Evolutionary ecology of the bark beetles Ips typographus and Pityogenes chalcographus.

Ips typographus (L.) and Pityogenes chalcographus (L.) (Coleoptera: Curculionidae) are two common bark beetle species on Norway spruce in Eurasia. Multiple biotic and abiotic factors affect the life cycles of these two beetles, shaping their ecology and evolution. In this article, we provide a comprehensive and comparative summary of selected life-history traits. We highlight similarities and differences in biotic factors, like host range, interspecific competition, host colonization, reproductive behaviour and fungal symbioses. Moreover, we focus on the species' responses to abiotic factors and compare their temperature-dependent development and flight behaviour, cold adaptations and diapause strategies. Differences in biotic and abiotic traits might be the result of recent, species-specific evolutionary histories, particularly during the Pleistocene, with differences in glacial survival and postglacial recolonization. Finally, we discuss future research directions to understand ecological and evolutionary pathways of the two bark beetle species, for both basic research and applied forest management.

Testing the potential contribution of Wolbachia to speciation when cytoplasmic incompatibility becomes associated with host-related reproductive isolation.

Endosymbiont-induced cytoplasmic incompatibility (CI) may play an important role in arthropod speciation. However, whether CI consistently becomes associated or coupled with other host-related forms of reproductive isolation (RI) to impede the transfer of endosymbionts between hybridizing populations and further the divergence process remains an open question. Here, we show that varying degrees of pre- and postmating RI exist among allopatric populations of two interbreeding cherry-infesting tephritid fruit flies (Rhagoletis cingulata and R. indifferens) across North America. These flies display allochronic and sexual isolation among populations, as well as unidirectional reductions in egg hatch in hybrid crosses involving southwestern USA males. All populations are infected by a Wolbachia strain, wCin2, whereas a second strain, wCin3, only co-infects flies from the southwest USA and Mexico. Strain wCin3 is associated with a unique mitochondrial DNA haplotype and unidirectional postmating RI, implicating the strain as the cause of CI. When coupled with nonendosymbiont RI barriers, we estimate the strength of CI associated with wCin3 would not prevent the strain from introgressing from infected southwestern to uninfected populations elsewhere in the USA if populations were to come into secondary contact and hybridize. In contrast, cytoplasmic-nuclear coupling may impede the transfer of wCin3 if Mexican and USA populations were to come into contact. We discuss our results in the context of the general paucity of examples demonstrating stable Wolbachia hybrid zones and whether the spread of Wolbachia among taxa can be constrained in natural hybrid zones long enough for the endosymbiont to participate in speciation.

Comparative genome sequencing reveals insights into the dynamics of Wolbachia in native and invasive cherry fruit flies.

Wolbachia is a maternally inherited obligate endosymbiont that can induce a wide spectrum of effects in its host, ranging from mutualism to reproductive parasitism. At the genomic level, recombination within and between strains, transposable elements, and horizontal transfer of strains between host species make Wolbachia an evolutionarily dynamic bacterial system. The invasive cherry fruit fly Rhagoletis cingulata arrived in Europe from North America ~40 years ago, where it now co-occurs with the native cherry pest R. cerasi. This shared distribution has been proposed to have led to the horizontal transfer of different Wolbachia strains between the two species. To better understand transmission dynamics, we performed a comparative genome study of the strain wCin2 in its native United States and invasive European populations of R. cingulata with wCer2 in European R. cerasi. Previous multilocus sequence genotyping (MLST) of six genes implied that the source of wCer2 in R. cerasi was wCin2 from R. cingulata. However, we report genomic evidence discounting the recent horizontal transfer hypothesis for the origin of wCer2. Despite near identical sequences for the MLST markers, substantial sequence differences for other loci were found between wCer2 and wCin2, as well as structural rearrangements, and differences in prophage, repetitive element, gene content, and cytoplasmic incompatibility inducing genes. Our study highlights the need for whole-genome sequencing rather than relying on MLST markers for resolving Wolbachia strains and assessing their evolutionary dynamics.

Host plant-related genomic differentiation in the European cherry fruit fly, Rhagoletis cerasi.

