Phylogeography of the widespread marine invader Microcosmus squamiger (Ascidiacea) reveals high genetic diversity of introduced populations and non-independent colonizations

The spread of non-indigenous species into new marine habitats represents an increasing threat to global diversity. Genetic techniques provide basic understanding of the invasion processes. The ascidian Microcosmus squamiger is considered to be native to Australia, having been spread worldwide via transoceanic vessels. It has successfully invaded artificial and natural habitats where it has become a pest. We studied phylogeography and genetic structure of 12 M. squamiger populations, including samples from its native range (Australia) and introduced populations from the Indian, Pacific, and Atlantic oceans, as well as the Mediterranean Sea. We amplified 574 bp of the mitochondrial COI gene in 258 individuals and found a total of 52 haplotypes. A haplotype tree revealed two main groups of haplotypes. The relative frequency of each group of haplotypes, multidimensional scaling, and analysis of molecular variance showed important differences between the western Australia localities and the remaining ones (eastern Australia and introduced populations). Furthermore, we found that the colonization of the different areas by M. squamiger has not occurred independently, as many introduced populations shared some low frequency alleles. A nested clade analysis showed a global pattern of restricted gene flow with isolation by distance, although we found episodes of long-distance dispersal in some clades. A contiguous range expansion was detected between Australian populations. We conclude that M. squamiger is native to Australia and has most likely expanded its range of distribution sequentially through worldwide shipping, especially from the harbours of the more populated eastern Australia. In introduced populations, we found a high genetic diversity which suggests enhanced invasive potential. Consequently, there is a need to control this species, as it outcompetes local biota and is an economic threat.     

Elevated mtDNA diversity in introduced populations of Cynotilapia afra (Günther 1894) in Lake Malawi National Park is evidence for multiple source populations and hybridization

Genetic variation in many invasive species shows little or no signs of a founder event, suggesting that high genetic diversity may facilitate establishment success. The rocky‐shore, plankton‐feeding cichlid fish Cynotilapia afra is endemic to Lake Malawi, but naturally absent from many suitable sites. In the 1960s, this species was introduced to the southern areas of the lake, presumably as a result of the aquarium fish trade. It has now become established on a number of rocky areas within the Lake Malawi National Park. Here, we analysed DNA sequence variation in the mitochondrial control region of six native and four introduced populations of C. afra, and three populations of the closely‐related and hybridizing Pseudotropheus zebra. In contrast to previous studies of Lake Malawi rock dwelling cichlids, network analyses suggested that native populations of C. afra showed high levels of lineage sorting in mtDNA. Introduced populations showed higher sequence and haplotype diversity than their native counterparts. Our analyses suggested that the elevated gene diversity was largely attributed to the fact that the introduced C. afra populations were derived from several genetically distinct and geographically separate populations, and to a lesser extent because of introgressive hybridization with native P. zebra. The establishment and spread of C. afra may be partly because of its ability to occupy a vacant ecological niche, but it may also have been facilitated by its enhanced genetic diversity.     

Relationships among native and introduced populations of the Argentine ant (Linepithema humile) and the source of introduced populations

The Argentine ant (Linepithema humile) is a damaging invasive species that has become established in many Mediterranean-type ecosystems worldwide. To identify likely sources of introduced populations we examined the relationships among native Linepithema populations from Argentina and Brazil and introduced populations of L. humile using mitochondrial cytochrome b sequence data and nuclear microsatellite allele frequencies. The mitochondrial phylogeny revealed that the populations in Brazil were only distantly related to both the introduced populations and the native populations in Argentina, and confirmed that populations in Brazil, previously identified as L. humile, are likely a different species. The microsatellite-based analysis provided resolution among native and introduced populations of L. humile that could not be resolved using the mitochondrial sequences. In the native range, colonies that were geographically close to one another tended to be genetically similar, whereas more distant colonies were genetically different. Most samples from the introduced range were genetically similar, although some exceptions were noted. Most introduced populations were similar to native populations from the southern Rio Parana and were particularly similar to a population from Rosario, Argentina. These findings implicate populations from the southern Rio Parana as the most likely source of introduced populations. Moreover, these data suggest that current efforts to identify natural enemies of the Argentine ant for biological control should focus on native populations in the southern Rio Parana watershed.     

