ICES Journal of Marine Science: Journal du Conseil Advance Access originally published online on March 11, 2008
ICES Journal of Marine Science: Journal du Conseil 2008 65(3):443-452; doi:10.1093/icesjms/fsn018
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This article appears in the following ICES Journal of Marine Science issue: 4th International Zooplankton Production Symposium: Human and Climate Forcing of Zooplankton Populations [View the issue table of contents]
Recent moon jelly (Aurelia sp.1) blooms in Korean coastal waters suggest global expansion: examples inferred from mitochondrial COI and nuclear ITS-5.8S rDNA sequences
1 Department of Chemistry, and the National Research Lab of Marine Molecular and Environmental Bioscience, College of Natural Sciences, Hanyang University, Seoul 133–791, South Korea
2 Department of Molecular and Environmental Bioscience, Graduate School, Hanyang University, Seoul 133–791, South Korea
3 South Sea Institute, Korea Ocean Research and Development Institute, Geoje 656–830, South Korea
4 Ocean Research Team National Fisheries Research and Development Institute, Busan 619–902, South Korea
5 Marine Ecology Research Team, National Fisheries Research and Development Institute, Busan 619–902, South Korea
Correspondence to J-S. Lee: tel: +82 2 22200769; fax: +82 2 22999450; e-mail: jslee2{at}hanyang.ac.kr
Ki, J-S., Hwang, D-S., Shin, K., Yoon, W. D., Lim, D., Kang, Y. S., Lee, Y., and Lee, J-S. 2008. Recent moon jelly (Aurelia sp.1) blooms in Korean coastal waters suggest global expansion: examples inferred from mitochondrial COI and nuclear ITS-5.8S rDNA sequences. – ICES Journal of Marine Science, 65: 443–452.The moon jelly Aurelia was found recently in Korean coastal environments, and its dense blooms caused economic losses for fisheries and power plants. The species is tentatively recognized as Aurelia aurita; yet, its identity and origin remain elusive. To find reliable molecular evidence for its identity, we determined the DNA sequence of the mitochondrial cytochrome c oxidase subunit I gene and nuclear ITS-5.8S rDNA of specimens collected from different Korean locations. We compared the nuclear and mitochondrial DNA data among specimens and demonstrated that all Korean Aurelia have an identical genotype. BLAST searches demonstrated that the Korean Aurelia matched the previously designated Aurelia sp.1. Parsimony and relevant phylogenetic analyses of the genus Aurelia demonstrated that the genotypes of Korean, Japanese, and Californian Aurelia sp.1 were nearly identical (>99.6% similarity), whereas they were significantly different (<84.1% similarity) from other Aurelia. This suggests that Aurelia sp.1, which occur in the three regions, are descendants of a single population and may have dispersed from one location. However, the dispersal time and origin of Aurelia sp.1 still remain uncertain.
Keywords: Aurelia, blooms, COI sequence, exotic species, moon jelly, phylogenetic comparison
Received 2 August 2007; accepted 26 December 2007; advance access publication 11 March 2008.
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Introduction |
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 Top Introduction Material and methodsResults and discussionReferences |
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The moon jelly (Aurelia spp.) belongs in the phylum Cnidaria (Scyphozoa) and is one of the most widely distributed species of jellyfish (Arai, 1996). Traditionally, classification of adult jellyfish has been based on morphological features and body measurements. However, identification of the fragile moon jelly is not easy because the shape of its medusae is highly variable, and the moon jelly has different life stages. Mainly as a result of the high morphological variability in the medusa,
12 Aurelia species or subspecies have been described (Mayer, 1910; Kramp, 1961). However, only two of them are recognized as distinct species: Aurelia aurita and Aurelia limbata (Russell, 1970). To resolve morphological ambiguities, molecular tools have been applied to various aquatic organisms (e.g. Dawson and Jacobs, 2001; Collins et al., 2006). These tools allow the differentiation of cryptogenic taxa, the identification of source populations and vectors, and the assessment of the extent and impacts of invasions (Holland, 2000; Wares et al., 2002). The molecular phylogeny of the moon jelly has been studied extensively, based on the nuclear ribosomal DNA (rDNA) and mitochondrial 16S or cytochrome c oxidase subunit I (COI) genes that have led to taxonomic revisions. For example, Schroth et al. (2002) suggested historical speciation events and the reconstruction of at least seven different species within Aurelia, based on the phylogenetic patterns derived from mitochondrial 16S and nuclear rDNA data from 66 specimens collected worldwide. Dawson and colleagues (Dawson and Jacobs, 2001; Dawson, 2003; Dawson et al., 2005) proposed a more acceptable taxonomic system for the moon jelly by combining its macro-morphology and internal transcribed spacer (ITS), as well as partial COI gene sequences. The global phylogeny of the genus Aurelia reveals at least 16 phylogenetic branches, i.e. 13 cryptic species including three recognized species (A. aurita, A. limbata, and Aurelia labiata). Most of them appear to be restricted regionally. However, several species such as Aurelia sp.1 and A. aurita have disjointed distributions, which may be a characteristic of globally expanded species (Dawson et al., 2005). Based on the molecular data, at least 13 species are distinguishable.