Elucidating the mechanisms and conditions facilitating the formation of biodiversity are central topics in evolutionary biology. A growing number of studies imply that divergent ecological selection may often play a critical role in speciation by counteracting the homogenising effects of gene flow. Several examples involve phytophagous insects, where divergent selection pressures associated with host plant shifts may generate reproductive isolation, promoting speciation. Here, we use ddRADseq to assess the population structure and to test for host-related genomic differentiation in the European cherry fruit fly, Rhagoletis cerasi (L., 1758) (Diptera: Tephritidae). This tephritid is distributed throughout Europe and western Asia, and has adapted to two different genera of host plants, Prunus spp. (cherries) and Lonicera spp. (honeysuckle). Our data imply that geographic distance and geomorphic barriers serve as the primary factors shaping genetic population structure across the species range. Locally, however, flies genetically cluster according to host plant, with consistent allele frequency differences displayed by a subset of loci between Prunus and Lonicera flies across four sites surveyed in Germany and Norway. These 17 loci display significantly higher FST values between host plants than others. They also showed high levels of linkage disequilibrium within and between Prunus and Lonicera flies, supporting host-related selection and reduced gene flow. Our findings support the existence of sympatric host races in R. cerasi embedded within broader patterns of geographic variation in the fly, similar to the related apple maggot, Rhagoletis pomonella, in North America.

Turning one into five: Integrative taxonomy uncovers complex evolution of cryptic species in the harvester ant Messor "structor".

Seed harvesting ants are ecosystem engineers that shape vegetation, nutrient cycles, and microclimate. Progress in ecological research is, however, slowed down by poor species delimitation. For example, it has not been resolved to date, how many species the European harvester ant Messor "structor" (Latreille, 1798) represents. Since its first description, splitting into additional taxa was often proposed but not accepted later on due to inconsistent support from morphology and ecology. Here, we took an iterative integrative-taxonomy approach - comparing multiple, independent data sets of the same sample - and used traditional morphometrics, Wolbachia symbionts, mitochondrial DNA, amplified fragment length polymorphism, and ecological niche modelling. Using the complementarity of the data sets applied, we resolved multiple, strong disagreements over the number of species, ranging from four to ten, and the allocation of individuals to species. We consider most plausible a five-species hypothesis and conclude the taxonomic odyssey by redescribing Messor structor, M. ibericus Santschi, 1925, and M. muticus (Nylander, 1849) stat.rev., and by describing two new species, M. ponticus sp.n. and M. mcarthuri sp.n. The evolutionary explanations invoked in resolving the various data conflicts include pronounced morphological crypsis, incomplete lineage-sorting or ongoing cospeciation of endosymbionts, and peripatric speciation - these ants' significance to evolutionary biology parallels that to ecology. The successful solution of this particular problem illustrates the usefulness of the integrative approach to other systematic problems of comparable complexity and the importance of understanding evolution to drawing correct conclusions on species' attributes, including their ecology and biogeography.

Spatial spread of Wolbachia in Rhagoletis cerasi populations.

The bacterial endosymbiont Wolbachia has been used to control insect pests owing to its ability to manipulate their life history and suppress infectious diseases. Therefore, knowledge on Wolbachia dynamics in natural populations is fundamental. The European cherry fruit fly, Rhagoletis cerasi, is infected with the Wolbachia strain wCer2, mainly present in southern and central European populations, and is currently spreading into wCer2-uninfected populations driven by high unidirectional cytoplasmic incompatibility. Here, we describe the distribution of wCer2 along two transition zones where the infection is spreading into wCer2-uninfected R. cerasi populations. Fine-scale sampling of 19 populations in the Czech Republic showed a smooth decrease of wCer2 frequency from south to north within a distance of less than 20 km. Sampling of 12 Hungarian populations, however, showed a sharp decline of wCer2 infection frequency within a few kilometres. We fitted a standard wave equation to our empirical data and estimated a Wolbachia wave speed of 1.9 km yr-1 in the Czech Republic and 1.0 km yr-1 in Hungary. Considering the univoltine life cycle and limited dispersal ability of R. cerasi, our study highlights a rapid Wolbachia spread in natural host populations.

Reproductive Manipulators in the Bark Beetle Pityogenes chalcographus (Coleoptera: Curculionidae)-The Role of Cardinium, Rickettsia, Spiroplasma, and Wolbachia.

