Tracking the origin of the American colonization by Drosophila subobscura: genetic comparison between Eastern and Western Mediterranean populations

Several pieces of evidence indicate a Mediterranean origin of the colonization of America by Drosophila subobscura . To ascertain whether the origin was from the Eastern or the Western Mediterranean region, samples from Barcelona (Spain) and Mt Parnes (Greece) were collected and O chromosomal inversion polymorphism and lethal genes were analysed. The frequencies of lethal chromosomes were 0.244 ± 0.039 in Barcelona and 0.336 ± 0.043 in Mt Parnes, consistent with the expectations for large populations located in the central area of the species distribution. Lethal genes seem to be distributed at random along the O chromosome in both populations. The intra-populational allelism frequencies of Barcelona and Mt Parnes were 0.016 ± 0.007 and 0.012 ± 0.005 respectively. Thus, the estimates of the effective population size were high in both populations (between 6964 and 13 004 in Barcelona and 11 874 to 26 828 in Mt Parnes). The cases of allelism in Mt Parnes were observed only between individual lethal genes, but in Barcelona some concatenated clusters of allelism were detected. This pattern of allelism can be explained by synthetic lethality, hybrid dysgenesis, the induction of recurrent lethal mutations by different factors or an effect of microdifferentiation in subpopulations. In both populations, a reduction in fitness in the heterozygotes for lethal genes has been detected. Furthermore, the estimates of the migration coefficient (between 0.0085 and 0.0120 in Barcelona, and 0.0057 and 0.0087 in Mt Parnes) confirm the existence of gene flow between Palearctic populations of D. subobscura . Our lethal genes and chromosomal inversion results are consistent with a Mediterranean origin of the colonization of America by D. subobscura , but are inconclusive with regard to the identification of the population from which these colonizers came.     

Colonization of America by Drosophila subobscura: association between Odh gene haplotypes, lethal genes and chromosomal arrangements

The colonization of America by Drosophila subobscura has been a unique exper iment in nature that has allowed us to explore the effects of evolution on a continental scale. To analyze this evolutionary event, nucleotide sequences of the Odh (Octanol dehydrogenase) gene were obtained for 43 lethal chromosomal lines from colonizing populations of North America and 5 from South America, in addition to 5 chromosomal lines from Europe with different viabilities and 2 from laboratory marker stocks. Since 10 different Odh haplotypes were found in America, the minimum number of colonizers would be 5 (or 3 mated females). Only one Odh haplotype was found in American O(5) inversions confirming that only one copy of this inversion was included among the sample of colonizers. The same Odh haplotypes were detected in association with the same chromosomal arrangements and with identical lethal genes in both North and South America indicating that exactly the same chromosome types reached both hemispheres. These observations indicate that the two continental colonizations are not independent. They are derived from the same colonization event. The population from which the colonization started should contain the O(5) inversion, a non-negligible frequency of the O(3+4+7) arrangement and all other arrangements found in America. So far the only populations that fulfill all these requirements are those from Greece, indicating that these populations can be considered good candidates as a starting point for an in depth analysis of the origin of the American colonization by D. subobscura.     

Colonization History,Host Distribution,Anthropogenic Influence and Landscape Features Shape Populations of White Pine Blister Rust,an Invasive Alien Tree Pathogen

White pine blister rust is caused by the fungal pathogen Cronartium ribicola J.C. Fisch (Basidiomycota, Pucciniales). This invasive alien pathogen was introduced into North America at the beginning of the 20th century on pine seedlings imported from Europe and has caused serious economic and ecological impacts. In this study, we applied a population and landscape genetics approach to understand the patterns of introduction and colonization as well as population structure and migration of C. ribicola. We characterized 1,292 samples of C. ribicola from 66 geographic locations in North America using single nucleotide polymorphisms (SNPs) and evaluated the effect of landscape features, host distribution, and colonization history on the structure of these pathogen populations. We identified eastern and western genetic populations in North America that are strongly differentiated. Genetic diversity is two to five times higher in eastern populations than in western ones, which can be explained by the repeated accidental introductions of the pathogen into northeastern North America compared with a single documented introduction into western North America. These distinct genetic populations are maintained by a barrier to gene flow that corresponds to a region where host connectivity is interrupted. Furthermore, additional cryptic spatial differentiation was identified in western populations. This differentiation corresponds to landscape features, such as mountain ranges, and also to host connectivity. We also detected genetic differentiation between the pathogen populations in natural stands and plantations, an indication that anthropogenic movement of this pathogen still takes place. These results highlight the importance of monitoring this invasive alien tree pathogen to prevent admixture of eastern and western populations where different pathogen races occur.     

