Chaetognaths are a small marine phylum living in various habitats, but most of them are among the most abundant planktonic organisms [1]. Their body is constituted of three parts, the head, trunk and tail, separated by septa [2]. These animals are protandric hermaphrodites; the ovaries lie in the trunk on both sides of the gut, while the testis are in the tail (reviewed in [3]). Their phylogenetic position remains enigmatic, although recent molecular analyses suggest a protostome affinity [4-7]. Casanova et al. [8] showed that chaetognaths can be considered as a model animal.

The chaetognath genomes exhibit several molecular singularities including paralog of both ribosomal RNA genes and ribosomal protein genes [9-11] and conservation of extremely divergent paralogous sequences suggesting a low rate of gene conversion [12]. Moreover, in situ hybridizations have shown that each type of both 18S and 28S rRNA paralogs is important for specific cellular functions [13-15]. The causes of these features are unknown, even if an alloploid event has been suggested [12]. As it is well known that mobile genetic elements (MGEs, also called transposable elements) can strongly impact genome evolution, knowledge of these elements about few is known in chaetognath [16] could be a fruitful contribution.

MGEs are ubiquitous in a wide range of living organisms; however, they make up a large fraction of genome sizes which is evident through the C-values of only pluricellular eukaryotes. These elements, which can transpose from one location to another within the genome, are known to be one of the causes of large scale genome reorganization [17]. Although regarded as a selfish DNA with negative impact on the host, MGEs have been shown to contribute significantly to gene evolution [18]. Now these elements are regarded as one of the principal forces driving the evolution of eukaryotic genomes [19,20]. Due to the great number of known MGEs (several thousands) and as new types of mobile repeats are discovered at a rapid rate, a unified classification system for eukaryotic transposable elements has recently been proposed, designed on the basis of the transposition mechanism, sequence similarities and structural relationships [21]. MGEs are divided into two classes. Class I retrotransposons replicate via an RNA intermediate; the key enzyme of this mechanism is the reverse transcriptase (RT), each complete replication cycle produces one new copy. Class II transposons, which are out of our topic, move as a DNA segment by a classical ‘‘cut-and-paste’’ mechanism.

Retrotransposons have been divided into five orders on the basis of their mechanistic features, organization and reverse transcriptase (RT) phylogeny: LTR-retrotransposons, DIRS-like elements, Penelope-like elements, LINEs and SINEs. The LTR-retrotransposons are retrovirus-like elements containing long terminal repeat (LTR) and ORFs for at least gag, a structural protein for virus-like particles, and pol. Pol encodes an aspartic proteinase (PR), reverse transcriptase, RNase H, and a DDE integrase (IN). LTR retrotransposons also contain specific signals for packaging, dimerization, reverse transcription and integration. The two main superfamilies Gypsy and Copia, differing in the relative order of integrase and RT domains. According to Wicker et al. [21], the other members are retrovirus, endogenous retroviruses (ERVs) and the Bel–Pao subfamily containing MGEs structurally similar to Gypsy or Copia elements but exhibit differences on RT phylogenies. Evolutionarily, LTR retrotransposons are closely related to retroviruses. Retroviruses have a viral lifestyle through acquisition of an envelope protein added to various regulatory proteins. Retroviruses are distributed widely among vertebrates and may also occur in some invertebrates, for example, some members of the Gypsy family found in dipteran insects are able to infect new individuals [22]. Retrovirus can also be transformed into an LTR retrotransposon-like through inactivation or deletion of the domains that enable extracellular mobility and can only be inherited by vertical transmission through the germ line, this is the case of the endogenous retroviruses even if some of them can always be transmitted horizontally.

Penelope-like elements (PLEs) represent a new order of retroelements identified in more than 80 species including unicellular animals, fungi and plants [23]. These elements code for a protein that represents a fusion between a reverse transcriptase and a GIY-YIG endonuclease. They encode an RT that is more closely related to telomerase than to the RT from LTR retrotransposons or LINEs; moreover, members of this order have also LTR-like sequences that can be in a direct or an inverse orientation. The principal other retrotransposon orders are the LINEs and the SINEs. The LINEs lack LTRs, can reach several kilobases in length, and are found in all eukaryotic kingdoms. Autonomous LINEs encode at least an RT and a nuclease in their pol ORF for transposition, they often display a poly(A) tail at their 3' end. The SINE elements are non-autonomous elements and as they do not contain RT gene, they rely on the activity of RT proteins encoded by LINEs to retrotranspose [24]. They originated from accidental retrotransposition of various polymerase III transcripts and possess an internal III promoter, allowing them to be expressed. SINEs belong to retrosequences, a group containing all the sequences arisen by reverse transcription of ribosomal, messenger and small stable RNAs [25,26].