Heritable bacterial endosymbionts can alter the biology of numerous arthropods. They can influence the reproductive outcome of infected hosts, thus affecting the ecology and evolution of various arthropod species. The spruce bark beetle Pityogenes chalcographus (L.) (Coleoptera: Curculionidae: Scolytinae) was reported to express partial, unidirectional crossing incompatibilities among certain European populations. Knowledge on the background of these findings is lacking; however, bacterial endosymbionts have been assumed to manipulate the reproduction of this beetle. Previous work reported low-density and low-frequency Wolbachia infections of P. chalcographus but found it unlikely that this infection results in reproductive alterations. The aim of this study was to test the hypothesis of an endosymbiont-driven incompatibility, other than Wolbachia, reflected by an infection pattern on a wide geographic scale. We performed a polymerase chain reaction (PCR) screening of 226 individuals from 18 European populations for the presence of the endosymbionts Cardinium, Rickettsia, and Spiroplasma, and additionally screened these individuals for Wolbachia. Positive PCR products were sequenced to characterize these bacteria. Our study shows a low prevalence of these four endosymbionts in P. chalcographus. We detected a yet undescribed Spiroplasma strain in a single individual from Greece. This is the first time that this endosymbiont has been found in a bark beetle. Further, Wolbachia was detected in three beetles from two Scandinavian populations and two new Wolbachia strains were described. None of the individuals analyzed were infected with Cardinium and Rickettsia. The low prevalence of bacteria found here does not support the hypothesis of an endosymbiont-driven reproductive incompatibility in P. chalcographus.

The hitchhiker's guide to Europe: the infection dynamics of an ongoing Wolbachia invasion and mitochondrial selective sweep in Rhagoletis cerasi.

Wolbachia is a maternally inherited and ubiquitous endosymbiont of insects. It can hijack host reproduction by manipulations such as cytoplasmic incompatibility (CI) to enhance vertical transmission. Horizontal transmission of Wolbachia can also result in the colonization of new mitochondrial lineages. In this study, we present a 15-year-long survey of Wolbachia in the cherry fruit fly Rhagoletis cerasi across Europe and the spatiotemporal distribution of two prevalent strains, wCer1 and wCer2, and associated mitochondrial haplotypes in Germany. Across most of Europe, populations consisted of either 100% singly (wCer1) infected individuals with haplotype HT1, or 100% doubly (wCer1&2) infected individuals with haplotype HT2, differentiated only by a single nucleotide polymorphism. In central Germany, singly infected populations were surrounded by transitional populations, consisting of both singly and doubly infected individuals, sandwiched between populations fixed for wCer1&2. Populations with fixed infection status showed perfect association of infection and mitochondria, suggesting a recent CI-driven selective sweep of wCer2 linked with HT2. Spatial analysis revealed a range expansion for wCer2 and a large transition zone in which wCer2 splashes appeared to coalesce into doubly infected populations. Unexpectedly, the transition zone contained a large proportion (22%) of wCer1&2 individuals with HT1, suggesting frequent intraspecific horizontal transmission. However, this horizontal transmission did not break the strict association between infection types and haplotypes in populations outside the transition zone, suggesting that this horizontally acquired Wolbachia infection may be transient. Our study provides new insights into the rarely studied Wolbachia invasion dynamics in field populations.

Genetic Analyses Suggest Separate Introductions of the Pine Pathogen Lecanosticta acicola Into Europe.

Lecanosticta acicola is a heterothallic ascomycete that causes brown spot needle blight on native and nonnative Pinus spp. in many regions of the world. In this study we investigated the origin of European L. acicola populations and estimated the level of random mating of the pathogen in affected areas. Part of the elongation factor 1-α gene was sequenced, 11 microsatellite regions were screened, and the mating type idiomorphs were determined for 201 isolates of L. acicola collected from three continents and 17 host species. The isolates from Mexico and Guatemala were unique, highly diverse and could represent cryptic species of Lecanosticta. The isolates from East Asia formed a uniform and discrete group. Two distinct populations were identified in both North America and Europe. Approximate Bayesian computation analyses strongly suggest independent introductions of two populations from North America into Europe. Microsatellite data and mating type distributions indicated random recombination in the populations of North America and Europe. Its intercontinental introduction can most likely be explained as a consequence of the movement of infected plant material. In contrast, the spread of L. acicola within Europe appears to be primarily due to conidial dispersion and probably also ascospore dissemination.

