Incipient allochronic speciation in the pine processionary moth (Thaumetopoea pityocampa, Lepidoptera, Notodontidae)

A plausible case of allochronic differentiation, where barrier to gene flow is primarily attributable to a phenological shift, was recently discovered in Portugal for the pine processionary moth Thaumetopoea pityocampa. Previous results suggested that the observed 'summer population' (SP) originated from the sympatric winter population (WP). Our objectives were to finely analyse these patterns and test their stability in time, through field monitoring and genetic analyses of larvae and adults across different years. Reproductive activity never overlapped between SP and WP. Microsatellites showed a clear differentiation of the SP, consistent with a strong reduction in gene flow owing to the phenological shift. Assignment tests suggested that some individuals shift from the SP to the WP phenology, causing some hybridization. We discuss these patterns and their maintenance over time. This could be a first stage of allochronic speciation, and SP should be considered as a distinct phenological race.     

Evidence for low‐level hybridization between two allochronic populations of the pine processionary moth,Thaumetopoea pityocampa (Lepidoptera: Notodontidae)

Divergence between populations sharing the same habitat can be initiated by different reproductive times, leading to allochronic differentiation. A spatially localized allochronic summer population (SP) of the pine processionary moth Thaumetopoea pityocampa, recently discovered in Portugal, occurs in sympatry with the local winter population (WP). We examined the level of genetic differentiation between the two populations and estimated the current gene flow within the spatial framework of their co‐occurrence. Mitochondrial data indicated that the two sympatric populations were genetically closer than other WP populations. Conversely, microsatellite genotyping uncovered greater differentiation between the two sympatric populations than between allopatric ones. While male trapping confirmed that reproduction of SP and WP occurred at distinct times, clustering approaches demonstrated the presence of a few LateSP individuals emerging within the WP flight period, although genetically identified as SP. We also identified rare recent hybridization events apparently occurring mainly in the margins of the current SP range. The ongoing gene flow detected between the ancestral and the emerging allochronic populations revealed an incomplete reproductive isolation, which must therefore be taken into account and integrated with studies focussed on ecological drivers, so that a complete understanding of the ongoing speciation process might be achieved.     

Phenotypic divergence in reproductive traits of a moth population experiencing a phenological shift

Allochrony that is reproductive isolation by time may further lead to divergence of reproductive adaptive traits in response to different environmental pressures over time. A unique “summer” population of the pine processionary moth Thaumetopoea pityocampa, reproductively isolated from the typical winter populations by allochronic differentiation, is here analyzed. This allochronically shifted population reproduces in the spring and develops in the summer, whereas “winter” populations reproduce in the late summer and have winter larval development. Both summer and winter populations coexist in the same pine stands, yet they face different climatic pressures as their active stages are present in different seasons. The occurrence of significant differences between the reproductive traits of the summer population and the typical winter populations (either sympatric or allopatric) is thus hypothesized. Female fecundity, egg size, egg covering, and egg parasitism were analyzed showing that the egg load was lower and that egg size was higher in the summer population than in all the studied winter populations. The scales that cover the egg batches of T. pityocampa differed significantly between populations in shape and color, resulting in a looser and darker covering in the summer population. The single specialist egg parasitoid species of this moth was almost missing in the summer population, and the overall parasitism rates were lower than in the winter population. Results suggest the occurrence of phenotypic differentiation between the summer population and the typical T. pityocampa winter populations for the life‐history traits studied. This work provides an insight into how ecological divergence may follow the process of allochronic reproductive isolation.     

Incipient allochronic speciation by climatic disruption of the reproductive period

Disruptive selection of life-cycle timing may cause temporal isolation directly and, ultimately, allochronic speciation. Despite the fact that segregation of the reproductive period among related species has been broadly observed across taxa, it remains controversial whether temporal isolation can function as the primary process of speciation. In the Japanese winter geometrid moth Inurois punctigera, allochronic divergence has resulted from climatic disruption of the reproductive period. In habitats with severe midwinter, two sympatric groups of moth reproduce allochronically in early and late winter. These groups are genetically diverging sister lineages and now co-occur allochronically throughout Japan. By contrast, in habitats with milder midwinter these lineages form a continuous adult period and gene flow has been facilitated between the lineages. These results, together with the fact that there is no difference in larval host use, indicate that temporal isolation has been the sole mechanism for allochronic isolation in colder habitats and that allochrony is not a by-product of other adaptations. Thus, the allochronic divergence of sympatric I. punctigera populations represents an incipient speciation process driven by midwinter disruption of the reproductive period.     

