Local Host Populations Research Articles

The Role of the Host Local Population in the Process of Migrants’ Adaptation

Social and psychological adaptation in a new country can be more or less successful depending on a number of factors. Research into behavioral patterns of migrants residing in Russia has revealed differences in their acculturation strategies and other expressions of migrant ethnic identity. These is a noticeable dependence on types of migrant settlement which can be compact or widespread. Local host population also demonstrates an unequal attitude toward compactly and diffusely settled migrants. This paper describes an attempt to find and analyze correlations between the length of residing in a new ethno-cultural environment, involvement in regional ethnic communities, contact with the host population, and ethnic identity. Differences in adaptation strategies develop three main ethnic identity models. Each model reflects general indicators of migrant adjustability to a new environment and new life.

New Mycobacterium tuberculosis Beijing clonal complexes in China revealed by phylogenetic and Bayesian population structure analyses of 24-loci MIRU-VNTRs

Beijing lineage of Mycobacterium tuberculosis constitutes the most predominant lineage in East Asia. Beijing epidemiology, evolutionary history, genetics are studied in details for years revealing probable origin from China followed by worldwide expansion, partially linked to higher mutation rate, hypervirulence, drug-resistance, and association with cases of mixed infections. Considering huge amount of data available for 24-loci Mycobacterial Interspersed Repetitive Units-Variable Number of Tandem Repeats, we performed detailed phylogenetic and Bayesian population structure analyses of Beijing lineage strains in mainland China and Taiwan using available 24-loci MIRU-VNTR data extracted from publications or the SITVIT2 database (n = 1490). Results on genetic structuration were compared to previously published data. A total of three new Beijing clonal complexes tentatively named BSP1, BPS2 and BSP3 were revealed with surprising phylogeographical specificities to previously unstudied regions in Sichuan, Chongqing and Taiwan, proving the need for continued investigations with extended datasets. Such geographical restriction could correspond to local adaptation of these “ecological specialist” Beijing isolates to local human host populations in contrast with “generalist pathogens” able to adapt to several human populations and to spread worldwide.

The diversity and impact of hookworm infections in wildlife.

Hookworms are blood-feeding nematodes that parasitize the alimentary system of mammals. Despite their high pathogenic potential, little is known about their diversity and impact in wildlife populations. We conducted a systematic review of the literature on hookworm infections of wildlife and analyzed 218 studies qualitative and quantitatively. At least 68 hookworm species have been described in 9 orders, 24 families, and 111 species of wild mammals. Black bears, red foxes, and bobcats harbored the highest diversity of hookworm species and Ancylostoma pluridentatum, A. tubaeforme, Uncinaria stenocephala and Necator americanus were the hookworm species with the highest host diversity index. Hookworm infections cause anemia, retarded growth, tissue damage, inflammation and significant mortality in several wildlife species. Anemia has been documented more commonly in canids, felids and otariids, and retarded growth only in otariids. Population- level mortality has been documented through controlled studies only in canines and eared seals although sporadic mortality has been noticed in felines, bears and elephants. The main driver of hookworm pathogenic effects was the hookworm biomass in a population, measured as prevalence, mean burden and hookworm size (length). Many studies recorded significant differences in prevalence and mean intensity among regions related to contrasts in local humidity, temperature, and host population density. These findings, plus the ability of hookworms to perpetuate in different host species, create a dynamic scenario where changes in climate and the domestic animal-human-wildlife interface will potentially affect the dynamics and consequences of hookworm infections in wildlife.

Analysis of climatic factors involved in the BTV-1 incursion in Central Italy in 2014.

In 2012, six years after the previous epidemic, Bluetongue virus serotype 1 (BTV-1) re-emerged in Sardinia causing a limited number of outbreaks. Due to impossibility of implementing a vaccination campaign, the BTV-1 then spread all over the island in 2013 with about 7,000 outbreaks and, in September 2013, the virus reached Central Italy, with a limited number of outbreaks located along the Tyrrhenian coast. The surveillance system in place in Italy detected viral circulation during the following winter, when a few seroconversions were notified. Starting from mid July 2014, a huge number of outbreaks were reported and the disease spread toward inland territories, affecting Umbria, Abruzzo and Marche. In 2014, BTV-1 affected areas where Culicoides species belonging to the Obsoletus and Pulicaris complexes were identified as main vectors. The analysis of temperature and rainfall in Central Italy revealed a significant warmer winter (2013-2014) and a cooler and rainy summer season (2014). These climatic aspects might have certainly favored the overwintering of the virus in local vector or host populations in the Tyrrhenian coast, and, then, the spread of the virus to the rest of Central Italy. However, the heavy circulation of BTV-1 and the severity of clinical outbreaks recorded leave a number of 'open questions' that are currently under investigations.