Lethal genes in O5, chromosomes of Drosophila subobscura from Europe and America

In populations of D. subobscura , a species that is know for its high chromosomal polymorphism, the O₅ inversion has a rather erratic frequency distribution in the Palearctic region. An O₅ lethal chromosomal line obtained from a Balkan population near Zanjic (South Adriatic, Montenegro, Yugoslavia) was tested for lethal allelism with other O₅ lethal chromosomal lines derived from American (USA and Chile) colonizing populations, and from the French population of Taulé. No allelism was found between the Balkan lethal gene and those from America and France. Thus, the lethal genes of the O₅ inversions are not of the same origin and it is most probable that the American colonizations did not start from the Zanjic population. The general difference in the chromosomal inversion polymorphism corroborates this conclusion. The cytological analysis confirms the assumption that all O₅ chromosomes studied are identical with respect to breakage points.     

The barriers to oceanic island radiation in bryophytes: insights from the phylogeography of the moss Grimmia montana

Aim In contrast to angiosperms, bryophytes do not appear to have radiated in Macaronesia and the western Mediterranean. We evaluate if: (1) the apparent lack of radiation in bryophytes reflects our failure to recognize cryptic endemic species; (2) bryophytes are characterized by extremely low evolutionary rates; or (3) bryophytes have a high dispersal ability, which prevents genetic isolation. Location Worldwide, with a special emphasis on Macaronesia and the western Mediterranean. Methods Three chloroplast regions were sequenced from samples of the moss Grimmia montana from its entire distribution range. Network analyses, Fst and Nst statistics were used to describe and interpret the phylogeographical signal in the data. Results Despite significant phylogeographical signal in the chloroplast genome, which demonstrates limits to gene flow at the continental scale, repeated sister group relationships observed among accessions from different geographical areas suggest recurrent colonization patterns. These observations are consistent with mounting evidence that intercontinental distributions exhibited by many bryophyte species result from long‐distance dispersal rather than continental drift. Madeiran and western Mediterranean island haplotypes are either shared by, or closely related to, European and North American ones. Fst values between Madeira, western Mediterranean islands, North America and Europe are not significantly different from zero, and suggest that Madeira and the south‐western Mediterranean are subject to strong transatlantic gene flow. By contrast, haplotypes found in the Canary Islands are shared or closely related to those of populations from south‐western Europe or southern Africa. Main conclusions Multiple origins and colonization events are not consistent with the hypothesis of a relictual origin of the Macaronesian moss flora. One possible reason for the failure of taxa that experienced multiple colonization events to radiate is niche pre‐emption. We suggest that strong gene flow, coupled with the occupancy of all suitable niches, either by earlier conspecific colonizers or by other species, could be the mechanism preventing island radiation in G. montana and other cryptogams with high long‐distance dispersal abilities.     

Colonization of America by Drosophila subobscura: spatial and temporal lethal-gene allelism

About twenty years ago Drosophila subobscura, a western Palearctic species, colonized both North and South America. Lethal genes in the O chromosome has been subject to much research. Lethal gene allelisms between American populations far away have been studied. These allelisms were not negligible, but all cases were due to the lethal gene completely associated to the O5 chromosomal inversion. Here we analyze the lethal genes in a new American population of D. subobscura (Centralia, Washington), located fairly close to a previously studied population (Bellingham, Washington) and separated in space and time with other American populations (Gilroy I and II in California and Santiago de Chile). The frequencies of lethal and semilethal genes of Centralia were 16.9+/-4.6 and 6.2+/-3.0, respectively. The intrapopulational allelism of Centralia was 0.122+/-0.036. Interpopulational allelisms were studied using the lethal genes from the populations separated in space and time from Centralia. The interpopulational allelisms between Centralia and Gilroy I (California) and between Centralia and Bellingham (Washington) were higher than the intrapopulational allelism (0.155+/-0.032 and 0.153+/-0.024, respectively). In all these cases allelism was due to a complete association between a lethal gene and the O5 chromosomal inversion. Accordingly, no other lethal genes are shared in these populations.     