A global issue was recently identified: the rapid expansion of moon jellies' geographic range and spread across oceanic regions from a single origin (e.g. Dawson et al., 2005). This could be by natural as well as anthropogenic means. Moon jellies can be introduced accidentally as organisms growing on ship hulls or in ballast water, or released unintentionally (or intentionally) by the aquarium trade and aquaculture (Holland, 2000; Grosholz, 2002). Several negative impacts caused by the introduction of exotic jellyfish species have been observed, including severe blooms, which can cause decreases in the abundance of endemic gelatinous plankton (Mills, 2001). In addition, jellyfish introductions can be hazardous to bathers because of their sting, and to fishers because they clog fishing nets (Mills, 2001).
In Korean coastal waters, a species of the moon jelly Aurelia is recognized as one of the most economically harmful animals to date, and its dense blooms have damaged commercial fisheries when swarms of medusae impeded trawling (data from the National Fisheries Research and Development Institute; http://nfrdi.re.kr/). In addition, these organisms have clogged seawater intakes, shutting down power plants located on the east coast of the Korean peninsula. There is no record of the moon jelly's existence before 2000. It remains therefore unresolved whether the Korean moon jelly is an endemic or an introduced species. Furthermore, based on specific morphological characteristics, it was presumed to be A. aurita. However, its identity remains elusive because of morphological similarities with closely related species.
In this paper, we determined the complete DNA sequences of the mitochondrial COI gene and the ITS-5.8S rDNA from a Korean moon jelly specimen, and compared them with previously reported DNA sequences of diverse Aurelia species and subspecies to determine their identity. In addition, we investigated partial COI gene and ITS-5.8S rDNA of several Korean Aurelia specimens collected from three geographically separated coastal areas at different times of the year. Furthermore, we investigated the biogeographic relationships of Aurelia occurring in Korea and other countries, using various molecular characteristics such as sequence similarity, parsimony, and phylogenetic analyses.
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Material and methods |
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TopIntroductionMaterial and methodsResults and discussionReferences |
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Specimens of Aurelia were collected from different localities in southern (Geoje-do, 34°59'33''N 128°40'31''E), southeastern (Busan, 35°12'58''N 129°13'41''E), and western (Incheon, 37°26'23''N 126°22'40''E) Korean coastal waters (Figure 1). They were immediately preserved in absolute ethanol to dehydrate, washed several times, and stored at room temperature until use. Genomic DNA was isolated from the stored tissues, using a treatment of proteinase K with chloroform extraction and isopropanol precipitation, followed by the protocol described in Lee (2000). In addition, the manually isolated DNA was purified with the DNeasy tissue kit (Qiagen, Valencia, CA, USA) according to the manufacturer's instructions. To amplify the mitochondrial COI gene from Aurelia sp., PCR was used with a set of primers (JF-F1, 5'-AGC AAG CCC ATT ATA CAA AAG GTA C-3'; JF-R1, 5'-CGA CAA ACA TTA TTT GAT CRT GAA G-3'). Primers were designed based on a comparison between A. aurita (GenBank number DQ787873 [GenBank] ) and a coral mtDNA sequence (GenBank number DQ304771 [GenBank] ). A volume of 25 µl of PCR reactions was carried out in 1x PCR buffer (10 mM Tris–HCl, 50 mM KCl, 1.5 mM MgCl2, 0.001% gelatine, pH 8.3) with <0.1 µg of genomic DNA as a template, 200 µM each of the four dNTPs, 0.5 µM of each primer, and 0.2 units of LA Taq polymerase (TaKaRa, Japan), using iCycler (Bio-Rad, CA, USA). PCR thermocycling parameters were as follows: 95°C for 5 min; 35 cycles of denaturation at 95°C for 20 s; annealing at 55°C for 30 s and extension at 72°C for 60 s; and a final extension at 72°C for 5 min. The PCR products (2 µl) were analysed by 1.0% agarose gel electrophoresis according to a standard method.