Deciphering the demographic history of allochronic differentiation in the pine processionary moth Thaumetopoea pityocampa

Understanding the processes of adaptive divergence, which may ultimately lead to speciation, is a major question in evolutionary biology. Allochronic differentiation refers to a particular situation where gene flow is primarily impeded by temporal isolation between early and late reproducers. This process has been suggested to occur in a large array of organisms, even though it is still overlooked in the literature. We here focused on a well‐documented case of incipient allochronic speciation in the winter pine processionary moth Thaumetopoea pityocampa. This species typically reproduces in summer and larval development occurs throughout autumn and winter. A unique, phenologically shifted population (SP) was discovered in 1997 in Portugal. It was proved to be strongly differentiated from the sympatric “winter population” (WP), but its evolutionary history could only now be explored. We took advantage of the recent assembly of a draft genome and of the development of pan‐genomic RAD‐seq markers to decipher the demographic history of the differentiating populations and develop genome scans of adaptive differentiation. We showed that the SP diverged relatively recently, that is, few hundred years ago, and went through two successive bottlenecks followed by population size expansions, while the sympatric WP is currently experiencing a population decline. We identified outlier SNPs that were mapped onto the genome, but none were associated with the phenological shift or with subsequent adaptations. The strong genetic drift that occurred along the SP lineage certainly challenged our capacity to reveal functionally important loci.     

Parallel allochronic divergence in a winter moth due to disruption of reproductive period by winter harshness

The disruption of reproductive timing by climatic harshness may result in the temporal isolation of conspecific populations and, ultimately, in speciation. However, whether temporal isolation alone can act as the force initiating speciation and how often the same type of climatic disruption results in the divergence of allochronic populations in a lineage are largely unknown. The reproductive period of the winter geometrid moth Inurois punctigera is separated into early and late winter in habitats with severe winters, but not in habitats with mild winters, suggesting that the reproductive season is disrupted by the harshness of the mid-winter period. Here, we show that sympatric pairs of early- and late-winter populations that differ in origin exist in different regions, suggesting a parallel divergence of reproductive timing. In each region, significant genetic differentiation exists between these early- and late-winter populations, suggesting that the temporal reproductive isolation has persisted. Moreover, we demonstrate that the temporal isolation, in comparison with geographic isolation, contributes greatly to the genetic differentiation among geographic and temporal populations by an analysis of molecular variance and by a comparison of genetic differentiations (F(ST) ) between geographic populations with and without difference in reproductive season. Our results suggest that adaptive divergence of allochronically reproducing populations has occurred independently in different regions, implying the generality of the role of temporal isolation in initiating speciation in a winter moth lineage.     

The role of allochrony in speciation

The importance of sympatric speciation – the evolution of reproductive isolation between codistributed conspecific individuals – in generating biodiversity is highly controversial. Allochrony, or differences in breeding time (phenology) between conspecific individuals, has the potential to lead to reproductive isolation and therefore speciation. We critically review the literature to test the importance of allochronic speciation over the three timescales over which allochrony can occur – over the day, between seasons or between years – and explore what is known about genomic mechanisms underlying allochrony in the diverse taxa in which it is found. We found that allochrony can be a key contributor to reproductive isolation, especially if populations have little overlap in breeding time and therefore little potential for gene flow, and may sometimes be the initial or key driver of speciation. Shifts in phenology can be caused by several factors, including a new ecological opportunity, environmental change, or reinforcement. The underlying genomic basis of allochrony has been studied mostly in insects, highlighting the need for genomic studies in other taxa; nonetheless, results to date indicate that several cases of allochrony involve changes in circadian genes. This review provides the first comprehensive discussion of the role of allochrony in speciation and demonstrates that allochrony as a contributor to divergence may be more widespread than previously thought. Understanding genomic changes and adaptations allowing organisms to breed at new times may be key in the light of phenological changes required under climate change.     

