Deciphering the worldwide invasion of the Asian long-horned beetle: A recurrent invasion process from the native area together with a bridgehead effect

Retracing introduction routes is crucial for understanding the evolutionary processes involved in an invasion, as well as for highlighting the invasion history of a species at the global scale. The Asian long‐horned beetle (ALB) Anoplophora glabripennis is a xylophagous pest native to Asia and invasive in North America and Europe. It is responsible for severe losses of urban trees, in both its native and invaded ranges. Based on historical and genetic data, several hypotheses have been formulated concerning its invasion history, including the possibility of multiple introductions from the native zone and secondary dispersal within the invaded areas, but none have been formally tested. In this study, we characterized the genetic structure of ALB in both its native and invaded ranges using microsatellites. In order to test different invasion scenarios, we used an approximate Bayesian “random forest” algorithm together with traditional population genetics approaches. The strong population differentiation observed in the native area was not geographically structured, suggesting complex migration events that were probably human‐mediated. Both native and invasive populations had low genetic diversity, but this characteristic did not prevent the success of the ALB invasions. Our results highlight the complexity of invasion pathways for insect pests. Specifically, our findings indicate that invasive species might be repeatedly introduced from their native range, and they emphasize the importance of multiple, human‐mediated introductions in successful invasions. Finally, our results demonstrate that invasive species can spread across continents following a bridgehead path, in which an invasive population may have acted as a source for another invasion

Traçage des routes d’invasion d’un insecte ravageur polyphage : le cas du capricorne asiatique Anoplophora glabripennis

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Comparative assessment of SSR and SNP markers for inferring the population genetic structure of the common fungus Armillaria cepistipes

During the last years, simple sequence repeats (SSRs, also known as microsatellites) and single-nucleotide polymorphisms (SNPs) have become the most popular molecular markers for describing neutral genetic variation in populations of a wide range of organisms. However, only a limited number of studies has focused on comparing the performance of these two types of markers for describing the underlying genetic structure of wild populations. Moreover, none of these studies targeted fungi, the group of organisms with one of the most complex reproductive strategies. We evaluated the utility of SSRs and SNPs for inferring the neutral genetic structure of Armillaria cepistipes (basidiomycetes) at different spatial scales. For that, 407 samples were collected across a small (150 km(2)) area in the Ukrainian Carpathians and a large (41 000 km(2)) area in the Swiss Alps. All isolates were analyzed at 17 SSR loci distributed throughout the whole genome and at 24 SNP loci located in different single-copy conserved genes. The two markers showed different patterns of structure within the two spatial scales studied. The multi-allelic SSR markers seemed to be best suited for detecting genetic structure in indigenous fungal populations at a rather small spatial scale (radius of ~50-100 km). The pattern observed at SNP markers rather reflected ancient divergence of distant (~1000 km) populations that in addition are separated by mountain ranges. Despite these differences, both marker types were suitable for detecting the weak genetic structure of the two A. cepistipes populations investigated

An easy and robust method for isolation and validation of single-nucleotide polymorphic markers from a first Erysiphe alphitoides draft genome

Isolating genetic markers is often costly and time-consuming for non-model fungal species. However, these markers are of primary importance to identify the origin of invasive species and to infer their reproductive mode and dispersal ability. We slightly modified a recent molecular method to quickly isolate and validate single-nucleotide polymorphism (SNP) markers, from a first Erysiphe alphitoides draft genome, one of the main causal agent of oak powdery mildew in Europe. Although the draft assembly was strongly fragmented (555,289 contigs), we successfully isolated 1700 SNPs from 75 single-copy genes conserved in most fungal genomes. Ninety percent of them allowed to clearly distinguish the two main Erysiphe species reported on European oaks: E. alphitoides and E. quercicola. Thirty-six SNPs, located in distinct genes, were then validated using a strategy of MassArray genotyping on 95 E. alphitoides isolates sampled in Europe. This genotyping showed that only monospore isolates had the expected haploid signature, whereas direct genotyping from field leaves showed signature of mixed infection. Considering haploid isolates, these markers led to the first results of population genetic diversity, and suggested that E. quercicola may have a more asexual reproduction than its sister species, E. alphitoides