High mitochondrial DNA diversity of an introduced alien carnivore: comparison of feral and ranch American mink Neovison vison in Poland

Aim Invasive alien species usually exhibit very high adaptation and rapid evolution in a new environment, but they often have low levels of genetic diversity (invasive species paradox). Genetic variation and population genetic structure of feral American mink, Neovison vison, in Poland was investigated to explain the invasion paradox and to assess current gene flow. Furthermore, the influence of mink farming on adaptation of the feral population was evaluated by comparing the genetic structure of feral and ranch mink. Location Samples from feral mink were collected in 11 study areas in northern and central Poland and from ranch mink at 10 farms distributed throughout the country. Methods A 373‐bp‐long mtDNA control region fragment was amplified from 276 feral and 166 ranch mink. Results Overall, 31 haplotypes, belonging to two groups from genetically diverse sources, were detected: 11 only in feral mink, 12 only in ranch mink and eight in both. The genetic differentiation of feral mink from the trapping sites was high, while that among ranch mink from various farms was moderate. There was no significant relationship between genetic and geographic distance. The number of trapping sites where given haplotypes occurred correlated with the number of farms with these haplotypes. The mink from two sites were the most divergent, both from all other feral mink and from ranch mink. Comparison of mtDNA and microsatellite differentiation suggests male‐biased dispersal in this species. Main conclusions American mink in Poland exhibit high genetic diversity and originate from different source populations of their native range. The process of colonization was triggered by numerous escapees from various farms and by immigrants from Belarus. The genetic structure of local feral mink populations was shaped by the founder effect and multiple introductions. The genomic admixture that occurred during mixing of different populations might have increased the fitness of individuals and accelerated the invasiveness of this species.     

The population genetics of a biological control introduction: mitochondrial DNA and microsatellie variation in native and introduced populations of Aphidus ervi, a parisitoid wasp

Introductions of biological control agents may cause bottlenecks in population size despite efforts to avoid them. We examined the population genetics of Aphidius ervi (Hymenoptera: Braconidae), a parasitoid that was introduced to North America from Western Europe in 1959 to control pea aphids. To explore the phylogeographical relationships of A. ervi we sequenced 1249 bp of mitochondrial DNA (mtDNA) from 27 individuals from the native range and 51 individuals from the introduced range. Most individuals from Western Europe, the Middle East and North America shared one of two common haplotypes, consistent with the known history of the introduction. However, some A. ervi from the Pacific Northwest have a haplotype that is most similar to haplotypes found in Japan, raising the possibility of a second accidental introduction. To examine population structure and assess whether a bottleneck occurred upon introduction to North America, we assayed variation at 5 microsatellite loci in 62 individuals from 2 native populations and 230 individuals from 6 introduced populations. Introduced samples had fewer rare alleles than native samples (F1,34 = 13.5, P = 0.0008), but heterozygosity did not differ significantly. These results suggest that a mild bottleneck occurred in spite of the introduction of over 1000 individuals. Using a hierarchical Bayesian approach, the founding population size was estimated to be 245 individuals. amova showed significant genetic differentiation between the European and North American samples, and a Bayesian assignment approach clustered individuals into four groups, with most European individuals in one group and most North American individuals in the other three. These results highlight that genetic changes are associated with founder events in rapidly growing natural populations, even when the founding population size is relatively large.     

Molecular evidence for multiple introductions of garlic mustard (Alliaria petiolata, Brassicaceae) to North America

Invasive species offer excellent model systems for studying rapid evolutionary change. In this context, molecular markers play an important role because they provide information about pathways of introduction, the amount of genetic variation introduced, and the extent to which founder effects and inbreeding after population bottlenecks may have contributed to evolutionary change. Here, we studied microsatellite variation in eight polymorphic loci among and within 27 native and 26 introduced populations of garlic mustard (Alliaria petiolata), a European herb which is a current serious invader in North American deciduous forests. Overall, introduced populations were genetically less diverse. However, considerable variability was present and when compared to the probable source regions, no bottleneck was evident. Observed heterozygosity was very low and resulted in high inbreeding coefficients, which did not differ significantly between native and introduced populations. Thus, selfing seems to be equally dominant in both ranges. Consequently, there was strong population differentiation in the native (F(ST) = 0.704) and the introduced (F(ST) = 0.789) ranges. The high allelic diversity in the introduced range strongly suggests multiple introductions of Alliaria petiolata to North America. Out of six European regions, the British Isles, northern Europe, and central Europe had significantly higher proportions of alleles, which are common to the introduced range, and are therefore the most probable source regions. The genetic diversity established by multiple introductions, and the lack of inbreeding depression in this highly selfing species, may have contributed to the invasion success of Alliaria petiolata.     