O chromosome inversion polymorphism in Northern and Atlantic Europe and its implications in the American colonization by Drosophila subobscura

The Western Mediterranean populations of D. subobscura have a very similar chromosomal inversion polymorphism to the American colonizing populations, with the exception of the O₅ inversion. This inversion, which is found in appreciable frequencies in colonizing populations and is distributed according to a significant latitudinal cline, both in North and South America, has never been reported in that Palearctic region. Therefore, an analysis of the distribution of this inversion, along a latitudinal cline, has been carried out in Europe. The O Chromosome inversion polymorphism has been studied in seven populations: Gävle and Lilla Edet (Sweden), Gesten (Denmark), Ter-Apel (Holland) and Crézy, Aizenay and Taulé (France). The O₅ inversion was only detected in three populations and in low frequencies: Gävle (3.7%), Lilla Edet (1.8 %) and Taulé (1.2 %). However, none of these three populations has all the inversions found in the American populations. The clinal distribution of some O chromosomal arrangements has also been studied according to latitude and the temporal changes of this polymorphism.     

Phylogeography of the common shrimp, Crangon crangon (L.) across its distribution range

The common or brown shrimp Crangon crangon (L.) is a highly abundant and important taxon, both ecologically and commercially, yet knowledge on its population structure and historical biogeography is limited. We studied population genetic structure across the distribution range of this species by sequencing a 388 bp fragment of the cytochrome-c-oxidase I gene for 140 individuals from 25 locations. Strong population structuring and high levels of genetic diversity were observed. Four main phylogroups were uncovered: northeastern Atlantic, western Mediterranean, Adriatic Sea and Black Sea. Gene flow of these shrimp across known oceanographical barriers (e.g., the Strait of Gibraltar and/or Oran-Almeria front, Sicilian Straits, and Turkish Straits) is severely restricted. The oldest and most variable populations currently inhabit the western Mediterranean. The observed absence of structure across the entire northeastern Atlantic shelf is proposed not to be due to gene flow, but to relatively recent colonization following the glacial cycles of the late Pleistocene. Black Sea shrimp are currently disconnected from Mediterranean populations, and colonization is inferred, on the basis of coalescent analysis, to have happened relatively recently, but possibly earlier than 7000 years ago. We postulate the hypothesis that C. crangon survived the last brackish-water (<7 per thousand) period inside the Black Sea and/or one of the adjacent inland seas. We conclude that (1) common shrimp populations from different basins are strongly differentiated, (2) gene flow across basins is probably very limited, and (3) the biogeographic history of the taxon is largely in accordance with the geographic history of its distribution range. This study provides further evidence that high population connectivity of marine species (e.g., by policy makers) should not be assumed.     

Genetic structure of the star sea squirt, Botryllus schlosseri, introduced in southern European harbours

The introduction of new genetic variants or species is often caused by maritime transport between harbours. Botryllus schlosseri is a cosmopolitan ascidian species that is found in both harbours and open shore habitats. In order to determine the influence of ship traffic on the genetic structure and phylogeography of B. schlosseri in southern Europe, we analyzed the variability of a fragment of the mitochondrial gene cytochrome c oxidase subunit I (COI). We sampled seven Atlanto-Mediterranean harbour populations and three open-shore populations. In addition, we sequenced some colonies from the US-Atlantic coast and from other Mediterranean localities to perform phylogenetic analyses. Although the number of polymorphic sites recorded (25.8%) was within the range observed in other population studies based on ascidian COI sequences, the haplotypic diversity (16 haplotypes out of 181 sequences) was much lower. Moreover, a lack of intermediate haplotypes was observed. This pattern of high nucleotide diversity and low haplotype diversity was consistent with introduction events of a few divergent haplotypes. We found a strong genetic structure in the study populations. Gene flow was only appreciable between some harbour populations. Harbour- and open-shore populations were well differentiated, although there was no evidence for isolation by distance. A nested clade analysis pointed to long-distance colonization, possibly coupled with subsequent fragmentation, as the underlying process. Our results suggest that B. schlosseri entered the study area via harbour-hopping, possibly through recurrent introduction events. The haplotypes from North America and most of the European ones were grouped in the same phylogenetic clade. This suggests occasional gene flow between both continents, probably through ship transport.     