Temperature niche shift observed in a Lepidoptera population under allochronic divergence

A process of adaptive divergence for tolerance to high temperatures was identified using a rare model system, consisting of two sympatric populations of a Lepidoptera (Thaumetopoea pityocampa) with different life cycle timings, a 'mutant' population with summer larval development, Leiria SP, and the founder natural population, having winter larval development, Leiria WP. A third, allopatric population (Bordeaux WP) was also studied. First and second instar larvae were experimentally exposed to daily-cycles of heat treatment reaching maximum values of 36, 38, 40 and 42 °C; control groups placed at 25 °C. A lethal temperature effect was only significant at 42 °C, for Leiria SP, whereas all temperatures tested had a significant negative effect upon Leiria WP, thus indicating an upper threshold of survival c.a. 6 °C above that of the WP. Cox regression model, for pooled heat treatments, predicted mortality hazard to increase for Leiria WP (+108%) and Bordeaux WP (+78%) in contrast to Leiria SP; to increase by 24% for each additional °C; and to decrease by 53% from first to second instar larvae. High variability among individuals was observed, a population characteristic that may favour selection and consequent adaptation. Present findings provide an example of ecological differentiation, following a process of allochronic divergence. Results further contribute to a better understanding of the implications of climate change for ecological genetics.     

Experimental evidence for heritable reproductive time in 2 allochronic populations of pine processionary moth

Phenology allows organisms to overcome seasonally variable conditions through life‐cycle adjustment. Changes in phenology can drastically modify the evolutionary trajectory of a population, while a shift in the reproductive time may cause allochronic differentiation. The hypothesis of heritable reproductive time was experimentally tested, by studying a unique population of the pine processionary moth Thaumetopoea pityocampa (Den. & Schiff.) which has a shifted phenology, and however co‐occurs with the typical population following the classical life cycle. When populations of both types were reared under controlled conditions, the reproductive time was maintained asynchronous, as observed in the field. The shifted population was manipulated in the laboratory to reproduce later than usual, yet the offspring emerged in the next year at the expected dates thus “coming back” to the usual cycle. Hybrids from crosses performed between the 2 populations showed an intermediate phenology. From the emergence times of parents and offspring, a high heritability of the reproductive time (h = 0.76) was observed. The offspring obtained from each type of cross was genetically characterized using microsatellite markers. Bayesian clustering analysis confirmed that hybrids can be successfully identified and separated from the parental genetic classes by genotyping. Findings support the hypothesis that, for this particular population, incipient allochronic speciation is due to a heritable shift in the reproductive time that further causes assortative mating and might eventually cause ecological adaptation/maladaptation in response to environmental changes.     

Phylogenetic and molecular evidence for allochronic speciation in gall-forming aphids (Pemphigus)

Sympatric populations can diverge when variation in phenology or life cycle causes them to mate at distinctly different times. We report patterns consistent with this process (allochronic speciation) in North American gall-forming aphids, in the absence of a host or habitat shift. Pemphigus populi-transversus Riley and P. obesinymphae Aoki form a monophyletic clade within the North American Pemphigus group. They are sympatric on the eastern cottonwood, Populus deltoides (Salicaceae), but have distinctly different life cycles, with sexual stages offset by approximately six months. Field evidence indicates that intermediate phenotypes do not commonly occur, and mitochondrial and bacterial endosymbiont DNA sequences show no maternal gene flow between the two species. Because a genetically distinct population of P. obesinymphae occurs in the southwestern United States on Populus fremontii, we consider the possibility of an initial allopatric phase in the divergence. We discuss the likely origins of the host use patterns in P. obesinymphae, and the larger sequence of evolutionary changes that likely led to the sympatric divergence of P. populi-transversus and P. obesinymphae. A plausible interpretation at this stage of investigation is that a shift in timing of the life cycle in an ancestral population, correlated with an underlying phenological complexity in its host plant, spurred divergence between the incipient species.     

Population structure and dynamics of sympatric Apodemus species (Rodentia: Muridae)

This paper describes the population biology of sympatric populations of Apodemus sylvaticus and A. flavicollis in western England. Annual changes in population siie closely resembled those seen in allopatric populations such that simultaneous peaks in number occurred only in autumn and early winter. Numbers of A. sylvaticus were low and stable in early summer increasing rapidly in late summer and autumn and remaining high throughout winter before a sharp decline in spring. Numbers of A. flavicollis increased after the start of reproduction and continued to rise throughout summer and autumn. The period of major decline of this species was during early winter. Densities of A. sylvaticus and A. flavicollis were comparable with those cited in allopatric studies. Spring populations of both species consisted of cohorts first appearing in the late summer and autumn of the previous year. Few individuals present in spring survived the summer. Reproduction of A. flavicollis commenced four to six weeks before that of A. sylvaticus . Seasonal variation in mean weight of adult males and females also indicates that those changes, due to the acquisition of secondary sex characteristics and reproductive maturity, occur earlier in A. flavicollis than in A. sylvaticus . Data presented contain no evidence of interspecific competition and it is concluded that interspecific differences, chiefly asynchrony in reproduction and annual population cycles, contribute to their stable coexistence.     

