Horizontal transfer of non-LTR retrotransposons in vertebrates

Since their discovery in family Bovidae (bovids), Bov-B LINEs, believed to be order-specific SINEs, have been found in all ruminants and recently also in Viperidae snakes. The distribution and the evolutionary relationships of Bov-B LINEs provide an indication of their origin and evolutionary dynamics in different species. The evolutionary origin of Bov-B LINE elements has been shown unequivocally to be in Squamata (squamates). The horizontal transfer of Bov-B LINE elements in vertebrates has been confirmed by their discontinuous phylogenetic distribution in Squamata (Serpentes and two lizard infra-orders) as well as in Ruminantia, by the high level of nucleotide identity, and by their phylogenetic relationships. The direction of horizontal transfer from Squamata to the ancestor of Ruminantia is evident from the genetic distances and discontinuous phylogenetic distribution of Bov-B LINE elements. The ancestral snake lineage (Boidae) has been recognized as a possible donor of Bov-B LINE elements to Ruminantia. The timing of horizontal transfer has been estimated from the distribution of Bov-B LINE elements in Ruminantia and the fossil data of Ruminantia to be 40–50mya. The phylogenetic relationships of Bov-B LINE elements from the various Squamata species agrees with that of the species phylogeny, suggesting that Bov-B LINE elements have been stably maintained by vertical transmission since the origin of Squamata in the Mesozoic era. This revised version was published online in July 2006 with corrections to the Cover Date.     

Molecular evolution of Bov-B LINEs in vertebrates

Since their discovery in family Bovidae (bovids), Bov-B LINEs, believed to be order-specific SINEs, have been found in all ruminants and recently also in Viperidae snakes. The distribution and the evolutionary relationships of Bov-B LINEs provide an indication of their origin and evolutionary dynamics in different species. The evolutionary origin of Bov-B LINE elements has been shown unequivocally to be in Squamata (squamates). The horizontal transfer of Bov-B LINE elements in vertebrates has been confirmed by their discontinuous phylogenetic distribution in Squamata (Serpentes and two lizard infra-orders) as well as in Ruminantia, by the high level of nucleotide identity, and by their phylogenetic relationships. The direction of horizontal transfer from Squamata to the ancestor of Ruminantia is evident from the genetic distances and discontinuous phylogenetic distribution of Bov-B LINE elements. The ancestor of Colubroidea snakes has been recognized as a possible donor of Bov-B LINE elements to Ruminantia. The timing of horizontal transfer has been estimated from the distribution of Bov-B LINE elements in Ruminantia and the fossil data of Ruminantia to be 40-50 My ago. The phylogenetic relationships of Bov-B LINE elements from the various Squamata species agrees with that of the species phylogeny, suggesting that Bov-B LINE elements have been stably maintained by vertical transmission since the origin of Squamata in the Mesozoic era.     

植物基因组中的非LTR反转录转座子SINEs和LINEs

反转录转座子是基因组进化的推动者之一。分为LTR和非LTR两种类型。前者是真核基因组的主要组分,结构和转座方式与逆转录病毒类似。后者是最初发现于动物基因组新近发现在植物基因组中也广泛存在的新型重复序列,包括LINEs(long interspersed nuclear elements)和SINEs(short interspersed nuclear elements)两个亚型。它们大多因自身或受宿主基因组的调控而失去转座活性。其转座机理目前还不十分清楚,推测LINEs可以自主转座,SINEs依赖其他转座子被动转座。种系分析认为LINEs可能是最古老的反转录转座子,SINEs的起源未知。文章对以上内容进行了归纳和讨论。     

Bov-B-mobilized SINEs in vertebrate genomes

Two new short retroposon families (SINEs) have been found in the genome of springhare Pedetes capensis (Rodentia). One of them, Ped-1, originated from 5S rRNA, while the other one, Ped-2, originated from tRNA-derived SINE ID. In contrast to most currently active mammalian SINEs mobilized by L1 long retrotransposon (LINE), Ped-1 and Ped-2 are mobilized by Bov-B, a LINE family of the widely distributed RTE clade. The 3' part of these SINEs originates from two sequences in the 5' and 3' regions of Bov-B. Such bipartite structure of the LINE-derived part has been revealed in all Bov-B-mobilized SINEs known to date (AfroSINE, Bov-tA, Mar-1, and Ped-1/2), which distinguishes them from other SINEs with only a 3' LINE-derived part. Structural analysis and the distribution of Bov-B LINEs and partner SINEs supports the horizontal transfer of Bov-B, while the SINEs emerged independently in lineages with this LINE.     