Rehabilitation and release of orphaned Eurasian lynx (Lynx lynx) in Europe: Implications for management and conservation.

Rehabilitation of injured or immature individuals has become an increasingly used conservation and management tool. However, scientific evaluation of rehabilitations is rare, raising concern about post-release welfare as well as the cost-effectiveness of spending scarce financial resources. Over the past 20 years, events of juvenile Eurasian lynx presumably orphaned have been observed in many European lynx populations. To guide the management of orphaned lynx, we documented survival, rehabilitation and fate after the release and evaluated the potential relevance of lynx orphan rehabilitation for population management and conservation implications. Data on 320 orphaned lynx was collected from 1975 to 2022 from 13 countries and nine populations. The majority of orphaned lynx (55%) were taken to rehabilitation centres or other enclosures. A total of 66 orphans were released back to nature. The portion of rehabilitated lynx who survived at least one year after release was 0.66. Release location was the best predictor for their survival. Of the 66 released lynx, ten have reproduced at least once (8 females and 2 males). Conservation implications of rehabilitation programmes include managing genetic diversity in small, isolated populations and reintroducing species to historical habitats. The lynx is a perfect model species as most reintroduced populations in Central Europe show significantly lower observed heterozygosity than most of the autochthonous populations, indicating that reintroduction bottlenecks, isolation and post-release management have long-term consequences on the genetic composition of populations. The release of translocated orphans could be a valuable contribution to Eurasian lynx conservation in Europe. It is recommended to release orphans at the distribution edge or in the frame of reintroduction projects instead of a release in the core area of a population where it is not necessary from a demographic and genetic point of view. Rehabilitation programmes can have conservation implications that extend far beyond individual welfare benefits.

Divergent evolutionary histories of two sympatric spruce bark beetle species.

Ips typographus and Pityogenes chalcographus are two sympatric Palearctic bark beetle species with wide distribution ranges. As both species are comparable in biology, life history, and habitat, including sharing the same host, Picea abies, they provide excellent models for applying a comparative approach in which to identify common historical patterns of population differentiation and the influence of species-specific ecological characteristics. We analysed patterns of genetic diversity, genetic structure and demographic history of ten I. typographus and P. chalcographus populations co-distributed across Europe using both COI and ITS2 markers. Rather than similarities, our results revealed striking differences. Ips typographus was characterised by low genetic diversity, shallow population structure and strong evidence that all extant haplogroups arose via a single Holocene population expansion event. In contrast, genetic variation and structuring were high in P. chalcographus indicating a longer and more complex evolutionary history. This was estimated to be five times older than I. typographus, beginning during the last Pleistocene glacial maximum over 100 000 years ago. Although the expansions of P. chalcographus haplogroups also date to the Holocene or just prior to its onset, we show that these occurred from at least three geographically separated glacial refugia. Overall, these results suggest that the much longer evolutionary history of P. chalcographus greatly influenced the levels of phylogeographic subdivision among lineages and may have led to the evolution of different life-history traits which in turn have affected genetic structure and resulted in an advantage over the more aggressive I. typographus.

Evidence for a recent horizontal transmission and spatial spread of Wolbachia from endemic Rhagoletis cerasi (Diptera: Tephritidae) to invasive Rhagoletis cingulata in Europe.

The widespread occurrence of Wolbachia in arthropods and nematodes suggests that this intracellular, maternally inherited endosymbiont has the ability to cross species boundaries. However, direct evidence for such a horizontal transmission of Wolbachia in nature is scarce. Here, we compare the well-characterized Wolbachia infection of the European cherry fruit fly, Rhagoletis cerasi, with that of the North American eastern cherry fruit fly, Rhagoletis cingulata, recently introduced to Europe. Molecular genetic analysis of Wolbachia based on multilocus sequence typing and the Wolbachia surface protein wsp showed that all R. cingulata individuals are infected with wCin2 identical to wCer2 in R. cerasi. In contrast, wCin1, a strain identical to wCer1 in R. cerasi, was present in several European populations of R. cingulata, but not in any individual from the United States. Surveys of R. cingulata from Germany and Hungary indicated that in some populations, the frequency of wCin1 increased significantly in just a few years with at least two independent horizontal transmission events. This is corroborated by the analysis of the mitochondrial cytochrome oxidase II gene that showed association of wCin1 with two distinct haplotypes in Germany, one of which is also infected with wCin1 in Hungary. In summary, our study provides strong evidence for a very recent inter-specific Wolbachia transmission with a subsequent spatial spread in field populations.