Migration, Prospecting, Dispersal? What Host Movement Matters for Infectious Agent Circulation?

Spatial disease ecology is emerging as a new field that requires the integration of complementary approaches to address how the distribution and movements of hosts and parasites may condition the dynamics of their interactions. In this context, migration, the seasonal movement of animals to different zones of their distribution, is assumed to play a key role in the broad scale circulation of parasites and pathogens. Nevertheless, migration is not the only type of host movement that can influence the spatial ecology, evolution, and epidemiology of infectious diseases. Dispersal, the movement of individuals between the location where they were born or bred to a location where they breed, has attracted attention as another important type of movement for the spatial dynamics of infectious diseases. Host dispersal has notably been identified as a key factor for the evolution of host-parasite interactions as it implies gene flow among local host populations and thus can alter patterns of coevolution with infectious agents across spatial scales. However, not all movements between host populations lead to dispersal per se. One type of host movement that has been neglected, but that may also play a role in parasite spread is prospecting, i.e., movements targeted at selecting and securing new habitat for future breeding. Prospecting movements, which have been studied in detail in certain social species, could result in the dispersal of infectious agents among different host populations without necessarily involving host dispersal. In this article, we outline how these various types of host movements might influence the circulation of infectious disease agents and discuss methodological approaches that could be used to assess their importance. We specifically focus on examples from work on colonial seabirds, ticks, and tick-borne infectious agents. These are convenient biological models because they are strongly spatially structured and involve relatively simple communities of interacting species. Overall, this review emphasizes that explicit consideration of the behavioral and population ecology of hosts and parasites is required to disentangle the relative roles of different types of movement for the spread of infectious diseases.

Genetic, ecological and geographic covariables explaining host range and specificity of a microsporidian parasite.

Parasites often have a smaller geographic distribution than their hosts. Common garden infection trials can untangle the role that historical contingencies, ecological conditions and the genetic constitution of local host populations play in limiting parasite geographic range; however, infection trials usually overestimate the range of hosts in which a parasite could naturally persist. This study overcomes that problem by using multigeneration, long-term persistence experiments. We study the microsporidian parasite Hamiltosporidium tvaerminnensis in monoclonal populations of Daphnia magna from 43 widely spread sites. The parasite persisted well in hosts collected from its natural geographic range, but demonstrated long-term persistence in only a few host genotypes outside this range. Genetic distance between hosts from the parasite's origin site and newly tested host populations correlated negatively with parasite persistence. Furthermore, the parasite persisted only in host populations from habitats with a high likelihood of drying up in summer, although we excluded environmental variation in our experiments. Together, our results suggest that host genetic factors play the dominant role in explaining the limited geographic range of parasites and that these genetic differences covary with geographic distance and the habitat type the host is adapted to.

The scaling of parasite biomass with host biomass in lake ecosystems: are parasites limited by host resources?

The standing crop biomass of different populations or trophic levels reflects patterns of energy flow through an ecosystem. The contribution of parasites to total biomass is often considered negligible; recent evidence suggests otherwise, although it comes from a narrow range of natural systems. Quantifying how local parasite biomass, whether that of a single species or an assemblage of species sharing the same host, varies across localities with host population biomass, is critical to determine what constrains parasite populations. We use an extensive dataset on all free‐living and parasitic metazoan species from multiple sites in New Zealand lakes to measure parasite biomass and test how it covaries with host biomass. In all lakes, trematodes had the highest combined biomass among parasite taxa, ranging from about 0.01 to 0.25 g m−2, surpassing the biomass of minor free‐living taxa. Unlike findings from other studies, the life stage contributing the most to total trematode biomass was the metacercarial stage in the second intermediate host, and not sporocysts or rediae within snail first intermediate hosts, possibly due to low prevalence and small snail sizes. For populations of single parasite species, we found no relationship between host and parasite biomass for either juvenile or adult nematodes. In contrast, all life stages of trematodes had local biomasses that correlated positively with those of their hosts. For assemblages of parasite species sharing the same host, we found strong relationships between local host population biomass and the total biomass of parasites supported. In these host–parasite biomass relationships, the scaling factor (slope in log‐log space) suggests that parasites may not be making full use of available host resources. Host populations appear capable of supporting a little more parasite biomass, and may be open to expansion of existing parasites or invasion by new ones.