Four cleaved amplified polymorphic sequence (CAPS) markers for the detection of the Juglans ailantifolia chloroplast in putatively native J. cinerea populations

Hybridization between butternut (Juglans cinerea), a forest tree native to eastern North America, and Japanese walnut (J. ailantifolia), a tree tolerant to the lethal fungal disease butternut canker, casts doubt on the genetic identity of the remaining butternuts. We report a diagnostic test to distinguish the J. cinerea chloroplast from the J. ailantifolia chloroplast using cleaved amplified polymorphic sequences resolvable in 1.5% agarose gels. J. ailantifolia maternal ancestry in naturally regenerated stands provides a site selection criterion for studies of introgression dynamics when the non-native parent and the hybrids tolerate a disease to which the native species is susceptible.     

Inferring the origin of populations introduced from a genetically structured native range by approximate Bayesian computation: case study of the invasive ladybird Harmonia axyridis

Correct identification of the source population of an invasive species is a prerequisite for testing hypotheses concerning the factors responsible for biological invasions. The native area of invasive species may be large, poorly known and/or genetically structured. Because the actual source population may not have been sampled, studies based on molecular markers may generate incorrect conclusions about the origin of introduced populations. In this study, we characterized the genetic structure of the invasive ladybird Harmonia axyridis in its native area using various population genetic statistics and methods. We found that native area of H. axyridis most probably consisted of two geographically distinct genetic clusters located in eastern and western Asia. We then performed approximate Bayesian computation (ABC) analyses on controlled simulated microsatellite data sets to evaluate (i) the risk of selecting incorrect introduction scenarios, including admixture between sources, when the populations of the native area are genetically structured and sampling is incomplete and (ii) the ability of ABC analysis to minimize such risks by explicitly including unsampled populations in the scenarios compared. Finally, we performed additional ABC analyses on real microsatellite data sets to retrace the origin of biocontrol and invasive populations of H. axyridis, taking into account the possibility that the structured native area may have been incompletely sampled. We found that the invasive population in eastern North America, which has served as the bridgehead for worldwide invasion by H. axyridis, was probably formed by an admixture between the eastern and western native clusters. This admixture may have facilitated adaptation of the bridgehead population.     

Long-term variability and density dependence in Hudson River Dreissena populations

1. We used a 27-year record of Dreissena populations in the freshwater tidal Hudson River to describe interannual variation in population density, body size, and body condition; estimate long-term variation in recruitment, survivorship, and shell growth; and assess possible controls on the populations.
2. Dreissena populations in the Hudson have been highly variable, with interannual ranges of c. 100-fold in abundance and biomass, and 7-fold in mean body mass. This large interannual variation arises from both long-term trends and 2–5-year cycles.
3. Long-term trends include the 2008 appearance of the quagga mussel (Dreissena rostriformis), which still forms a small part (<10%) of the dreissenid community, and a decline in zebra mussel body size. The decline in body size was caused by a long-term decline in adult survivorship rather than a decline in rates of shell growth. We could detect no long-term trends in adult abundance or spread of Dreissena onto soft sediments in the Hudson.
4. We observed persistent, strong cycles in adult abundance and body size. These were driven by the appearance and decay of eight dominant year classes over the 27 years of our study, and were a result of temporal variation in recruitment rather than temporal variation in survivorship. The observed strongly irregular recruitment appears to arise from strong adult–larval interactions, and is consistent with previous simulation model results showing that interactions between adults and larvae can drive persistent cycling.
5. We found evidence that negative density dependence affects recruitment, somatic growth, and body condition of Dreissena in the Hudson. Warm summers may also cause high adult mortality.
6. We put our results into the context of a conceptual model of Dreissena population dynamics, and argue that neither the dynamics nor the controls of populations of these important invaders is known satisfactorily.

