Showing papers by "Pierre-and-Marie-Curie University published in 1999"

Measurements of Omega and Lambda from 42 High-Redshift Supernovae

Abstract: We report measurements of the mass density, Omega_M, and cosmological-constant energy density, Omega_Lambda, of the universe based on the analysis of 42 Type Ia supernovae discovered by the Supernova Cosmology Project. The magnitude-redshift data for these SNe, at redshifts between 0.18 and 0.83, are fit jointly with a set of SNe from the Calan/Tololo Supernova Survey, at redshifts below 0.1, to yield values for the cosmological parameters. All SN peak magnitudes are standardized using a SN Ia lightcurve width-luminosity relation. The measurement yields a joint probability distribution of the cosmological parameters that is approximated by the relation 0.8 Omega_M - 0.6 Omega_Lambda ~= -0.2 +/- 0.1 in the region of interest (Omega_M <~ 1.5). For a flat (Omega_M + Omega_Lambda = 1) cosmology we find Omega_M = 0.28{+0.09,-0.08} (1 sigma statistical) {+0.05,-0.04} (identified systematics). The data are strongly inconsistent with a Lambda = 0 flat cosmology, the simplest inflationary universe model. An open, Lambda = 0 cosmology also does not fit the data well: the data indicate that the cosmological constant is non-zero and positive, with a confidence of P(Lambda > 0) = 99%, including the identified systematic uncertainties. The best-fit age of the universe relative to the Hubble time is t_0 = 14.9{+1.4,-1.1} (0.63/h) Gyr for a flat cosmology. The size of our sample allows us to perform a variety of statistical tests to check for possible systematic errors and biases. We find no significant differences in either the host reddening distribution or Malmquist bias between the low-redshift Calan/Tololo sample and our high-redshift sample. The conclusions are robust whether or not a width-luminosity relation is used to standardize the SN peak magnitudes.

Prochlorococcus, a Marine Photosynthetic Prokaryote of Global Significance

Abstract: The minute photosynthetic prokaryote Prochlorococcus, which was discovered about 10 years ago, has proven exceptional from several standpoints. Its tiny size (0.5 to 0.7 μm in diameter) makes it the smallest known photosynthetic organism. Its ubiquity within the 40°S to 40°N latitudinal band of oceans and its occurrence at high density from the surface down to depths of 200 m make it presumably the most abundant photosynthetic organism on Earth. Prochlorococcus typically divides once a day in the subsurface layer of oligotrophic areas, where it dominates the photosynthetic biomass. It also possesses a remarkable pigment complement which includes divinyl derivatives of chlorophyll a (Chl a) and Chl b, the so-called Chl a2 and Chl b2, and, in some strains, small amounts of a new type of phycoerythrin. Phylogenetically, Prochlorococcus has also proven fascinating. Recent studies suggest that it evolved from an ancestral cyanobacterium by reducing its cell and genome sizes and by recruiting a protein originally synthesized under conditions of iron depletion to build a reduced antenna system as a replacement for large phycobilisomes. Environmental constraints clearly played a predominant role in Prochlorococcus evolution. Its tiny size is an advantage for its adaptation to nutrient-deprived environments. Furthermore, genetically distinct ecotypes, with different antenna systems and ecophysiological characteristics, are present at depth and in surface waters. This vertical species variation has allowed Prochlorococcus to adapt to the natural light gradient occurring in the upper layer of oceans. The present review critically assesses the basic knowledge acquired about Prochlorococcus both in the ocean and in the laboratory.

Enumeration of Marine Viruses in Culture and Natural Samples by Flow Cytometry

Abstract: Flow cytometry (FCM) was successfully used to enumerate viruses in seawater after staining with the nucleic acid-specific dye SYBR Green-I. The technique was first optimized by using the Phaeocystis lytic virus PpV-01. Then it was used to analyze natural samples from different oceanic locations. Virus samples were fixed with 0.5% glutaraldehyde and deep frozen for delayed analysis. The samples were then diluted in Tris-EDTA buffer and analyzed in the presence of SYBR Green-I. A duplicate sample was heated at 80°C in the presence of detergent before analysis. Virus counts obtained by FCM were highly correlated to, although slightly higher than, those obtained by epifluorescence microscopy or by transmission electron microscopy (r 5 0.937, n 5 14, and r 5 0.96, n 5 8, respectively). Analysis of a depth profile from the Mediterranean Sea revealed that the abundance of viruses displayed the same vertical trend as that of planktonic cells. FCM permits us to distinguish between at least two and sometimes three virus populations in natural samples. Because of its speed and accuracy, FCM should prove very useful for studies of virus infection in cultures and should allow us to better understand the structure and dynamics of virus populations in natural waters.