Small-scale genetic structure of Gracilaria salicornia and its red algal parasite, G. babae (Gracilariaceae, Rhodophyta), in Malaysia

Abstract The present study compares genetic structure from the plastid rbcL, mitochondrial cox1, and nuclear internal transcribed spacer (ITS) sequences among 13 populations of Gracilaria salicornia and its red algal parasite, G. babae, in Malaysia. Owing to the unique biology of the red algal parasite, identical phylogeographies were inferred from the rbcL and cox1 sequences in both G. babae and its host, G. salicornia. Despite the low genetic variation, rbcL and cox1 showed a concordant phylogeographic split that largely corresponds to Peninsular Malaysia and East Malaysia, except for a sample from East Malaysia that was consistently grouped with samples from the peninsula. On the other hand, G. salicornia was separated from G. babae in the midpoint-rooted phylogeny inferred from ITS data despite the failure to retrieve the two taxa in reciprocal monophyly. Gracilaria salicornia was less diverse and lacked apparent geographic structure. In contrast, the ITS data revealed considerable genetic diversity and fine-scale genetic structuring in G. babae within Malaysian waters, probably as a result of adaptation to the local host population.

Genetic, morphological and pathogenic diversity in the canker pathogen Cryptodiaporthe melanocraspeda on Banksia

Diversity within and between Cryptodiaporthe melanocraspeda populations on Banksia spp. and variation in susceptibility within B. coccinea that resulted from pathogen-induced selection, was investigated. Genetic distances between C. melanocraspeda isolate populations were much less than those between Cryptodiaporthe spp. Spore size and rates of lesion extension of isolate populations of C. melanocraspeda formed continua that reflected intraspecific variation within a species. There were two isozyme groups amongst the populations of C. melanocraspeda from different Banksia hosts. Populations of C. melanocraspeda from the five geographic clusters could be divided into three geographic isozyme groups. Pycnidiospore size of populations of C. melanocraspeda isolates from different Banksia hosts formed a continuum from relatively short-thin spores for isolates from B. sessilis and relatively long-wide spores for isolates from B. baxteri. The rate of lesion extension formed a continuum, with ranking of isolates from slowest to fastest rates differing on the four hosts inoculated. Isolate populations varied in their pattern of adaptation to local Banksia host populations. Although intraspecific variation in the response of B. coccinea to C. melanocraspeda was mainly skewed to highly susceptible trees, there would be the potential for the conservation of diversity through single-tree selection for resistance and breeding.

Life-long shedding of Puumala hantavirus in wild bank voles (Myodes glareolus).

The knowledge of viral shedding patterns and viraemia in the reservoir host species is a key factor in assessing the human risk of zoonotic viruses. The shedding of hantaviruses (family Bunyaviridae) by their host rodents has widely been studied experimentally, but rarely in natural settings. Here we present the dynamics of Puumala hantavirus (PUUV) shedding and viraemia in naturally infected wild bank voles (Myodes glareolus). In a monthly capture-mark-recapture study, we analysed 18 bank voles for the presence and relative quantity of PUUV RNA in the excreta and blood from 2 months before up to 8 months after seroconversion. The proportion of animals shedding PUUV RNA in saliva, urine and faeces peaked during the first month after seroconversion, but continued throughout the study period with only a slight decline. The quantity of shed PUUV in reverse transcription quantitative PCR (RT-qPCR) positive excreta was constant over time. In blood, PUUV RNA was present for up to 7 months but both the probability of viraemia and the virus load declined with time. Our findings contradict the current view of a decline in virus shedding after the acute phase and a short viraemic period in hantavirus infection - an assumption widely adopted in current epidemiological models. We suggest the life-long shedding as a means of hantaviruses to survive over host population bottlenecks, and to disperse in fragmented habitats where local host and/or virus populations face temporary extinctions. Our results indicate that the kinetics of pathogens in wild hosts may differ considerably from those observed in laboratory settings.

