Mechanisms of invasive population establishment and spread of pinewood nematodes in China

This paper summarizes the results of our study of the pinewood nematode (Bursaphelenchus xylophilus). By population genetic analysis, it was determined that there was no genetic bottle caused by the founder effect and genetic drift in the Chinese invasive population. Multiple invasions with large amounts of nematodes from different sources led to rich genetic diversity in the invasive population. Keeping high genetic diversity in the invasive process may be one of the genetic mechanisms in its successful invasion. By testing interspecies competition, it was shown that, with high fecundity and a strong competitive ability, B. xylophilus outcompeted the native species B. mucronatus in the natural ecosystem during the invasion process. Competitive displacement may be one of the ecological mechanisms of B. xylophilus’s invasion. In addition, an unequal interspecific hybridization with introgress was in favor of the invasive species which also accelerated the replacement of B. mucronatus by B. xylophilus. The structures, functions and evolutions of a few important genes that are closely related to the ecological adaptation of pinewood nematodes were studied to explore the molecular mechanism of its ecological adaptations. Further, the resistance and resilience mechanism of the pine ecosystem invaded by pinewood nematodes was also investigated. The results of these studies uncovered a portion of the genetic and ecological mechanisms of PWN’s successful invasion and laid a foundation for further study to obtain a comprehensive interpretation of the mechanisms of the nematode invasion. The results also provided a scientific basis for effectively controlling the occurrence and spread of pine wilt disease which is caused by nematodes. Various aspects requiring further investigation are considered. Supported by the National Key Basic Research and Development Program of China (Grant Nos. 2009CB119200 and 2002CB111400).     

No evolutionary shift in the mating system of north American Ambrosia artemisiifolia (Asteraceae) following its introduction to China

The mating system plays a key role during the process of plant invasion. Contemporary evolution of uniparental reproduction (selfing or asexuality) can relieve the challenges of mate limitation in colonizing populations by providing reproductive assurance. Here we examined aspects of the genetics of colonization in Ambrosia artemisiifolia, a North American native that is invasive in China. This species has been found to possess a strong self-incompatibility system and have high outcrossing rates in North America and we examined whether there has been an evolutionary shift towards the dependence on selfing in the introduced range. Specifically, we estimated outcrossing rates in one native and five invasive populations and compared levels of genetic diversity between North America and China. Based on six microsatellite loci we found that, like the native North American population, all five Chinese populations possessed a completely outcrossing mating system. The estimates of paternity correlations were low, ranging from 0.028-0.122, which suggests that populations possessed ~8-36 pollen donor parents contributing to each maternal plant in the invasive populations. High levels of genetic diversity for both native and invasive populations were found with the unbiased estimate of gene diversity ranging from 0.262-0.289 for both geographic ranges based on AFLP markers. Our results demonstrate that there has been no evolutionary shift from outcrossing to selfing during A. artemisiifolia's invasion of China. Furthermore, high levels of genetic variation in North America and China indicate that there has been no erosion of genetic variance due to a bottleneck during the introduction process. We suggest that the successful invasion of A. artemisiifolia into Asia was facilitated by repeated introductions from multiple source populations in the native range creating a diverse gene pool within Chinese populations.     

Genetic diversity and population structure of invasive and native populations of Erigeron canadensis L.

小蓬草入侵地和原产地种群的遗传多样性和种群结构 外来入侵植物对全球生物多样性造成了危害。小蓬草(Erigeron canadensis L.)是危害最为严重的外来农业杂草之一,代表了洲际入侵的典型例子。本研究利用10个多态性SSR位点,分别对采自中国江苏和浙江省的入侵地和采自美国阿拉巴马州的原产地各5个种群、共计312个植株的基因型进行了遗传多样性和遗传结构分析。结果表明,江苏省和浙江省的入侵种群显示出与阿拉巴马州原产地种群相似的遗传多样性,表明入侵期间没有严重的遗传瓶颈。利用STRUCTURE对种群结构的分析结果显示,种群之间分化较低,在原产地和入侵范围内均只仅检测到两个基因群。在入侵种群中观察到的遗传多样性较高,表明在入侵初期可能存在多次引入或引入了遗传背景不同的繁殖体。上述研究为阐明小蓬草这一全球有害杂草在中国东部的入侵动态提供了新的证据。在防除实践中,应注意防范小蓬草入侵种群和本地种群之间的种子基因流传播,阻止除草剂抗性植株的引入和扩散。     

