Mixed infection,risk projection,and misdirection: Interactions among pathogens alter links between host resources and disease

A growing body of literature links resources of hosts to their risk of infectious disease. Yet most hosts encounter multiple pathogens, and projections of disease risk based on resource availability could be fundamentally wrong if they do not account for interactions among pathogens within hosts. Here, we measured infection risk of grass hosts (Avena sativa) exposed to three naturally co‐occurring viruses either singly or jointly (barley and cereal yellow dwarf viruses [B/CYDVs]: CYDV‐RPV, BYDV‐PAV, and BYDV‐SGV) along experimental gradients of nitrogen and phosphorus supply. We asked whether disease risk (i.e., infection prevalence) differed in single versus co‐inoculations, and whether these differences varied with rates and ratios of nitrogen and phosphorus supply. In single inoculations, the viruses did not respond strongly to nitrogen or phosphorus. However, in co‐inoculations, we detected illustrative cases of 1) resource‐dependent antagonism (lower prevalence of RPV with increasing N; possibly due to competition), 2) resource‐dependent facilitation (higher prevalence of SGV with decreasing N:P; possibly due to immunosuppression), and 3) weak or no interactions within hosts (for PAV). Together, these within‐host interactions created emergent patterns for co‐inoculated hosts, with both infection prevalence and viral richness increasing with the combination of low nitrogen and high phosphorus supply. We demonstrate that knowledge of multiple pathogens is essential for predicting disease risk from host resources and that projections of risk that fail to acknowledge resource‐dependent interactions within hosts could be qualitatively wrong. Expansions of theory from community ecology theory may help anticipate such relationships by linking host resources to diverse pathogen communities.     

Transmission routes maintaining a viral pathogen of steelhead trout within a complex multi‐host assemblage

This is the first comprehensive region wide, spatially explicit epidemiologic analysis of surveillance data of the aquatic viral pathogen infectious hematopoietic necrosis virus (IHNV) infecting native salmonid fish. The pathogen has been documented in the freshwater ecosystem of the Pacific Northwest of North America since the 1950s, and the current report describes the disease ecology of IHNV during 2000–2012. Prevalence of IHNV infection in monitored salmonid host cohorts ranged from 8% to 30%, with the highest levels observed in juvenile steelhead trout. The spatial distribution of all IHNV‐infected cohorts was concentrated in two sub‐regions of the study area, where historic burden of the viral disease has been high. During the study period, prevalence levels fluctuated with a temporal peak in 2002. Virologic and genetic surveillance data were analyzed for evidence of three separate but not mutually exclusive transmission routes hypothesized to be maintaining IHNV in the freshwater ecosystem. Transmission between year classes of juvenile fish at individual sites (route 1) was supported at varying levels of certainty in 10%–55% of candidate cases, transmission between neighboring juvenile cohorts (route 2) was supported in 31%–78% of candidate cases, and transmission from adult fish returning to the same site as an infected juvenile cohort was supported in 26%–74% of candidate cases. The results of this study indicate that multiple specific transmission routes are acting to maintain IHNV in juvenile fish, providing concrete evidence that can be used to improve resource management. Furthermore, these results demonstrate that more sophisticated analysis of available spatio‐temporal and genetic data is likely to yield greater insight in future studies.     

Investigating the effects of age‐related spatial structuring on the transmission of a tick‐borne virus in a colonially breeding host

Higher pathogen and parasite transmission is considered a universal cost of colonial breeding due to the physical proximity of colony members. However, this has rarely been tested in natural colonies, which are structured entities, whose members interact with a subset of individuals and differ in their infection histories. We use a population of common guillemots, Uria aalge, infected by a tick‐borne virus, Great Island virus, to explore how age‐related spatial structuring can influence the infection costs borne by different members of a breeding colony. Previous work has shown that the per‐susceptible risk of infection (force of infection) is different for prebreeding (immature) and breeding (adult) guillemots which occupy different areas of the colony. We developed a mathematical model which showed that this difference in infection risk can only be maintained if mixing between these age groups is low. To estimate mixing between age groups, we recorded the movements of 63 individually recognizable, prebreeding guillemots in four different parts of a major colony in the North Sea during the breeding season. Prebreeding guillemots infrequently entered breeding areas (in only 26% of watches), though with marked differences in frequency of entry among individuals and more entries toward the end of the breeding season. Once entered, the proportion of time spent in breeding areas by prebreeding guillemots also varied between different parts of the colony. Our data and model predictions indicate low levels of age‐group mixing, limiting exposure of breeding guillemots to infection. However, they also suggest that prebreeding guillemots have the potential to play an important role in driving infection dynamics. This highlights the sensitivity of breeding colonies to changes in the behavior of their members—a subject of particular importance in the context of global environmental change.     

