The discovery of three genetic races of the dwarf mistletoe Arceuthobium americanum (Viscaceae) provides insight into the evolution of parasitic angiosperms

A population genetic approach was used to explore the evolutionary biology of the parasitic angiosperm Arceuthobium americanum Nutt. ex Engelm. (Viscaceae). Arceuthobium americanum infects three principal hosts and has the most extensive geographical range of any North American dwarf mistletoe. Based on the lack of apparent morphological and phenological differences between populations of A. americanum, past researchers have found no evidence for recognizing infraspecific taxa. In this study, molecular analysis using amplified fragment length polymorphism (AFLP) analysis indicated that A. americanum is divided into three distinct genetic races, each associated with a different host taxon in regions of allopatry: (i) Pinus banksiana in western Canada; (ii) Pinus contorta var. murrayana in the Sierra Nevada and Cascade Mountain ranges in the western US; and (iii) Pinus contorta var. latifolia in the western US and Canada. These observations suggest that host identity, geographical isolation and environmental factors have contributed to race formation in A. americanum. The lack of fine-scale patterning within each of the A. americanum races is attributed to random dispersal of seeds over long distances by animal vectors. Historical factors such as glaciations and founder events have also influenced structuring and genetic diversity in A. americanum populations. Given sufficient time, it is possible that these races will become reproductively isolated and undergo speciation.     

Large-scale asymmetric introgression of cytoplasmic DNA reveals Holocene range displacement in a North American boreal pine complex

Jack pine (Pinus banksiana) and lodgepole pine (Pinus contorta var. latifolia) are two North American boreal hard pines that hybridize in their zone of contact in western Canada. The main objective of this study was to characterize their patterns of introgression resulting from past and recent gene flow, using cytoplasmic markers having maternal or paternal inheritance. Mitochondrial DNA (mtDNA) and chloroplast DNA (cpDNA) diversity was assessed in allopatric populations of each species and in stands from the current zone of contact containing morphological hybrids. Cluster analyses were used to identify genetic discontinuities among groups of populations. A canonical analysis was also conducted to detect putative associations among cytoplasmic DNA variation, tree morphology, and site ecological features. MtDNA introgression was extensive and asymmetric: it was detected in P. banksiana populations from the hybrid zone and from allopatric areas, but not in P. contorta populations. Very weak cpDNA introgression was observed, and only in P. banksiana populations. The mtDNA introgression pattern indicated that central Canada was first colonized by migrants from a P. contorta glacial population located west of the Rocky Mountains, before being replaced by P. banksiana migrating westward during the Holocene. In contrast, extensive pollen gene flow would have erased the cpDNA traces of this ancient presence of P. contorta. Additional evidence for this process was provided by the results of canonical analysis, which indicated that the current cpDNA background of trees reflected recent pollen gene flow from the surrounding dominant species rather than historical events that took place during the postglacial colonization.     

A morphometric study of the geographic variation in Pinus contorta (Pinaceae)

This morphometric study of the geographic variation in Pinus contorta is based on 93 provenances cultivated in northern Jutland, Hjardemål Klitplantage, 57'04"08'48'E (Arboretum trial no. F275). The numerical methods employed were common principal components analysis (CPCA), non-metric multidimensional scaling (MDSCALE), minimum spanning trees (MST), and the neighbour-joining method (NJOIN). Although it is not always possible to draw a very clear line between the coastal var. contorta and the inland var. latifolia of P. contorta subsp. contorta, these varieties are distinct in their extreme forms. Critical provenances of both varieties seem to originate from the transition zone between the two taxa. The Californian or Sierra Nevada provenances of f contorta subsp. murrayana are very distinct from the Oregon provenances. Apparently most of the inland Oregon provenances of P. contorta are more or less intermediate between var. latifolia and subsp. murmyana s.str. The NJOIN method of phylogenetic reconstruction supports the division of P. contorta into two major groups or subspecies: 1. subsp. contorta, divided into var. contorta and var. latifolia, and 2. subsp. murrayana.     

Host dispersal as the driver of parasite genetic structure: a paradigm lost?

