PwRn1, a novel Ty3/gypsy-like retrotransposon of Paragonimus westermani: molecular characters and its differentially preserved mobile potential according to host chromosomal polyploidy
© Bae et al; licensee BioMed Central Ltd. 2008
Received: 14 May 2008
Accepted: 14 October 2008
Published: 14 October 2008
Retrotransposons have been known to involve in the remodeling and evolution of host genome. These reverse transcribing elements, which show a complex evolutionary pathway with diverse intermediate forms, have been comprehensively analyzed from a wide range of host genomes, while the information remains limited to only a few species in the phylum Platyhelminthes.
A LTR retrotransposon and its homologs with a strong phylogenetic affinity toward CsRn1 of Clonorchis sinensis were isolated from a trematode parasite Paragonimus westermani via a degenerate PCR method and from an insect species Anopheles gambiae by in silico analysis of the whole mosquito genome, respectively. These elements, designated PwRn1 and AgCR-1 – AgCR-14 conserved unique features including a t-RNATrp primer binding site and the unusual CHCC signature of Gag proteins. Their flanking LTRs displayed >97% nucleotide identities and thus, these elements were likely to have expanded recently in the trematode and insect genomes. They evolved heterogeneous expression strategies: a single fused ORF, two separate ORFs with an identical reading frame and two ORFs overlapped by -1 frameshifting. Phylogenetic analyses suggested that the elements with the separate ORFs had evolved from an ancestral form(s) with the overlapped ORFs. The mobile potential of PwRn1 was likely to be maintained differentially in association with the karyotype of host genomes, as was examined by the presence/absence of intergenomic polymorphism and mRNA transcripts.
Our results on the structural diversity of CsRn1-like elements can provide a molecular tool to dissect a more detailed evolutionary episode of LTR retrotransposons. The PwRn1-associated genomic polymorphism, which is substantial in diploids, will also be informative in addressing genomic diversification following inter-/intra-specific hybridization in P. westermani populations.
Retrotransposons, which comprise a major portion of eukaryotic genomes, replicate progeny copies into new genomic loci via reverse transcription of an RNA intermediate . As intragenomic parasites, retrotransposons have been known as a potent causative agent involved in various harmful biological processes such as insertional mutagenesis  and ectopic recombination . Conversely, cumulative data have evidenced that these elements play significant roles in the formation and maintenance of host chromosomes . The sporadic expansion of retrotransposons can also introduce genomic/phenotypic variants, which lead to speciation in the long evolutionary terms [5, 6]. Retrotransposons are subdivided into two large categories, long-terminal-repeat (LTR) and non-LTR retrotransposons, according to overall structures, and the elements with LTR seem to be the most abundant type in invertebrates. The LTR retrotransposons include multiple groups, as have been distinguished by comparing their own gene products [7, 8].
In addition to the typical LTR retrotransposons, multiple elements with unusual structural features have recently been isolated from various genomes, such as CsRn1 of Clonorchis sinensis , Xena of Takifugu rubripes , Gmr1 of Gadus morhua  and Saci-2 of Schistosoma mansoni . The CsRn1 and Saci-2 elements were found to encode Gag with unique motifs of CHCC and CCCH, respectively, instead of the conventional CCHC. Gmr1 produced a Pol protein, in which the functional protein domains lie in an order identical to those of Ty1/copia elements (protease [PR]-integrase [IN]-reverse transcriptase [RT]-RNase H [RH]). Retrotransposons of the most ancient Xena group had a single open reading frame (ORF) containing a RT domain but lacking any other enzymatic domain. Taken together, these facts have suggested that the category and evolutionary episode of the diverse reverse-transcribing elements are more complex than currently understood.
Paragonimus westermani is a hermaphroditic, digenetic trematode that lives as adult in the lungs of carnivorous mammals. This parasite causes pulmonary and cerebral diseases in humans and is one of the most medically important flukes. The natural populations of P. westermani in northeast Asian countries have three different levels of polyploidy in their genomic contents, i.e., di-, tri- and tetrapolyploidy, displaying variations in morphology, allozyme patterns, and nucleotide sequences of ribosomal and mitochondrial DNAs . The Paragonimus genome contains retrotransposons, which belong to various retrotransposon groups including the CsRn1 clade . CsRn1-like retrotransposons have been detected largely in trematodes and insects [9, 12, 15]. With the unique Gag motif and amino acid conservation, members of the clade showed heterogeneity in their coding profiles. Furthermore, Boudicca and Saci-3 of S. mansoni contain a third ORF resembling envelope protein (ENV) of errantivirus and retrovirus [12, 16].