A Wide Variety of Mutations in the Parkin Gene Are Responsible for Autosomal Recessive Parkinsonism in Europe

Abstract: Autosomal recessive juvenile parkinsonism (AR-JP, PARK2; OMIM 602544), one of the monogenic forms of Parkinson's disease (PD), was initially described in Japan. It is characterized by early onset (before age 40), marked response to levodopa treatment and levodopa-induced dyskinesias, The gene responsible for AR-JP was recently identified and designated parkin, We have analysed the 12 coding exons of the parkin gene in 35 mostly European families with early onset autosomal recessive parkinsonism. In one family, a homozygous deletion of exon 4 could be demonstrated. By direct sequencing of the exons in the index patients of the remaining 34 families, eight previously undescribed point. mutations (homozygous or heterozygous) were detected in eight families that included 20 patients, The mutations segregated with the disease in the families and were not detected on 110-166 control chromosomes. Four mutations caused truncation of the parkin protein. Three were frameshifts (202-203delAG, 255delA and 321-322insGT) and one a nonsense mutation (Trp453Stop). The other four were missense mutations (Lys161Asn, Arg256Cys, Arg275Trp and Thr415Asn) that probably affect amino acids that are important for the function of the parkin protein, since they result in the same phenotype as truncating mutations or homozygous exon deletions. Mean age at onset was 38 +/- 12 years, but onset up to age 58 was observed. Mutations in the parkin gene are therefore not invariably associated with early onset parkinsonism. In many patients, the phenotype is indistinguishable from that of idiopathic PD. This study has shown that a wide variety of different mutations in the parkin gene are a common cause of autosomal recessive parkinsonism in Europe and that different types of point mutations seem to be more frequently responsible for the disease phenotype than are deletions.

A front-tracking algorithm for accurate representation of surface tension

Abstract: We present a front tracking algorithm for the solution of the 2D incompressible Navier-Stokes equations with interfaces and surface forces. More particularly, we focus our attention on the accurate description of the surface tension terms and the associated pressure jump. We consider the stationary Laplace solution for a bubble with surface tension. A careful treatment of the pressure gradient terms at the interface allows us to reduce the spurious currents to the machine precision. Good results are obtained for the oscillation of a capil- lary wave compared with the linear viscous theory. A classical test of Rayleigh-Taylor instability is presented.

Orbital controls on the El Niño/Southern Oscillation and the tropical climate

Abstract: The global synchroneity of glacial-interglacial events is one of the major problems in understanding the link between Milankovitch forcing and the climate of the late Quaternary. In this study we isolate a part of the climate system, the tropical Pacific, and test its sensitivity to changes in solar forcing associated with changes in the Earth's orbital parameters. We use a simplified coupled ocean-atmosphere model that is run for the past 150,000 years and forced with Milankovitch changes in the solar insolation. This system responds primarily to the precessional cycle in solar forcing and is capable of generating a mean response to the changes in the seasonal distribution of solar radiation even while the annual mean insolation is roughly constant. The mean response to the precessional forcing is due to an interaction between an altered seasonal cycle and the El Nifio/Southern Oscillation (ENSO). Changes in the ENSO behavior result in a mean tropical climate change. The hypothesis is advanced that such a change in the tropical climate can generate a globally synchronous climate response to Milankovitch forcing.

Improved energy estimates for interior penalty, constrained and discontinuous Galerkin methods for elliptic problems. Part I

Abstract: Three Galerkin methods using discontinuous approximation spaces are introduced to solve elliptic problems. The underlying bilinear form for all three methods is the same and is nonsymmetric. In one case, a penalty is added to the form and in another, a constraint on jumps on each face of the triangulation. All three methods are locally conservative and the third one is not restricted. Optimal a priori hp error estimates are derived for all three procedures.