Understanding traits influencing the distribution of genetic diversity has major ecological and evolutionary implications for host–parasite interactions. The genetic structure of parasites is expected to conform to that of their hosts, because host dispersal is generally assumed to drive parasite dispersal. Here, we used a meta‐analysis to test this paradigm and determine whether traits related to host dispersal correctly predict the spatial co‐distribution of host and parasite genetic variation. We compiled data from empirical work on local adaptation and host–parasite population genetic structure from a wide range of taxonomic groups. We found that genetic differentiation was significantly lower in parasites than in hosts, suggesting that dispersal may often be higher for parasites. A significant correlation in the pairwise genetic differentiation of hosts and parasites was evident, but surprisingly weak. These results were largely explained by parasite reproductive mode, the proportion of free‐living stages in the parasite life cycle and the geographical extent of the study; variables related to host dispersal were poor predictors of genetic patterns. Our results do not dispel the paradigm that parasite population genetic structure depends on host dispersal. Rather, we highlight that alternative factors are also important in driving the co‐distribution of host and parasite genetic variation.     

Genetic and morphometric variation in the Holarctic helminth parasite Andrya arctica (Cestoda, Anoplocephalidae) in relation to the divergence of its lemming hosts (Dicrostonyx spp.)

Andrya arctica is a cestode parasite of the family Anoplocephalidae (Cyclophyllidea), parasitizing lemmings of the genus Dicrostonyx throughout the Holarctic region. The population structure of this intestinal parasite was studied from eight different regions, six of which represented different genetic entities of lemming hosts. Molecular sequence tagged site markers and minisatellite fingerprints as well as morphology and morphometries were used to reveal the population structure of A. arctica in the Holarctic region. The results suggest that the evolutionary history of this cestode species has included different processes acting on different geographical regions. On the Siberian mainland (host D. torquatus), the division of the parasites into different genetic entities agreed perfectly with the chromosomal races of the lemming hosts that points towards a shared evolutionary history between the host and the parasite ('cospeciation'). The main phylogenetic split of Dicrostonyx between Eurasia and North America was not, however, observed in A. arctica. This suggests that in the Nearctic (host D. groenlandicus) the parasite has remained relatively unmodified because of the large cohesive populations ('coadaptation'). The uniqueness of the Greenland population, and possibly also that of the Wrangel Island, can be explained by peripheral isolation, refugial effects or founder effects.     

Modelling the evolution of common cuckoo host‐races: speciation or genetic swamping?

Co‐evolutionary arms races have provided clear evidence for evolutionary change, especially in host–parasite systems. The evolution of host‐specific races in the common cuckoo (Cuculus canorus), however, is also an example where sexual conflict influences the outcome. Cuckoo females benefit from better adaptation to overcome host defences, whereas cuckoo males face a trade‐off between the benefits of better adaptation to a host and the benefits of multiple mating with females from other host‐races. The outcome of this trade‐off might be genetic differentiation or prevention of it by genetic swamping. We use a simulation model to test which outcome is more likely with three sympatric cuckoo host‐races. We assume a cost for cuckoo chicks that express a host adaptation allele not suited to their foster host species and that cuckoo males that switch to another host‐race experience either a fitness benefit or cost. Over most of the parameter space, cuckoo male host‐race fidelity increases significantly with time, and gene flow between host‐races ceases within a few thousand to a hundred thousand generations. Our results hence support the idea that common cuckoo host‐races might be in the incipient stages of speciation.     

Replicated population divergence caused by localized coevolution? A test of three hypotheses in the red crossbill‐lodgepole pine system

Several lines of evidence support the hypothesis that local populations of red crossbills (Loxia curvirostra complex) enter into a predator-prey arms race with lodgepole pine (Pinus contorta latifolia) in the absence of competing pine squirrels (Tamiasciurus hudsonicus). Nevertheless, the alternative hypotheses that neutral evolution or factors other than squirrels have caused crossbill population differentiation have not been thoroughly tested. We compared crossbill and pine cone morphology between island populations where squirrels are absent or present, and mainland sites where squirrels are present, in order to distinguish among these hypotheses. All comparisons supported an effect of squirrel absence, not island status, on crossbill and cone morphology. Hence our results provide further evidence that strong localized coevolutionary interactions in a geographic mosaic have driven adaptive population differentiation. In addition, vocal differentiation of crossbills was related to the absence of squirrels, but not to island status. As morphological and vocal differentiation is correlated with reproductive isolation in crossbills, the geographic mosaic of coevolution also seems to promote ecological speciation.     