Effect of Temperature and Photoperiod on Development, Survival, and Growth Rate of Mealworms, Tenebrio molitor.

Insects are a potential substitute for conventional meat and can be part of a sustainable human diet due to their valuable nutrients and relatively low environmental production impact. One species that is already produced for human consumption and livestock feed is the mealworm, i.e., larvae of Tenebrio molitor. Knowledge of the effects of temperature, and particularly photoperiod, on mealworm development is scarce, but crucial for the improvement of rearing. Therefore, the effects of three temperatures (20 °C, 25 °C, and 30 °C), in combination with three photoperiods (long-day-16 h:8 h light:dark; short-day-8 h:16 h light:dark, and constant darkness) on mealworm survival, developmental time, and growth rate were tested. We describe a significant effect of temperature on survival rate, developmental time, and growth rate. Furthermore, significant effects of photoperiod on developmental time and growth rate were found. At 25 and 30 °C and constant darkness, the highest survival and growth rate, along with the shortest developmental time, were observed. Our data can be used to improve the mass rearing of mealworms for an efficient production of food and feed.

Facultative and obligate diapause phenotypes in populations of the European spruce bark beetle Ips typographus.

The bark beetle Ips typographus is the most destructive insect pest in Norway spruce-dominated forests. Its potential to establish multiple generations per year (multivoltinism) is one major trait that makes this beetle a severe pest. Ips typographus enters diapause to adjust its life cycle to seasonally changing environments. Diapause is characterized by developmental and reproductive arrest; it prolongs generation time and thus affects voltinism. In I. typographus a facultative, photoperiod-regulated diapause in the adult stage has been described. In addition, the presence of an obligate, photoperiod-independent, diapause has been hypothesized. The diapause phenotype has important implications for I. typographus voltinism, as populations with obligate diapausing individuals would be univoltine. To test for the presence of different I. typographus diapause phenotypes, we exposed Central and Northern European individuals to a set of photoperiodic treatments. We used two ovarian traits (egg number and vitellarium size) that are associated with gonad development, to infer reproductive arrest and thus diapause. We found a distinct effect of photoperiod on ovarian development, with variable responses in Central and Northern European beetles. We observed obligate diapausing (independent of photoperiod) individuals in Northern Europe, and both facultative (photoperiod-regulated) as well as obligate diapausing individuals in Central Europe. Our results show within-species variation for diapause induction, an adaptation to match life cycles with seasonally fluctuating environmental conditions. As the diapause phenotype affects the potential number of generations per season, our data are the basis for assessing the risk of outbreaks of this destructive bark beetle. SUPPLEMENTARY INFORMATION: The online version contains supplementary material available at 10.1007/s10340-021-01416-w.

Assessment of drivers of spatial genetic variation of a ground-dwelling bird species and its implications for conservation.

In modern wildlife ecology, spatial population genetic methods are becoming increasingly applied. Especially for animal species in fragmented landscapes, preservation of gene flow becomes a high priority target in order to restore genetic diversity and prevent local extinction. Within Central Europe, the Alps represent the core distribution area of the black grouse, Lyrurus tetrix. At its easternmost Alpine range, events of subpopulation extinction have already been documented in the past decades. Molecular data combined with spatial analyses can help to assess landscape effects on genetic variation and therefore can be informative for conservation management. Here, we addressed whether the genetic pattern of the easternmost Alpine black grouse metapopulation system is driven by isolation by distance or isolation by resistance. Correlative ecological niche modeling was used to assess geographic distances and landscape resistances. We then applied regression-based approaches combined with population genetic analyses based on microsatellite data to disentangle effects of isolation by distance and isolation by resistance among individuals and subpopulations. Although population genetic analyses revealed overall low levels of genetic differentiation, the ecological niche modeling showed subpopulations to be clearly delimited by habitat structures. Spatial genetic variation could be attributed to effects of isolation by distance among individuals and isolation by resistance among subpopulations, yet unknown effects might factor in. The easternmost subpopulation was the most differentiated, and at the same time, immigration was not detected; hence, its long-term survival might be threatened. Our study provides valuable insights into the spatial genetic variation of this small-scale metapopulation system of Alpine black grouse.