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The entire ITS containing 5.8S rDNA was amplified from the same genomic DNA material from the Aurelia specimens, followed by conventional PCR protocols with its flanking 18S and 28S rDNA-target primers (JF-18F1750, 5'- AAA GTC GTA ACA AGG TTT CCG-3'; JF-28R765, 5'-TTG GTC CGT GTT TCA AGA CG-3').
All PCR products of the COI gene were subcloned into pCR2.1 vectors (Invitrogen, CA, USA), and additional PCR products from geographic samples were cloned into identical vectors. All the subcloned DNAs were sequenced according to the manufacturer's suggested protocol with commercial primers (T7 and M13R) and nested flanking sequencing primers for primer walking in both directions. Labelled DNA fragments were analysed on a Model 3700 automated DNA sequencer (Applied Biosystems, CA, USA). All the PCR amplicons of ITS-5.8S rDNA gene were purified with the QIAquick PCR purification kit (Qiagen GmbH, Germany), and were directly sequenced using the identical PCR and additional nested primers (e.g. JF-28R1, 5'-GCG AAT TGT AGT CTC GAG AAG CGT-3'; JF-5.8R1, 5'-GAA TCA TCG AAT CTT TGA ACG C-3'), followed by the sequencing method described above. Editing and contiguous assembly of the DNA sequence fragments were carried out with Sequencher 4.7 (Gene Codes, MI, USA).
For comparative molecular features, complete COI and ITS-5.8S DNA sequences determined in the present study (Table 1) were aligned with other Aurelia sequences retrieved from GenBank/DDBJ/EMBL, using Clustal W version 1.84 (Thompson et al., 1994). For precise comparisons within the genus Aurelia, the data matrixes were constructed with well-defined DNA sequences, excluding sequence data that contained ambiguous nucleotide readings such as N, W, K, M, or S (designated as degeneracy symbols by the International Union of Pure and Applied Chemistry). Overall, genetic characteristics such as parsimony-informative (P-I), transversion, transition, conserved, and variable sites were calculated using MEGA 3.1 (Kumar et al., 2001). Molecular similarities of each gene between subspecies or subgenus of Aurelia were measured separately in BioEdit 5.0.6 (North Carolina State University). The uncorrected pairwise (p-) distance was calculated when investigating nucleotide substitutions.