Mitochondrial DNA variation in Spanish populations of Ceratitis capitata (Wiedemann) (Tephritidae) and the colonization process

Abstract: Mitochondrial DNA variation in the Mediterranean fruit fly Ceratitis capitata (Wiedemann, Diptera: Tephritidae) was studied in three natural populations from southern, central and eastern Spain by means of restriction fragment length polymorphisms using 22 restriction endonucleases. Nine different haplotypes were found based upon the restriction patterns of the seven polymorphic endonucleases, providing a measure of discrimination between populations (N_ST = 0.2462, F_ST = 0.154). The observed distribution of haplotypes, corroborated by a parsimonious unrooted tree, suggests an ancient origin for haplotype VII, and a first step in the colonization of the Iberian Peninsula and subsequently the northern and eastern Mediterranean basin, through the Straits of Gibraltar. No relationship can be established between the colonization process for Europe and America.     

The east‐west‐north colonization history of the Mediterranean and Europe by the coastal plant Carex extensa (Cyperaceae)

Coastal plants are ideal models for studying the colonization routes of species because of the simple linear distributions of these species. Carex extensa occurs mainly in salt marshes along the Mediterranean and European coasts. Variation in cpDNA sequences, amplified fragment length polymorphisms (AFLPs) and simple sequence repeats (SSRs) of 24 populations were analysed to reconstruct its colonization history. Phylogenetic relationships indicate that C. extensa together with the South American Carex vixdentata and the southern African Carex ecklonii form a monophyletic group of halophilic species. Analyses of divergence times suggest that early lineage diversification may have occurred between the late Miocene and the late Pliocene (Messinian crisis). Phylogenetic and network analyses of cpDNA variation revealed the monophyly of the species and an ancestral haplotype contained in populations of the eastern Mediterranean. The AFLP and SSR analyses support a pattern of variation compatible with these two lineages. These analyses also show higher levels of genetic diversity and differentiation in the eastern population group, which underwent an east‐to‐west Mediterranean colonization. Quaternary climatic oscillations appear to have been responsible for the split between these two lineages. Secondary contacts may have taken place in areas near the Ligurian Sea in agreement with the gene flow detected in Corsican populations. The AFLP and SSR data accord with the ‘tabula rasa’ hypothesis in which a recent and rapid colonization of northern Europe took place from the western Mediterranean after the Last Glacial Maximum. The unbalanced west‐east vs. west‐north colonization may be as a result of ‘high density blocking’ effect.     

Genetic diversity and gene flow between the wild olive (oleaster, Olea europaea L.) and the olive: several Plio-Pleistocene refuge zones in the Mediterranean basin suggested by simple sequence repeats analysis

Aim The oleaster is believed to have originated in the eastern Mediterranean, implying that those in the western Mediterranean basin could be feral. Several studies with different molecular markers (isozymes, random amplified polymorphic DNA, amplified fragment length polymorphism) have shown a cline between the eastern and the western populations, which supports this hypothesis. To reconstruct the post‐glacial colonization history and establish a relationship between olive and oleaster populations in the Mediterranean basin, analyses were carried out on the genetic variation of chloroplast DNA (chlorotype) and at 12 unlinked simple sequence repeat (SSR) loci, sampling a total of 20 oleaster groves. Location This is the first known large‐scale molecular study of SSR loci based on samples of both oleasters and cultivars from the entire Mediterranean basin. Methods Samples were taken from 166 oleasters in 20 groves of modern populations, and 40 cultivars to represent molecular diversity in the cultivated olive. The Bayesian method and admixture analysis were used to construct the ancestral populations (RPOP; reconstructed panmictic oleaster populations) and to estimate the proportion of each RPOP in each tree. If one tree can be assigned to two or more RPOPs, it can be regarded as a product of hybridization between trees from different populations (i.e. admix origin). Results On this first examination of the SSR genetic diversity in the olive and oleaster, it was found to be structured in seven RPOPs in both eastern and western populations. Based on different population genetic methods, it was shown that: (1) oleasters are equally present in the eastern and the western Mediterranean, (2) are native, and (3) are not derived from cultivars. Chlorotypes (one and three in the eastern and western Mediterranean, respectively) revealed fruit displacement for the oleasters. Main conclusions Oleaster genetic diversity is divided into seven regions that could overlay glacial refuges. The gradient, or cline, of genetic diversity revealed by chloroplast and SSR molecular markers was explained by oleaster recolonization of the Mediterranean basin from refuges after the last glacial event, located in both eastern and western regions. It is likely that gene flow has occurred in oleasters mediated by cultivars spread by human migration or through trade. Animals may have helped spread oleasters locally, but humans have probably transported olives but not oleaster fruits over long distances. We found that cultivars may have originated in several RPOPs, and thus, some may have a more complex origin than expected initially.     

