Pathways of spread of the introduced ascidian Styela clava (Tunicata) in Northern Europe, as revealed by microsatellite markers

Isolation and Characterization of Polysaccharides from the Ascidian Styela clava

Styela clava is an edible sea squirt farmed in Korea that has gradually invaded other seas, negatively impacting the ecology and economy of coastal areas. Extracts from S. clava have shown wide bioactivities, and ascidians have the unique capability among animals of biosynthesizing cellulose. Thus, S. clava is a relevant candidate for valorization. Herein, we aimed at surveying and characterizing polysaccharides in both tunic and flesh of this ascidian. To this end, we enzymatically hydrolyzed both tissues, recovering crystalline cellulose from the tunic with high aspect ratios, based on results from microscopy, X-ray diffraction, and infrared spectroscopy analyses. Alkaline hydroalcoholic precipitation was applied to isolate the polysaccharide fraction that was characterized by gel permeation chromatography (with light scattering detection) and NMR. These techniques allowed the identification of glycogen in the flesh with an estimated Mw of 7 MDa. Tunic polysaccharides consisted of two fractions of different Mw. Application of Diffusion-Ordered NMR allowed spectroscopically separating the low-molecular-weight fraction to analyze the major component of an estimated Mw of 40–66 kDa. We identified six different sugar residues, although its complexity prevented the determination of the complete structure and connectivities of the residues. The two more abundant residues were N-acetylated and possibly components of the glycosaminoglycan-like (GAG-like) family, showing the remaining similarities to sulfated galactans. Therefore, Styela clava appears as a source of nanocrystalline cellulose and GAG-like polysaccharides.

Extensive gene flow of white-backed planthopper in the Greater Mekong Subregion as revealed by microsatellite markers

The white-backed planthopper (WBPH), Sogatella furcifera (Horváth), is a destructive pest of rice in the Greater Mekong Subregion (GMS) countries including Cambodia, Laos, Myanmar, Thailand, Vietnam, and China’s Yunnan Province. Our previous study not only confirmed the immigration sources of the WBPH in China’s Yunnan Province were from Myanmar, Vietnam, and Laos, but also indicated that Cambodia was likely an additional migration source. To further clarify the migration sources and patterns of the WBPH in the GMS, we investigated the genetic structure of 42 WBPH populations using microsatellite loci markers. The analysis of genetic diversity, heterozygosity deficit, and heterozygosity excess based on the nuclear markers suggest that there is extensive gene flow between the 42 sampled populations from the GMS. The genetic structure confirmed the immigration sources of WBPH as revealed by mitochondrial markers and trajectory analyses methods in previous studies. These findings will aid in the sustainable regional management of this insect pest in the GMS.

Stochasticity in space, persistence in time: genetic heterogeneity in harbour populations of the introduced ascidian Styela plicata

Spatio-temporal changes in genetic structure among populations provide crucial information on the dynamics of secondary spread for introduced marine species. However, temporal components have rarely been taken into consideration when studying the population genetics of non-indigenous species. This study analysed the genetic structure of Styela plicata, a solitary ascidian introduced in harbours and marinas of tropical and temperate waters, across spatial and temporal scales. A fragment of the mitochondrial gene Cytochrome Oxidase subunit I (COI) was sequenced from 395 individuals collected at 9 harbours along the NW Mediterranean coast and adjacent Atlantic waters (> 1,200 km range) at two time points 5 years apart (2009 and 2014). The levels of gene diversity were relatively low for all 9 locations in both years. Analyses of genetic differentiation and distribution of molecular variance revealed strong genetic structure, with significant differences among many populations, but no significant differences among years. A weak and marginally significant correlation between geographic distance and gene differentiation was found. Our results revealed spatial structure and temporal genetic homogeneity in S. plicata, suggesting a limited role of recurrent, vessel-mediated transport of organisms among small to medium-size harbours. Our study area is representative of many highly urbanized coasts with dense harbours. In these environments, the episodic chance arrival of colonisers appears to determine the genetic structure of harbour populations and the genetic composition of these early colonising individuals persists in the respective harbours, at least over moderate time frames (five years) that encompass ca. 20 generations of S. plicata.