Temperature,size, reproductive allocation,and life‐history evolution in a gregarious caterpillar

The present study aimed to investigate the relationship between growth rate, final mass, and larval development, as well as how this relationship influences reproductive trade‐offs, in the context of a gregarious life‐style and the need to keep an optimal group size. We use as a model two sympatric populations of the pine processionary moth Thaumetopoea pityocampa, which occur in different seasons and thus experience different climatic conditions. Thaumetopoea pityocampa is a strictly gregarious caterpillar throughout the larval period, which occurs during winter in countries all over the Mediterranean Basin. However, in 1997, a population in which larval development occurs during the summer was discovered in Portugal, namely the summer population (SP), as opposed to the normal winter population (WP), which coexists in the same forest feeding on the same host during the winter. Both populations were monitored over 3 years, with an assessment of the length of the larval period and its relationship with different climatic variables, final mass and adult size, egg size and number, colony size, and mortality at different life stages. The SP larval period was reduced as a result of development in the warmer part of the year, although it reached the same final mass and adult size as the WP. Despite an equal size at maturity, a trade‐off between egg size and number was found between the two populations: SP produced less but bigger eggs than WP. This contrasts with the findings obtained in other Lepidoptera species, where development in colder environments leads to larger eggs at the expense of fecundity, but corroborates the trend found at a macro‐geographical scale for T. pityocampa, with females from northern latitudes and a colder environment producing more (and smaller) eggs. The results demonstrate the importance of the number of eggs in cold environments as a result of an advantage of large colonies when gregarious caterpillars develop in such environments, and these findings are discussed in accordance with the major theories regarding size in animals. © 2011 The Linnean Society of London, Biological Journal of the Linnean Society, 2012, 105 , 340–349.     

Reproductive character displacement and the genetics of gamete recognition in tropical sea urchins

Reproductive character displacement occurs when sympatric and allopatric populations of a species differ in traits crucial to reproduction, and it is commonly thought of as a signal of selection acting to limit hybridization. Most documented cases of reproductive character displacement involve characters that are poorly understood at the genetic level, and rejecting alternative hypotheses for biogeographic shifts in reproductive traits is often very difficult. In sea urchins, the gamete recognition protein bindin evolves under positive selection when species are broadly sympatric, suggesting character displacement may be operating in this system. We sampled sympatric and allopatric populations of two species in the sea urchin genus Echinometra for variation in bindin and for the mitochondrial cytochrome oxidase I to examine patterns of population differentiation and molecular evolution at a reproductive gene. We found a major shift in bindin alleles between central Pacific (allopatric) and western Pacific (sympatric) populations of E. oblonga. Allopatric populations of E. oblonga are polyphyletic with E. sp. C at bindin, whereas sympatric populations of the two species are reciprocally monophyletic. There is a strong signal of positive selection (P(N)/P(S) = 4.5) in the variable region of the first exon of bindin, which is associated with alleles found in sympatric populations of E. oblonga. These results indicate that there is a strong pattern of reproductive character displacement between E. oblonga and E. sp. C and that the divergence is driven by selection. There is much higher population structure in sympatric populations at the bindin locus than at the neutral mitochondrial locus, but this difference is not seen in allopatric populations. These data suggest a pattern of speciation driven by selection for local gamete coevolution as a result of interactions between sympatric species. Although this pattern is highly suggestive of speciation by reinforcement, further research into hybrid fitness and egg-sperm interactions is required to address this potential mechanism for character displacement.     

Sympatric speciation: when is it possible?