Maintaining genetic diversity and population panmixia through dispersal and not gene flow in a holocyclic heteroecious aphid species

Heteroecious holocyclic aphids exhibit both sexual and asexual reproduction and alternate among primary and secondary hosts. Most of these aphids can feed on several related hosts, and invasions to new habitats may limit the number of suitable hosts. For example, the aphid specialist Aphis glycines survives only on the primary host buckthorn (Rhamnus spp.) and the secondary host soybean (Glycine max) in North America where it is invasive. Owing to this specialization and sparse primary host distribution, host colonization events could be localized and involve founder effects, impacting genetic diversity, population structure and adaptation. We characterized changes in the genetic diversity and structure across time among A. glycines populations. Populations were sampled from secondary hosts twice in the same geographical location: once after secondary colonization (early season), and again immediately before primary host colonization (late season). We tested for evidence of founder effects and genetic isolation in early season populations, and whether or not late-season dispersal restored genetic diversity and reduced fragmentation. A total of 24 single-nucleotide polymorphisms and 6 microsatellites were used for population genetic statistics. We found significantly lower levels of genotypic diversity and more genetic isolation among early season collections, indicating secondary host colonization occurred locally and involved founder effects. Pairwise F(ST) decreased from 0.046 to 0.017 in early and late collections, respectively, and while genetic relatedness significantly decreased with geographical distance in early season collections, no spatial structure was observed in late-season collections. Thus, late-season dispersal counteracts the secondary host colonization through homogenization and increases genetic diversity before primary host colonization.     

Population structure of a parasitic plant and its perennial host

Characterization of host and parasite population genetic structure and estimation of gene flow among populations are essential for the understanding of parasite local adaptation and coevolutionary interactions between hosts and parasites. We examined two aspects of population structure in a parasitic plant, the greater dodder (Cuscuta europaea) and its host plant, the stinging nettle (Urtica dioica), using allozyme data from 12 host and eight parasite populations. First, we examined whether hosts exposed to parasitism in the past contain higher levels of genetic variation. Second, we examined whether host and parasite populations differ in terms of population structure and if their population structures are correlated. There was no evidence that host populations differed in terms of gene diversity or heterozygosity according to their history of parasitism. Host populations were genetically more differentiated (F(ST) = 0.032) than parasite populations (F(ST) = 0.009). Based on these F(ST) values, gene flow was high for both host and parasite. Such high levels of gene flow could counteract selection for local adaptation of the parasite. We found no significant correlation between geographic and genetic distance (estimated as pairwise F(ST)), either for the host or for the parasite. Furthermore, host and parasite genetic distance matrices were uncorrelated, suggesting that sites with genetically similar host populations are unlikely to have genetically similar parasite populations.     

Host-dependent genetic structure of parasite populations: differential dispersal of seabird tick host races

Abstract Despite the fact that parasite dispersal is likely to be one of the most important processes influencing the dynamics and coevolution of host-parasite interactions, little information is available on the factors that affect it. In most cases, opportunities for parasite dispersal should be closely linked to host biology. Here we use microsatellite genetic markers to compare the population structure and dispersal of two host races of the seabird tick Ixodes uriae at the scale of the North Atlantic. Interestingly, tick populations showed high within-population genetic variation and relatively low population differentiation. However, gene flow at different spatial scales seemed to depend on the host species exploited. The black-legged kittiwake ( Rissa tridactyla ) had structured tick populations showing patterns of isolation by distance, whereas tick populations of the Atlantic puffin ( Fratercula arctica ) were only weakly structured at the largest scale considered. Host-dependent rates of tick dispersal between colonies will alter infestation probabilities and local dynamics and may thus modify the adaptation potential of ticks to local hosts. Moreover, as I. uriae is a vector of the Lyme disease agent Borrelia burgdorferi sensu lato in both hemispheres, the large-scale movements of birds and the subsequent dispersal of ticks will have important consequences for the dynamics and coevolutionary interactions of this microparasite with its different vertebrate and invertebrate hosts.     

Population genetic structure of Tomicus piniperda L. (Curculionidae: Scolytinae) on different pine species and validation of T. destruens (Woll.)