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Phylogenetic analysis of the genus Aurelia was carried out with the COI gene sequences from the data in Table 1. In all, 55 DNA sequences were aligned as described above, and the alignment results were adjusted manually. Only those positions that could be unambiguously aligned were used for the analysis. This yielded 625 of the 658 alignment positions for the partial COI sequence. Modeltest 3.07 (Posada and Crandall, 1998) was used to find the optimal model of DNA substitution for maximum-likelihood (ML) construction. As the best-fit model for this dataset, the GTR+I+G model was chosen by both hierarchical likelihood ratio tests (hLRTs) and the Akaike information criterion (AIC), respectively. The Bayesian analysis was implemented with MrBayes version 3.1.2 (Huelsenbeck and Ronquist, 2001), using the molecular model selected by AIC, namely a GTR nucleotide substitution model, with among-site rate variation modelled with a proportion of sites being invariable, while the rates for variable sites were drawn from a gamma distribution. The Markov chain Monte Carlo (MCMC) process was set to four chains, and 1 000 000 generations were conducted and sampled every 100 generations. After analysis, the
rst 1000 trees were deleted as burn-in, and the consensus tree was constructed. Bayesian posterior probabilities (>0.50) were indicated at each branch node. The phylogenetic trees were visualized with TreeView version 1.6.6 (Page, 1996). Additionally, the phylogeography of conspecific Aurelia members was studied with ITS-5.8S rDNA sequences. In all, 20 DNA sequences were available for analysis, using the method described above. In this analysis, 982 of the 1008 alignment positions for the complete ITS-5.8S rDNA sequence were used for the subsequent analysis. Modeltest 3.07 was used to find the optimal model of DNA substitution for ML construction and suggested the GTR+G model with –ln L = 6821 as the best-fit model for the ITS-5.8S dataset by AIC. An ML tree of Aurelia was constructed with the selected GTR+G model in PAUP* 4.0b10 (Swofford, 2002), using the following likelihood settings determined from the above Modeltest: base frequencies A = 0.2558, C = 0.2307, G = 0.2404; base substitution rates AC = 0.8610, AG = 1.9527, AT = 1.5856, CG = 0.7921, CT = 2.6291; assumed proportion of invariable sites = 0.0001; and gamma distribution shape parameter = 0.3406. Bootstrap analyses with 100 replicates were conducted to determine the robustness of the clades. Further Bayesian analysis of the identical ITS-5.8S dataset was implicated with MrBayes version 3.1.2, followed by the above method used in COI analysis.
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Results and discussion |
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TopIntroductionMaterial and methodsResults and discussionReferences |
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Complete COI sequence of Aurelia sp.1
The complete sequence DNA of a Korean Aurelia COI gene is 1581 bp in length (Table 2). The representative sequence has been deposited in GenBank (Table 1). In this study, we compared Korean Aurelia with A. aurita, because the latter had been considered previously as being identical with the Korean moon jelly. A BLAST search of the GenBank database revealed the COI sequence determined here was closely matched with those of the Aurelia sp.1 COI sequence from Lake Macquarie (GenBank number DQ787873 [GenBank] ), Perth (AY903163 [GenBank] ), and Tokyo Bay (AY903126 [GenBank] ). Comparison of 658 sites revealed that it was nearly identical, with 99.9% DNA similarity. Therefore, we were able to identify the Korean moon jelly as Aurelia sp.1 and used this designation hereafter. The COI gene of the Korean Aurelia sp.1 coded 526 amino acids, of which the start and stop codons were recorded as "ATG" and "TAA", respectively. The GC ratio of the COI gene was 37.6%. In addition, GC composition of each first, second, and third codon position in COI gene was 43.7%, 40.6%, and 28.3%, respectively. Interestingly, the third position was significantly lower in GC content in both Aurelia sp.1 and A. aurita (Table 2).
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Until now, the COI sequences of Aurelia spp. available in public databases, except for A. aurita, are slightly less than 700 bp in length. Here, we are the first to report on a complete COI DNA sequence of Aurelia sp.1 from a Korean specimen collected from Incheon coastal waters. It allows us to make more detailed comparisons between Aurelia sp.1 and A. aurita. By a comparison of COI amino acid sequences, Aurelia sp.1 was found to differ from A. aurita with 98.0% similarity (Table 2), whereas the DNA sequence comparison had 84.8% similarity. In addition, the stop codon was different from A. aurita, "TAG", whereas the start codon, "ATG", in the protein coding sequence was identical in both species (Table 2). As pointed out previously, robust morphological characters are rare in Aurelia (Greenberg et al., 1996), making it difficult to distinguish the various Aurelia species. However, using molecular signatures (e.g. Dawson and Jacobs, 2001), it is possible to resolve discrepancies between A. aurita and Aurelia sp.1 and so to distinguish the two species.
In this study, we also included some Aurelia specimens collected from geographically distinct areas (Figure 1). Based on our findings, we sequenced partial COI gene sequences (around 500 bp) from five specimens collected in Incheon coastal waters (37°26'23''N 126°22'40''E), where Aurelia occurred densely, and found all the sequences were exactly identical with GenBank number EU010386 [GenBank] , which suggested that the single bloom of Aurelia in Incheon was genetically homogenous. Furthermore, partial COI sequences of some specimens, including two polyps and one ephyra-staged sample, collected from Busan, were also identical with other specimens in our collections. This suggested that all Aurelia found in three different areas of Korea might have a mitochondrial COI genotype that is identical with Aurelia sp.1.