Colonization of the Americas by Drosophila Subobscura: Lethal-Gene Allelism and Association with Chromosomal Arrangements

Drosophila subobscura is a Palearctic species that has recently colonized the Americas. It was first found in 1978 in Puerto Montt, Chile, and in 1982 in Port Townsend, WA. The colonization and rapid expansion of the species in western South and North America provides distinctive opportunities for investigating the process of evolution in action. The inversion polymorphism in the O chromosome from populations of central California and northern Washington, separated by 1300 km, corresponds to a previously observed latitudinal cline, also observed in Europe. Recessive lethal genes are not randomly distributed among the chromosomal arrangements. The incidence of lethal allelism is high, yielding unrealistically low estimates of the effective size of these populations (on the order of 1000 individuals). The high incidence of lethal allelism is likely to be a consequence of the low number of the American colonizers (on the order of 10-100 individuals), but the persistence of the allelism over several years suggests that some lethal-carrying chromosomes may be heterotic owing to shared associations between lethal and other genes.     

Genetic structure of insular Mediterranean populations of the house mouse

The genetic structure of six western Mediterranean island populations of Mus musculus domesticus were investigated by means of an electrophoretic analysis at 34 loci and compared to that of five neighbouring mainland populations. No reduction in variability (H = 0.09 for both island and mainland samples) was observed in the island populations except for a very small island (6 ha), in which one-third of the variability was lost. Patterns of colonization inferred from a Wagner cluster analysis and the distribution of rare variants suggest that, although these island populations are clearly related to European mainland mice, their genetic structure is the result of multiple founding events from sources dispersed throughout the Mediterranean Basin. Furthermore, the presence of the same rare alleles in Sardinia, Corsica and Piana, suggest that the three islands share a common history of colonization.
Estimates of genetic distance and gene flow indicate that the level of genie differentiation is greater between island and mainland populations that between the latter due to geographic isolation. Multiple founder events and post-colonization evolution are the factors that best explain the observed levels of genie variability and differentiation in these Mediterranean islands.     

Distribution of polymorphic gene duplication at theGpdh locus in natural populations ofDrosophila melanogaster

Summary The glycerol-3-phosphate dehydrogenase (GPDH, E. C. 1.1.1.8) gene ofDrosophila melanogaster contains a tandem duplication of a 4.5-kb-long DNA fragment. Survey of theGpdh gene region by the Southern blot analysis revealed the following features of this gene duplication: (1) The duplication was not observed in chromosome lines that carryIn(2L)t, a cosmopolitan chromosomal inversion in this species. The duplication and the inversion are in linkage disequilibrium. (2) The duplication is polymorphic in the Japan and US natural populations examined. Its frequency is 0.26 on an average inIn(2L)t-free chromosomes. (3) Triplication is absent or has not become frequent in the populations surveyed. Possible evolutionary factors of this duplication polymorphism are discussed.     

Genetic differentiation of Mediterranean horse mackerel (Trachurus mediterraneus) populations as revealed by mtDNA PCR-RFLP analysis