Genetic diversity and population structure of Tomicus piniperda was assessed using mitochondrial sequences on 16 populations sampled on 6 pine species in France. Amplifications of Internal transcribed space 1 (ITS1) were also performed. Our goals were to determine the taxonomic status of the Mediterranean ecotype T. piniperda destruens, and to test for host plant or geographical isolation effect on population genetic structure. We showed that T. piniperda clusters in two mtDNA haplotypic groups. Clade A corresponds to insects sampled in continental France on Pinus sylvestris, P. pinaster and P. uncinata, whereas clade B gathers the individuals sampled in Corsica on P. pinaster and P. radiata and in continental France on P. pinea and P. halepensis. Insects belonging to clade A and clade B also consistently differ in the length of ITS1. Individuals belonging to both clades were found once in sympatry on P. pinaster. Genetic distances between clades are similar to those measured between distinct species of Tomicus. We concluded that clade B actually corresponds to the destruens ecotype and forms a good species, T. destruens. Analyses of molecular variance ( amova ) were conducted separately on T. destruens and T. piniperda to test for an effect of either geographical isolation or host species. Interestingly, the effect of host plant was significant for T. piniperda only, while the effect of geographical isolation was not. Pine species therefore seems to act as a significant barrier to gene flow, even if host race formation is not observed. These results still need to be confirmed by nuclear markers.     

Population structure of a vector‐borne plant parasite

Parasites are among the most diverse groups of life on Earth, yet complex natural histories often preclude studies of their speciation processes. The biology of parasitic plants facilitates in situ collection of data on both genetic structure and the mechanisms responsible for that structure. Here, we studied the role of mating, dispersal and establishment in host race formation of a parasitic plant. We investigated the population genetics of a vector‐borne desert mistletoe (Phoradendron californicum) across two legume host tree species (Senegalia greggii and Prosopis velutina) in the Sonoran desert using microsatellites. Consistent with host race formation, we found strong host‐associated genetic structure in sympatry, little genetic variation due to geographic site and weak isolation by distance. We hypothesize that genetic differentiation results from differences in the timing of mistletoe flowering by host species, as we found initial flowering date of individual mistletoes correlated with genetic ancestry. Hybrids with intermediate ancestry were detected genetically. Individuals likely resulting from recent, successful establishment events following dispersal between the host species were detected at frequencies similar to hybrids between host races. Therefore, barriers to gene flow between the host races may have been stronger at mating than at dispersal. We also found higher inbreeding and within‐host individual relatedness values for mistletoes on the more rare and isolated host species (S. greggii). Our study spanned spatial scales to address how interactions with both vectors and hosts influence parasitic plant structure with implications for parasite virulence evolution and speciation.     

Evolution of a pathogen population in host mixtures: rate of emergence of complex races

 The development of a pathogen population in a crop varietal mixture was studied with an epidemic simulator based on the model EPIMUL. The pathogen population was composed of simple races, able to develop on only one genetic component of the mixture, and a complex race, which developed on all mixture components. The complex race was modelled with a very low initial frequency compared to simple races, to simulate the emergence of a complex race in the field. Several successive epidemics were simulated as if the pathogen population reproduced on the same plot for several years. The development of the complex race on the simulated plot was either focal or uniform. The effects of the cost of virulence, of density dependence and of differential adaptation to host genetic background on the simple race-complex race competition were studied. Experimentally measured values of the cost of virulence and differential adaptation were incorporated into the model, and both factors were shown to greatly reduce the increase in frequency of the complex race over time. Density dependance also influenced race competition, but mainly for high values of the parameter. Our results suggest that the cost of virulence is probably not the only mechanism that may influence the simple race-complex race competition in host mixtures. In our simulations, differences in the spatial distribution of the initial inoculum between parasite races led to large differences in their final frequencies. Thus, more investigations, including randomized disease distributions, would be of interest to judge the potential importance of spatial effects in the field. Received: 4 October 1996 / Accepted: 3 January 1997     

Genetic variation in postfire aspen seedlings in yellowstone national park

A rare episode of regeneration of aspen (Populus tremuloides Michx.) by seeds occurred in Yellowstone National Park (YNP), Wyoming, USA, following extensive fires that occurred in 1988. In 1997, we sampled 410 aspen seedlings from 23 local populations distributed widely across YNP to determine how genetic diversity varies with elevation, substrate, plant competition, ungulate browsing, and geographical location. We employed 132 randomly amplified polymorphic DNA (RAPD) markers based on six primers to show genetic relationships within and among the postfire aspen seedling populations. Measures of genetic variation, including estimates of percentage polymorphic loci, expected heterozygosity, and Nei's FST, indicated that most of the variation occurred within rather than among local populations. There was no indication of geographical differentiation among sampled populations based on hierarchal estimates of Nei's FST, neighbour-joining, or correlations between genetic distance and geographical distance. Even genetically distant populations shared nearly 90% of the same markers. Within plots, the amount of genetic variation decreased slightly in response to increased percentage vegetative cover, mean seedling basal diameter, and mean seedling height. Geological substrate, density of lodgepole pine (Pinus contorta var. latifolia Dougl.) seedlings, browsing intensity, and elevation were not significantly related to levels of genetic variation within the seedling plots. These data suggest that genetic variation and geographical structure among seedling populations may occur over time as the transition from seedling-dominated stands to clone-dominated stands occurs.     