Population genetic structure of a common host predicts the spread of white‐nose syndrome,an emerging infectious disease in bats

Landscape complexity influences patterns of animal dispersal, which in turn may affect both gene flow and the spread of pathogens. White‐nose syndrome (WNS) is an introduced fungal disease that has spread rapidly throughout eastern North America, causing massive mortality in bat populations. We tested for a relationship between the population genetic structure of the most common host, the little brown myotis (Myotis lucifugus), and the geographic spread of WNS to date by evaluating logistic regression models of WNS risk among hibernating colonies in eastern North America. We hypothesized that risk of WNS to susceptible host colonies should increase with both geographic proximity and genetic similarity, reflecting historical connectivity, to infected colonies. Consistent with this hypothesis, inclusion of genetic distance between infected and susceptible colonies significantly improved models of disease spread, capturing heterogeneity in the spatial expansion of WNS despite low levels of genetic differentiation among eastern populations. Expanding our genetic analysis to the continental range of little brown myotis reveals strongly contrasting patterns of population structure between eastern and western North America. Genetic structure increases markedly moving westward into the northern Great Plains, beyond the current distribution of WNS. In western North America, genetic differentiation of geographically proximate populations often exceeds levels observed across the entire eastern region, suggesting infrequent and/or locally restricted dispersal, and thus relatively limited opportunities for pathogen introduction in western North America. Taken together, our analyses suggest a possibly slower future rate of spread of the WNS pathogen, at least as mediated by little brown myotis.     

Three‐year surveillance (2016–2018) of chigger mites vector for tsutsugamushi disease in the Hwaseong‐Si area of Gyeonggi‐Do,Republic of Korea

Owing to climate change, the global resurgence of vector‐borne infectious diseases has emerged as a critical public health issue. Orientia tsutsugamushi is the etiological agent of tsutsugamushi disease (scrub typhus) a mite‐borne acute febrile disease occurring in the Asia‐Pacific region. We investigated the prevalence of tsutsugamushi disease transmitted by chigger mite vectors living on rodents. Using sticky‐type chigger traps for three months during 2016–2018, 1,057 chigger mites were collected (chigger mite index, 1.31) from four locations in the Hwaseong‐si area of Gyeonggi‐do, Republic of Korea. Five species distributed among three genera were identified. In addition, 94 rodents were captured (collection rate: 7.83%) using Sherman live traps over the course of three months (April, October, and November) during 2016–2017. Three rodent species were captured and identified and the striped field mouse (Apodemus agrarius) was the dominant rodent host species in the surveyed area. A total of 10,469 ectoparasitic chigger mites were recovered from the 94 rodents, from which 13 species distributed among four genera were identified. Of the 5,250 chigger mites examined, Leptotrombidium pallidum was most abundant (n = 2,558), followed by L. orientale, L. scutellare, L. zetum, Euschoengastia koreaensis, L. subintermedium, and Neotrombicula tamiyai. Of the examined chigger mites, no groups recovered from rodent hosts tested positive for O. tsutsugamushi. This study provides fundamental regional information on vector‐borne disease data collection in the Hwaseong‐si area, Gyeonggi‐do, and will further contribute to formulating disease control and prevention strategies.     

Towards a resource‐based habitat approach for spatial modelling of vector‐borne disease risks

Given the veterinary and public health impact of vector‐borne diseases, there is a clear need to assess the suitability of landscapes for the emergence and spread of these diseases. Current approaches for predicting disease risks neglect key features of the landscape as components of the functional habitat of vectors or hosts, and hence of the pathogen. Empirical–statistical methods do not explicitly incorporate biological mechanisms, whereas current mechanistic models are rarely spatially explicit; both methods ignore the way animals use the landscape (i.e. movement ecology). We argue that applying a functional concept for habitat, i.e. the resource‐based habitat concept (RBHC), can solve these issues. The RBHC offers a framework to identify systematically the different ecological resources that are necessary for the completion of the transmission cycle and to relate these resources to (combinations of) landscape features and other environmental factors. The potential of the RBHC as a framework for identifying suitable habitats for vector‐borne pathogens is explored and illustrated with the case of bluetongue virus, a midge‐transmitted virus affecting ruminants. The concept facilitates the study of functional habitats of the interacting species (vectors as well as hosts) and provides new insight into spatial and temporal variation in transmission opportunities and exposure that ultimately determine disease risks. It may help to identify knowledge gaps and control options arising from changes in the spatial configuration of key resources across the landscape. The RBHC framework may act as a bridge between existing mechanistic and statistical modelling approaches.     