The genomic distribution of CsRn1-like elements have been well described in C. sinensis and S. mansoni, with their unique structural features [9, 12, 16]. Nevertheless, molecular information on this distinctive clade is highly limited to address their evolutionary episodes. In this study, we isolated a novel CsRn1-like LTR retrotransposon in the P. westermani genome and analyzed its intra- and inter-genomic distribution patterns among the parasite populations with different karyotes. Differential replication activity of the Paragonimus element was examined by detecting the presence or absence of mRNA transcripts and intergenomic polymorphism introduced by the element. The molecular characters of multiple retrotransposons homologous to the element were also analyzed from the genomes of an African malaria mosquito Anopheles gambiae, which had recently been released , and a fruit fly Drosophila melanogaster, in order to gain more detailed information on the invertebrate-specific CsRn1 clade. The heterogeneous expression strategies were found to be substantial within the unique clade, and the possible evolutionary relationships between elements with distinctive coding profiles were shown by phylogenetic analyses.
Isolation of CsRn1-like retrotransposons from P. westermani and A. gambiae
CsRn1-like retrotransposons of Anopheles gambiae isolated in this study
Contig No. (nucleotide position)a
Nucleotide lengths (bp)
LTR identity (%)
Functional motifs and structural features of PwRn1 and AgCRs
TSD detected in all of the CsRn1-like elements were found to be 4 bp, although there seemed to be no specific or preferential nucleotide in selecting the insertional targets among copies of each element and among the AgCR types (Table 1). The short inverted repeats of 3 bp, found at both ends of LTR in most retrotransposons (5'-TGT...ACA-3') , were slightly modified either to 5'-TGT...AAA-3' or to 5'-TGT...ATA-3' (Additional file 1). The LTR pairs flanking each of the AgCR elements displayed >97% nucleotide identities, suggestive of their recent expansion in the mosquito genome . The putative primer-binding site of these elements for the synthesis of the first cDNA strand (5'-TGGTGAG/CCCCGT/A-3') was complementary to the nucleotides at the 3'-end of bovine t-RNATrp. An additional priming site (polypurine tract) for the synthesis of the second cDNA strand was also found in the direct upstream region of 3'-LTR.
The internal regions of these elements contained ORFs with different expression strategies for their autonomous retrointegration. PwRn1 had an ORF similar to that of CsRn1, in which the gag and pol genes were fused together in a single frame (Figure 1). Of the 14 AgCR elements analyzed, nine elements utilized two ORFs overlapped by -1 frameshifting to encode the Gag and Pol proteins, while the other six elements contained two ORFs with an identical reading frame, which were intervened by a short nucleotide sequence ranging from 60 to 165 bp (Figure 2). The amino acid sequences of Gag proteins in retrotransposons generally diverge very rapidly so that they display a low level of sequence identity to one another, except for the functional Cys-His signature (CX2CX4HX4C; CCHC) . The proteins, however, showed a high degree of amino acid conservation over their entire lengths in these CsRn1- like retrotransposons. They had the unique CHCC signature (CX2HX9CX3C) in the C-terminal regions, while it was imperfect in 3 elements, AgCR-5, -7 and -9. The α-helix-rich secondary structures of the Gag proteins were similar to those of the other Ty3/gypsy-like LTR retrotransposons (Additional file 2). The enzymatic domains contained in the Pol proteins were also well conserved with the conventional motifs/signatures: DT/SG tripeptide of PR, F/YXDD tetrapeptide of RT, DAS tripeptide of RH, and HHCC and DDE signatures of IN (data not shown). A series of LTR retrotransposons have an additional motif in the C-terminal region of IN, which is implicated in non-specific binding to the target site and/or to LTR via the best-conserved GPY/F residues [21, 22]. This motif could also be detected in the corresponding proteins of PwRn1 and AgCR s with the exceptions of AgCR-7 and -14. Neither the chromatin organization modifier domain (chromodomain) nor ENV-like protein was detected in these elements.