Subordinators: Examples and Applications

Abstract: 0. Foreword 1. Elements on subordinators 1.1. Definitions and first properties 1.2. The Levy-Khintchine formula 1.3. The renewal measure 1.4. The range of a subordinator 2. Regenerative property 2.1. Regenerative sets 2.2. Connection with Markov processes 3. Asymptotic behaviour of last passage times 3.1. Asymptotic behaviour in distribution 3.1.1. The self-similar case 3.1.2. The Dynkin-Lamperti theorem 3.2. Asymptotic sample path behaviour 4. Rates of growth of local time 4.1. Law of the iterated logarithm 4.2. Modulus of continuity 5. Geometric properties of regenerative sets 5.1. Fractal dimensions 5.1.1. Box-counting dimension 5.1.2. Hausdorff and packing dimensions 5.2. Intersections with a regenerative set 5.2.1. Equilibrium measure and capacity 5.2.2. Dimension criteria 5.2.3. Intersection of independant regenerative sets 6. Burgers equation with Brownian initial velocity 6.1. Burgers equation and the Hopf-Cole Solution 6.2. Brownian initial velocity 6.3. Proof of the theorem 7. Random covering 7.1. Setting 7.2. The Laplace exponent of the uncovered set 7.3. Some properties of the uncovered set 8. Levy processes 8.1. Local time at a fixed point 8.2. Local time at the supremum 8.3. The spectrally negative case 8.4. Bochner’s subordination for Levy processes 9. Occupation times of a linear Brownian motion 9.1. Occupation times and subordinators 9.2. Levy measure and Laplace exponent 9.2.1. Levy measure via excursion theory 9.2.2. Laplace exponent via the Sturm-Liouville equation 9.2.3. Spectral representation of the Laplace exponent 9.3. The zero set of a one-dimensional diffusion References

Experimental observation of localized optical excitations in random metal-dielectric films

Abstract: Localization of optical excitations within subwavelength areas of a random metal-dielectric film is observed using near-field scanning optical microscopy. This effect is attributed to Anderson localization of surface plasmon modes in a semicontinuous metal film. The localized modes are seen as giant fluctuations of local electric fields spatially concentrated in ``hot'' spots, where the fields are much greater than the applied field. The local near-field spectra consisting of strong resonance peaks are detected and shown to depend markedly on the sample site probed. The observed spectral peaks correspond to localized modes of random metal-dielectric films.

Interleukin 17, a T-cell-derived cytokine, promotes tumorigenicity of human cervical tumors in nude mice.

Abstract: Interleukin (IL) 17 is a proinflammatory cytokine secreted mainly by activated human memory CD4 T cells that induces IL-6, IL-8, and nitric oxide. Because IL-6 and IL-8 have been implicated in the pathogenesis of cervical cancer, we investigated the action of IL-17 on human cervical tumor cell lines in vitro and in vivo. We showed that in vitro, IL-17 increases IL-6 and IL-8 secretion by cervical carcinoma cell lines at both protein and mRNA levels. No direct effect of IL-17 on in vitro proliferation of cervical tumor cell lines could be demonstrated. However, two cervical cell lines transfected with a cDNA encoding IL-17 exhibited a significant increase in tumor size as compared to the parent tumor when transplanted in nude mice. This enhanced tumor growth elicited by IL-17 was associated with increased expression of IL-6 and macrophage recruitment at the tumor site. A potential role of IL-17 in modulation of the human cervical tumor phenotype was also supported by its expression on the cervical tumor in patients with CD4 infiltration. IL-17 therefore behaves like a T-cell-specific cytokine with paradoxical tumor-promoting activity. This may partially explain previous reports concerning the deleterious effect of CD4 T cells in cancer.

Parasitism, host immune function, and sexual selection.

Abstract: Parasite-mediated sexual selection may arise as a consequence of 1) females avoiding mates with directly transmitted parasites, 2) females choosing less-parasitized males that provide parental care of superior quality, or 3) females choosing males with few parasites in order to obtain genes for parasite resistance in their offspring. Studies of specific host-parasite systems and comparative analyses have revealed both supportive and conflicting evidence for these hypotheses. A meta-analysis of the available evidence revealed a negative relationship between parasite load and the expression of male secondary sexual characters. Experimental studies yielded more strongly negative relationships than observations did, and the relationships were more strongly negative for ectoparasites than for endoparasites. There was no significant difference in the magnitude of the negative effect for species with and without male parental care, or between behavioral and morphological secondary sexual characters. There was a ...