Host‐associated divergence in sympatric host races of the leaf beetle Lochmaea capreae: implications for local adaptation and reproductive isolation

Ecological specialization is widely recognized as a major determinant of the emergence and maintenance of biodiversity. We studied two critical facets of specialization – local adaptation and habitat choice – in the host races of the leaf beetle Lochmaea capreae on willow and birch. Our results revealed that there is asymmetric disruptive selection for host use traits, and host races achieved different adaptive sets of life history traits through association with their host plant. Beetles from each host race exhibited food and oviposition preference for their own host plant. Reciprocal transplant displayed significant variation in host acceptance and performance: all families from the willow race rejected the alternative host plant before initiation of feeding and all died on this host plant. By contrast, all families from the birch race accepted willow for feeding, but they consumed less and performed less well. Intriguingly, families that performed well on birch also performed well on willow, suggesting positive genetic correlation rather than genetic trade‐offs. Our results suggest that the major proximal determinant of host specialization in the willow race is the behavioural acceptance of a plant rather than the toxicity of the food resource. However, in the birch race a combination of behavioural host acceptance and performance may play a role in specialization. Our study sheds light on the mechanisms by which divergent host adaptation might influence the evolution of reproductive isolation between herbivorous populations.     

Recent evolution of host-associated divergence in the seabird tick Ixodes uriae

Ecological interactions are an important source of rapid evolutionary change and thus may generate a significant portion of novel biodiversity. Such changes may be particularly prevalent in parasites, where hosts can induce strong selection for adaptation. To understand the relative frequency at which host-associated divergences occur, it is essential to examine the evolutionary history of the divergence process, particularly when it is occurring over large geographical scales where both geographical and host-associated isolation may playa part. In this study, we use population genetics and phylogeography to study the evolutionary history of host-associated divergence in the seabird tick Ixodes uriae (Acari, Ixodidae). We compare results from microsatellite markers that reflect more ecological timescales with a conserved mitochondrial gene (COIII) that reflects more ancient divergence events. Population structure based on microsatellites showed clear evidence of host-associated divergence in all colonies examined. However, isolated populations of the same host type did not always group together in overall analyses and the genetic differentiation among sympatric host races was highly variable. In contrast, little host or geographical structure was found for the mitochondrial gene fragment. These results suggest that host race formation in I. uriae is a recent phenomenon, that it may have occurred several times and that local interactions are at different points in the divergence process. Rapid divergence in I. uriae implies a strong interaction with its local host species, an interaction that will alter the ecological dynamics of the system and modify the epidemiological landscape of circulating micropathogens.     

粉蝶盘绒茧蜂中国和荷兰种群学习行为及EAG反应的比较

昆虫的取食危害可导致植物释放的挥发物发生质和量的变化,天敌利用植物在受到植食性昆虫的危害后释放的挥发性物质来寻找寄主,这种植物挥发物在寄生蜂寻找寄主过程中扮演者重要的角色。粉蝶盘绒茧蜂(Cotesia glomerata)是菜粉蝶(pieris rapae)主要的内寄生蜂,是优良的蜂种,在生物防治上具有广阔的应用前景。分别以中国种群和荷兰种群的粉蝶盘绒茧蜂为研究对象,利用Y型嗅觉仪和触角电位(Electroantennogram,EAG)技术,比较了不同地理种群粉蝶盘绒茧蜂雌蜂的学习行为和触角电生理反应差异。旨在明确虫害植株释放的挥发物在两个地理种群中的作用。Y型嗅觉仪试验结果表明,雌蜂不同的学习经历会影响其对寄主植株的趋性行为。没有学习经历的中国种群雌蜂对虫害甘蓝挥发物较虫害紫甘蓝和虫害羽衣甘蓝挥发物表现出更强地趋性行为,无学习经历荷兰种群对虫害甘蓝挥发物同样表现出更强地趋性行为,中国种群3次间隔产卵和3次间隔气味学习使其对经历过的寄主植物挥发物趋性明显提高。荷兰种群3次间隔气味学习也会提高该蜂对经历寄主挥发物的趋性。EAG试验结果显示,两个地理种群雌蜂对所测定三类挥发物(体积比10-1)的反应强弱依次为:C6化合物>脂肪族化合物>萜类化合物;且在所测定的6种化合物剂量范围内,两个地理种群雌蜂均与trans-2--己醛和乙酸己酯(体积比10-4—10-1),壬醛、月桂烯和里那醇(体积比10-4—10-2)化合物的活性成正相关。结果表明,中国和荷兰种群雌蜂均表现出较为一致的学习能力和电生理反应。研究显示粉蝶盘绒茧蜂雌蜂可能利用甘蓝挥发物作为利它素而寻觅适宜的寄主,学习经历会明显提高该蜂对经历的寄主挥发物的趋性的行为。实验结果为菜粉蝶的生物防治提供理论基础。     