Specificity and transmission mosaic of ant nest-wall fungi.

Mutualism, whereby species interact to their mutual benefit, is extraordinary in a competitive world. To recognize general patterns of origin and maintenance from the plethora of mutualistic associations proves a persisting challenge. The simplest situation is believed to be that of a single mutualist specific to a single host, vertically transmitted from one host generation to the next. We characterized ascomycete fungal associates cultured for nest architecture by the ant subgenera Dendrolasius and Chthonolasius. The ants probably manage their fungal mutualists by protecting them against fungal competitors. The ant subgenera display different ant-to-fungus specificity patterns, one-to-two and many-to-one, and we infer vertical transmission, in the latter case overlaid by horizontal transmission. Possible evolutionary trajectories include a reversal from fungiculture by other Lasius subgenera and inheritance of fungi through life cycle interactions of the ant subgenera. The mosaic indicates how specificity patterns can be shaped by an interplay between host life-cycles and transmission adaptations.

Integrative taxonomy: a multisource approach to exploring biodiversity.

Good alpha taxonomy is central to biology. On the basis of a survey of arthropod studies that used multiple disciplines for species delimitation, we evaluated the performance of single disciplines. All included disciplines had a considerable failure rate. Rigor in species delimitation can thus be increased when several disciplines chosen for complementarity are used. We present a flexible procedure and stopping rule for integrative taxonomy that uses the information from different disciplines separately. Disagreement among disciplines over the number and demarcation of species is resolved by elucidating and invoking evolutionary explanations for disagreement. With the identification of further promising study organisms and of new questions for in-depth analysis, evolutionary biology should profit from integrative taxonomy. An important rationale is clarity in researcher bias in the decision-making process. The success of integrative taxonomy will further increase through methodological progress, taxonomic training of evolutionary biologists, and balanced resource allocation.

Enterobacter sp. AA26 as a Protein Source in the Larval Diet of Drosophila suzukii.

The Spotted-Wing Drosophila fly, Drosophila suzukii, is an invasive pest species infesting major agricultural soft fruits. Drosophila suzukii management is currently based on insecticide applications that bear major concerns regarding their efficiency, safety and environmental sustainability. The sterile insect technique (SIT) is an efficient and friendly to the environment pest control method that has been suggested for the D. suzukii population control. Successful SIT applications require mass-rearing of the strain to produce competitive and of high biological quality males that will be sterilized and consequently released in the wild. Recent studies have suggested that insect gut symbionts can be used as a protein source for Ceratitis capitata larval diet and replace the expensive brewer's yeast. In this study, we exploited Enterobacter sp. AA26 as partial and full replacement of inactive brewer's yeast in the D. suzukii larval diet and assessed several fitness parameters. Enterobacter sp. AA26 dry biomass proved to be an inadequate nutritional source in the absence of brewer's yeast and resulted in significant decrease in pupal weight, survival under food and water starvation, fecundity, and adult recovery.

Abandoning aggression but maintaining self-nonself discrimination as a first stage in ant supercolony formation.

An ant supercolony is a very large entity with very many queens. Although normal colonies of small extent and few queens remain distinct, a supercolony is integrated harmoniously over a large area [1, 2]. The lack of aggression is advantageous: Aggression is costly, involving direct and indirect losses and recognition errors [3, 4]. Indeed, supercolonial ants are among the ecologically most successful organisms [5-7]. But how supercolonies arise remains mysterious [1, 2, 8]. Suggestions include that reduced within-colony relatedness or reduced self-nonself discrimination would foster supercolony formation [1, 2, 5, 7, 9-12]. However, one risks confusing correlation and causality in deducing the evolution from distinct colonies to supercolonies when observing established supercolonies. It might help to follow up observations of another lack of aggression, that between single-queened colonies in some ant species. We show that the single-queened Lasius austriacus lacks aggression between colonies and occasionally integrates workers across colonies but maintains high within-colony relatedness and self-nonself discrimination. Provided that the ecological framework permits, reduced aggression might prove adaptive for any ant colony irrespective of within-colony relatedness. Abandoning aggression while maintaining discrimination might be a first stage in supercolony formation. This adds to the emphasis of ecology as central to the evolution of cooperation in general [13].