Chronic brain inflammation and persistent herpes simplex virus 1 thymidine kinase expression in survivors of syngeneic glioma treated by adenovirus-mediated gene therapy: implications for clinical trials.

Abstract: The long-term consequences of adenovirus-mediated conditional cytotoxic gene therapy for gliomas remain uncharacterized. We report here detection of active brain inflammation 3 months after successful inhibition of syngeneic glioma growth. The inflammatory infiltrate consisted of activated macrophages/microglia and astrocytes, and T lymphocytes positive for leucosyalin, CD3 and CD8, and included secondary demyelination. We detected strong widespread herpes simplex virus 1 thymidine kinase immunoreactivity and vector genomes throughout large areas of the brain. Thus, patient evaluation and the design of clinical trials in ongoing and future gene therapy for brain glioblastoma must address not only tumor-killing efficiency, but also long-term active brain inflammation, loss of myelin fibers and persistent transgene expression.

New Class I and II HLA Alleles Strongly Associated with Opposite Patterns of Progression to AIDS

Abstract: The genetics of resistance to infection by HIV-1 cohort consists of 200 slow and 75 rapid progressors to AIDS corresponding to the extremes of HIV disease outcome of 20,000 Caucasians of European descent. A comprehensive analysis of HLA class I and class II genes in this highly informative cohort has identified HLA alleles associated with fast or slow progression, including several not described previously. A quantitative analysis shows an overall HLA influence independent of and equal in magnitude (for the protective effect) to the effect of the CCR5-Delta32 mutation. Among HLA class I genes, A29 (p = 0.001) and B22 (p < 0.0001) are significantly associated with rapid progression, whereas B14 (p = 0.001) and C8 (p = 0.004) are significantly associated with nonprogression. The class I alleles B27, B57, C14 (protective), and C16, as well as B35 (susceptible), are also influential, but their effects are less robust. Influence of class II alleles was only observed for DR11. These results confirm the influence of the immune system on disease progression and may have implications on peptide-based vaccine development.

Femtosecond dynamics of hydrogen bonds in liquid water : a real time study

Abstract: A pump-probe experiment is described to study femtosecond dynamics of hydrogen bonds in liquid water. The key element of the experimental setup is a laser source emitting 150 fs pulses in the 2.5– 4.4 mm spectral region, at a 10 mJ power level. The OH-stretching band is recorded for different excitation frequencies and different pump-probe delay times. Time-dependent solvatochromic shifts are observed and are interpreted with the help of statistical mechanics of nonlinear optical processes. Using these spectral data, the OH · · · O motions are “photographed” in real time. [S0031-9007(98)08298-2]

Tom22 is a multifunctional organizer of the mitochondrial preprotein translocase

Abstract: Mitochondrial preproteins are imported by a multisubunit translocase of the outer membrane (TOM), including receptor proteins and a general import pore. The central receptor Tom22 binds preproteins through both its cytosolic domain and its intermembrane space domain and is stably associated with the channel protein Tom40 (refs 11-13). Here we report the unexpected observation that a yeast strain can survive without Tom22, although it is strongly reduced in growth and the import of mitochondrial proteins. Tom22 is a multifunctional protein that is required for the higher-level organization of the TOM machinery. In the absence of Tom22, the translocase dissociates into core complexes, representing the basic import units, but lacks a tight control of channel gating. The single membrane anchor of Tom22 is required for a stable interaction between the core complexes, whereas its cytosolic domain serves as docking point for the peripheral receptors Tom20 and Tom70. Thus a preprotein translocase can combine receptor functions with distinct organizing roles in a multidomain protein.

The 1997 spectroscopic GEISA databank

Abstract: The current version GEISA-97 of the computer-accessible database system GEISA (Gestion et Etude des Informations Spectroscopiques Atmospheriques: Management and Study of Atmospheric Spectroscopic Information) is described. This catalogue contains 1,346,266 entries. These are spectroscopic parameters required to describe adequately the individual spectral lines belonging to 42 molecules (96 isotopic species) and located between 0 and 22,656 cm-1. The featured molecules are of interest in studies of the terrestrial as well as the other planetary atmospheres, especially those of the Giant Planets. GEISA-97 contains also a catalog of absorption cross-sections of molecules such as chlorofluorocarbons which exhibit unresolvable spectra. The modifications and improvements made to the earlier edition (GEISA-92) and the data management software are described. GEISA-97 and the associated management software are accessible from the ARA/LMD (Laboratoire de Meteorologie Dynamique du CNRS, France) web site: http://ara01.polytechnique.fr/registration.

