In field experiments manipulating generalist pathogens and host community composition, the presence of a highly susceptible reservoir species drove disease dynamics in multiple nonreservoir species, sometimes decreasing their abundance through apparent competition. The dynamics of generalist pathogens in multispecies host communities remain a major frontier for disease ecology. Of particular interest are how host community structure controls pathogen transmission and how disease spread feeds back to influence the host community. Pathogen spillover occurs when epidemics in a host population are driven not by transmission within that population but by transmission from a reservoir population. Here we review examples of spillover in pathogens infecting humans, domesticated animals, and crops, noting that most empirical evidence for spillover results from nonmanipulative, observational studies. We then present results from two field experiments utilizing an experimentally tractable model system of annual wild grasses and a generalist virus, the barley yellow dwarf virus. In these experiments, the presence of a highly susceptible reservoir species, Avena fatua (wild oats), greatly increased pathogen prevalence in several other species. This result demonstrates pathogen spillover and illustrates the crucial role of host community structure in controlling the dynamics of generalist pathogens. Further, pathogen spillover from A. fatua decreased the abundance of two other host species through pathogen-mediated apparent competition. Thus, our results provide experimental support for theoretical predictions of strong feedbacks between host community structure and generalist disease dynamics.

/pdf/pathogen-spillover-in-disease-epidemics-4xh824gf6g.pdf

Pathogen spillover in disease epidemics

In this paper, I review the relevance of the niche to biogeography, and what biogeography may tell us about the niche. The niche is defined as the combination of abiotic and biotic conditions where a species can persist. I argue that most biogeographic patterns are created by niche differences over space, and that even 'geographic barriers' must have an ecological basis. However, we know little about specific ecological factors underlying most biogeographic patterns. Some evidence supports the importance of abiotic factors, whereas few examples exist of large-scale patterns created by biotic interactions. I also show how incorporating biogeography may offer new perspectives on resource-related niches and species interactions. Several examples demonstrate that even after a major evolutionary radiation within a region, the region can still be invaded by ecologically similar species from another clade, countering the long-standing idea that communities and regions are generally 'saturated' with species. I also describe the somewhat paradoxical situation where competition seems to limit trait evolution in a group, but does not prevent co-occurrence of species with similar values for that trait (called here the 'competition-divergence-co-occurrence conundrum'). In general, the interface of biogeography and ecology could be a major area for research in both fields.

/pdf/the-niche-biogeography-and-species-interactions-49l10w0fm0.pdf

The niche, biogeography and species interactions

A study was conducted to assess the breed resistance against nematode infections in Santa Ines, Ile de France and Suffolk male lambs over a 9-month period in Sao Paulo state, Brazil. Lambs were born during the winter (year 2000) and were weaned at 2 months of age. The animals were then housed and treated with anthelmintics to eliminate natural infections by gastrointestinal nematodes. In late October 2000, lambs were placed in a paddock, where they stayed until August of the following year. Fecal and blood samples were taken from each animal every 2 weeks. On the same day, a pasture sample was collected to determine the number of infective larvae on the herbage. To prevent deaths, individual treatment with anthelmintics was provided to lambs with fecal egg counts (FEC) higher than 4000 eggs per gram (EPG) or with a packed cell volume (PCV) lower than 21%. In August 2001, all animals were slaughtered and the worms present in samples of the gastrointestinal contents were identified and counted. Most of the Suffolk and Ile de France sheep received three to six anthelmintic treatments over a period of 7 months, while most of the Santa Ines were not treated. Reductions in PCV and plasma protein values associated with high FEC and worm burdens were recorded, particularly, in Suffolk and Ile de France lambs. Haemonchus contortus and Oesophagostomum columbianum burdens and number of nodular lesions caused in the large intestine by O. columbianum larvae were significantly lower in Santa Ines sheep. All three breeds showed similar Trichostrongylus colubriformis worm burdens. The relative resistance of Santa Ines young male sheep was superior to that of Suffolk and Ile de France sheep.

Resistance of Santa Ines, Suffolk and Ile de France sheep to naturally acquired gastrointestinal nematode infections.

