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- Senckenbergische Naturforschende Gesellschaft (25) (remove)
The Asian bush mosquito (Aedes japonicus japonicus, Theobald 1901) is an invasive culicid species which originates in Asia but is nowadays present in northern America and Europe. It is a competent vector for several human disease pathogens. In addition to the public health threat, this invasive species may also be an ecological threat for native container-breeding mosquitoes which share a similar larval habitat. Therefore, it is of importance to gain knowledge on ecological and eco-toxicological features of the Asian bush mosquito. However, optimal laboratory feeding conditions have not yet been established. Standardized feeding methods will be needed in assessing the impact of insecticides or competitional strength of this species. To fill this gap, we performed experiments on food quality and quantity for Ae. j. japonicus larvae. We found out that the commercial fish food TetraMin (Tetra, Melle, Germany) in a dose of 10 mg per larva is the most suitable food tested. We also suggest a protocol with a feeding sequence of seven portions for all larval stages of this species.
Active species reintroduction is an important conservation tool when aiming for the restoration of biological communities and ecosystems. The effective monitoring of reintroduction success is a crucial factor in this process. Here, we used a combination of environmental DNA (eDNA) techniques and species distribution models (SDMs) to evaluate the success of recent reintroductions of the freshwater fish Alburnoides bipunctatus in central Germany. We built SDMs without and with eDNA presence data to locate further suitable reintroduction sites and potentially overlooked populations of the species. We successfully detected eDNA of A. bipunctatus at all reintroduction sites, as well as several adjacent sites mostly in downstream direction, which supports the success of reintroduction efforts. eDNA‐based species detection considerably improved SDMs for A. bipunctatus, which allowed to identify species presence in previously unknown localities. Our results confirm the usefulness of eDNA techniques as standard tool to monitor reintroduced fish populations. We propose that combining eDNA with SDMs is a highly effective approach for long‐term monitoring of reintroduction success in aquatic species.
Bears are iconic mammals with a complex evolutionary history. Natural bear hybrids and studies of few nuclear genes indicate that gene flow among bears may be more common than expected and not limited to polar and brown bears. Here we present a genome analysis of the bear family with representatives of all living species. Phylogenomic analyses of 869 mega base pairs divided into 18,621 genome fragments yielded a well-resolved coalescent species tree despite signals for extensive gene flow across species. However, genome analyses using different statistical methods show that gene flow is not limited to closely related species pairs. Strong ancestral gene flow between the Asiatic black bear and the ancestor to polar, brown and American black bear explains uncertainties in reconstructing the bear phylogeny. Gene flow across the bear clade may be mediated by intermediate species such as the geographically wide-spread brown bears leading to large amounts of phylogenetic conflict. Genome-scale analyses lead to a more complete understanding of complex evolutionary processes. Evidence for extensive inter-specific gene flow, found also in other animal species, necessitates shifting the attention from speciation processes achieving genome-wide reproductive isolation to the selective processes that maintain species divergence in the face of gene flow.
Background: The invasive temperate mosquito Aedes japonicus japonicus is a potential vector for various infectious diseases and therefore a target of vector control measures. Even though established in Germany, it is unclear whether the species has already reached its full distribution potential. The possible range of the species, its annual population dynamics, the success of vector control measures and future expansions due to climate change still remain poorly understood. While numerous studies on occurrence have been conducted, they used mainly presence data from relatively few locations. In contrast, we used experimental life history data to model the dynamics of a continuous stage-structured population to infer potential seasonal densities and ask whether stable populations are likely to establish over a period of more than one year. In addition, we used climate change models to infer future ranges. Finally, we evaluated the effectiveness of various stage-specific vector control measures.
Results: Aedes j. japonicus has already established stable populations in the southwest and west of Germany. Our models predict a spread of Ae. j. japonicus beyond the currently observed range, but likely not much further eastwards under current climatic conditions. Climate change models, however, will expand this range substantially and higher annual densities can be expected. Applying vector control measures to oviposition, survival of eggs, larvae or adults showed that application of adulticides for 30 days between late spring and early autumn, while ambient temperatures are above 9 °C, can reduce population density by 75%. Continuous application of larvicide showed similar results in population reduction. Most importantly, we showed that with the consequent application of a mixed strategy, it should be possible to significantly reduce or even extinguish existing populations with reasonable effort.