First Insights into the Genetic Diversity of the Pinewood Nematode in Its Native Area Using New Polymorphic Microsatellite Loci

The pinewood nematode, Bursaphelenchus xylophilus, native to North America, is the causative agent of pine wilt disease and among the most important invasive forest pests in the East-Asian countries, such as Japan and China. Since 1999, it has been found in Europe in the Iberian Peninsula, where it also causes significant damage. In a previous study, 94 pairs of microsatellite primers have been identified in silico in the pinewood nematode genome. In the present study, specific PCR amplifications and polymorphism tests to validate these loci were performed and 17 microsatellite loci that were suitable for routine analysis of B. xylophilus genetic diversity were selected. The polymorphism of these markers was evaluated on nematodes from four field origins and one laboratory collection strain, all originate from the native area. The number of alleles and the expected heterozygosity varied between 2 and 11 and between 0.039 and 0.777, respectively. First insights into the population genetic structure of B. xylophilus were obtained using clustering and multivariate methods on the genotypes obtained from the field samples. The results showed that the pinewood nematode genetic diversity is spatially structured at the scale of the pine tree and probably at larger scales. The role of dispersal by the insect vector versus human activities in shaping this structure is discussed.     

Invasion genetics of <Emphasis Type="Italic">Senecio vulgaris</Emphasis>: loss of genetic diversity characterizes the invasion of a selfing annual,despite multiple introductions

Genetic variation in invasive populations is affected by a variety of processes including stochastic forces, multiple introductions, population dynamics and mating system. Here, we compare genetic diversity between native and invasive populations of the selfing, annual plant Senecio vulgaris to infer the relative importance of genetic bottlenecks, multiple introductions, post-introduction genetic drift and gene flow to genetic diversity in invasive populations. We scored multilocus genotypes at eight microsatellite loci from nine native European and 19 Chinese introduced populations and compared heterozygosity and number of alleles between continents. We inferred possible source populations for introduced populations by performing assignment analyses and evaluated the relative contributions of gene flow and genetic drift to genetic diversity based on correlations of pairwise genetic and geographic distance. Genetic diversity within Chinese populations was significantly reduced compared to European populations indicating genetic bottlenecks accompanying invasion. Assignment tests provided support for multiple introductions with populations from Central China and southwestern China descended from genotypes matching those from Switzerland and the UK, respectively. Genetic differentiation among populations in China and Europe was not correlated with geographic distance. However, European populations exhibited less variation in the relation between G _ST and geographical distance than populations in China. These results suggest that gene flow probably plays a more significant role in structuring genetic diversity in native populations, whereas genetic drift appears to predominate in introduced populations. High rates of selfing in Chinese populations may restrict opportunities for pollen-mediated gene flow. Repeated colonization-extinction cycles associated with ongoing invasion is likely to maintain low genetic diversity in Chinese populations.     