The evolutionary history of the largest salamander family (Plethodontidae) is characterized by extreme morphological homoplasy. Analysis of the mechanisms generating such homoplasy requires an independent molecular phylogeny. To this end, we sequenced 24 complete mitochondrial genomes (22 plethodontids and two outgroup taxa), added data for three species from GenBank, and performed partitioned and unpartitioned Bayesian, maximum likelihood, and maximum parsimony phylogenetic analyses. We explored four dataset partitioning strategies to account for evolutionary process heterogeneity among genes and codon positions, all of which yielded increased model likelihoods and decreased numbers of supported nodes in the topologies (Bayesian posterior probability >0.95) relative to the unpartitioned analysis. Our phylogenetic analyses yielded congruent trees that contrast with the traditional morphology-based taxonomy; the monophyly of three of four major groups is rejected. Reanalysis of current hypotheses in light of these evolutionary relationships suggests that (i) a larval life history stage reevolved from a direct-developing ancestor multiple times; (ii) there is no phylogenetic support for the “Out of Appalachia” hypothesis of plethodontid origins; and (iii) novel scenarios must be reconstructed for the convergent evolution of projectile tongues, reduction in toe number, and specialization for defensive tail loss. Some of these scenarios imply morphological transformation series that proceed in the opposite direction than was previously thought. In addition, they suggest surprising evolutionary lability in traits previously interpreted to be conservative.

/pdf/morphological-homoplasy-life-history-evolution-and-324avox49n.pdf

Morphological homoplasy, life history evolution, and historical biogeography of plethodontid salamanders inferred from complete mitochondrial genomes

1 Individuals and populations T H Clutton-Brock and J M Pemberton 2 The sheep of St Kilda T H Clutton-Brock, J M Pemberton, T N Coulson, IR Stevenson and A D C MacColl 3 Population dynamics in Soay sheep T H Clutton-Brock, T N Coulson, A D C MacColl, A Illius M Forchhammer K Wilson, J Lindstrom, M J Crawley and S D Albon 4 Vegetation and sheep population dynamics M J Crawley, S D Albon, D R Bazely, J Milner, J G Pilkington and A L Tuke 5 Parasites and their impact K Wilson, B T Grenfell, J G Pilkington, H E G Boyd and F M D Gulland 6 Mating patterns and male breeding success J M Pemberton, D W Coltman, J A Smith and D A Bancroft 7 Selection on phenotype J M Milner, S D Albon, L E B Kruuk and J M Pemberton 8 Molecular genetic variation and selection on genotype J M Pemberton, D W Coltman, D R Bancroft, J A Smith and S Paterson 9 Adaptive reproductive strategies I R Stevenson, P Marrow, B T Preston, J M Pemberton and K Wilson 10 The causes and consequences of instability T H Clutton-Brock Appendix 1 The flora of St Kilda M J Crawley Appendix 2 Inheritance of coat colour and horn type in Hirta Soay sheep D W Coltman and J M Pemberton References

/pdf/soay-sheep-dynamics-and-selection-in-an-island-population-2czoymuyrv.pdf

Soay Sheep: Dynamics and Selection in an Island Population

Faecal egg counts and live weights were measured on approximately 200 predominantly twin-born Scottish Blackface lambs each year for 3 years, from 1 to 6 1'lonths of age. Measurements lvere made at 4-ti'eek mt/:'ltlals following anthelmintic treatment. Heritability estimates (with s.e.s) of log transformed faecal egg count at each age were 0.01,0.00,0.12 (0.10), 0.14 (0.12), 0.15 (0.07) ard 0.22 (0.13), for ages 1 to 6 months respectively. Therefore, genetic variation exists for acquired but apparentl~ not for innate resistance to infection. Maternal common environmental effects (with s.e.s) were 0.36 (0.11), 0.2b (0.05), 0.27 (0.09),0.06 (0.08), 0.15 (0.09) and 0.16 (0.08), for ages 1 to 6 months respectively. Genetic correlatiqns between faecal egg counts in Iambs older than 3 mpnths u'ere not significantly less than 1.0, indicating that faecal egg counts at different ages are expressions of the same trait. Phenotypic correlations between faecal egg colfnts were generally positive but small. Measurement ,error contributed one-third of the observed variation for indrvidual egg counts. The heritability of mean faecal egg count from.3 to 6 months was 0:33 (s.e. 0.15), .indicatin~ ~hat selection decisions can be ma~e mo~e accurately using multIple egg counts per anImal. PhenotypIc correlatIons between faecal egg counts and live weIght were generally negative but close to zero. However, genetic correlati~ns between faecal egg counts and live weight in lambs older than 3 months were close to -1.0, indicating that r~sistance to gastrointestinal parasites may be an important genetic determinant of growth rate in this environment.