Invasion Genetics of Woolly Apple Aphid (Hemiptera: Aphididae) in China

In China, the woolly apple aphid, Eriosoma lanigerum (Hausmann), was first detected as an invasive species during the 1910s to 1930s, restricted to Shandong, Liaoning, and Yunnan Provinces. However, since the 1990s, the pest has spread into many other areas of China. To determine the possible spread routes of the recently established populations, the genetic diversity and genetic structure of 24 populations in 10 provinces were analyzed using eight microsatellite loci. Analyses using STRUCTURE software identified two genetic clusters overall. Three populations from Yunnan and Xinjiang consisted of individuals originating from a single cluster. Nineteen populations from eight northern provinces consisted only of individuals from another cluster, which formed a single large and panmictic population, resembling a distinct "supercolony" in Northern China. The other two populations from Yunnan consisted of individuals from both clusters. The possible routes of spread of the recently established populations of E. lanigerum in China were revealed as follows: 1) the populations in Northern China (including these from Henan, Hebei, Shanxi, Shannxi, Jiangsu, and Gansu) may have been introduced from Shandong or Liaoning Provinces; 2) the populations in Yunnan consisted of an early-established population and a population introduced secondarily from Shandong or neighboring areas, indicating that the population in Yunnan has at least two sources; and 3) the recently established populations of E. lanigerum in Xinjiang might not have been introduced from the "supercolony" in Northern China. Knowledge of these routes of spread is useful for avoiding further dissemination and/or additional introductions.

Harbor networks as introduction gateways: contrasting distribution patterns of native and introduced ascidians

Harbors and marinas are well known gateways for species introductions in marine environments but little work has been done to ascertain relationships between species diversity, harbor type, and geographic distance to uncover patterns of secondary spread. Here, we sampled ascidians from 32 harbors along ca. 300 km of the NW Mediterranean coast and investigated patterns of distribution and spread related to harbor type (marina, fishing, commercial) and geographic location using multivariate techniques. In total, 28 ascidians were identified at the species level and another 9 at the genus level based on morphology and genetic barcoding. Eight species were assigned to introduced forms, 15 were given native status and 5 were classified as cryptogenic. Aplidium accarense was reported for the first time in the Mediterranean Sea and was especially abundant in 23 of the harbors. Introduced and cryptogenic species were abundant in most of the surveyed harbors, while native forms were rare and restricted to a few harbors. Significant differences in the distribution of ascidians according to harbor type and latitudinal position were observed. These differences were due to the distribution of introduced species. We obtained a significant correlation between geographic distance and ascidian composition, indicating that closely located harbors shared more ascidian species among them. This study showed that harbors act as dispersal strongholds for introduced species, with native species only appearing sporadically, and that harbor type and geographic location should also be considered when developing management plans to constrain the spread of non-indigenous species in highly urbanized coastlines.

High genetic diversity and absence of founder effects in a worldwide aquatic invader

The introduced oriental shrimp Palaemon macrodactylus has recently become widespread in temperate estuaries worldwide. However, this recent worldwide spread outside of its native range arises after a previous introduction to the US Pacific coast, where it was restricted for more than 30 years. Using a phylogeographic approach, the present work investigates the genetic history of the invasion of this decapod worldwide. Japan acted as the main native source area for worldwide introduced populations, but other native areas (likely South Korea and China) may act as source populations as well. The recently introduced European and NW Atlantic populations result from colonization from both Japan and an unknown area of the native range, although colonization from the NE Pacific could not be ruled out. Most introduced populations had higher haplotypic diversity than most native populations. P. macrodactylus has a strong potential to become one of the most widespread introduced species and may become the dominant estuarine shrimp in Europe. The ecological and economic consequences of this invasion remain to be thoroughly evaluated.

Spatial genetic heterogeneity in populations of a newly invasive whitefly in china revealed by a nation-wide field survey

Background

Even though introductions of exotic species provide ready-made experiments of rapid evolution, few studies have examined the genetic structure of an exotic species shortly after its initial introduction and subsequent spread. To determine the genetic structure of its populations during the initial introduction, we investigated the invasive sweet potato whitefly (Bemisia tabaci Q, commonly known as B. tabaci biotype Q) in China, which was introduced in approximately 2003. A total of 619 B. tabaci Q individuals in 20 provinces throughout China were collected and analyzed using five microsatellite loci.