Phylogenetic analysis of CsRn1-like retrotransposons
Genomic distribution and mobile potential of PwRn1
Since the first detection of LTR retrotransposons with an unusual capsid protein in a trematode and insect species [9, 24], a series of similar elements have been isolated from genomes of the other trematodes and insects. They share unique properties such as PBS for t-RNATrp, 4 bp of TSD and tight amino acid conservation in the encoded proteins. The most conspicuous feature, nevertheless, can be found in the amino acid sequences of Gag proteins. In general, the first ORF contained in various retrotransposons diverge rapidly so that they display low levels of sequence identity to one another, except for the active Cys-His motif (CX2CX4HX4C) . However, the proteins of CsRn1-like elements have been known to conserve a novel sequence motif of CX2HX9CX3C (CHCC), instead of the conventional CCHC motif. These facts have suggested that members of this clade are originated from an intermediate form of reverse-transcribing elements, which have emerged in the common ancestor of the monophylyl ecdysozoan and lophotrochozoan animals. Retrotransposons characterized from the lung fluke and mosquito possessed all of these features and thus, further supported the notion that these elements are selectively expanded in the lower animal taxa.
The Gag protein of retrotransposons functions as a nucleocapsid protein during the assembly of virus-like particle (VLP), which is thought to be an important intermediate in retrotransposition . The origin and functional relevance of Gag found in the CsRn1-like elements could not be properly addressed, because neither cellular nor the other retrotransposon-related protein was detected during homolog searches based on the BLAST algorithm and Hidden Markov models. Considering the well-conserved, α-helix-rich secondary structure (Additional file 2) and the active replication of these elements including CsRn1 , the proteins seem to have nucleic acid-binding capacity to form VLPs. Malik and Eickbush  have proposed the chimeric origin of LTR retrotransposons and retroviruses, between a preexisting element and non-LTR element/host gene. This may reflect the genetic flexibility endogenous in the mobile elements to generate novel functional/structural variants. A gag gene equipped in the preexisting element is likely to have been substituted by a presently unidentified host gene with the unusual motif during the evolutionary start point of CsRn1-like elements.
Different expression strategies were observed among the CsRn1-like elements (Figure 1 and 2; see also ). A single primary transcript is responsible for the production of multiple retrotransposon proteins, which are essential for the autonomous replication of the corresponding element, and is subjected to ribosomal frameshifting to maintain a proper ratio between the Gag and Pol proteins . Genetic events including insertion/deletion of single nucleotide around the frameshifting region are occasionally attributable to the generation of retrotransposons with a single fused ORF. The Gag and Pol proteins are produced by proteolytic cleavage of a single translation product in the cases of single-ORF variants [28, 29]. The variant form will replace the previous one in the host genome, via the highly efficient VLP formation . The structural variation was not detected with the multiple PwRn1 copies retrieved by PCR. The ancient copies with two overlapped ORFs might have decayed in the Paragonimus genome, mainly due to the low propagation rate. Otherwise, diagnostic base substitutions in priming regions of the variant copies might lead to a failure in the amplification of the corresponding DNA segments.
Retrotransposons generally show very low retrotransposition and excision rates, and their expansion is largely restricted to germ cells and/or an early developmental stage . These elements, however, become unstable under specific circumstances and can rapidly expand in the host genomes by increasing their replication frequency. The mobile potential of PwRn1 seemed to be differentially preserved among P. westermani populations; the element maintained its activity in diploids, while a majority of the activity had been suppressed in triploid individuals (Figures 5 and 6). In association with the origin of the triploid worms, an allopolyploidic hybridization occurred in ancestral diploids at the intra- or interspecies level has been suggested as a relevant genomic event ( and references therein). IRAP markers shared in the diploid and triploid worms are likely to represent genomic loci flanked by PwRn1 copies in the putative ancestral diploids, and may provide further genomic evidence supporting the proposed mechanism. Together with biotic/abiotic stresses such as aridity , polyploidization can boost the mobile potential of various retrotransposons [31, 32]. Therefore, it was unexpected to find that PwRn1 is less active in triploid genomes. Host genome surveillance for retrotransposons is likely to be reinforced in association with an increase in genomic dosage . Alternatively, PwRn1 itself would give a feedback effect as a mechanism to compensate for the amplified copy number, following the hybridization process. Phylogenetic and comparative analyses with the multiple PwRn1 copy sequences will be informative to elucidate the evolutionary status/mode of the element among the Paragonimus populations with different polyploidy.