/pdf/genetic-parameters-for-faecal-egg-count-following-mixed-4x7srvzbgb.pdf

Genetic parameters for faecal egg count following mixed, natural, predominantly Ostertagia circumcincta infection and relationships with live weight in young lambs

Nematodes are a major cause of disease and death in humans, domestic animals and wildlife. Understanding why some individuals suffer severely whereas others exposed to the same infection remain healthy may assist in the development of rational and sustainable strategies to control infection. Here, using a quantitative genetic analysis of the parasitic nematode population that had accumulated naturally in lambs, we find no apparent influence of host genetics on nematode numbers but an extremely strong influence on average worm length and fecundity. Our results indicate that in growing lambs the main manifestation of genetic resistance is the control of worm fecundity.

/pdf/how-hosts-control-worms-9i7986p193.pdf

How hosts control worms

The impact of mixed, nematode infection upon a group of animals will depend upon the number of nematodes present, how they are distributed among hosts and whether individuals that are heavily parasitized with one species are more likely to be heavily parasitized with other species. A survey of over 500 six-month-old, Scottish Blackface lambs from a single farm in Southwest Strathclyde identified 7 different categories of nematodes in the abomasum and small intestine. There were considerable differences among years and among nematodes in the prevalence and mean intensity of infection. Ostertagia circumcincta was present in nearly all lambs and judged by prevalence and intensity is one of the most successful of all parasitic nematodes. Each category of nematodes had a skewed distribution; most animals had relatively few worms but a small proportion had many worms. The variance of the number of nematodes in each category were approximately equal to the square of the mean. The counts of adult O. circumcincta followed a negative binomial distribution, but the negative binomial distribution did not provide a good description of the observed values for the other species. These other species had a lower prevalence and possibly some sheep were not exposed to infection. There was no significant genetic variation among lambs in the number of nematodes present and therefore the differences among these lambs were unlikely to be a consequence of genetic differences in host susceptibility. Lambs with increased numbers of one species were more likely to be have increased numbers of the other species, but the correlations were weak and may reflect covariation in exposure to different parasites.

The processes influencing the distribution of parasitic nematodes among naturally infected lambs

The relationship between Ostertagia (Teladorsagia) circumcincta and sheep is one of the best understoodhost-parasite relationships in any species. The key components of resistance have been quantified, the extent of genetic control has been established for lambs, and methods now exist to breed lambs which will be both more resistant to worms and more productive than unselected lambs. A major gene for resistance has been identified within or around the major histocompatibility complex, and this gene appears to be the strongest yet identified for resistance to any parasite species. The most important mechanisms of resistance are local IgA responses which regulate worm fecundity and immediate hypersensitivity responses which regulate worm burdens. IgA responses develop before effective immediate hypersensitivity responses. Good simulation models now exist to predict, for example, outbreaks of disease and the response of sheep to selection. The challenge now is to use our improved understanding of the population biology to develop even better simulation models and to produce expert systems based on these models which can be used by veterinarians and others to determine optimal procedures for individual farms to control disease and reduce sub-clinical economic losses.

The genetic basis of resistance to Ostertagia circumcincta in lambs

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Genetic parameters for faecal egg count following mixed, natural, predominantly Ostertagia circumcincta infection and relationships with live weight in young lambs

How hosts control worms

The processes influencing the distribution of parasitic nematodes among naturally infected lambs

The genetic basis of resistance to Ostertagia circumcincta in lambs