Results

The introduced populations of B. tabaci Q in China represent eight genetic clusters with different geographic distributions. The populations in Yunnan Province, where B. tabaci Q was first detected, are genetically different from the other populations in China.

Conclusion

The introduced populations of B. tabaci Q in China have high spatial genetic heterogeneity. Additional research is required to determine whether the heterogeneity results from multiple introductions, rapid evolution following one or few introductions, or some combination of multiple introductions and rapid evolution. The heterogeneity, however, is inconsistent with a single introduction at Yunnan Province, where B. tabaci Q was first detected, followed by spread.

Isolation with differentiation followed by expansion with admixture in the tunicate Pyura chilensis

Background

Pyura chilensis, a tunicate commercially exploited as food resource in Chile, is subject to management strategies, including restocking. The goal of this study was to examine the genetic structure of P. chilensis using information from a mitochondrial gene (Cytochrome Oxidase I, COI) and a nuclear gene (Elongation 1 alpha, EF1a), to characterize the geographic distribution of genetic diversity and differentiation, and to identify the main processes that have shaped it. We analyzed 268 and 208 sequences of COI and EF1a, respectively, from samples of eight local populations covering ca. 1800 km.

Results

For Pyura chilensis, partial sequences of the gene COI revealed three highly supported haplogroups that diverged 260000 to 470000 years ago. Two haplogroups currently are widely distributed and sympatric, while one is dominant only in Los Molinos (LM, 39°50′S). The two widespread COI haplogroups underwent a geographic expansion during an interglacial period of the Late Pleistocene ca. 100000 years ago. The nuclear gene was less divergent and did not resolve the COI haplogroups. Bayesian clustering of the nuclear gene’s SNPs revealed that individuals from the two widespread COI haplogroups were mostly assigned to two of the three detected clusters and had a marked degree of admixture. The third cluster predominated in LM and showed low admixture. Haplotypic diversity of both genes was very high, there was no isolation by distance, and most localities were genetically undifferentiated; only LM was consistently differentiated with both genes analyzed.

Conclusions

Pyura chilensis has less genetic structure than expected given its life history, which could be a consequence of dispersal on ship hulls. The only differentiated local population analyzed was LM. Coincidentally, it is the one furthest away from main maritime routes along the coast of Chile.

The use of mitochondrial and nuclear markers allowed detection of divergent mitochondrial haplogroups in P. chilensis, two of which revealed nuclear admixture. The genetic structure of P. chilensis has likely been shaped by Pleistocene’s climatic effect on sea level leading to population contraction with isolation, followed by geographic range expansions with concomitant secondary contact and admixture.

Intracoastal shipping drives patterns of regional population expansion by an invasive marine invertebrate

Understanding the factors contributing to expansion of nonnative populations is a critical step toward accurate risk assessment and effective management of biological invasions. Nevertheless, few studies have attempted explicitly to test hypotheses regarding factors driving invasive spread by seeking correlations between patterns of vector movement and patterns of genetic connectivity. Herein, we describe such an attempt for the invasive tunicate Styela clava in the northeastern Pacific. We utilized microsatellite data to estimate gene flow between samples collected throughout the known range of S. clava in the region, and assessed correlation of these estimates with patterns of intracoastal commercial vessel traffic. Our results suggest that recent shipping patterns have contributed to the contemporary distribution of genetic variation. However, the analysis also indicates that other factors-including a complex invasion history and the influence of other vectors-have partially obscured genetic patterns associated with intracoastal population expansion.