There have been various reports on the comparison of P. westermani genomes to address the issue regarding the origin of polyploid genomes (reviewed in ). Molecular data such as the nucleotide sequences of mitochondrial and ribosomal DNAs dichotomized the genomes into the northeast (China, Japan, Korea and Taiwan) and south (the Philippines, Thailand and Malaysia) Asian groups, but failed in establishing more detailed relationships among the northeast populations with different polyploidy . The retrointegration of retrotransposons is an irreversible event and the integration site is randomly selected [35, 36]. These characteristics of mobile elements are particularly suitable for the comparative population studies at the intra- and interspecies levels [37, 38]. In this study, we provided molecular evidence demonstrating genomic conservation and/or diversification between Korean diploids and triploids by analyzing intergenomic distributions of PwRn1 (Figure 5). The data suggests that IRAP and locus-specific typing of active retrotransposons can be an informative genetic marker in exploring the flow of haplotypes among P. westermani populations, and in elucidating the controversial hypotheses on the origin of the triploid populations. In plants, transposable elements are believed to be reactivated very early following polyploid formation and thus contribute to genetic diversity in the polyploidy species . However, understanding on the role of these elements in polyploidy evolution is just beginning to emerge. The polyploid genomes are less common in animals than in plants. Our results on the suppressed mobile activity of PwRn1 in the triploid Paragonimus genomes can boost research to examine the evolutionary impacts of retrotransposons in polyploid animal genomes.
With the completion of genome projects from numerous eukaryotic organisms, comprehensive studies on the mobile genetic elements have been conducted with the whole genomic sequences. These studies have broadened our knowledge on the dynamics and impacts of retrotransposons in the host genomes. Despite the rapid accumulation of data, however, only limited information is currently available from a few species in the wide phylum Platyhelminthes. It is apparent that these invertebrate animal genomes contain diverse retrotransposons and that these elements are actively involved in the remodeling and diversification of their host genomes. Our results on the CsRn1-like elements of P. westermani and A. gambiae, especially in association with the diversified expression strategies, can make a significant step toward a better understanding of evolutionary episode within the unique CsRn1 clade of LTR retrotransposons, which is specifically expanded in the lower animal taxa. The differentially preserved mobile potential of PwRn1 and resulting genomic polymorphism in diploid individuals will also be helpful in designing a comparative genomic study with the northeast Asian populations of P. westermani.
Experimental dogs were orally challenged with the metacercariae of P. westermani, which were collected from naturally infected crayfish in endemic areas of Korea (Haenam, 2n; Bogil-do and Youngam, 3n) and China (Nanchang, Jiangxi Province, 2n) . Five months after the infections, adult worms of the parasite were harvested from the dogs' lungs. The worms were washed five times with physiological saline at 4°C and used for the extraction of DNA and RNA with the Wizard DNA Purification Kit (Promega, Madison, WI, USA) and TriZol reagents (Invitrogen, Carlsbad, CA, USA), according to the manufacturers' instructions. Materials from diploid worms (Haenam, Korea) were commonly used in this study; otherwise the sources were specifically indicated. The use of animals was approved by the Animal Ethics Committee of Korea Food and Drug Administration (protocol number NIH-05-09).
Isolation of a CsRn1-like retrotransposon from P. westermani
The retrotransposon-related sequences were retrieved via a degenerate PCR method from the P. westermani genome, as described previously . Of the sequences obtained, a clone (Pw-D-13), that showed a significant degree of sequence identity with CsRn1 was selected for further characterization. Genomic DNA library of the parasite constructed using the lambda FIX II vector system (Stratagene, La Jolla, CA. USA) was screened with the DNA probe according to the standard procedure of plaque-lift hybridization. The inserts of two positive clones were amplified by long-range PCR using primers designed from the vector regions (5'-CTAATACGACTCACTATAGGGCGTCG-3' and 5'-CCCTCACTAAAGGGAGTCGACTCG-3') and LA Taq polymerase, under the standard cycle condition (Takara, Shiga, Japan). The amplified products were digested with Kpn I and Xho I, and were cloned into pBluescript II SK(-) phagemid (Stratagene) for sequencing. The nucleotide sequences were automatically determined with the ABI PRISM 377 DNA Sequencer (Applied Biosystems, Foster City, CA, USA) and the BigDye Terminator Cycle Sequencing Reaction Kit (Perkin Elmer Corporation, Foster City, CA, USA). To ensure the accuracy of the reactions, nucleotide sequences from both strands were determined. Contig sequences were obtained by overlapping the sequence information. These contigs were then compared with one another to determine the full-unit of a retrotransposon encompassing the Pw-D-13 clone, which was designated P. westermani retrotransposon 1 (PwRn1).