ITS-5.8S rDNA of Korean Aurelia
To further study their genotypes, we sequenced the nuclear ITS-5.8S rDNA genes and the flanking regions from the Korean Aurelia specimens, which were used in the COI analyses. All the specimens have identical genotypes. A total length of the ITS-5.8S rDNA from the Korean Aurelia spp. was 272 bp (ITS1), 158 bp (5.8S rDNA), and 278 bp (ITS2), respectively. Nucleotide frequencies of complete rDNA were recorded at A, 25.8%; T, 27.4%; G, 24.6%; C, 22.2%, respectively. The present ITS-5.8S rDNA sequence was compared with other Aurelia rDNA sequences in the NCBI database. A BLAST search confirmed that the Korean Aurelia examined matched the previously designated Aurelia sp.1 (GenBank accession number AY935214
[GenBank]
and AY935203
[GenBank]
). This agrees with the present COI analysis and confirms that the Korean moon jelly was Aurelia sp.1 among the diverse Aurelia members classified by Dawson et al. (2005).
In addition, we found that all Aurelia blooming in different areas of Korea have an identical genotype (i.e. Aurelia sp.1) when compared with ITS-5.8S rDNA sequences (Table 1). This was consistent with the COI gene comparison. These results suggest that the Aurelia blooming in Korean coastal areas were identical with Aurelia sp.1.
Molecular phylogeny of Aurelia
Mitochondrial DNA, which is transferred to descendants through maternal lineages, is useful to estimate genetic diversity and phylogenetic relationships. Particularly, the COI gene is highly conserved within the same species but variable among species, so can be used for phylogenetic studies and serve as DNA barcodes (that is, species identification tags).We examined the phylogenetic relationships of conspecifics of Aurelia using their partial COI DNA sequences (Table 1). In an unrooted Bayesian tree (Figure 2a), we detected 13 distinct clades including A. aurita, A. limbata, A. labiata, and ten unassigned Aurelia, with overall well-supported branch confidence. The same tree with unscaled branches (Figure 2b) showed the origins of specimens and that the 13 Aurelia spp. were clustered according to their species identities rather than to the isolation localities. For example, A. aurita forms a clade that includes specimens from Sweden, Turkey, Finland, the UK, and the USA, and also forms a sister group relationship with A. labiata with 0.97 of posterior probability (PP). Aurelia sp.1 forms a clade including Japanese, Korean, USA, and Australian specimens with a high PP value (0.99), and has a sister group relationship with both A. limbata and Aurelia sp.10. Taking into account the branch topology of the Bayesian analysis and the genetic distance between the three identified species (A. aurita, A. limbata, and A. labiata), it is possible to consider the ten unassigned Aurelia spp. as truly distinct species, as proposed by Dawson et al. (2005). Therefore, existing ecological and systematic descriptions of Aurelia may need to be revised in view of these molecular data.
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Beyond the taxonomy of Aurelia, an additional Bayesian tree of Aurelia sp.1 showed that expected changes per site were
20 times lower in Aurelia sp.1 than in the genus Aurelia (Figure 2c). Here, we included COI sequences from 23 different sampling sites in four countries (Japan, Korea, USA, and Australia). As noted previously, all the Korean specimens formed a clade because their genotype is identical, and clustered with a Japanese specimen (isolated from Tokyo Bay, GenBank number AY903116
[GenBank]
; Figure 2d). In this tree, four different Japanese specimens (collected from Ondo Strait, Miyazu Bay, Tokyo Bay, Sakata Bay) differed slightly, based on their origins. The specimen from Sakata Bay clustered with specimens from the US and Australia, with 0.84 PP value (Figure 2c and d). However, phylogenetic resolution in the Aurelia sp.1 COI analysis was low because of the conservative nature of the COI gene that sometimes does not reflect the relationships between subspecies. In contrast, ITS rDNA regions are useful for defining intraspecific differences, because they are less subject to functional constraints, so evolve more rapidly (Bena et al., 1998). The ITS regions are highly variable within Aurelia (see parsimony analysis below), so their sequences may be useful in studying the relationships among subspecies. Phylogenetic analysis of the ITS-5.8S rDNA demonstrated that the 13 species included here with 18 sequences from different localities were separated from each other, and all clades within the same species were well supported by bootstrap (BP) and PP values (Figure 3). All the Korean Aurelia specimens formed a clade with other Aurelia sp.1 collected in Japan and the US (California), and the tree branch was separated from the other Aurelia members (1.0 PP, 100 BP).