基于线粒体COI基因的桔小实蝇种群遗传分化研究

【目的】推测桔小实蝇Bactrocera dorsalis在中国的扩散路径和新发生地区的入侵来源。【方法】本研究测序获得来自中国、泰国、日本、老挝、孟加拉国和美国等地31个种群的192头桔小实蝇个体的COI序列（1 496 bp,占COI基因全长97.3%）,并以软件DnaSP 5.0, MEGA 6.0和Arlequin 3.5等完成各种群的遗传多样性、种群间的遗传分化以及单倍型分析。【结果】所测31个桔小实蝇种群总体表现出较高水平的核苷酸多样性（0.00663）和高水平的单倍型多样性（0.98069）。以F-统计法度量种群间遗传分化程度, 结果显示中国桔小实蝇地理种群间遗传分化较弱, 中国种群与泰国、日本、老挝、孟加拉国、美国种群间的遗传分化程度不同, 中国种群与美国种群及日本种群的遗传分化最大。而Mantel检验发现,中国、泰国、日本、老挝、孟加拉国和美国种群间的遗传分化与空间距离有关（R=0.670, P<0.0001）, 中国种群间的遗传分化不是地理隔离所造成的（R=0.038, P=0.534）。中国种群与泰国、日本、老挝、孟加拉国和美国种群间不存在共享单倍型。根据种群系统发育树,可将中国原发生地区种群划分为西南、东南两大分支。中性检验和错配分析结果均表明桔小实蝇曾存在大规模的扩张。【结论】桔小实蝇以东南地区和西南地区为源头向中国内陆扩散,其中广东、福建、广西和贵州种群为中国内陆种群较为有影响力的源头。根据遗传多样性、遗传分化、单倍型分析,推测新发生地区桔小实蝇的来源, 例如安徽合肥桔小实蝇种群主要来源于福建长乐、广东珠海和上海。     

基于微卫星标记对中国Q型烟粉虱早期入侵种群与B型烟粉虱种群的遗传结构分析（英文）

2003年首次在云南昆明发现Q型烟粉虱Bemisia tabaci (Gennadius)传入中国。随后几年时间内, 它在许多省份逐年取代了B型烟粉虱种群。2008年后,Q型烟粉虱基本上成为了中国多数省份农区的优势生物型。为了进一步揭示Q型烟粉虱在中国快速扩散以及取代B型烟粉虱的遗传学基础, 本研究利用11个微卫星位点分析并比较了2003年中国云南昆明Q型烟粉虱入侵种群及其他地点的11个B型入侵种群, 西班牙2个Q型土著种群, 以色列1个Q型入侵种群, 以色列1个B型土著种群, 以及西班牙、 美国与澳大利亚的5个B型入侵种群的遗传结构。结果表明, 中国Q型烟粉虱早期种群（云南昆明种群）可能来自于西部地中海地区。中国B型烟粉虱种群遗传多样性高于西班牙、 澳大利亚、 美国B型种群, 中国B型可能存在多次传入或某个混合种群的再次传入。相对于原产地种群, 中国Q型烟粉虱早期入侵种群与B型烟粉虱种群遗传多样性并没有明显降低, 表明Q型与B型烟粉虱种群可能经历了较小的瓶颈效应或奠基者效应。中国Q型烟粉虱早期入侵种群遗传多样性高于B型烟粉虱种群, Q型烟粉虱这种较高的遗传多样性可能为其较强的生态适应性提供了遗传基础, 有利于Q型烟粉虱在新的环境下快速扩散并取代B型烟粉虱。     

Microsatellite variation in the pinewood nematode, Bursaphelenchus xylophilus (Steiner and Buhrer) Nickle in South Korea

The pinewood nematode Bursaphelenchus xylophilus is the causative agent of pine wilt disease, which has caused heavy economic losses to the South Korean forest industry. In this study, we investigated the genetic variation among South Korean pinewood nematodes using newly developed microsatellite loci. In order to ensure sufficient templates for the amplification of multiple loci required for individual identification, we employed an amplifying step of restricted fragments during the microsatellite development procedure. We found atypical genetic patterns in this non-native pest species: high allelic diversity and population structure. The large number of alleles may be the result of continuous and/or large-scaled introduction, which apparently went unnoticed before the first official report of pine wilt disease in Korea in 1998, or may come from gene pools of closely related species through genetic introgression after hybridization. Ecological properties of this species, such as a vector-mediated life cycle, may have contributed to its population structure, which may be enhanced by governmental efforts to prevent dispersal of this disease. As a geographic population structure was not observed, geographic patterns of genetic variation appear to be more affected by anthropogenic mediation than by natural dispersion through vector insects. And genotypes of Korean populations were compared to genotypes found in neighboring countries such as China and Japan.     