In silico identification of insect retrotransposons homologous to CsRn1
Whole genomic sequences of A. gambiae and D. melanogaster deposited in the GenBank were surveyed by BLAST algorithms using the amino acid sequences of CsRn1 Gag protein, which showed a unique Cys motif , as a query. The hits showing amino acid positive values greater than 45% throughout the whole query sequence were subject to further analyses to determine the full-units of retrotransposons encompassing each of the gag genes. Multiple scaffold sequences were compared with one another and then, a common genetic element contained within them was isolated. The terminal repeats flanking a protein-encoding internal region were determined by analyzing the sequences with bl2seq at National Center for Biotechnology Information (NCBI, http://www.ncbi.nlm.nih.gov/) and further verified by recognizing the duplicated target sequences from the direct upstream and downstream regions of the elements. BLAST searches with the entire nucleotide sequences of the elements were followed to confirm their integrity and to retrieve additional copy sequences from the identical databases. The coding profiles and conserved protein domains were predicted using ORF Finder at NCBI and InterProScan programs at European Bioinformatics Institute (EBI, http://www.ebi.ac.uk/Tools/InterProScan), respectively. The homology patterns were also determined by a series of BLAST searches against the non-redundant genomic/protein databases of the GenBank.
Southern and dot-blot hybridization of PwRn1
Genomic DNAs isolated from P. westermani adult worms (5 μg per each) were digested with restriction enzymes, Acs I, Sac I and Sfu I. After being separated on a 0.8% agarose gel, the restricted DNA fragments were transferred onto a nylon membrane (Hybond-N+; Amersham Pharmacia Biotech, Uppsala, Sweden) by capillary action in 10 × SSC. The blots were hybridized with the Pw-D-13 probe enzymatically labeled with the ECL Direct Labeling Kit (Amersham Pharmacia Biotech). The labeling, hybridizing, and signal detection conditions were determined under the manufacturer's instructions. For stringency washing, the membrane was washed twice in 0.1 × SSC containing 6 M urea and 0.4% SDS at 42°C for 20 min, and twice in 2 × SSC at room temperature for 5 min.
Various amounts of the Pw-D-13 DNA fragment were blotted onto a nylon membrane, according to the standard procedure of dot-blot hybridization. The membrane was hybridized with the genomic probe of P. westermani, which was prepared by sonicating the genomic DNA into small DNA fragments between 0.5 and 2 kb. For a single-copy control, a portion of the Paragonimus cysteine protease gene [GenBank:U70537], with a size of approximately 700 bp, was amplified from the parasite genome by PCR using a gene-specific primer pair (5'-TCAGTTGTCTTGTTGTCGTGG-3' and 5'-TGCCTGTTTCTCCTCATTCTTG-3') and blotted onto the membrane. Probe labeling and hybridizing conditions were identical to those for Southern blot hybridization. The intensities of signals were measured using the LAS-1000plus system (FUJIFILM, Tokyo, Japan) and compared with those of the control gene.
Reverse-transcription PCR (RT-PCR)
Total RNAs were extracted from the diploid and triploid adult worms (10 worms per each) and treated with the RNase-free DNase (GIBCO BRL, Rockvile, MD, USA). Reverse transcription and following amplification of PwRn1 transcripts were carried out with the total RNAs and retrotransposon-specific primers, using RNA PCR Kit (AMV) (Ver. 2.1; Takara) under the manufacturer's instructions. The primers used were as follows: 5'-AGAGAGATGTGGCTACAGCG-3' and 5'-GTTTAAGTCGTGGACCTCAG-3' for gag; 5'-GCAGGTTGACACCAGACAAG-3' and 5'-ATCGGTTAACGGTCGGATAC-3' for rt; 5'-GTCGATGCAATCCGTTGGAC-3' and 5'-CCAGTGCACCCTACGACCTG-3' for in; 5'-GGCCATGTACGTTGCTATCC-3' and 5'-CAGAGAGAACAGTGTTGGCG-3' for β-actin gene. The absence of any contaminating DNA was confirmed by preparing reactions without reverse transcriptase during the first round of cDNA synthesis and the β-actin gene was selected for a house-keeping gene control. The reaction products were resolved by agarose gel electrophoresis.