This paper is written to compare the results of theoretical investigations of sympatric speciation with the relevant experimental data. We understand sympatric speciation as a formation of species out of a population whose spatial structure is not important genetically. A necessary prerequisite for speciation is an action of disruptive selection on sufficiently polymorphic traits. The present analysis confirms the view that such a selection is ecologically realistic. The genetical part of speciation begins with a development of reproductive isolation between those individuals that are opposed in some characters. It is shown that selection for reproductive isolation may be quite strong. Extinction of intermediate individuals, which completes speciation, proceeds under a wide range of conditions, including those when the newly formed species differ in quantitative characters, though most of the genes arc likely to remain the same in both species. The whole process seems possible if differences in several (up to 10) loci are sufficient to adapt the forming species to different niches and to establish reproductive isolation. It is shown that populations with bimodal distributions of some genetically determined quantitative characters can have a considerable life-time. Such distributions may be formed either as a transition stage of sympatric speciation or represent a stationary state under conditions close to those necessary to complete speciation. They are very important for experimental investigations. Sympatric speciation always follows the same principal course; it does not contradict the idea of a genome coadaptedness. The occurrence of sympatric speciation is different for different taxa depending rather on how frequently populations are subjected to the appropriate kind of selection than on their ability to obey it.     

Rapid evolution and specialization following host colonization in a yucca moth

Host shifts followed by specialization can result in sympatric genetic differentiation, and may have fuelled the diversification of phytophagous insects. This study examines a recent colonization of a non‐native host by Prodoxus quinquepunctellus (Lepidoptera: Prodoxidae). Allozyme differentiation was detected among different host feeding populations, yet was nearly absent among similar host feeding populations in sympatry. Geographical patterns of allozyme variation showed a much higher level of population structure among populations feeding on the derived host. Conversely, mtDNA haplotype frequencies were nearly homogeneous in the derived populations compared to the ancestral populations, suggesting a bottleneck and/or rapid fixation of haplotypes following host colonization. Moth emergence coincided with host plant flowering, and phenological differences between host species translated into allochronic isolation between populations feeding on different hosts. Derived moth populations also differed significantly in three ovipositor characters from ancestral populations. These findings suggest rapid host‐specific genetic differentiation, and specialization of moth emergence time and ovipositor morphology following host colonization.     

Divergence before the host shift? Prezygotic reproductive isolation among three varieties of a specialist fly on a single host plant

1. Although divergence via host‐plant shifting is a common theme in the speciation of some phytophagous insects, it is not clear whether host shifts are typically initiators of speciation or if they instead contribute to divergence events already in progress. While host shifts appear to be generally associated with speciation events for flies in the genus Strauzia, three sympatric varieties of the sunflower fly [Strauzia longipennis (Wiedemann)] co‐occur on the same host plant in the Midwestern United States and may have evolved reproductive barriers without a host shift. 2. The strength of two prezygotic reproductive barriers was compared among the three S. longipennis varieties: one barrier that is often associated with divergent ecological selection (allochronic isolation), and another that is more likely to be independent of ecological selection (pre‐copulatory sexual isolation). The presence and relative strength of each barrier between fly varieties were evaluated using microsatellites, no choice mating experiments, studies of allochronic isolation, and field collection data. 3. Evidence for both allochronic isolation and pre‐copulatory sexual isolation was detected between the three varieties of S. longipennis. The measure of isolation calculated for each barrier between the three varieties was lower than measures calculated between different species of Strauzia found on different hosts, suggesting that subsequent host shifts may increase the degree of reproductive isolation. For Strauzia and other specialist insects, some reproductive isolation may evolve prior to, and indeed may facilitate, host shifts.     

Evidence for sympatric speciation by host shift in the sea

The genetic divergence and evolution of new species within the geographic range of a single population (sympatric speciation) contrasts with the well-established doctrine that speciation occurs when populations become geographically isolated (allopatric speciation). Although there is considerable theoretical support for sympatric speciation, this mode of diversification remains controversial, at least in part because there are few well-supported examples. We use a combination of molecular, ecological, and biogeographical data to build a case for sympatric speciation by host shift in a new species of coral-dwelling fish (genus Gobiodon). We propose that competition for preferred coral habitats drives host shifts in Gobiodon and that the high diversity of corals provides the source of novel, unoccupied habitats. Disruptive selection in conjunction with strong host fidelity could promote rapid reproductive isolation and ultimately lead to species divergence. Our hypothesis is analogous to sympatric speciation by host shift in phytophagous insects except that we propose a primary role for intraspecific competition in the process of speciation. The fundamental similarity between these fishes and insects is a specialized and intimate relationship with their hosts that makes them ideal candidates for speciation by host shift.     