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Based on all available information, we find that Aurelia sp.1 generally occurs in temperate regions (Korea, Japan, Europe, Australia, and North America) rather than tropical and cold seas, whereas A. aurita was frequently found in the cold–temperate North Atlantic, although it also occurs as far as Boston Harbor, US, and the Bosphorus, Turkey (Figure 3). Schroth et al. (2002) demonstrated that A. aurita occurs in the cold–temperate North Atlantic, A. labiata in the cold–temperate eastern North Pacific, and A. limbata in the northern polar oceans, respectively. The latter two are present in very restricted areas, whereas A. aurita and Aurelia sp.1 are dispersed in coastal areas worldwide.
Parsimony analyses of COI and ITS-5.8S rDNA
By numerical comparison of Aurelia COI gene sequences (Table 3), we found that similarities within different specimens of the same species were great (Aurelia sp.1, >99.3% similarity, <0.0098 genetic distance; Aurelia sp.7, 99.6%, 0.0024); similarities between interspecies were significantly smaller (<85.0%, >0.1449). In addition, the Korean, Japanese, and Californian Aurelia sp.1 COI sequences are nearly identical (99.6% DNA similarity, <0.0098 genetic distance). In contrast, similarities between our Aurelia sp.1 and the other Aurelia spp. were generally small (<84.1% similarity). Similarly, comparisons of Aurelia ITS-5.8S, which were determined from the same specimens used in COI sequencing listed in Table 1, revealed results similar to the COI comparisons (Table 3). Specifically, the greatest similarity (99.7%) was recorded between our specimens and Aurelia sp.1 (AY935214
[GenBank]
; Miyazu Bay, Japan), followed by 99.4% with Aurelia sp.1 (AY935203
[GenBank]
; California). Overall, DNA sequence similarities between ours and those reported for other Aurelia spp. were <80% (mostly <70%). In this context, the current data on Aurelia sp.1 were significantly different from those of A. aurita (AY935206
[GenBank]
, 76.4%), A. labiata (AY935202
[GenBank]
, 70.8%), and A. limbata (AY935215
[GenBank]
, 79.5%), as reported previously (Dawson et al., 2005).
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Parsimony analyses of the COI gene and ITS rDNA, including additional 5.8S (Table 4), revealed that the transition:transversion ratio (Ts/Tv) was high in the coding COI gene (2.25) rather than in the non-coding ITS region (0.76). High p-distance, in contrast, was recorded at ITSs (0.314) when compared with that in the COI gene (0.211). Additionally, analyses on P-I sites revealed that P-I sites were higher in the ITS rDNA (48.2% in only ITS, 40.7% in ITS-5.8S rDNA) than in the COI gene (33.3%). Variation in the ITS non-coding rDNA was
1.45 times greater than that of the coding COI genes, as judged by a comparison of the P-I sites. These observations suggest that the variations in COI and ITS DNA sequences are sufficient to allow us to discriminate distinct species, sometimes at the subspecies level.
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Implications of global expansion
The morphological identification of the fragile moon jelly and its relatives is not easy because their medusae and other life stages are highly variable. Correct and rapid identification of these species is crucial to understanding their global expansion and to controlling blooms to protect fisheries and the biodiversity of endemic species (Mills, 2001). To date, molecular tools (e.g. in situ hybridization, species-specific PCR, restriction fragment length polymorphism, DNA sequencing) are increasingly being applied to recognize and distinguish particularly cryptic organisms. DNA sequencing generally provides the most accurate means for identification (Dawson and Jacobs, 2001; Dawson, 2003). The complete COI sequence of Aurelia sp.1 reported here will provide an additional option of molecular signatures and/or DNA barcodes.