     

Two invasive populations of the dry rot fungus Serpula lacrymans show divergent population genetic structures

The dry rot fungus Serpula lacrymans is a devastating basidiomycete occurring in wooden constructions in temperate regions worldwide. In this study, we compare the genetic structures of two invasive populations from Europe and Japan. Microsatellite data from 14 loci and DNA sequences from four loci demonstrated that the two populations were highly differentiated. Significant isolation by distance effect was observed in Europe and Japan. Higher genetic variation was observed within the Japanese population than within the European population, corresponding with the observed higher richness of vegetative compatibility types in Japan, indicating that there has been a higher level of gene flow from the Asian source populations to Japan than to Europe. The European population is genetically more homogenous with only six detected vegetative compatibility types. Various tests indicate that both the European and the Japanese populations have gone through population bottlenecks prior to population expansion. No identical multi‐locus genotypes were observed within Japan and very few within Europe, indicating limited clonal dispersal. Deviations from Hardy Weinberg expectations were observed both in Europe and Japan and heterozygote excess were observed at several loci, especially in Europe. Possible explanations for this pattern are discussed.     

Species introduced from nearby sources have a more homogenizing effect than species from distant sources: evidence from plants and fishes in the USA

Biotic homogenization is occurring in many biota as widespread introduced species are replacing unique native species. Although efforts to document homogenization have increased, no studies have explicitly compared the homogenizing effects of species introduced from distant areas to the homogenizing effects of species introduced from more proximate areas. The author analysed three data sets, at different scales and in different taxa, that distinguish species introduced from distant sources (e.g. outside the US) from species introduced from less distant sources (e.g. within the United States). These data include: plant introductions among eight major US cities and fish introductions among 12 US states and among 10 watersheds from New York state. The authors found that, for all data sets, species introduced from less distant sources (within the US) have a greater homogenizing effect on community composition than species from more distant sources (outside the US). In agreement with other studies, the author also find that, in terms of absolute numbers, introductions from nearby sources are far more frequent than introduction of species from distant sources. While tentative, these findings point out the importance of considering species introduced from nearby areas (e.g., extralimital native species) when discussing biotic homogenization from human activities.     

A model for range expansion of an introduced species: the common waxbill Estrilda astrild in Portugal

Abstract. The common waxbill Estrilda astrild was first introduced to Portugal from Africa in 1964, and has spread across much of the country and into Spain. We modelled the expansion of the common waxbill on a 20 × 20 km UTM grid in 4‐year periods from 1964 to 1999. The time variation of the square root of the occupied area shows that this expansion process is stabilizing in Portugal, and reasons for this are discussed. Several methods used to model biological expansions are not appropriate for the present case, because little quantitative data are available on the species ecology and because this expansion has been spatially heterogeneous. Instead, colonization on a grid was modelled as a function of several biophysical and spatio‐temporal variables through the fitting of a multivariate autologistic equation. This approach allows examination of the underlying factors affecting the colonization process. In the case of the common waxbill it was associated positively with its occurrence in adjacent cells, and affected negatively by altitude and higher levels of solar radiation.     

Diversity and distribution of genetic variation in gammarids: Comparing patterns between invasive and non‐invasive species

Biological invasions are worldwide phenomena that have reached alarming levels among aquatic species. There are key challenges to understand the factors behind invasion propensity of non‐native populations in invasion biology. Interestingly, interpretations cannot be expanded to higher taxonomic levels due to the fact that in the same genus, there are species that are notorious invaders and those that never spread outside their native range. Such variation in invasion propensity offers the possibility to explore, at fine‐scale taxonomic level, the existence of specific characteristics that might predict the variability in invasion success. In this work, we explored this possibility from a molecular perspective. The objective was to provide a better understanding of the genetic diversity distribution in the native range of species that exhibit contrasting invasive propensities. For this purpose, we used a total of 784 sequences of the cytochrome c oxidase subunit I of mitochondrial DNA (mtDNA‐COI) collected from seven Gammaroidea, a superfamily of Amphipoda that includes species that are both successful invaders (Gammarus tigrinus, Pontogammarus maeoticus, and Obesogammarus crassus) and strictly restricted to their native regions (Gammarus locusta, Gammarus salinus, Gammarus zaddachi, and Gammarus oceanicus). Despite that genetic diversity did not differ between invasive and non‐invasive species, we observed that populations of non‐invasive species showed a higher degree of genetic differentiation. Furthermore, we found that both geographic and evolutionary distances might explain genetic differentiation in both non‐native and native ranges. This suggests that the lack of population genetic structure may facilitate the distribution of mutations that despite arising in the native range may be beneficial in invasive ranges. The fact that evolutionary distances explained genetic differentiation more often than geographic distances points toward that deep lineage divergence holds an important role in the distribution of neutral genetic diversity.     

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。