Advances in the treatment of acute graft‐versus‐host disease

Allogeneic hematopoietic stem cell transplantation (HSCT) has been widely used for the treatment of hematologic malignant and non‐malignant hematologic diseases and other diseases. However, acute graft‐versus‐host disease (GVHD) is a life‐threatening complication of allogeneic transplantation. Acute GVHD may occur in 30% of transplant recipients, which is a syndrome of erythematous skin eruption, cholestatic liver disease and intestinal dysfunction, resulting from the activation of donor T lymphocytes by host antigen‐presenting cells, resulting in an immune‐mediated inflammatory response. Recent scientific advances in the understanding of the pathogenesis involved in the development of acute GVHD and clinical investigation have provided more effective therapeutic strategies for acute GVHD. This review focuses on major scientific and clinical advances in the treatment of acute GVHD.     

The role of drought as a determinant of hemorrhagic disease in the eastern United States

Bluetongue virus and epizootic hemorrhagic disease (HD) virus are globally distributed, vector‐borne viruses that infect and cause disease in domestic and wild ruminant species. The forces driving increases in resulting HD may be linked to weather conditions and increasing severity has been noted in northerly latitudes. We evaluated the role of drought severity in both space and time on changes in HD reports across the eastern United States for a recent 15 year period. The objectives of this study were to: (a) develop a spatiotemporal model to evaluate if drought severity explains changing patterns of HD presence; and (b) determine whether this potential risk factor varies in importance over the present range of HD in the eastern United States. Historic data (2000–2014) from an annual HD presence–absence survey conducted by the Southeastern Cooperative Wildlife Disease Study and from the United States Drought Monitor were used for this analysis. For every county in 23 states and for each of 15 years, data were based on reported drought status for August, wetland cover, the physiographic region, and the status of HD in the previous year. We used a generalized linear mixed model to explain HD presence and evaluated spatiotemporal predictors across the region. We found that drought severity was a significant predictor of HD presence and the significance of this relationship was dependent on latitude. In more northerly latitudes, where immunological naivety is most likely, we demonstrated the increasing strength of drought severity as a determinant of reported HD and established the importance of variation in drought severity as a risk factor over the present range of HD in the eastern United States. Our research provides spatially explicit evidence for the link between climate forces and emerging disease patterns across latitude for a globally distributed disease.     

Open questions and recent advances in the control of a multi‐host infectious disease: animal tuberculosis

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The vector ecology of introduced Culex quinquefasciatus populations,and implications for future risk of West Nile virus emergence in the Galápagos archipelago

Culex quinquefasciatus Say (Diptera: Culicidae), an important vector of West Nile virus (WNV) in the U.S.A., was first detected on the Galápagos Islands (Ecuador) in the 1980s. However, little is known of its ecology, distribution or capacity for arbovirus transmission in the Galápagos. We characterize details of lifecycle (including gonotrophic period), temporal abundance, spatial distribution, vector competence and host‐feeding behaviour. Culex quinquefasciatus was detected on five islands of the Galápagos during 2006–2011. A period of 7–14 days was required for egg–adult emergence; water salinity above 5 ppt was demonstrated to hinder larval development. Blood‐meal analysis indicated feeding on reptiles, birds and mammals. Assessment of WNV vector competency of Galápagos C. quinquefasciatus showed a median infectious dose of 7.41 log₁₀ plaque‐forming units per millilitre and evidence of vertical transmission (minimal filial infection rate of 3.7 per 1000 progeny). The distribution of C. quinquefasciatus across the archipelago could be limited by salt intolerance, and its abundance constrained by high temperatures. Feeding behaviour indicates potential to act as a bridge vector for transmission of pathogens across multiple taxa. Vertical transmission is a potential persistence mechanism for WNV on Galápagos. Together, our results can be used for epidemiological assessments of WNV and target vector control, should this pathogen reach the Galápagos Islands.     