The mermithid species Isomermis lairdi (Nematoda, Mermithidae), previously only known in Africa, found in Europe

The present work contributs to the knowledge on the aquatic mermithids (Nematoda, Mermithidae) occurring in black flies – an insufficiently studied group of parasitic nematodes. Isomermis lairdi Mondet, Poinar & Bernadou, 1977, described from larvae of Simulium damnosum Theobald, 1903 in Western Africa, is reported to occur in Bulgaria. The species was isolated from larvae of Simulium ornatum Meigen, 1818 in a local population of simuliids in a mountain stream near Jeleznitsa Village, Sofia district. Postparasitic juveniles of mermithids were obtained from the hosts and reared to the adult stage. The species was identified by morphological and morphometrical characters of postparasitic juveniles, and of male and female individuals. In the summer of 2012 a relatively high rate of mermithid infection in a local host population was detected (prevalence up to 44.1%). In August of the next year host abundance had considerably declined and other simuliid species, Simulium variegatum Meigen, 1818 and Simulium reptans (Linnaeus, 1758), predominated in the investigated locality. In West Africa, Isomermis lairdi is considered to be a potential biological agent for reducing the population density of the Simulium damnosum complex – the main vector of human onchocerciasis. In Europe, species of the Simulium ornatum complex are among the vectors of onchocerciasis of cattle and deer. The mermithids presumably play a certain role in the epidemiology of these diseases. A brief discussion on the taxonomy of the genus Isomermis Coman, 1953, and of the feasibility of molecular methods in mermithid taxonomy is provided. The species Isomermis lairdi is reported for the first time from Europe.

Experimental coevolution: rapid local adaptation by parasites depends on host mating system.

Host-parasite interactions can drive rapid, reciprocal genetic changes (coevolution), provided both hosts and parasites have high heritabilities for resistance/infectivity. Similarly, the host's mating system should also affect the rate of coevolutionary change in host-parasite interactions. Using experimental coevolution, we determined the effect of obligate outcrossing verses partial self-fertilization (mixed mating) on the rate of evolutionary change in a nematode host (Caenorhabditis elegans) and its bacterial parasite (Serratia marcescens). Bacterial populations were derived from a common ancestor. We measured the effects of host mating system on host adaptation to the parasite. We then determined the extent of parasite adaptation to their local host populations. Obligately outcrossing hosts exhibited more rapid adaptation to parasites than did mixed mating hosts. In addition, most of the parasites became adapted to infecting their local hosts, but parasites from obligately outcrossing hosts showed a greater level of local adaptation. These results suggest that host populations evolved along separate trajectories and that outcrossing host populations diverged further than partially selfing populations. Finally, parasites tracking outcrossing host populations diverged further than parasites tracking the partially selfing host populations. These results show that the evolutionary trajectories of both hosts and parasites can be shaped by the host's mating system.

Data-driven modeling to assess receptivity for Rift Valley Fever virus.

Rift Valley Fever virus (RVFV) is an enzootic virus that causes extensive morbidity and mortality in domestic ruminants in Africa, and it has shown the potential to invade other areas such as the Arabian Peninsula. Here, we develop methods for linking mathematical models to real-world data that could be used for continent-scale risk assessment given adequate data on local host and vector populations. We have applied the methods to a well-studied agricultural region of California with 1 million dairy cattle, abundant and competent mosquito vectors, and a permissive climate that has enabled consistent transmission of West Nile virus and historically other arboviruses. Our results suggest that RVFV outbreaks could occur from February–November, but would progress slowly during winter–early spring or early fall and be limited spatially to areas with early increases in vector abundance. Risk was greatest in summer, when the areas at risk broadened to include most of the dairy farms in the study region, indicating the potential for considerable economic losses if an introduction were to occur. To assess the threat that RVFV poses to North America, including what-if scenarios for introduction and control strategies, models such as this one should be an integral part of the process; however, modeling must be paralleled by efforts to address the numerous remaining gaps in data and knowledge for this system.