Atomic Structure and Vibrational Properties of Icosahedral B 4 C Boron Carbide

Abstract: The atomic structure of icosahedral ${\mathrm{B}}_{4}\mathrm{C}$ boron carbide is determined by comparing existing infrared absorption and Raman diffusion measurements with the predictions of accurate ab initio lattice-dynamical calculations performed for different structural models, a task presently beyond x-ray and neutron diffraction ability. By examining the inter- and intraicosahedral contributions to the stiffness we show that, contrary to recent conjectures, intraicosahedral bonds are harder.

Mitochondrial DNA repairs double-strand breaks in yeast chromosomes

Abstract: The endosymbiotic theory for the origin of eukaryotic cells proposes that genetic information can be transferred from mitochondria to the nucleus of a cell, and genes that are probably of mitochondrial origin have been found in nuclear chromosomes. Occasionally, short or rearranged sequences homologous to mitochondrial DNA are seen in the chromosomes of different organisms including yeast, plants and humans. Here we report a mechanism by which fragments of mitochondrial DNA, in single or tandem array, are transferred to yeast chromosomes under natural conditions during the repair of double-strand breaks in haploid mitotic cells. These repair insertions originate from noncontiguous regions of the mitochondrial genome. Our analysis of the Saccharomyces cerevisiae mitochondrial genome indicates that the yeast nuclear genome does indeed contain several short sequences of mitochondrial origin which are similar in size and composition to those that repair double-strand breaks. These sequences are located predominantly in non-coding regions of the chromosomes, frequently in the vicinity of retrotransposon long terminal repeats, and appear as recent integration events. Thus, colonization of the yeast genome by mitochondrial DNA is an ongoing process.

Memories in sand: experimental tests of construction history on stress distributions under sandpiles.

Abstract: We report experiments on piles of cohesionless granular materials showing the effect of construction history on static stress distributions. Stresses under piles are monitored by sensitive capacitive techniques. The piles are formed either by pouring granular material from a funnel with a small outlet (localized source), or from a large sieve (homogeneous rain). Localized sources yield stress profiles with a clear stress dip near the center of the pile; the homogeneous rain profiles have no stress dip. We show that the stress profiles scale linearly with the pile height. Experiments on wedge-shaped piles show similar but weaker effects.

Immunocompetence and condition-dependent sexual advertisement in male house sparrows (Passer domesticus)

Abstract: Summary1. We tested the condition-dependent hypothesis of sexual advertisement in housesparrows (Passer domesticus). Male house sparrows have a bib of black featherswhich serves as both a badge of social status and as a cue for female choice. Wemanipulated environmental conditions during the premoult period of juvenilehouse sparrows kept in outdoor aviaries. Birds were assigned to two treatments dif-fering in the amount of dietary proteins, which are known to a•ect the expressionof immune response in birds. We tested whether birds in the protein-rich grouphad better immune responses and developed larger bibs than birds reared on a pro-tein-poor diet. We also checked whether immune response was a predictor of survi-val and parasite resistance.2. Individuals with higher cellular immune response at capture had greater prob-ability to survive during the 3months of the experiment, and they had a higherprobability to recover from infection with Haemoproteus sp. (a blood parasite).Conversely, birds with high immunoglobulin concentrations at capture had ahigher probability of mortality.3. Birds on the protein-rich diet had a higher cellular immune response comparedto birds in the protein-poor treatment. Humoral immune response showed theopposite pattern, being higher for birds in the protein-poor treatment. We did notfind any e•ect of food quality on the development of the badge, assessed as the sizeof the trait and its colour properties.4. In conclusion, our results support the view that immune defences are importantfor survival and parasite resistance in natural populations, and that they might becostly to produce. On the other hand, we did not find support for the condition-dependent hypothesis of sexual advertisement, suggesting that the badge may notbe a costly trait to produce. However, badge size could reflect other aspects of con-dition. The kind of pigments involved in colour signals may be the key factor deter-mining the production costs of such traits.Key-words: immune response, immunoglobulins, parasite resistance, sexual signals,survival, T-cell response.Journal of Animal Ecology (1999) 68, 1225–1234Introduction