A previous study using degenerate primers gave positive results during screening of Sagitta sp. for LINE-like and Gypsy-like reverse transcriptases, and also for Mariner-like transposases; however, only two LINE elements have been sequenced [16]. As in this phylum retrovirus-like sequences have never been documented, analysis of a published expressed sequence tag (EST) collection [6] of the chaetognath Spadella cephaloptera has been made.

Present analysis of a chaetognath EST library allowed to find thirteen retrotransposon sequences. Retrotransposons comprise 0.11% and 0.54 % of respectively the total ESTs and of the number of ESTs assigned to protein gene transcripts. In other animals, the corresponding values are generally higher, e.g., 1.0% in venom glands of a fish [53], from 4 to 14 according to the stages in a platyhelminth [54], and 14% in mouse oocytes [55]. However, the chaetognath value is similar to the 0.12% reported from the survey of ESTs from many plant species [56] but higher values could be found [57]. These transcriptional differences could be due to severe controls of the retrotransposon expression in higher plants, in which LTR-retrotransposons alone can comprise 50–90% of the genome [58] and probably also in juvenile chaetognaths. However, EST collection can exhibit several bias; indeed, ESTs are short single-pass sequence reads derived from cDNA clones selected randomly from cDNA libraries, and in contrast to genomic sequences, they are generally of low quality, poorly annotated, and highly redundant. Common EST features include ~2% sequence error rate with high frequency of insertions and deletions, redundancy of sequences derived from highly expressed genes, and low representation of genes expressed at low levels. Moreover, ESTs may derive from unspliced immature mRNAs, alternative splicing and polyadenylation sites, cloning artifacts (chimerisms), and mitochondrial transcripts [59]. In addition, it should be kept in mind that only transcription and ultimate integration in tissues giving rise to gametes is heritable and the chaetognath EST collection was derived from whole animals.

In spite the low number of ESTs encoding retrotransposons found in the chaetognath library studied, members of three out of four orders of autonomous retrotransposons have been found [21]; the lacking order is the one containing the DIRS-like elements. The higher number of transcripts has been found for the Gypsy retrovirus-like superfamily which could constitute a deep-branching clade according to phylogenetic analyses. The origin of chaetognaths remains obscure, but fossil evidences suggest that this phylum was widespread and diverse in the earliest Cambrian [60], and the difficulties of the phylogenetic position of this taxon is probably partly due to its divergence at an early stage from the primitive ancestor of the Bilateria. Moreover, as it is well known [61], present retrovirus-like phylogenies suggest the polyphyly of these elements.

In addition, one deduced amino acid sequence of two similar ESTs exhibits sequence homology RT of Bel-Pao LTR-retrotransposons issuing from deuterostomian and platyhelminthes. Moreover, phylogenies suggest the presence of active LINEs in the chaetognath genome; however, the level of homology between these sequences and the closer published sequences is very low. Phylogenetic analyses of LINE-like RT sequences suggested that sequences of the chaetognath Sagitta sp. group with those of Lophotrochozoa (i.e., Nemertea, Mollusca, Gastrotricha, Annelida, Echiura and Rotifera) [16], whereas in our phylogenies, the chaetognath LINE elements group with vertebrae and insect sequences.

Alignments of other predicted EST sequences with the closer orthologs obtained using Blast analyses show that the chaetognath genome also contains active elements which exhibit sequence homology with RT of Penelope-like elements issuing from deuterostomian, platyhelminthes and insects [39,62,63]. However, all our analyses show that the chaetognath retrotransposon sequences are not phylogenetically informative; this is principally due to the short size of the regions analyzed. Moreover, both in plants and animals numerous evidences of horizontal transmission have been published [64-66]; however, the basal positions of chaetognath sequences in most of our phylogenetic trees seem to exclude horizontal transfers.