West Nile Virus Prevalence across Landscapes Is Mediated by Local Effects of Agriculture on Vector and Host Communities

Arthropod-borne viruses (arboviruses) threaten the health of humans, livestock, and wildlife. West Nile virus (WNV), the world’s most widespread arbovirus, invaded the United States in 1999 and rapidly spread across the county. Although the ecology of vectors and hosts are key determinants of WNV prevalence across landscapes, the factors shaping local vector and host populations remain unclear. Here, we used spatially-explicit models to evaluate how three land-use types (orchards, vegetable/forage crops, natural) and two climatic variables (temperature, precipitation) influence the prevalence of WNV infections and vector/host distributions at landscape and local spatial scales. Across landscapes, we show that orchard habitats were associated with greater prevalence of WNV infections in reservoirs (birds) and incidental hosts (horses), while increased precipitation was associated with fewer infections. At local scales, orchard habitats increased the prevalence of WNV infections in vectors (mosquitoes) and the abundance of mosquitoes and two key reservoir species, the American robin and the house sparrow. Thus, orchard habitats benefitted WNV vectors and reservoir hosts locally, creating focal points for the transmission of WNV at landscape scales in the presence of suitable climatic conditions.

Changing distributions of ticks: causes and consequences

Today, we are witnessing changes in the spatial distribution and abundance of many species, including ticks and their associated pathogens. Evidence that these changes are primarily due to climate change, habitat modifications, and the globalisation of human activities are accumulating. Changes in the distribution of ticks and their invasion into new regions can have numerous consequences including modifications in their ecological characteristics and those of endemic species, impacts on the dynamics of local host populations and the emergence of human and livestock disease. Here, we review the principal causes for distributional shifts in tick populations and their consequences in terms of the ecological attributes of the species in question (i.e. phenotypic and genetic responses), pathogen transmission and disease epidemiology. We also describe different methodological approaches currently used to assess and predict such changes and their consequences. We finish with a discussion of new research avenues to develop in order to improve our understanding of these host-vector-pathogen interactions in the context of a changing world.

Consequences of Landscape Fragmentation on Lyme Disease Risk: A Cellular Automata Approach

The abundance of infected Ixodid ticks is an important component of human risk of Lyme disease, and various empirical studies have shown that this is associated, at least in part, to landscape fragmentation. In this study, we aimed at exploring how varying woodland fragmentation patterns affect the risk of Lyme disease, through infected tick abundance. A cellular automata model was developed, incorporating a heterogeneous landscape with three interactive components: an age-structured tick population, a classical disease transmission function, and hosts. A set of simplifying assumptions were adopted with respect to the study objective and field data limitations. In the model, the landscape influences both tick survival and host movement. The validation of the model was performed with an empirical study. Scenarios of various landscape configurations (focusing on woodland fragmentation) were simulated and compared. Lyme disease risk indices (density and infection prevalence of nymphs) differed considerably between scenarios: (i) the risk could be higher in highly fragmented woodlands, which is supported by a number of recently published empirical studies, and (ii) grassland could reduce the risk in adjacent woodland, which suggests landscape fragmentation studies of zoonotic diseases should not focus on the patch-level woodland patterns only, but also on landscape-level adjacent land cover patterns. Further analysis of the simulation results indicated strong correlations between Lyme disease risk indices and the density, shape and aggregation level of woodland patches. These findings highlight the strong effect of the spatial patterns of local host population and movement on the spatial dynamics of Lyme disease risks, which can be shaped by woodland fragmentation. In conclusion, using a cellular automata approach is beneficial for modelling complex zoonotic transmission systems as it can be combined with either real world landscapes for exploring direct spatial effects or artificial representations for outlining possible empirical investigations.

Do host species evolve a specific response to slave-making ants?

BackgroundSocial parasitism is an important selective pressure for social insect species. It is particularly the case for the hosts of dulotic (so called slave-making) ants, which pillage the brood of host colonies to increase the worker force of their own colony. Such raids can have an important impact on the fitness of the host nest. An arms race which can lead to geographic variation in host defenses is thus expected between hosts and parasites. In this study we tested whether the presence of a social parasite (the dulotic ant Myrmoxenus ravouxi) within an ant community correlated with a specific behavioral defense strategy of local host or non-host populations of Temnothorax ants. Social recognition often leads to more or less pronounced agonistic interactions between non-nestmates ants. Here, we monitored agonistic behaviors to assess whether ants discriminate social parasites from other ants. It is now well-known that ants essentially rely on cuticular hydrocarbons to discriminate nestmates from aliens. If host species have evolved a specific recognition mechanism for their parasite, we hypothesize that the differences in behavioral responses would not be fully explained simply by quantitative dissimilarity in cuticular hydrocarbon profiles, but should also involve a qualitative response due to the detection of particular compounds. We scaled the behavioral results according to the quantitative chemical distance between host and parasite colonies to test this hypothesis.ResultsCuticular hydrocarbon profiles were distinct between species, but host species did not show a clearly higher aggression rate towards the parasite than toward non-parasite intruders, unless the degree of response was scaled by the chemical distance between intruders and recipient colonies. By doing so, we show that workers of the host and of a non-host species in the parasitized site displayed more agonistic behaviors (bites and ejections) towards parasite than toward non-parasite intruders.ConclusionsWe used two different analyses of our behavioral data (standardized with the chemical distance between colonies or not) to test our hypothesis. Standardized data show behavioral differences which could indicate qualitative and specific parasite recognition. We finally stress the importance of considering the whole set of potentially interacting species to understand the coevolution between social parasites and their hosts.