Droplet Spreading: Partial Wetting Regime Revisited

Abstract: We study the time evolution of a sessile liquid droplet, which is initially put onto a solid surface in a nonequilibrium configuration and then evolves towards its equilibrium shape. We adapt here the standard approach to the dynamics of mechanical dissipative systems, in which the driving force, i.e., the gradient of the system's Lagrangian function, is balanced against the rate of the dissipation function. In our case, the driving force is the loss of the droplet's free energy due to the increase of its base radius, whereas the dissipation occurs because of viscous flows in the core of the droplet and frictional processes in the vicinity of the advancing contact line, associated with attachment of fluid particles to solid. Within this approach, we derive closed-form equations for the evolution of the droplet's base radius and specify regimes at which different dissipation channels dominate. Our analytical predictions compare very well with experimental data.

Bolidomonas : a new genus with two species belonging to a new algal class,the bolidophyceae (heterokonta)

Abstract: A new algal class, the Bolidophyceae (Heterokonta), is described from one genus, Bolidomonas, gen nov, and two species, Bolidomonas pacifica, sp nov and Bolidomonas mediterranea, sp nov, isolated from the equatorial Pacific Ocean and the Mediterranean Sea, respectively Both species are approximately 12 mm in diameter and have two unequal flagella; the longer flagellum bears tubular hairs, whereas the shorter is smooth The flagellar basal apparatus is restricted to two basal bodies, and there is no transitional helix Cells are naked, devoid of walls or siliceous structures The internal cellular organization is simple with a single plastid containing a ring genophore and a girdle lamella, one mitochondrion with tubular cristae, and one Golgi apparatus close to the basal bodies The Mediterranean and the Pacific species differ in the insertion angle between their flagella and their pattern of swimming, these differences possibly being linked to each other Analyses of the SSU rDNA gene place the two strains as a sister group to the diatoms Moreover, pigment analyses confirm this position, as fucoxanthin is found as the major carotenoid in both lineages These data strongly suggest that the ancestral heterokont that gave rise to the diatom lineage was probably a biflagellated unicell

Local maladaptation in the anther-smut fungus microbotryum violaceum to its host plant silene latifolia: evidence from a cross-inoculation experiment.

Abstract: Conventional wisdom holds that parasites evolve more rapidly than their hosts and are therefore locally adapted, that is, better at exploiting sympatric than allopatric hosts. We studied local adaptation in the insect-transmitted fungal pathogen Microbotryum violaceuin and its host plant Silene latifolia. Infection success was tested in sympatric (local) and allopatric (foreign) combinations of pathogen and host from 14 natural populations from a metapopulation. Seedlings from up to 10 seed families from each population were exposed to sporidial suspensions from each of four fungal strains derived from the same population, from a near-by population (< 10 km distance), and from two populations at an intermediate (< 30 km) and remote (< 170 km) distance, respectively. We obtained significant pathogen X plant interactions in infection success (proportion of diseased plants) at both fungal population and strain level. There was an overall pattern of local maladaptation of this pathogen: average fungal infection success was significantly lower on sympatric hosts (mean proportion of diseased plants = 0.32 + 0.03 SE) than on allopatric hosts (0.40 + 0.02). Five of the 14 fungal populations showed no strong reduction in infection success on sympatric hosts, and three even tended to perform better on sympatric hosts. This pattern is consistent with models of time-lagged cycles predicting patterns of local adaptation in host-parasite systems to emerge only on average. Several factors may restrict the evolutionary potential of this pathogen relative to that of its host. First, a predominantly selfing breeding system may limit its ability to generate new virulence types by sexual recombination, whereas the obligately outcrossing host S. latifolia may profit from rearrangement of resistance alleles by random mating. Second, populations often harbor only a few infected individuals, so virulence variation may be further reduced by drift. Third, migration rates among host plant populations are much higher than among pathogen populations, possibly because pollinators prefer healthy over diseased plants. Migration among partly isolated populations may therefore introduce novel host plant resistance variants more often than novel parasite virulence variants. That migration contributes to the coevolutionary dynamics in this system is supported by the geographic pattern of infectivity. Infection success increased over the first 10-km range of host-pathogen population distances, which is likely the natural range of gene exchange.