Association between microsatellites and MGEs has been reported in a variety of organisms including plants and animals [34-36]. These MGEs could be DNA transposons, but many of them are retroelements. This results in microsatellites that have similar or nearly identical flanking regions that share great homology with MGEs, suggesting these last elements can be involved in the genesis and genomic spread of microsatellites in organisms as diverse as animals [34,35,67-69] and plants [70]. This intimate association between microsatellite repeats and retrotransposons has also been put to good use for develop a method named REMAP for genotyping and fingerprinting analyses [71]. The microsatellite–MGEs association could be a molecular symbiosis between two types of genomic sequences. Indeed, the two main molecular pathways of this mutual aid are by means of transposition and recombination. MGEs can not generally invade some chromosomic regions, due, e.g., to the absence of insertion sites or the presence of euchromatin. Moreover, MGEs can be strongly negatively regulated by various mechanisms, they even can encode their own negative trans-regulator [72, 73]. Recombination-related events due to microsatellites, such as unequal crossing over, allow the expansion of MGEs present in the microsatellite flanking regions, even when they are inactive. Contrarily, microsatellites flanking MGEs sequences could be multiplicated and dispersed in the genome during transposition processes. Transposition of additional regions including functional genes are known for both DNA transposons for which complex transposons probably evolve by transposition of homologous insertion sequences to nearby sites within a DNA molecule [74], and retrotransposons which can mediate sequence transduction [75]. Moreover, the 3’ end region of several non LTR-retrotransposons can be implicated as a major source for formation of adenine-rich microsatellites [67]. The potential molecular symbiosis between MGEs and microsatellites show that the behavior and the evolution of repetitive sequences can only be understood within a larger genomic context.

Our research of ESTs containing telomeric-like sequences reveals the presence of two types of telomeric motifs. The vertebrate motif (TTAGGG) is dominant; it constitutes an ancestral motif of telomeres in bilaterian animals and possibly also in the superclade including animals, fungi and amoebozoans [50,76]. More surprisingly, the probably ancestral Arthropoda motif (TTAGG)n [76] has also been found in ~30% of the cases, suggesting that it is not a type of degenerate TTAGGG repeats. Moreover, no sequence of retrotransposons or RT genes has been found in the telomeric regions.

The ability of various factors to stimulate MGE activity was first proposed by McClintock [77], and one of us (EF) has been involved in this field of research for a long time. With regard to LTR retrotransposons and retrovirus, chemical and physical agents have been shown to induce transcription and transposition [78-92]. Non-LTR retrotransposons also respond to these stress [93]. Moreover, expression of retrotransposon promoters are wound-inducible [94,95] and retrotransposons can be activated during viral, bacterial, fungal or parasitic attacks [96-98]. As retrotransposition burst can be an indicator of the stress genomic response, activation of MGEs will be investigated in chaetognaths, because this taxon seems very resistant. Indeed, chaetognaths do acquire eukaryotic and bacterial parasites but not very frequently [99,100]. There is no host-specific parasite known in chaetognaths, which is remarkable for such an old group [60]. Moreover, chaetognaths exhibit a great antibacterial activity [101] and in laboratory, no beheaded chaetognaths, which can survive some 30 days after decapitation and are able to mature spermatozoans and to mate with normal specimens; have exhibited bacterial infestation [102].

In spite the low number of retrotransposon ESTs found in a juvenile chaetognath collection added to the small size of the sequences analyzed, this study suggests that chaetognath retrovirus-like retrotranposons could constitute deep-branching clades. The origin of these elements could correspond to the origin of the phylum; indeed, fossils of chaetognath grasping spines support the hypothesis that this animal taxon was present in the Cambrian times or even earlier. Moreover, studies on chaetognath retrotransposons could highlight future research in the exciting domain of the evolutionary origin of the retrovirus. Lastly, owing to the role of LTR-retrotransposons on genome structure, evolution and function, entire elements will be cloned and characterized, starting from the retrotransposon fragments shown in this study. Moreover, activation of these elements during normal development and in situations of stress, including pathogen attack will be sought.