Monocyte subsets in bone marrow grafts may contribute to a low incidence of acute graft‐vs‐host disease for young donors

Young donors are associated with a lower cumulative incidence of acute graft‐vs‐host disease (aGVHD) after allogenic haematopoietic stem cell transplantation (allo‐HSCT) than old donors. Although grafts are harvested from healthy donors, it is unclear whether donor age is associated with aGVHD occurrence owing to its effect on cell compositions in grafts. Moreover, the differences in monocyte subsets in grafts between young and old donors and the association between monocyte subsets in bone marrow (BM) grafts and aGVHD remain to be elucidated. In the current study, non‐classical monocytes and the CD4⁺/CD8⁺ T cell ratio were remarkably decreased in BM grafts in donors <30 years old. Multivariate analysis further revealed that the level of non‐classical monocytes in BM grafts (≥0.31 × 10⁶/kg) was an independent risk factor for the occurrence of II‐IV aGVHD. In summary, our data indicate that non‐classical monocytes in BM grafts may help identify patients at high risk for aGVHD after allo‐HSCT. Although further validation is required, our results suggest that the low level of non‐classical monocytes and a low ratio of CD4⁺/CD8⁺ T cell in BM grafts may be correlated with the lower incidence of aGVHD in young donors.     

The influence of host and non‐host companion plants on the behaviour of pest insects in field crops

Companion plants grown as ‘trap crops’ or ‘intercrops’ can be used to reduce insect infestations in field crops. The ways in which such reductions are achieved are being described currently using either a chemical approach, based on the ‘push‐pull strategy’, or a biological approach, based on the ‘appropriate/inappropriate landing theory’. The chemical approach suggests that insect numbers are reduced by chemicals from the intercrop ‘repelling’ insects from the main crop, and by chemicals from the trap‐crop ‘attracting’ insects away from the main crop. This approach is based on the assumptions that (1) plants release detectable amounts of volatile chemicals, and (2) insects ‘respond’ while still some distance away from the emitting plant. We discuss whether the above assumptions can be justified using the ‘appropriate/inappropriate landing theory’. Our tenet is that specialist insects respond only to the volatile chemicals released by their host plants and that these are released in such small quantities that, even with a heightened response to such chemicals, specialist insects can only detect them when a few metres from the emitting plant. We can find no robust evidence in the literature that plant chemicals ‘attract’ insects from more than 5 m and believe that ‘trap crops’ function simply as ‘interception barriers’. We can also find no evidence that insects are ‘repelled’ from landing on non‐host plants. Instead, we believe that ‘intercrops’ disrupt host‐plant finding by providing insects with a choice of host (appropriate) and non‐host (inappropriate) plant leaves on which to land, as our research has shown that, for intercropping to be effective, insects must land on the non‐host plants. Work is needed to determine whether non‐host plants are repellent (chemical approach) or ‘non‐stimulating’ (biological approach) to insects.     

The absence of concordant population genetic structure in the black‐tailed prairie dog and the flea,Oropsylla hirsuta,with implications for the spread of Yersinia pestis

The black‐tailed prairie dog (Cynomys ludovicianus) is a keystone species on the mid‐ and short‐grass prairies of North America. The species has suffered extensive colony extirpations and isolation as a result of human activity including the introduction of an exotic pathogen, Yersinia pestis, the causative agent of sylvatic plague. The prairie dog flea, Oropsylla hirsuta, is the most common flea on our study colonies in north‐central Montana and it has been shown to carry Y. pestis. We used microsatellite markers to estimate the level of population genetic concordance between black‐tailed prairie dogs and O. hirsuta in order to determine the extent to which prairie dogs are responsible for dispersing this potential plague vector among prairie dog colonies. We sampled fleas and prairie dogs from six prairie dog colonies in two regions separated by about 46 km. These colonies were extirpated by a plague epizootic that began months after our sampling was completed in 2005. Prairie dogs showed significant isolation‐by‐distance and a tendency toward genetic structure on the regional scale that the fleas did not. Fleas exhibited higher estimated rates of gene flow among prairie dog colonies than the prairie dogs sampled from the same colonies. While the findings suggested black‐tailed prairie dogs may have contributed to flea dispersal, we attributed the lack of concordance between the population genetic structures of host and ectoparasite to additional flea dispersal that was mediated by mammals other than prairie dogs that were present in the prairie system.     