Patterns of genetic diversity reveal multiple introductions and recurrent founder effects during range expansion in invasive populations of Geranium carolinianum (Geraniaceae)

Genetic diversity, and thus the adaptive potential of invasive populations, is largely based on three factors: patterns of genetic diversity in the species'' native range, the number and location of introductions and the number of founding individuals per introduction. Specifically, reductions in genetic diversity (‘founder effects'') should be stronger for species with low within-population diversity in their native range and few introductions of few individuals to the invasive range. We test these predictions with Geranium carolinianum, a winter annual herb native to North America and invasive in China. We measure the extent of founder effects using allozymes and microsatellites, and ask whether this is consistent with its colonization history and patterns of diversity in the native range. In the native range, genetic diversity is higher and structure is lower than expected based on life history traits. In China, our results provide evidence for multiple introductions near Nanjing, Jiangsu province, with subsequent range expansion to the west and south. Patterns of genetic diversity across China reveal weak founder effects that are driven largely by low-diversity populations at the expansion front, away from the introduction location. This suggests that reduced diversity in China has resulted from successive founder events during range expansion, and that the loss of genetic diversity in the Nanjing area was mitigated by multiple introductions from diverse source populations. This has implications for the future of G. carolinianum in China, as continued gene flow among populations should eventually increase genetic diversity within the more recently founded populations.     

Genetic diversity and colony breeding structure in native and introduced ranges of the Formosan subterranean termite, Coptotermes formosanus

The Formosan subterranean termite, Coptotermes formosanus is recognized as one of the most important invasive pest species. Originating from China, C. formosanus has spread to many tropical and subtropical regions around the globe in the last 400 years, including Japan, Hawaii and the continental USA. Although the current distribution is well documented, information about the patterns of invasion and effects of introduction on the population genetics of this species is largely lacking. We analyzed the genetic structure of populations from two native populations (Guangdong and Hunan provinces, China) and two introduced populations (Maui and Kauai, Hawaii) using microsatellite genotyping. We also reanalyzed published data of additional populations from China, Japan, Hawaii, and the continental USA. The population from Hunan, the earliest introduction outside of China (Japan) and the first introduction to the continental USA (South Carolina) showed little genetic similarity with any of the native or introduced populations investigated. However, populations from Oahu (HI), New Orleans (LA) and Rutherford County (NC) showed close similarity. In general, genetic patterns suggest multiple introductions to the USA, with, for example, two separate introductions to the island of Maui. Bottleneck effects were detected in almost all recent introductions (after 1940). All populations in the introduced range showed lower genetic diversity than those in the native range. However, this low genetic diversity did not result in the formation of polygynous supercolonies as has been described for other invasive termite and ant species.     

Gene Flow and Maintenance of Genetic Diversity in Invasive Mosquitofish (Gambusia holbrooki)

Genetic analyses contribute to studies of biological invasions by mapping the origin and dispersal patterns of invasive species occupying new territories. Using microsatellite loci, we assessed the genetic diversity and spatial population structure of mosquitofish (Gambusia holbrooki) that had invaded Spanish watersheds, along with the American locations close to the suspected potential source populations. Mosquitofish populations from the Spanish streams that were studied had similar levels of genetic diversity to the American samples; therefore, these populations did not appear to have undergone substantial losses of genetic diversity during the invasion process. Population structure analyses indicated that the Spanish populations fell into four main clusters, which were primarily associated with hydrography. Dispersal patterns indicated that local populations were highly connected upstream and downstream through active dispersal, with an average of 21.5% fish from other locations in each population. After initially introducing fish to one location in a given basin, such dispersal potential might contribute to the spread and colonization of suitable habitats throughout the entire river basin. The two-dimension isolation-by-distance pattern here obtained, indicated that the human-mediated translocation of mosquitofish among the three study basins is a regular occurrence. Overall, both phenomena, high natural dispersal and human translocation, favor gene flow among river basins and the retention of high genetic diversity, which might help retain the invasive potential of mosquitofish populations.     