Inter-retrotransposon amplified polymorphism (IRAP)
Primers were designed to match the 5'- and 3'-ends of a consensus PwRn1 LTR sequence in the outward directions, respectively (LTR-5R, 5'-AGGCAGGCCAGTGAAATCT-3' and LTR-3F, 5'-CGACTTAGTGCAACGAGCAC-3'). Genomic DNAs were individually extracted from diploid and triploid worms. The IRAP PCR was performed with reaction mixtures containing 20 ng of the genomic DNA, 1 μM of each primer, 0.2 mM of each dNTP precursors, 1.6 mM of MgCl2, and 1.25 units of Taq polymerase (Takara) in 2 mM Tris-HCl buffer (pH 8.0). PCR cycling parameters were as follows: 4 min at 94°C; 35 cycles of 40 sec at 96°C, 40 sec at 56°C, and 2 min at 72°C; 10 min at 72°C. The PCR products were analyzed by electrophoresis on 2% agarose gels (NuSieve 3:1 agarose; Cambrex Bio Science, Rockland, ME, USA) and visualized by staining with ethidium bromide.
Pol protein sequences of LTR retrotransposons were aligned with ClustalX and optimized with GeneDoc programs. The regions corresponding to each of the RT, RH and IN domain sequences were selected from the alignment, in order to increase the analytical resolution, compared to that obtained by using RT sequences alone [22, 40]. The resulting concatenates comprising approximately 510 amino acid positions were adopted in a phylogenetic analysis for the construction of maximum likelihood tree using the quartet method implemented in TREE-PUZZLE (Ver. 5.2). The analytical options were as follows: JTT model for substitution, estimation of invariant site proportion from the input data, 1,000 puzzling steps assuming rate heterogeneity with eight gamma categories (the gamma distribution parameter alpha was estimated from the data set). Indels between pairs of sequences were regarded as missing data. Tree construction was performed by the neighbor-joining method. Gag sequences of the CsRn1-clade members were similarly examined by the phylogenetic approach. The phylogenetic trees were displayed by the TreeView program.
inter-retrotransposon amplified polymorphism
long terminal repeat
open reading frame
target site duplication
This work was supported by a grant from the Korea Science and Engineering Foundation (KOSEF; 1999 2-208-004 5) to SYC and partly by a National Research and Development Program of the National Institute of Health (the Anti-Communicable Diseases Control Program, NIH 348-6111-215) to YAB.
- Boeke JD, Garfinkel DJ, Styles CA, Fink GR: Ty elements transpose through an RNA intermediate. Cell. 1985, 40: 491-500.PubMedView ArticleGoogle Scholar
- Jurka J, Kapitonov VV: Sectorial mutagenesis by transposable elements. Genetica. 1999, 107: 239-248.PubMedView ArticleGoogle Scholar
- Lim JK, Simmons MJ: Gross chromosome rearrangements mediated by transposable elements in Drosophila melanogaster. Bioessays. 1994, 16: 269-275.PubMedView ArticleGoogle Scholar
- Hudakova S, Michalek W, Presting GG, ten Hoopen R, dos Santos K, Jasencakova Z, Schubert I: Sequence organization of barley centromeres. Nucleic Acids Res. 2001, 29: 5029-5035.PubMedPubMed CentralView ArticleGoogle Scholar
- McDonald JF: Macroevolution and retroviral elements. Bioscience. 1990, 40: 183-191.View ArticleGoogle Scholar
- Kidwell MG, Lisch D: Transposable elements as sources of variation in animals and plants. Proc Natl Acad Sci USA. 1997, 94: 7704-7711.PubMedPubMed CentralView ArticleGoogle Scholar
- Malik HS, Eickbush TH: Phylogenetic analysis of ribonuclease H domains suggests a late, chimeric origin of LTR retrotransposable elements and retroviruses. Genome Res. 2001, 11: 1187-1197.PubMedView ArticleGoogle Scholar