Conclusion: Our study provides valuable insights into the mechanisms concerning the establishment of stable populations in invasive species. In order to minimise the hazard to public health, we recommend vector control measures to be applied in ‘high risk areas’ which are predicted to allow establishment of stable populations to establish.
Background: As ectothermic animals, temperature influences insects in almost every aspect. The potential disease spreading Asian bush mosquito (Aedes japonicus japonicus) is native to temperate East Asia but invasive in several parts of the world. We report on the previously poorly understood temperature-dependence of its life history under laboratory conditions to understand invasion processes and to model temperature niches.
Results: To evaluate winter survival, eggs were exposed between 1 day and 14 days to low temperatures (5 °C, 0 °C, -5 °C and -9 °C). Hatching success was drastically decreased after exposure to 0 °C and -5 °C, and the minimal hatching success of 0% was reached at -9 °C after two days. We then exposed larvae to 14 temperatures and assessed their life trait parameters. Larval survival to adulthood was only possible between 10 °C and 31 °C. Based on this, we modelled the optimal (25 °C), minimal (7 °C) and maximal (31 °C) temperature for cumulative female survival. The time to adult emergence ranges from 12 days to 58 days depending on temperature. We used an age-at-emergence-temperature model to calculate the number of potential generations per year for the Asian bush mosquito in Germany with an average of 4.72 potential generations. At lower temperatures, individuals grew larger than at higher temperatures with female R1 length ranging from 3.04 ± 0.1 mm at 31 °C to 4.26 ± 0.2 mm at 15 °C.
Conclusions: Reduced egg hatch after exposure to sub-zero temperatures prohibits the establishment of the Asian bush mosquito in large parts of Germany. Larval overwintering is not possible at temperature ≤ 5 °C. The many potential generations displayed per year may contribute to the species’ invasion success. This study on the thermal ecology of the Asian bush mosquito adds to our knowledge on the temperature dependence of the species and data could be incorporated in epidemiological and population dynamic modelling.
Background: While research on the impact of global climate change (GCC) on ecosystems and species is flourishing, a fundamental component of biodiversity -- molecular variation -- has not yet received its due attention in such studies. Here we present a methodological framework for projecting the loss of intraspecific genetic diversity due to GCC.
Methods: The framework consists of multiple steps that and combines 1) hierarchical genetic clustering methods to define comparable units of inference, 2) species accumulation curves (SAC) to infer sampling completeness, and 3) species distribution modelling (SDM) to project the genetic diversity loss under GCC. We suggest procedures for existing data sets as well as specifically designed studies. We illustrate the approach with two worked examples from a land snail (Trochulus villosus) and a caddisfly (Smicridea (S.) mucronata).
Results: Sampling completeness was diagnosed on the third most coarse haplotype clade level for T. villosus and the second most coarse for S. mucronata. For both species, a substantial species range loss was projected under the chosen climate scenario. However, despite substantial differences in data set quality concerning spatial sampling and sampling depth, no loss of haplotype clades due to GCC was predicted for either species.
Conclusions: The suggested approach presents a feasible method to tap the rich resources of existing phylogeographic data sets and guide the design and analysis of studies explicitly designed to estimate the impact of GCC on a currently still neglected level of biodiversity.
Abstract
Divergence is mostly viewed as a progressive process often initiated by selection targeting individual loci, ultimately resulting in ever increasing genomic isolation due to linkage. However, recent studies show that this process may stall at intermediate stable equilibrium states without achieving complete genomic isolation. We tested the extent of genomic isolation between two recurrently hybridizing nonbiting midge sister taxa, Chironomus riparius and Chironomus piger, by analyzing the divergence landscape. Using a principal component‐based method, we estimated that only about 28.44% of the genomes were mutually isolated, whereas the rest was still exchanged. The divergence landscape was fragmented into isolated regions of on average 30 kb, distributed throughout the genome. Selection and divergence time strongly influenced lengths of isolated regions, whereas local recombination rate only had minor impact. Comparison of divergence time distributions obtained from several coalescence‐simulated divergence scenarios with the observed divergence time estimates in an approximate Bayesian computation framework favored a short and concluded divergence event in the past. Most divergence happened during a short time span about 4.5 million generations ago, followed by a stable equilibrium between mutual gene flow through ongoing hybridization for the larger part of the genome and isolation in some regions due to rapid purifying selection of introgression, supported by high effective population sizes and recombination rates.