Genetic variation of mitochondrial DNA in Chinese populations of Pectinophora gossypiella (Lepidoptera: Gelechiidae)

The pink bollworm Pectinophora gossypiella is an invasive pest insect that has successfully established populations in many cotton growing regions around the world. In this study, the genetic diversity and population structure of Chinese populations of P. gossypiella were evaluated using mitochondrial DNA sequence data (COII and Nad4). For comparison, individuals of Pakistan and America were also sequenced at the same two mtDNA regions. Extremely low genetic variation was observed in the two mitochondrial regions among all populations examined. Most of the populations harbored only one to two haplotypes. Although the Nad4 region showed relatively high haplotype diversity and nucleotide variation, ranging from 0.363 to 0.591 and from 0.00078 to 0.00140, respectively, there were only three haplotypes observed in this region. COII and Nad4 haplotype networks shaved one or two common haplotype(s) forming the center of a star-shaped phylogeny. Pairwise tests showed that most of the populations were not significantly differentiated from each other. The Chinese populations were differentiated from the Pakistani and American populations in the Nad4 region. The low level of population genetic variation of P. gossypiella is attributed to invasion bottlenecks, which may have been subsequently strengthened by its nonmigratory biology and the mosaic pattern of agricultural activities.     

A layover in Europe: Reconstructing the invasion route of asexual lineages of a New Zealand snail to North America

Non‐native invasive species are threatening ecosystems and biodiversity worldwide. High genetic variation is thought to be a critical factor for invasion success. Accordingly, the global invasion of a few clonal lineages of the gastropod Potamopyrgus antipodarum is thus both puzzling and has the potential to help illuminate why some invasions succeed while others fail. Here, we used SNP markers and a geographically broad sampling scheme (N = 1617) including native New Zealand populations and invasive North American and European populations to provide the first widescale population genetic assessment of the relationships between and among native and invasive P. antipodarum. We used a combination of traditional and Bayesian molecular analyses to demonstrate that New Zealand populations harbour very high diversity relative to the invasive populations and are the source of the two main European genetic lineages. One of these two European lineages was in turn the source of at least one of the two main North American genetic clusters of invasive P. antipodarum, located in Lake Ontario. The other widespread North American group had a more complex origin that included the other European lineage and two New Zealand clusters. Altogether, our analyses suggest that just a small handful of clonal lineages of P. antipodarum were responsible for invasion across continents. Our findings provide critical information for prevention of additional invasions and control of existing invasive populations and are of broader relevance towards understanding the establishment and evolution of asexual populations and the forces driving biological invasion.     

Molecular ecology of zebra mussel invasions

The invasion of the zebra mussel, Dreissena polymorpha, into North American waters has resulted in profound ecological disturbances and large monetary losses. This study examined the invasion history and patterns of genetic diversity among endemic and invading populations of zebra mussels using DNA sequences from the mitochondrial cytochrome oxidase I (COI) gene. Patterns of haplotype frequency indicate that all invasive populations of zebra mussels from North America and Europe originated from the Ponto-Caspian Sea region. The distribution of haplotypes was consistent with invasive populations arising from the Black Sea drainage, but could not exclude the possibility of an origin from the Caspian Sea drainage. Similar haplotype frequencies among North American populations of D. polymorpha suggest colonization by a single founding population. There was no evidence of invasive populations arising from tectonic lakes in Turkey, while lakes in Greece and Macedonia contained only Dreissena stankovici. Populations in Turkey might be members of a sibling species complex of D. polymorpha. Ponto-Caspian derived populations of D. polymorpha (theta = 0.0011) and Dreissena bugensis (one haplotype) exhibited low levels of genetic diversity at the COI gene, perhaps as a result of repeated population bottlenecks. In contrast, geographically isolated tectonic lake populations exhibited relatively high levels of genetic diversity (theta = 0.0032 to 0.0134). It is possible that the fluctuating environment of the Ponto-Caspian basin facilitated the colonizing habit of invasive populations of D. polymorpha and D. bugensis. Our findings were concordant with the general trend of destructive freshwater invaders in the Great Lakes arising from the Ponto-Caspian Sea basin.     

Reduction in post-invasion genetic diversity in <Emphasis Type="Italic">Crangonyx pseudogracilis</Emphasis> (Amphipoda: Crustacea): a genetic bottleneck or the work of hitchhiking vertically transmitted microparasites?