- Lloréns C, Futami R, Bezemer D, Moya A: The Gypsy database (GyDB) of mobile genetic elements. Nucleic Acids Res. 2008, 36: D38-D46.PubMedPubMed CentralView ArticleGoogle Scholar
- Bae YA, Moon SY, Kong Y, Cho SY, Rhyu MG: CsRn1, a novel active retrotransposon in a parasitic trematode, Clonorchis sinensis, discloses a new phylogenetic clade of Ty3/gypsy-like LTR retrotransposons. Mol Biol Evol. 2001, 18: 1474-1483.PubMedView ArticleGoogle Scholar
- Dalle Nogare DE, Clark MS, Elgar G, Frame IG, Poulter RT: Xena, a full-length basal retroelement from tetraodontid fish. Mol Biol Evol. 2002, 19: 247-255.PubMedView ArticleGoogle Scholar
- Butler M, Goodwin T, Poulter R: An unusual vertebrate LTR retrotransposon from the cod Gadus morhua. Mol Biol Evol. 2001, 18: 443-447.PubMedView ArticleGoogle Scholar
- DeMarco R, Kowaltowski AT, Machado AA, Soares MB, Gargioni C, Kawano T, Rodrigues V, Madeira AM, Wilson RA, Menck CF, Setubal JC, Dias-Neto E, Leite LC, Verjovski-Almeida S: Saci-1, -2, and -3 and Perere, four novel retrotransposons with high transcriptional activities from the human parasite Schistosoma mansoni. J Virol. 2004, 78: 2967-2978.PubMedPubMed CentralView ArticleGoogle Scholar
- Blair D, Xu ZB, Agatsuma T: Paragonimiasis and the genus Paragonimus. Adv Parasitol. 1999, 42: 113-222.PubMedView ArticleGoogle Scholar
- Bae YA, Kong Y: Divergent long-terminal-repeat retrotransposon families in the genome of Paragonimus westermani. Korean J Parasitol. 2003, 41: 221-231.PubMedPubMed CentralView ArticleGoogle Scholar
- Tubío JM, Naveira H, Costas J: Structural and evolutionary analyses of the Ty3/gypsy group of LTR retrotransposons in the genome of Anopheles gambiae. Mol Biol Evol. 2005, 22: 29-39.PubMedView ArticleGoogle Scholar
- Copeland CS, Brindley PJ, Heyers O, Michael SF, Johnston DA, Williams DL, Ivens AC, Kalinna BH: Boudicca, a retrovirus-like long terminal repeat retrotransposon from the genome of the human blood fluke Schistosoma mansoni. J Virol. 2003, 77: 6153-6166.PubMedPubMed CentralView ArticleGoogle Scholar
- Holt RA, Subramanian GM, Halpern A, Sutton GG, Charlab R, Nusskem DR, Wincker P, Clark AG, Ribeiro JM, Wides R: The genome sequence of the malaria mosquito Anopheles gambiae. Science. 2002, 298: 129-149.PubMedView ArticleGoogle Scholar
- Boeke JD, Stoye JP: Retrotransposons, endogenous retroviruses, and the evolution of retroelements. Edited by: Coffin JM, Hughes SH, Varmus HE. 1997, Cold Spring Harbor Laboratory pressGoogle Scholar
- Jordan IK, McDonald JF: Tempo and mode of Ty element evolution in Saccharomyces cerevisiae. Genetics. 1999, 151: 1341-1351.PubMedPubMed CentralGoogle Scholar
- Eickbush TH: Origin and evolutionary relationships of retroelements. Edited by: Morse SS. 1994, Raven pressGoogle Scholar
- Engelman A, Hickman AB, Craigie R: The core and carboxyl-terminal domains of the integrase protein of human immunodeficiency virus type 1 each contribute to nonspecific DNA binding. J Virol. 1994, 68: 5911-5917.PubMedPubMed CentralGoogle Scholar
- Malik HS, Eickbush TH: Modular evolution of the integrase domain in the Ty3/Gypsy class of LTR retrotransposons. J Virol. 1999, 73: 5186-5190.PubMedPubMed CentralGoogle Scholar
- Jurka J: Evolutionary impact of human Alu repetitive elements. Curr Opin Genet Dev. 2004, 14: 603-608.PubMedView ArticleGoogle Scholar
- Abe H, Ohbayashi F, Shimada T, Sugasaki T, Kawai S, Mita K, Oshiki T: Molecular structure of a novel gypsy -Ty3-like retrotransposon (Kabuki) and nested retrotransposable elements on the W chromosome of the silkworm Bombyx mori. Mol Gen Genet. 2000, 263: 916-924.PubMedView ArticleGoogle Scholar
- Roth JF: The yeast Ty virus-like particles. Yeast. 2000, 16: 785-95.PubMedView ArticleGoogle Scholar