Flaviviral disease mosquito vector surveillance in Incheon Metropolitan City and the Hwaseong area,Gyeonggi‐Do,Republic of Korea,in 2015

Owing to global climate change, the global resurgence of vector‐borne infectious diseases and their potential to inflict widespread casualties among human populations has emerged as a pivotal burden on public health systems. In this study, the prevalence of flaviviral diseases transmitted by mosquitoes, and their target vector diversity, abundance, and distribution was investigated to enable the mapping of hotspots for these diseases. For the surveillance of the vector mosquitoes carrying flaviviruses during April to November 2015, female mosquitoes were collected to study whether they carried pathogens from abroad at seven locations in Incheon Metropolitan City (Incheon) as a typical urban area and Hwaseong‐si (= city, Hwaseong) of Gyeonggi‐do (= province) as a rural area. A total of 15 species belonging to seven genera (29,102 female mosquitoes) were collected with black‐light and BG‐Sentinel? traps at a collection rate of 260 per trap/night from whole collection locations. The most collected mosquito species in Incheon were Aedes vexans nipponii (species ratio (SR), 29.9%) and the Culex pipiens complex (SR, 28.8%), followed by Anopheles sinensis s.l. (SR, 27.9%) and Ochlerotatus koreicus (SR, 7.1%). From the results of viral RNA detection, five flaviviruses were found in 20,981 individuals (excluding An. sinensis; 696 pools) in the Cx. pipiens complex and Ae. vexans nipponii. Three Japanese encephalitis virus (JEV)‐positive pools were from the Cx. pipiens complex, a Chaoyang virus pool was found from Ae. vexans nipponii, and the remaining unidentified flavivirus pool was from Cx. pipiens. The three JEV‐positive pools were phylogenetically grouped as genotype V. The results of our study demonstrate that enhanced monitoring and long‐term surveillance of these vector viruses are of great public health importance.     

Transformer‐4 version 2.0.1, a free multi‐platform software to quickly reformat genotype matrices of any marker type,and archive them in the Demiurge information system

Transformer‐4 version 2.0.1 (T4) is a multi‐platform freeware programmed in java that can transform a genotype matrix in Excel or XML format into the input formats of one or several of the most commonly used population genetic software, for any possible combination of the populations that the matrix contains. T4 also allows the users to (i) draw allozyme gel interpretations for any number of diploid individuals, and then generate a genotype matrix ready to be used by T4; and (ii) produce basic reports about the data in the matrices. Furthermore, T4 is the only way to optionally submit ‘genetic diversity digests’ for publication in the Demiurge online information system ( http://www.demiurge-project.org ). Each such digest undergoes peer‐review, and it consists of a geo‐referenced data matrix in the tfm4 format plus any ancillary document or hyperlink that the digest authors see fit to include. The complementarity between T4 and Demiurge facilitates a free, safe, permanent, and standardized data archival and analysis system for researchers, and may also be a convenient resource for scientific journals, public administrations, or higher educators. T4 and its converters are freely available (at, respectively, http://www.demiurge-project.org/download_t4 and http://www.demiurge-project.org/converterstore ) upon registration in the Demiurge information system ( http://demiurge-project.org/register ). Users have to click on the link provided on an account validation email, and accept Demiurge's terms of use (see http://www.demiurge-project.org/termsofuse ). A thorough user's guide is available within T4. A 3‐min promotional video about T4 and Demiurge can be seen at http://vimeo.com/29828406 .     

Field and climate‐based model for predicting the density of host‐seeking nymphal Ixodes scapularis,an important vector of tick‐borne disease agents in the eastern United States

Aim Ixodes scapularis is the most important vector of human tick‐borne pathogens in the United States, which include the agents of Lyme disease, human babesiosis and human anaplasmosis, among others. The density of host‐seeking I. scapularis nymphs is an important component of human risk for acquiring Borrelia burgdorferi, the aetiological agent of Lyme disease. In this study we used climate and field sampling data to generate a predictive map of the density of host‐seeking I. scapularis nymphs that can be used by the public, physicians and public health agencies to assist with the diagnosis and reporting of disease, and to better target disease prevention and control efforts. Location Eastern United States of America. Methods We sampled host‐seeking I. scapularis nymphs in 304 locations uniformly distributed east of the 100th meridian between 2004 and 2006. Between May and September, 1000 m² were drag sampled three to six times per site. We developed a zero‐inflated negative binomial model to predict the density of host‐seeking I. scapularis nymphs based on altitude, interpolated weather station and remotely sensed data. Results Variables that had the strongest relationship with nymphal density were altitude, monthly mean vapour pressure deficit and spatial autocorrelation. Forest fragmentation and soil texture were not predictive. The best‐fit model identified two main foci – the north‐east and upper Midwest – and predicted the presence and absence of I. scapularis nymphs with 82% accuracy, with 89% sensitivity and 82% specificity. Areas of concordance and discordance with previous studies were discussed. Areas with high predicted but low observed densities of host‐seeking nymphs were identified as potential expansion fronts. Main conclusions This model is unique in its extensive and unbiased field sampling effort, allowing for an accurate delineation of the density of host‐seeking I. scapularis nymphs, an important component of human risk of infection for B. burgdorferi and other I. scapularis‐borne pathogens.     