Parasites can strongly influence the success of biological invasions. However, as invading hosts and parasites may be derived from a small subset of genotypes in the native range, it is important to examine the distribution and invasion of parasites in the context of host population genetics. We demonstrate that invasive European populations of the North American Crangonyx pseudogracilis have experienced a reduction in post-invasion genetic diversity. We predict that vertically transmitted parasites may evade the stochastic processes and selective pressures leading to enemy release. As microsporidia may be vertically or horizontally transmitted, we compared the diversity of these microparasites in the native and invasive ranges of the host. In contrast to the reduction in host genetic diversity, we find no evidence for enemy release from microsporidian parasites in the invasive populations. Indeed, a single, vertically transmitted, microsporidian sex ratio distorter dominates the microsporidian parasite assemblage in the invasive range and appears to have invaded with the host. We propose that overproduction of female offspring as a result of parasitic sex ratio distortion may facilitate host invasion success. We also propose that a selective sweep resulting from the increase in infected individuals during the establishment may have contributed to the reduction in genetic diversity in invasive Crangonyx pseudogracilis populations.     

Successful worldwide invasion of the veined rapa whelk, <Emphasis Type="Italic">Rapana venosa</Emphasis>, despite a dramatic genetic bottleneck

Biological invasions represent an important component of global change, with potentially huge detrimental effects on native biological biodiversity and ecosystems. Knowledge about invasion history provides information about the invasion process and the origin and genetic composition of invading populations. To clarify the source and invasive routes of a successful world-wide invader, the veined rapa whelk, Rapana venosa, genetic variability of samples from five representative native populations from coasts of Japan and China and 13 worldwide invasive populations was analyzed using 11 nuclear microsatellite loci. A dramatic decrease of genetic variation was detected in the invasive populations compared with the native populations. The results demonstrated that R. venosa was capable of establishing itself in many areas despite a dramatic genetic bottleneck, suggesting that a remarkable reduction of genetic diversity is not a limiting factor for short-term success of this invasive species. Considering the lack of mitochondrial variation previously observed in the invasive populations, the dramatic genetic bottleneck and the allele distribution detected using microsatellites suggested that the original introduced Black Sea population could have been founded by very few individuals, perhaps only a single female and a single male. The initial invasive Black Sea population was likely an accidental introduction from Japan, and then invaded the Adriatic Sea by range expansion, which served as a source for subsequent invasive populations in Europe and America by various transport vectors. In addition, microsatellite alleles in the invasive populations showed a tendency to mutate with the addition or deletion of a single repeat, which is consistent with the stepwise mutation model. Our findings provide a good example of how an aquatic invader with a drastic genetic bottleneck and very low genetic diversity rapidly expands its geographical range.     

Multiple introductions boosted genetic diversity in the invasive range of black cherry (Prunus serotina; Rosaceae)

Background and Aims

Black cherry (Prunus serotina) is a North American tree that is rapidly invading European forests. This species was introduced first as an ornamental plant then it was massively planted by foresters in many countries but its origins and the process of invasion remain poorly documented. Based on a genetic survey of both native and invasive ranges, the invasion history of black cherry was investigated by identifying putative source populations and then assessing the importance of multiple introductions on the maintenance of gene diversity.

Methods

Genetic variability and structure of 23 populations from the invasive range and 22 populations from the native range were analysed using eight nuclear microsatellite loci and five chloroplast DNA regions.

Key Results

Chloroplast DNA diversity suggests there were multiple introductions from a single geographic region (the north-eastern United States). A low reduction of genetic diversity was observed in the invasive range for both nuclear and plastid genomes. High propagule pressure including both the size and number of introductions shaped the genetic structure in Europe and boosted genetic diversity. Populations from Denmark, The Netherlands, Belgium and Germany showed high genetic diversity and low differentiation among populations, supporting the hypothesis that numerous introduction events, including multiple individuals and exchanges between sites, have taken place during two centuries of plantation.