- Bae YA, Kong Y: Evolutionary courses of CsRn1 long-terminal-repeat retrotransposon and its heterogeneous integrations into the genome of the liver fluke, Clonorchis sinensis. Korean J Parasitol. 2003, 41: 209-219.PubMedPubMed CentralView ArticleGoogle Scholar
- Dinman JD, Wickner RB: Ribosomal frameshifting efficiency and gag/gag-pol ratio are critical for yeast M1 double-stranded RNA virus propagation. J Virol. 1992, 66: 3669-3676.PubMedPubMed CentralGoogle Scholar
- Levin HL, Weaver DC, Boeke JD: Novel gene expression mechanism in a fission yeast retroelement: Tf1 proteins are derived from a single primary translation product. EMBO J. 1993, 12: 4885-4895.PubMedPubMed CentralGoogle Scholar
- Kalmykova AI, Kwon DA, Rozovsky YM, Hueber N, Capy P, Maisonhaute C, Gvozdev VA: Selective expansion of the newly evolved genomic variants of retrotransposon 1731 in the Drosophila genomes. Mol Biol Evol. 2004, 21: 2281-2289.PubMedView ArticleGoogle Scholar
- Kalendar R, Tanskanen J, Immonen S, Nevo E, Schulman AH: Genome evolution of wild barley (Hordeum spontaneum) by BARE-1 retrotransposon dynamics in response to sharp microclimatic divergence. Proc Natl Acad Sci USA. 2000, 97: 6603-6607.PubMedPubMed CentralView ArticleGoogle Scholar
- Evgen'ev MB, Zelentsova H, Shostak N, Kozitsina M, Barskyi V, Lankenau DH, Corces VG: Penelope, a new family of transposable elements and its possible role in hybrid dysgenesis in Drosophila virilis. Proc Natl Acad Sci USA. 1997, 94: 196-201.PubMedPubMed CentralView ArticleGoogle Scholar
- Labrador M, Farre M, Utzet F, Fontdevila A: Interspecific hybridization increase transposition rates of Osvaldo. Mol Biol Evol. 1999, 16: 931-937.PubMedView ArticleGoogle Scholar
- Cam HP, Noma K, Ebina H, Levin HL, Grewal SI: Host genome surveillance for retrotransposons by transposon-derived proteins. Nature. 2008, 451: 431-436.PubMedView ArticleGoogle Scholar
- Iwagami M, Ho LY, Su K, Lai PF, Fukushima M, Nakano M, Blair D, Kawashima K, Agatsuma T: Molecular phylogeographic studies on Paragonimus westermani in Asia. J Helminthol. 2000, 74: 315-322.PubMedGoogle Scholar
- Okada N: SINEs. Curr Opin Genet Dev. 1991, 1: 498-504.PubMedView ArticleGoogle Scholar
- Kido Y, Saitoh M, Murata S, Okada N: Evolution of the active sequences of the Hpa I short interspersed elements. J Mol Evol. 1995, 41: 986-995.PubMedView ArticleGoogle Scholar
- Takahashi K, Terai Y, Nishida M, Okada N: A novel family of short interspersed repetitive elements (SINEs) from cichlids: the patterns of insertion of SINEs at orthologous loci support the proposed monophyly of four major groups of cichlid fishes in lake Tanganyika. Mol Biol Evol. 1998, 15: 391-407.PubMedView ArticleGoogle Scholar
- Nikaido M, Rooney AP, Okada N: Phylogenetic relationships among cetartiodactyls based on insertions of short and long interspersed elements: hippopotamuses are the closest extant relatives of whales. Proc Natl Acad Sci USA. 1999, 96: 10261-10266.PubMedPubMed CentralView ArticleGoogle Scholar
- Park GM, Lee KJ, Im KI, Park H, Yong TS: Occurrence of a diploid type and a new first intermediate host of a human lung fluke, Paragonimus westermani, in Korea. Exp Parasitol. 2001, 99: 206-212.PubMedView ArticleGoogle Scholar
- Xiong Y, Eickbush TH: Origin and evolution of retroelements based upon their reverse transcriptase sequences. EMBO J. 1990, 9: 3353-3362.PubMedPubMed CentralGoogle Scholar
- Comai L, Tyagi AP, Winter K, Holmes-Davis R, Reynolds SH, Stevens Y, Byers B: Phenotypic instability and rapid gene silencing in newly formed Arabidopsis allotetraploids. Plant Cell. 2000, 12: 1551-1568.PubMedPubMed CentralView ArticleGoogle Scholar
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