This study demonstrates that the moon jelly blooms in Korean coastal waters were caused by Aurelia sp.1. The species identity was strongly supported by a comparison of COI and ITS sequences (Table 3) and phylogenies (Figures 2 and 3). In addition, molecular comparison and parsimony analyses demonstrated that genotypes of the Korean, Japanese, and Californian Aurelia sp.1 were nearly identical (>99.6% similarity), whereas they were significantly different (<84.1 similarity) from other Aurelia. Phylogeny of the COI sequences demonstrated that slight differences among the Aurelia sp.1 were detected according to their geographic origins. However, by comparing the non-coding ITS, which evolves faster than the coding COI gene (Table 4), Aurelia sp.1, occurring in the three regions, reveal an identical genotype. Interestingly, we found only one base difference in the ITS-5.8S rDNA sequences between Korean and Californian specimens, and found that the difference was detected in a tract of poly(T), possibly caused by sequencing or PCR errors. This demonstrated that Aurelia sp.1 of both regions have exactly the same genotype in this respect. However, we found an insertion–deletion site (nucleotide sequence, TAA) at position 205–206 bp of Aurelia sp.1 from Incheon (GenBank number EU276014 [GenBank] ) in the ITS comparison of Korean and Japanese specimens. As noted previously (Figure 3), the moon jellyfish Aurelia sp.1 has the broadest geographic range of all species of Aurelia studied to date. It occurs at least in Japan, Australia, and the Atlantic, and along the Mediterranean coast of France, in San Francisco Bay (Greenberg et al., 1996; Dawson and Martin, 2001), and from Los Angeles to San Diego (Dawson and Jacobs, 2001; Schroth et al., 2002; Dawson, 2003). These results suggest that Aurelia sp.1 populations may have recently dispersed worldwide.
In eastern Asian coastal waters, the Korean and some Japanese moon jellies were assigned to Aurelia sp.1, using ITS and COI sequences. The moon jelly has also been observed in the East China Sea and in Taiwan coastal areas (Figure 3), but its identity remains unclear because its genetic data are not available. The identity of Aurelia sp. 1 in Korean and Japanese waters can be explained by the Kuroshio and the Tsushima currents, which flow from the South China Sea to the coasts of Korea and Japan. Dawson et al. (2005) used this kind of reasoning in trying to explain how Aurelia were spread from Japan northwards and eastwards by the Kuroshio Current in the North Pacific, as well as local mixing in the Yellow Sea, the East China Sea, and Japanese waters. The low genetic variation in ITS coupled with such a large geographic range that includes distinct populations from some of the major warm-temperate seaports of the world would suggest that Aurelia sp.1 may have dispersed recently across the Pacific; however, the dispersal time and origin of the Aurelia sp.1 remain uncertain.
Recently, Mills (2001) reviewed the worldwide occurrence of blooms and the dispersal of cryptic jellyfish, as well as their ecological and economic impacts. We found no record or monitoring data indicating the presence of moon jelly in Korean waters before 2000. Since then, two dense blooms have occurred. In this study, we found that the moon jelly blooms from different geographic areas of Korea had an identical genotype and should be assigned to Aurelia sp.1, based on the characteristics of mitochondrial COI and nuclear ITS-5.8S rDNA sequences. In addition, we demonstrated that Korean Aurelia sp.1 has a genotype identical to that of specimens from California and is slightly different from specimens collected in Japan. This suggests that Aurelia sp.1 in the three Korean regions is likely to have descended from the same population. This agrees with the conclusion of Dawson et al. (2005). To understand the origin and global expansion of Aurelia precisely, more detailed studies are necessary in future, including wider geographic sampling and intensive molecular analyses, such as population genetics.
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Acknowledgements |
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We thank two anonymous reviewers and Hans-Uwe Dahms for their comments on earlier versions of this manuscript. This work was supported by grants of the National Research Laboratory of KOSEF (2006) and the ETEP (2006) funded to J-SL. Also this work was partially supported by the funds of National Project on Marine Bioinvasion from MOMAF, Korea to YL and the Research on Mass Occurrence of Jellyfish and its Prevention from NFRDI to YL.
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References |
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TopIntroductionMaterial and methodsResults and discussionReferences |
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