Conclusions

This study postulates that the invasive black cherry has originated from east of the Appalachian Mountains (mainly the Allegheny plateau) and its invasiveness in north-western Europe is mainly due to multiple introductions containing high numbers of individuals.     

Evidence for multiple sources of invasion and intraspecific hybridization in Brachypodium sylvaticum (Hudson) Beauv. in North America

We compared the levels and distribution of genetic diversity in Eurasian and North American populations of Brachypodium sylvaticum (Huds.) Beauv. (false brome), a newly invasive perennial bunchgrass in western North America. Our goals were to identify source regions for invasive populations, determine the number of independent invasion events, and assess the possibility that postinvasion bottlenecks and hybridization have affected patterns of genetic diversity in the invaded range. We tested the hypothesis that this Eurasian grass was accidentally introduced into two areas in Oregon and one site in California by examining nuclear microsatellites and chloroplast haplotype variation in 23 introduced and 25 native populations. In the invaded range, there was significantly lower allelic richness (R(S)), observed heterozygosity (H(O)) and within-population gene diversity (H(S)), although a formal test failed to detect a significant genetic bottleneck. Most of the genetic variation existed among populations in the native range but within populations in the invaded range. All of the allelic variation in the invaded range could be explained based on alleles found in western European populations. The distribution of identified genetic clusters in the North American populations and the unique alleles associated with them is consistent with two historical introductions in Oregon and a separate introduction to California. Further analyses of population structure indicate that intraspecific hybridization among genotypes from geographically distinct regions of western Europe occurred following colonization in Oregon. The California populations, however, are more likely to be derived from one or perhaps several genetically similar regions in the native range. The emergence and spread of novel recombinant genotypes may be facilitating the rapid spread of this invasive species in Oregon.     

The oriental fruit fly, Bactrocera dorsalis, in China: origin and gradual inland range expansion associated with population growth

The oriental fruit fly, Bactrocera dorsalis, expanded throughout mainland China in the last century to become one of the most serious pests in the area, yet information on this process are fragmentary. Three mitochondrial genes (nad1, cytb and nad5) were used to infer the genetic diversity, population structure and demographic history of the oriental fruit fly from its entire distribution range in China. High levels of genetic diversity, as well as a significant correspondence between genetic and geographic distances, suggest that the invasion process might have been gradual, with no associated genetic bottlenecks. Three population groups could be identified, nevertheless the overall genetic structure was weak. The effective number of migrants between populations, estimated using the coalescent method, suggested asymmetric gene flow from the costal region of Guangdong to most inland regions. The demographic analysis indicates the oriental fruit fly underwent a recent population expansion in the Central China. We suggest the species originated in the costal region facing the South China Sea and gradually expanded to colonize mainland China, expanding here to high population numbers.     

Contrasting patterns of genetic diversity and spatial structure in an invasive symbiont-host association

Do host invaders and their associated symbiont co-invaders have different genetic responses to the same invasion process? To answer this question, we compared genetic patterns of native and exotic populations of an invasive symbiont-host association. This is an approach applied by very few studies, of which most are based on parasites with complex life cycles. We used the mitochondrial genetic marker cytochrome oxidase subunit I (COI) to investigate a non-parasitic freshwater ectosymbiont with direct life-cycle, low host specificity and well-documented invasion history. The study system was the crayfish Procambarus clarkii and its commensal ostracod Ankylocythere sinuosa, sampled in native (N American) and exotic (European) ranges. Results of analyses indicated: (1) higher genetic diversity in the symbiont than its host; (2) genetic diversity loss in the exotic range for both species, but less pronounced in the symbiont; (3) native populations genetically structured in space, with stronger patterns in the symbiont and (4) loss of spatial genetic structure in the exotic range in both species. The combination of historical, demographic and genetic data supports a higher genetic diversity of source populations and a higher propagule size that allowed the symbiont to overcome founder effects better than its host co-invader. Thus, the symbiont might be endowed with a higher adaptive potential to new hosts or off-host environmental pressures expected in the invasive range. We highlight the usefulness of this relatively unexplored kind of symbiont-host systems in the invasion context to test important ecological and evolutionary questions.