Origin of a sudden mass occurrence of the stolbur phytoplasma vector Hyalesthes obsoletus (Cixiidae) in Austria

The grapevine disease ‘bois noir’ is widespread in European viticulture, but in many regions there is a lack of correspondence between disease spread and abundance of the main insect vector, the planthopper Hyalesthes obsoletus. This was the situation in Austria until 2012, when a mass occurrence of the vector was observed on Urtica dioica, a new host plant for the vector and reservoir plant for the pathogen, stolbur phytoplasma, in this area. Here we analyse the origin of the Austrian vector populations using genetic markers. The origin was unambiguously assigned to two regional populations, and two causes for the population expansion: immigration of East Central European populations and local demographic expansion. The observed population increase was thus independent of phylogenetic ancestry, but linked to the host plant and the exchange of a specific stolbur phytoplasma strain between the two vector populations. These circumstances are identical to but independent of the emergence of bois noir west of the European Alps, where an exchange between other vector populations associated with U. dioica of another stolbur phytoplasma genotype has led to disease outbreaks. Combined, the independent outbreaks in Austria and Europe west of the Alps are suggestive of an active role for stolbur phytoplasma in the vector–plant interaction and thus the host distribution of the vector.     

Plague studies in California: a review of long‐term disease activity,flea‐host relationships and plague ecology in the coniferous forests of the Southern Cascades and northern Sierra Nevada mountains

We review 28 years of long‐term surveillance (1970–1997) for plague activity among wild rodents from ten locations within three coniferous forest habitat types in the northern Sierra Nevada and the Southern Cascade mountains of northeastern California. We identify rodent hosts and their fleas and document long‐term plague activity in each habitat type. The highest seroprevalence for Yersinia pestis occurred in the chipmunks, Tamias senex and T. quadrimaculatus, and the pine squirrel, Tamiasciurus douglasii. The most commonly infected fleas were Ceratophyllus ciliatus and Eumolpianus eumolpi from chipmunks and Oropsylla montana and O. idahoensis from ground squirrels. Serological surveillance demonstrated that populations of T. senex, T. quadrimaculatus and T. douglasii are moderately resistant to plague, survive infection, and are, therefore, good sentinels for plague activity. Recaptured T. senex and T. quadrimaculatus showed persistence of plague antibodies and evidence of re‐infection over a two year period. These rodent species, their fleas, and the ecological factors common to the coniferous forest habitats likely promote the maintenance of plague foci in northeastern California.     

Non‐random segregation of an autosomal gene in males of the flea beetle,Phyllotreta nemorum: implications for colonization of a novel host plant

The flea beetle, Phyllotreta nemorum (L.) (Coleoptera: Chrysomelidae: Alticinae), is currently expanding its host plant range in Europe. The ability to utilize a novel host plant, Barbarea vulgaris R. Br. (Brassicaceae), is controlled by major dominant genes named R‐genes. The present study used extensive crossing experiments to illustrate a peculiar mode of inheritance of the R‐gene in a population from Delemont (Switzerland). When resistant males from Delemont are mated with recessive females from a laboratory line, the female F₁ offspring contains the R‐allele and is able to utilize B. vulgaris, whereas the male offspring contains the r‐allele and is unable to utilize the plant. This outcome suggests X‐linkage of the R‐gene, but further crossing experiments demonstrated that this was not the case. When the R‐gene is present in offspring from males from a laboratory line that originates from Taastrup (Denmark), it is transmitted to female and male offspring in equal proportions as a normal autosomal gene. The results demonstrate a polymorphism in segregation patterns of an autosomal R‐gene in P. nemorum males. Males from Delemont contain a factor which causes non‐random segregation of the R‐gene (NRS‐factor). This factor is inherited patrilineally (from fathers to sons). Males with the NRS‐factor transmit the R‐gene to their female offspring, whereas males without the NRS‐factor transmit the R‐gene to female and male offspring in equal proportions. Various models for the non‐random segregation of autosomes in P. nemorum males are discussed – e.g., fusions between autosomes and sex chromosomes, and genomic imprinting. The implications of various modes of inheritance of R‐genes for the ability of P. nemorum populations to colonize novel patches of B. vulgaris are discussed.