal group to study, providing an informative, perhaps unique, model to illustrate the complexities of defining generalism versus specialism.

Opening the debate on MRI practitioner education - Is there a need for change?

RATIONALE: Magnetic Resonance Imaging (MRI) practitioners are traditionally radiographers who learn MRI experientially. This review opens the debate on whether this is the best way of learning MRI. It was conducted as part of a study exploring the value of direct-entry into MRI via a specialized undergraduate degree without first qualifying as a radiographer. METHOD: A narrative literature review using the search terms MRI, radiography education and specialist education (healthcare, radiography). FINDINGS: There is a lack of cohesive policy on how to educate imaging specialists. There is evidence that MRI practitioners lack knowledge and that current educational methods are failing. Direct entry via a specialized undergraduate degree is permitted in some countries, and research suggests that this is a beneficial way of learning MRI.

Host allometry influences the evolution of parasite host-generalism: theory and meta-analysis.

Parasites vary widely in the diversity of hosts they infect: some parasite species are specialists-infecting just a single host species, while others are generalists, capable of infecting many. Understanding the factors that drive parasite host-generalism is of basic biological interest, but also directly relevant to predicting disease emergence in new host species, identifying parasites that are likely to have unidentified additional hosts, and assessing transmission risk. Here, we use mathematical models to investigate how variation in host body size and environmental temperature affect the evolution of parasite host-generalism. We predict that parasites are more likely to evolve a generalist strategy when hosts are large-bodied, when variation in host body size is large, and in cooler environments. We then explore these predictions using a newly updated database of over 20 000 fish-macroparasite associations. Within the database we see some evidence supporting these predictions, but also highlight mismatches between theory and data. By combining these two approaches, we establish a theoretical basis for interpreting empirical data on parasites' host specificity and identify key areas for future work that will help untangle the drivers of parasite host-generalism.This article is part of the themed issue 'Opening the black box: re-examining the ecology and evolution of parasite transmission'.

Investigation of profession development status of intra-veineuse nurse team in Anhui province / 中国实用护理杂志

Objective To investigate the profession development status of intra-veineuse (IV) nurse team in Anhui province, provide the reference and baseline data on intravenous therapy management of province unity standard. Methods Using the cluster sampling method, the questionnaire designed by IV professional committee of Anhui province was used to investigate the status of IV nurse team in 137 hospitals. Results IV nurse in 35 hospitals (25.6%) was accepted the intravenous treatment knowledge training, however no specific provision for training time in 91 hospitals (66.4%). Organization in PICC evaluation was diversified, such as manufacturer (40 accounted for 29.2%), nursing association in province (city)( 38 accounted for 27.7%) and nursing department in hospital (36 accounted for 26.3%). 28 hospitals had PIVAS (20.4%). 62 hospitals established IV nurse team (45.3%), 87 hospitals established IV quality control system (63.5%). Implementation strength was good in the special inspection, the following feedback in standardized operation and adverse events (81 accounted for 59.1%, 72 accounted for 52.6%and 75 accounted for 54.7%, respectively), however, there was no enough emphasis on regular meeting, organization consultation and collection data of regular basis (59 accounted for 43.1%, 52 accounted for 38.0%and 32 accounted for 23.4%, respectively). Conclusions IV nurse team has been preliminarily established in Anhui province, however there are some problems, for example, specialist nurse training and examination is not unified, overall quality control system have not established, external conditions need to be improved. Related departments should actively formulate corresponding countermeasure to promote the healthy development of the IV nurse team.

Exposing the structure of an Arctic food web.

How food webs are structured has major implications for their stability and dynamics. While poorly studied to date, arctic food webs are commonly assumed to be simple in structure, with few links per species. If this is the case, then different parts of the web may be weakly connected to each other, with populations and species united by only a low number of links. We provide the first highly resolved description of trophic link structure for a large part of a high-arctic food web. For this purpose, we apply a combination of recent techniques to describing the links between three predator guilds (insectivorous birds, spiders, and lepidopteran parasitoids) and their two dominant prey orders (Diptera and Lepidoptera). The resultant web shows a dense link structure and no compartmentalization or modularity across the three predator guilds. Thus, both individual predators and predator guilds tap heavily into the prey community of each other, offering versatile scope for indirect interactions across different parts of the web. The current description of a first but single arctic web may serve as a benchmark toward which to gauge future webs resolved by similar techniques. Targeting an unusual breadth of predator guilds, and relying on techniques with a high resolution, it suggests that species in this web are closely connected. Thus, our findings call for similar explorations of link structure across multiple guilds in both arctic and other webs. From an applied perspective, our description of an arctic web suggests new avenues for understanding how arctic food webs are built and function and of how they respond to current climate change. It suggests that to comprehend the community-level consequences of rapid arctic warming, we should turn from analyses of populations, population pairs, and isolated predator-prey interactions to considering the full set of interacting species.

Palliative care: From oncology to all nursing arenas - Good practice or scaring the patients?

The aim of this paper is to encourage critical discussion of an individual's understanding of palliative care and compare this with a health care professional's understanding of palliative care. In doing this, the paper serves to illustrate the importance of words attached to services - so with palliative care - are we providing good care, or scaring the patients? The paper touches on the historical origins of palliative care as an adjunct of oncology, to a specialism in its own right and now as an integral part of all care - in a 'generalist palliative care nurse'. However, it is unlikely that patients and their families are aware of such developments and will see palliative care services as immediate end of life care. It is argued that whatever your thoughts on the use of the term 'palliative care', it is important to understand what has resonance for patients. You may not agree with this paper, and I do not expect all of you to agree, but I hope it has made you think. For those who strongly disagree, take heart from a quote by Frank A. Clark (1860-1936) 'We find comfort among those who agree with us - growth among those who don't'.

Do-or-die life cycles and diverse post-infection resistance mechanisms limit the evolution of parasite host ranges.

In light of the dynamic nature of parasite host ranges and documented potential for rapid host shifts, the observed high host specificity of most parasites remains an ecological paradox. Different variants of host-use trade-offs have become a mainstay of theoretical explanations of the prevalence of host specialism, but empirical evidence for such trade-offs is rare. We propose an alternative theory based on basic features of the parasite life cycle: host selection and subsequent intrahost replication. We introduce a new concept of effective burst size that accounts for the fact that successful host selection does not guarantee intrahost replication. Our theory makes a general prediction that a parasite will expand its host range if its effective burst size is positive. An in silico model of bacteria-phage coevolution verifies our predictions and demonstrates that the tendency for relatively narrow host ranges in parasites can be explained even in the absence of trade-offs.