Cuckoos versus hosts in insects and birds: adaptations,counter‐adaptations and outcomes

Avian parents and social insect colonies are victimized by interspecific brood parasites—cheats that procure costly care for their dependent offspring by leaving them in another species' nursery. Birds and insects defend themselves from attack by brood parasites; their defences in turn select counter‐strategies in the parasite, thus setting in motion antagonistic co‐evolution between the two parties. Despite their considerable taxonomic disparity, here we show striking parallels in the way that co‐evolution between brood parasites and their hosts proceeds in insects and birds. First, we identify five types of co‐evolutionary arms race from the empirical literature, which are common to both systems. These are: (a) directional co‐evolution of weaponry and armoury; (b) furtiveness in the parasite countered by strategies in the host to expose the parasite; (c) specialist parasites mimicking hosts who escape by diversifying their genetic signatures; (d) generalist parasites mimicking hosts who escape by favouring signatures that force specialization in the parasite; and (e) parasites using crypsis to evade recognition by hosts who then simplify their signatures to make the parasite more detectable. Arms races a and c are well characterized in the theoretical literature on co‐evolution, but the other types have received little or no formal theoretical attention. Empirical work suggests that hosts are doomed to lose arms races b and e to the parasite, in the sense that parasites typically evade host defences and successfully parasitize the nest. Nevertheless hosts may win when the co‐evolutionary trajectory follows arms race a, c or d. Next, we show that there are four common outcomes of the co‐evolutionary arms race for hosts. These are: (1) successful resistance; (2) the evolution of defence portfolios (or multiple lines of resistance); (3) acceptance of the parasite; and (4) tolerance of the parasite. The particular outcome is not determined by the type of preceding arms race but depends more on whether hosts or parasites control the co‐evolutionary trajectory: tolerance is an outcome that parasites inflict on hosts, whereas the other three outcomes are more dependent on properties intrinsic to the host species. Finally, our review highlights considerable interspecific variation in the complexity and depth of host defence portfolios. Whether this variation is adaptive or merely reflects evolutionary lag is unclear. We propose an adaptive explanation, which centres on the relative strength of two opposing processes: strategy‐facilitation, in which one line of host defence promotes the evolution of another form of resistance, and strategy‐blocking, in which one line of defence may relax selection on another so completely that it causes it to decay. We suggest that when strategy‐facilitation outweighs strategy‐blocking, hosts will possess complex defence portfolios and we identify selective conditions in which this is likely to be the case.     

Egg mimicry by the Pacific koel: mimicry of one host facilitates exploitation of other hosts with similar egg types

When brood parasites exploit multiple host species, egg rejection by hosts may select for the evolution of host‐specific races, where each race mimics a particular host's egg type. However, some brood parasites that exploit multiple hosts with the ability to reject foreign eggs appear to have only a single egg type. In these cases, it is unclear how the parasite egg escapes detection by its hosts. Three possible explanations are: 1) host‐specific races are present, but differences in egg morphology are difficult for the human eye to detect; 2) the brood parasite evolves a single egg type that is intermediate in appearance between the eggs of its hosts; 3) or the parasite evolves mimicry of one of its hosts, which subsequently allows it to exploit other species with similar egg morphology. Here we test these possibilities by quantifying parameters of egg appearance of the brood‐parasitic Pacific koel Eudynamys orientalis and seven of its hosts. Koel eggs laid in the nests of different hosts did not show significant differences in colour or pattern, suggesting that koels have not evolved host‐specific races. Koel eggs were similar in colour, luminance and pattern to the majority of hosts, but were significantly more similar in colour and luminance to one of the major hosts than to two other major hosts, supporting hypothesis 3. Our findings suggest that mimicry of one host can allow a brood parasite to exploit new hosts with similar egg morphologies, which could inhibit the evolution of host defences in naïve hosts.