Emergence of novel fungal pathogens by ecological speciation: importance of the reduced viability of immigrants

Expanding global trade and the domestication of ecosystems have greatly accelerated the rate of emerging infectious fungal diseases, and host-shift speciation appears to be a major route for disease emergence. There is therefore an increased interest in identifying the factors that drive the evolution of reproductive isolation between populations adapting to different hosts. Here, we used genetic markers and cross-inoculations to assess the level of gene flow and investigate barriers responsible for reproductive isolation between two sympatric populations of Venturia inaequalis, the fungal pathogen causing apple scab disease, one of the fungal populations causing a recent emerging disease on resistant varieties. Our results showed the maintenance over several years of strong and stable differentiation between the two populations in the same orchards, suggesting ongoing ecological divergence following a host shift. We identified strong selection against immigrants (i.e. host specificity) from different host varieties as the strongest and likely most efficient barrier to gene flow between local and emerging populations. Cross-variety disease transmission events were indeed rare in the field and cross-inoculation tests confirmed high host specificity. Because the fungus mates within its host after successful infection and because pathogenicity-related loci prevent infection of nonhost trees, adaptation to specific hosts may alone maintain both genetic differentiation between and adaptive allelic combinations within sympatric populations parasitizing different apple varieties, thus acting as a 'magic trait'. Additional intrinsic and extrinsic postzygotic barriers might complete reproductive isolation and explain why the rare migrants and F1 hybrids detected do not lead to pervasive gene flow across years.     

A phenological hypothesis on the thermophilous distribution of Pistacia lentiscus L.

In this study we present evidences of the importance of the phenological pattern on the distribution limits of the dioecious Pistacia lentiscus L. This species, though displaying a thermophilous distribution, has been proved to resist low freezing temperatures during winter. We try to explain this apparent paradox by studying the effects of an extreme cold event that occurred in December 2001 on a natural population of P. lentiscus in the NE of the Iberian Peninsula. In previous phenological studies conduced over 4 years, no spring branch damage was noticeable among population individuals. Female individuals of this species present a prolonged phenophase development which extends until early winter with the conclusion of fruit set. Therefore, we hypothesize that female plants would be more affected than male ones in terms of the following: (1) vegetative and reproductive organs survival; and (2) next-year vegetative growth and fruit production. To test this hypothesis, we selected 225 adult individuals (113 females and 112 males) in April 2002, and estimated their crown volume, percentage of frozen branches and reproductive buds, and the amount of 2001 fruits frozen. In June 2002 we evaluated, in the same individuals, the percentage of vegetative buds flushed into shoots and of reproductive buds producing infrutescences. Branch mortality was significantly higher in female plants and females with increased frost-damage displayed a higher amount of frozen fruits. The loss of reproductive buds caused a decrease in 2002 fruit production, while 2002 vegetative growth was unaffected by the degree of frost damage. These results verify most of the predictions of our hypothesis. Moreover, they suggest that the limited distribution of P. lentiscus in the cold areas of Mediterranean climate could be more determined by the long extent of the phenological activity of the crown than by its frost tolerance during winter.     

A second case of genetic host races in Rhagoletis? A population genetic comparison of sympatric host populations in the European cherry fruit fly,Rhagoletis cerasi

Despite an increasing acceptance in the biological community for sympatric speciation as a mode of species formation, well documented examples of sympatrically evolved ‘incipient species’ remain rare. The sympatric host races of apple maggot, Rhagoletis pomonella (Walsh), represent one of the most prominent case studies for sympatric speciation via a host shift. The European cherry fruit fly, R. cerasi (L.), shows strong ecological similarities to R. pomonella: (1) infestation of two different host plants, Lonicera xylosteum L. and Prunus avium L., and (2) divergent phenological and behavioral adaptations of flies on different hosts. The population genetic study presented here addresses whether the host associated populations of R. cerasi also represent genetically differentiated true host races. Out of a total of 29 allozyme loci examined, six were polymorphic and used to analyze six sympatric pairs of R. cerasi populations on Lonicera and Prunus from Switzerland and Germany. A direct comparison of allele frequencies between sympatric sites showed no pattern indicative of host races in R. cerasi. However, the hierarchical F‐statistic for one locus, mannose 6‐phosphate isomerase (Mpi), showed significant population differentiation that was in accordance with host race differentiation. Mpi is one of several loci that are also diagnostic for host race differentiation in R. pomonella. Results from Mpi suggest the formation of sympatric host races in R. cerasi, but additional polymorphic markers are necessary.