Complex genetic patterns in closely related colonizing invasive species

Anthropogenic activities frequently result in both rapidly changing environments and translocation of species from their native ranges (i.e., biological invasions). Empirical studies suggest that many factors associated with these changes can lead to complex genetic patterns, particularly among invasive populations. However, genetic complexities and factors responsible for them remain uncharacterized in many cases. Here, we explore these issues in the vase tunicate Ciona intestinalis (Ascidiacea: Enterogona: Cionidae), a model species complex, of which spA and spB are rapidly spreading worldwide. We intensively sampled 26 sites (N = 873) from both coasts of North America, and performed phylogenetic and population genetics analyses based on one mitochondrial fragment (cytochrome c oxidase subunit 3–NADH dehydrogenase subunit I, COX3-ND1) and eight nuclear microsatellites. Our analyses revealed extremely complex genetic patterns in both species on both coasts. We detected a contrasting pattern based on the mitochondrial marker: two major genetic groups in C. intestinalis spA on the west coast versus no significant geographic structure in C. intestinalis spB on the east coast. For both species, geo-graphically distant populations often showed high microsatellite-based genetic affinities whereas neighboring ones often did not. In addition, mitochondrial and nuclear markers provided largely inconsistent genetic patterns. Multiple factors, including random genetic drift associated with demographic changes, rapid selection due to strong local adaptation, and varying propensity for human-mediated propagule dispersal could be responsible for the observed genetic complexities.

Tracking invasion histories in the sea: facing complex scenarios using multilocus data

In recent years, new analytical tools have allowed researchers to extract historical information contained in molecular data, which has fundamentally transformed our understanding of processes ruling biological invasions. However, the use of these new analytical tools has been largely restricted to studies of terrestrial organisms despite the growing recognition that the sea contains ecosystems that are amongst the most heavily affected by biological invasions, and that marine invasion histories are often remarkably complex. Here, we studied the routes of invasion and colonisation histories of an invasive marine invertebrate Microcosmus squamiger (Ascidiacea) using microsatellite loci, mitochondrial DNA sequence data and 11 worldwide populations. Discriminant analysis of principal components, clustering methods and approximate Bayesian computation (ABC) methods showed that the most likely source of the introduced populations was a single admixture event that involved populations from two genetically differentiated ancestral regions - the western and eastern coasts of Australia. The ABC analyses revealed that colonisation of the introduced range of M. squamiger consisted of a series of non-independent introductions along the coastlines of Africa, North America and Europe. Furthermore, we inferred that the sequence of colonisation across continents was in line with historical taxonomic records - first the Mediterranean Sea and South Africa from an unsampled ancestral population, followed by sequential introductions in California and, more recently, the NE Atlantic Ocean. We revealed the most likely invasion history for world populations of M. squamiger, which is broadly characterized by the presence of multiple ancestral sources and non-independent introductions within the introduced range. The results presented here illustrate the complexity of marine invasion routes and identify a cause-effect relationship between human-mediated transport and the success of widespread marine non-indigenous species, which benefit from stepping-stone invasions and admixture processes involving different sources for the spread and expansion of their range.

The whereabouts of an ancient wanderer: global phylogeography of the solitary ascidian Styela plicata

Genetic tools have greatly aided in tracing the sources and colonization history of introduced species. However, recurrent introductions and repeated shuffling of populations may have blurred some of the genetic signals left by ancient introductions. Styela plicata is a solitary ascidian distributed worldwide. Although its origin remains unclear, this species is believed to have spread worldwide by travelling on ship's hulls. The goals of this study were to infer the genetic structure and global phylogeography of S. plicata and to look for present-day and historical genetic patterns. Two genetic markers were used: a fragment of the mitochondrial gene Cytochrome Oxidase subunit I (COI) and a fragment of the nuclear gene Adenine Nucleotide Transporter/ADP-ATP Translocase (ANT). A total of 368 individuals for COI and 315 for ANT were sequenced from 17 locations worldwide. The levels of gene diversity were moderate for COI to high for ANT. The Mediterranean populations showed the least diversity and allelic richness for both markers, while the Indian, Atlantic and Pacific Oceans had the highest gene and nucleotide diversities. Network and phylogenetic analyses with COI and ANT revealed two groups of alleles separated by 15 and 4 mutational steps, respectively. The existence of different lineages suggested an ancient population split. However, the geographic distributions of these groups did not show any consistent pattern, indicating different phylogeographic histories for each gene. Genetic divergence was significant for many population-pairs irrespective of the geographic distance among them. Stochastic introduction events are reflected in the uneven distribution of COI and ANT allele frequencies and groups among many populations. Our results confirmed that S. plicata has been present in all studied oceans for a long time, and that recurrent colonization events and occasional shuffling among populations have determined the actual genetic structure of this species.