Phylogeny and fitness of Vibrio fischeri from the light organs of Euprymna scolopes in two Oahu, Hawaii populations

The evolutionary relationship among Vibrio fischeri isolates obtained from the light organs of Euprymna scolopes collected around Oahu, Hawaii, were examined in this study. Phylogenetic reconstructions based on a concatenation of fragments of four housekeeping loci (recA, mdh, katA, pyrC) identified one monophyletic group ('Group-A') of V. fischeri from Oahu. Group-A V. fischeri strains could also be identified by a single DNA fingerprint type. V. fischeri strains with this fingerprint type had been observed to be at a significantly higher abundance than other strains in the light organs of adult squid collected from Maunalua Bay, Oahu, in 2005. We hypothesized that these previous observations might be related to a growth/survival advantage of the Group-A strains in the Maunalua Bay environments. Competition experiments between Group-A strains and non-Group-A strains demonstrated an advantage of the former in colonizing juvenile Maunalua Bay hosts. Growth and survival assays in Maunalua Bay seawater microcosms revealed a reduced fitness of Group-A strains relative to non-Group-A strains. From these results, we hypothesize that there may exist trade-offs between growth in the light organ and in seawater environments for local V. fischeri strains from Oahu. Alternatively, Group-A V. fischeri may represent an example of rapid, evolutionarily significant, specialization of a horizontally transmitted symbiont to a local host population.

2009 West Nile disease epidemic in Italy: First evidence of overwintering in Western Europe?

For the second consecutive year a West Nile disease (WND) epidemic has affected Italy causing disease in horses and humans. The infection re-occurred in the same places of the 2008 and moved westerly and southerly involving new areas and regions. The whole genome sequence of the Italian 2009 West Nile disease isolate (WNDV) was compared with those responsible for the 2008 WND outbreaks. The epidemiological findings of the two years of epidemic were compared as well. The high identity between 2008 and 2009 WNV strains (>99%), the earlier virus circulation in 2009 and the re-occurrence of the disease starting from the bordering infected areas reached by the infection in the previous year, strongly support the hypothesis of the overwintering of the virus and the endemisation to local host populations.

The common cuckoo Cuculus canorus is not locally adapted to its reed warbler Acrocephalus scirpaceus host

The obligate avian brood parasitic common cuckoo Cuculus canorus comprises different strains of females that specialize on particular host species by laying eggs of a constant type that often mimics those of the host. Whether cuckoos are locally adapted for mimicking populations of the hosts on which they are specialized has never been investigated. In this study, we first explored the possibility of local adaptation in cuckoo egg mimicry over a geographical mosaic of selection exerted by one of its main European hosts, the reed warbler Acrocephalus scirpaceus. Secondly, we investigated whether cuckoos inhabiting reed warbler populations with a broad number of alternative suitable hosts at hand were less locally adapted. Cuckoo eggs showed different degrees of mimicry to different reed warbler populations. However, cuckoo eggs did not match the egg phenotypes of their local host population better than eggs of other host populations, indicating that cuckoos were not locally adapted for mimicry on reed warblers. Interestingly, cuckoos exploiting reed warblers in populations with a relatively larger number of co-occurring cuckoo gentes showed lower than average levels of local adaptation in egg volume. Our results suggest that cuckoo local adaptation might be prevented when different cuckoo populations exploit more or fewer different host species, with gene flow or frequent host switches breaking down local adaptation where many host races co-occur.