Impact Summary
The process of speciation has fascinated biologists from early on. Prevailing theory suggested that gene flow among populations is the main obstacle for their divergence. Recently, it became clear that speciation with gene flow is possible under certain circumstances. However, it remains unclear how the divergence process proceeds in time, how widespread the phenomenon is, and whether it always and inevitably leads to complete isolation. Comparing the genomes of individuals of two regularly hybridizing sister taxa of nonbiting midges, we could show that they diverged during a short period millions of generations ago. Their divergence process apparently ceased before the entire genome was mutually isolated. The taxa remain distinct since, even though they share most of their genome. Our findings thus extend our view of the nature of species and the temporal dynamics of their divergence and describe novel approaches to analyze both current and past divergence processes.
In the course of global climate change, central Europe is experiencing more frequent and prolonged periods of drought. The drought years 2018 and 2019 affected European beeches (Fagus sylvatica L.) differently: even in the same stand, drought damaged trees neighboured healthy trees, suggesting that the genotype rather than the environment was responsible for this conspicuous pattern. We used this natural experiment to study the genomic basis of drought resistance with Pool-GWAS. Contrasting the extreme phenotypes identified 106 significantly associated SNPs throughout the genome. Most annotated genes with associated SNPs (>70%) were previously implicated in the drought reaction of plants. Non-synonymous substitutions led either to a functional amino acid exchange or premature termination. A SNP-assay with 70 loci allowed predicting drought phenotype in 98.6% of a validation sample of 92 trees. Drought resistance in European beech is a moderately polygenic trait that should respond well to natural selection, selective management, and breeding.
The gradual heterogeneity of climatic factors pose varying selection pressures across geographic distances that leave signatures of clinal variation in the genome. Separating signatures of clinal adaptation from signatures of other evolutionary forces, such as demographic processes, genetic drift, and adaptation to non-clinal conditions of the immediate local environment is a major challenge. Here, we examine climate adaptation in five natural populations of the harlequin fly Chironomus riparius sampled along a climatic gradient across Europe. Our study integrates experimental data, individual genome resequencing, Pool-Seq data, and population genetic modelling. Common-garden experiments revealed a positive correlation of population growth rates corresponding to the population origin along the climate gradient, suggesting thermal adaptation on the phenotypic level. Based on a population genomic analysis, we derived empirical estimates of historical demography and migration. We used an FST outlier approach to infer positive selection across the climate gradient, in combination with an environmental association analysis. In total we identified 162 candidate genes as genomic basis of climate adaptation. Enriched functions among these candidate genes involved the apoptotic process and molecular response to heat, as well as functions identified in other studies of climate adaptation in other insects. Our results show that local climate conditions impose strong selection pressures and lead to genomic adaptation despite strong gene flow. Moreover, these results imply that selection to different climatic conditions seems to converge on a functional level, at least between different insect species.
Genomic basis of ecological niche divergence among cryptic sister species of non-biting midges
(2013)
Background: There is a lack of understanding the evolutionary forces driving niche segregation of closely related organisms. In addition, pinpointing the genes driving ecological divergence is a key goal in molecular ecology. Here, larval transcriptome sequences obtained by next-generation-sequencing are used to address these issues in a morphologically cryptic sister species pair of non-biting midges (Chironomus riparius and C. piger).
Results: More than eight thousand orthologous open reading frames were screened for interspecific divergence and intraspecific polymorphisms. Despite a small mean sequence divergence of 1.53% between the sister species, 25.1% of 18,115 observed amino acid substitutions were inferred by α statistics to be driven by positive selection. Applying McDonald-Kreitman tests to 715 alignments of gene orthologues identified eleven (1.5%) genes driven by positive selection.
Conclusions: Three candidate genes were identified as potentially responsible for the observed niche segregation concerning nitrite concentration, habitat temperature and water conductivity. Additionally, signs of positive selection in the hydrogen sulfide detoxification pathway were detected, providing a new plausible hypothesis for the species’ ecological differentiation. Finally, a divergently selected, nuclear encoded mitochondrial ribosomal protein may contribute to reproductive isolation due to cytonuclear coevolution.