Global phylogeography of the widely introduced North West Pacific ascidian Styela clava

The solitary ascidian Styela clava Herdman, 1882 is considered to be native to Japan, Korea, northern China and the Russian Federation in the NW Pacific, but it has spread globally over the last 80 years and is now established as an introduced species on the east and west coasts of North America, Europe, Australia and New Zealand. In eastern Canada it reaches sufficient density to be a serious pest to aquaculture concerns. We sequenced a fragment of the cytochrome oxidase subunit I mitochondrial gene (COI) from a total of 554 individuals to examine the genetic relationships of 20 S. clava populations sampled throughout the introduced and native ranges, in order to investigate invasive population characteristics. The data presented here show a moderate level of genetic diversity throughout the northern hemisphere. The southern hemisphere (particularly New Zealand) displays a greater amount of haplotype and nucleotide diversity in comparison. This species, like many other invasive species, shows a range of genetic diversities among introduced populations independent of the age of incursion. The successful establishment of this species appears to be associated with multiple incursions in many locations, while other locations appear to have experienced rapid expansion from a potentially small population with reduced genetic diversity. These contrasting patterns create difficulties when attempting to manage and mitigate a species that continues to spread among ports and marinas around the world.

Looking at both sides of the invasion: patterns of colonization in the violet tunicate Botrylloides violaceus

Understanding the ecological and evolutionary forces that shape the genetic structure of invasive populations and facilitate their expansion across a large spectrum of environments is critical for the prediction of spread and management of ongoing invasions. Here, we study the dynamics of postestablishment colonization in the colonial ascidian Botrylloides violaceus, a notorious marine invader. After its initial introduction from the Northwest Pacific, B. violaceus spread rapidly along the Pacific and Atlantic coasts of North America, impacting both aquaculture facilities and natural ecosystems. We compare genetic diversity and patterns of gene flow among 25 populations (N=679) from the West and East coasts, and evaluate the contribution of sexual vs. asexual reproduction to this species' invasion success using data from the mitochondrial cytochrome c oxidase subunit I (COI) gene and 13 nuclear polymorphic microsatellite loci. Our results reveal contrasting patterns of spread in the coastal waters of North America. While the West coast was colonized by noncontiguous (long-distance) dispersal, the East coast invasion appears to have occurred through contiguous (stepping-stone) spread. Molecular data further indicate that although dispersal in colonial ascidians is predominantly achieved through sexually produced propagules, aquaculture practices such as high-pressure washing can facilitate fragmentation and potentially exacerbate infestations and spread via asexual propagules. The results presented here suggest that caution should be used against the general assumption that all invasions, even within a single species, exhibit similar patterns of colonization, as highly contrasting dynamics may transpire in different invaded ranges.

Invasion genetics of the Ciona intestinalis species complex: from regional endemism to global homogeneity

Determining the degree of population connectivity and investigating factors driving genetic exchange at various geographical scales are essential to understanding population dynamics and spread potential of invasive species. Here, we explore these issues in the highly invasive vase tunicate, Ciona intestinalis, a species whose invasion history has been obscured by its poorly understood taxonomy and population genetics. Recent phylogenetic and comparative genomic studies suggest that C. intestinalis is a cryptic species complex consisting of at least three species. We reconstructed phylogenies based on both mitochondrial (cytochrome c oxidase subunit 3--NADH dehydrogenase subunit 1 region and NADH dehydrogenase subunit 4 gene) and nuclear (internal transcribed spacer 1) sequences, results of which support four major phylogroups corresponding to the previously reported spA, spB and Ciona spp. (spC) as well as an undescribed cryptic species (spD). While spC and spD remain restricted to their native ranges in the Mediterranean Sea and Black Sea, respectively, the highly invasive species (spA and spB) have disjunct global distributions. Despite extensive interspecific divergences, we identified low phylogeographical structure within these two invasive species. Haplotype network analyses revealed comparatively limited mutation steps among haplotypes within each species. Population genetic analyses based on two mtDNA fragments and eight unlinked microsatellites illustrated relatively low population differentiation and high population connectivity at both regional and continental scales in the two invasive species. Human-mediated dispersal coupled with a high potential for natural dispersal is probably responsible for the observed genetic homogeneity.