The genetic population structure of Mediterranean horse mackerel, Trachurus mediterraneus , from seven locations throughout the Black, Marmara, Aegean and eastern Mediterranean seas was investigated using restriction fragment length polymorphism (RFLP) analysis of the mtDNA 16S rDNA region. An approximately 2000-bp segment was screened in 280 individuals using six restriction enzymes, resulting in 10 composite haplotypes. The most common haplotype was present in 56.42% individuals; the next most frequent haplotype was present in 22.85% individuals. Average haplotype diversity within samples was moderate (0.38), and nucleotide diversity was low (0.00435). Mean nucleotide divergence for the seven sampling sites was 0.0028. Nucleotide divergence among samples was moderate, with the highest value detected between the Aegean Sea (Izmir) and the eastern Black Sea (Trabzon) populations (0.007055), and the lowest (−0.000043) between the Marmara Sea (Adalar) and the western Black Sea (Sile) populations. In Monte Carlo pairwise comparisons of haplotype frequencies, the Sinop from the middle Black Sea, Trabzon from the eastern Black Sea, and Iskenderun Bay from the north-eastern Mediterranean Sea exhibited highly significant (P   <   0.001) geographical differentiation from each other and from all other populations. Mantel's test indicated that the nucleotide divergence among populations of T. mediterraneus was not significantly associated with their geographical isolation ( r  = −0.2963; P   >   0.05). Consequently, the mtDNA 16S rDNA region provided evidence for the existence of three distinct T. mediterraneus populations (Sinop, Trabzon and Iskenderun Bay) in the Black and north-eastern Mediterranean seas.     

Chromosomal inversion polymorphism leads to extensive genetic structure: a multilocus survey in Drosophila subobscura

The adaptive character of inversion polymorphism in Drosophila subobscura is well established. The O(ST) and O(3+4) chromosomal arrangements of this species differ by two overlapping inversions that arose independently on O(3) chromosomes. Nucleotide variation in eight gene regions distributed along inversion O(3) was analyzed in 14 O(ST) and 14 O(3+4) lines. Levels of variation within arrangements were quite similar along the inversion. In addition, we detected (i) extensive genetic differentiation between arrangements in all regions, regardless of their distance to the inversion breakpoints; (ii) strong association between nucleotide variants and chromosomal arrangements; and (iii) high levels of linkage disequilibrium in intralocus and also in interlocus comparisons, extending over distances as great as approximately 4 Mb. These results are not consistent with the higher genetic exchange between chromosomal arrangements expected in the central part of an inversion from double-crossover events. Hence, double crossovers were not produced or, alternatively, recombinant chromosomes were eliminated by natural selection to maintain coadapted gene complexes. If the strong genetic differentiation detected along O(3) extends to other inversions, nucleotide variation would be highly structured not only in D. subobscura, but also in the genome of other species with a rich chromosomal polymorphism.     

Low mitochondrial DNA variation and recent colonization of Scandinavia by the wood lemming Myopus schisticolor

The population genetic structure of wood lemmings ( Myopus schisticolor ) from Scandinavia, Finland and western Siberia was examined by restriction fragment length polymorphism of mitochondrial DNA (mtDNA) in 45 individuals from six localities. The 12 observed mtDNA haplotypes demonstrated a distinct phylogeographic pattern, suggesting that the postglacial colonization of Scandinavia by wood lemmings occurred from north-east. However, a very low level of haplotype and nucleotide diversity and a lack of geographical structure were found within Scandinavia. The limited mtDNA diversity in the Scandinavian populations probably reflects recent divergence in situ after colonization by a limited number of founders. Allozyme data support this scenario.     

Strait of Gibraltar: an effective gene-flow barrier for wind-pollinated Carex helodes (Cyperaceae) as revealed by DNA sequences, AFLP, and cytogenetic variation

The Strait of Gibraltar is the most important barrier disconnecting the landmasses of Europe and Africa on the western Mediterranean extreme. Carex helodes is a wind-pollinated species endemic to the western Mediterranean. Because molecular and cytogenetic data allow the inference of its evolutionary history, we analyzed variations in chromosome number, including meiotic chromosome behavior, amplified fragment length polymorphism (AFLP) fingerprints, and nucleotide substitutions in plastid and nuclear DNA sequences. Cytogeographic results showed that the African populations have stabilized at a single chromosome number of 2n = 74, whereas the most frequent cytotype in Iberia is 2n = 72. Phylogenetic reconstructions of 17 sequences from nine closely related species revealed that C. helodes is monophyletic and that the Moroccan populations are embedded in the Iberian lineages. The haplotype network is also consistent with a European origin of the northern African haplotype. AFLP analysis also revealed hierarchical levels of genetic variation compatible with a founder effect process responsible for the African populations. All sources of evidence support the hypothesis that the Strait of Gibraltar has been an effective gene-flow barrier, generating two isolated evolutionary lineages after their dispersal. Recent connections between the two lineages appear unlikely, whereas active gene flow occurs among populations within the two lineages.     

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.