A non-LTR retroelement extinction in Spermophilus tridecemlineatus

The typical mammalian genome is dominated by two types of transposable elements (TEs), the autonomous and non-autonomous non-LTR retrotransposons, i.e. LINEs and SINEs, and with few exceptions there is a sole active LINE family (L1). During an ongoing investigation of TEs in rodents we determined that overall transposon activity has been steadily declining in Spermophilus tridecemlineatus. More specifically, the typically ubiquitous L1 activity of mammals has decreased drastically within the last 26MY. Indeed, only three L1 insertions with intact ORF1 sequences were readily identifiable and no intact ORF2 sequences were identified. The last L1 and SINE insertions date to ~5.3MYA and 4MYA, respectively. Based on our inability to computationally identify recently inserted L1 elements we suggest that S. tridecemlineatus is experiencing a quiescence or extinction of non-LTR retrotransposon activity. Such a finding represents only the fourth instance of a loss of non-LTR retrotransposon activity identified in mammals and, as such, represents an important additional data point to guide our understanding of LINE dynamics in eutherians.     

Getting closer to a pre-vertebrate genome: the non-LTR retrotransposons of Branchiostoma floridae

Non-LTR retrotransposons are common in vertebrate genomes and although present in invertebrates they appear at a much lower frequency. The cephalochordate amphioxus is the closest living relative to vertebrates and has been considered a good model for comparative analyses of genome expansions during vertebrate evolution. With the aim to assess the involvement of transposable elements in these events, we have analysed the non-LTR retrotransposons of Branchiostoma floridae. In silico searches have allowed to reconstruct non-LTR elements of six different clades (CR1, I, L1, L2, NeSL and RTE) and assess their structural features. According to the estimated copy number of these elements they account for less than 1% of the haploid genome, which reminds of the low abundance also encountered in the urochordate Ciona intestinalis. Amphioxus (B. floridae) and Ciona share a pre-vertebrate-like organization for the non-LTR retrotransposons (<150 copies, < 1% of the genome) versus the complexity associated to higher vertebrates (Homo sapiens >1.3.10(6) copies, > 20% of the genome).     

RetroPred: A tool for prediction, classification and extraction of non-LTR retrotransposons (LINEs & SINEs) from the genome by integrating PALS, PILER, MEME and ANN

The problem of predicting non-long terminal repeats (LTR) like long interspersed nuclear elements (LINEs) and short interspersed nuclear elements (SINEs) from the DNA sequence is still an open problem in bioinformatics. To elevate the quality of annotations of LINES and SINEs an automated tool "RetroPred" was developed. The pipeline allowed rapid and thorough annotation of non-LTR retrotransposons. The non-LTR retrotransposable elements were initially predicted by Pairwise Aligner for Long Sequences (PALS) and Parsimonious Inference of a Library of Elementary Repeats (PILER). Predicted non-LTR elements were automatically classified into LINEs and SINEs using ANN based on the position specific probability matrix (PSPM) generated by Multiple EM for Motif Elicitation (MEME). The ANN model revealed a superior model (accuracy = 78.79 +/- 6.86 %, Q(pred) = 74.734 +/- 17.08 %, sensitivity = 84.48 +/- 6.73 %, specificity = 77.13 +/- 13.39 %) using four-fold cross validation. As proof of principle, we have thoroughly annotated the location of LINEs and SINEs in rice and Arabidopsis genome using the tool and is proved to be very useful with good accuracy. Our tool is accessible at http://www.juit.ac.in/RepeatPred/home.html.     

Phylogenetic determination of the pace of transposable element proliferation in plants: copia and LINE-like elements in Gossypium.

Transposable elements contribute significantly to plant genome evolution in myriad ways, ranging from local insertional mutations to global effects exerted on genome size through accumulation. Differential accumulation and deletion of transposable elements may profoundly affect genome size, even among members of the same genus. One example is that of Gossypium (cotton), where much of the 3-fold genome size variation is due to differential accumulation of one gypsy-like LTR retrotransposon, Gorge3. Copia and non-LTR LINE retrotransposons are also major components of the Gossypium genome, but unlike Gorge3, their extant copy numbers do not correlate with genome size. In the present study, we describe the nature and timing of transposition for copia and LINE retrotransposons in Gossypium. Our findings indicate that copia retrotransposons have been active in each lineage since divergence from a common ancestor, and that they have proliferated in a punctuated manner. However, the evolutionary history of LINEs contrasts markedly with that of the copia retrotransposons. Although LINEs have also been active in each lineage, they have accumulated in a stochastically regular manner, and phylogenetic analysis suggests that extant LINE populations in Gossypium are dominated by ancient insertions. Interestingly, the magnitude of transpositional bursts in each lineage corresponds directly with extant estimated copy number.     

Ribonucleoprotein formation by the ORF1 protein of the non-LTR retrotransposon Tx1L in Xenopus oocytes.

The Tx1L elements constitute a family of site-specific non-LTR retrotransposons found in the genome of the frog Xenopus laevis . The elements have two open reading frames (ORFs) with homology to proteins of retroviruses and other retroelements. This study demonstrates an expected activity of one of the element-encoded proteins. The RNA binding properties of ORF1p, the product of the first ORF of Tx1L, were examined after expression from RNA injected into Xenopus oocytes. Using sucrose gradient sedimentation and non-denaturing gel electrophoresis, we show that ORF1p associates with RNA in cytoplasmic ribonucleoprotein (RNP) particles. Discrete RNPs are formed with well-defined mobilities. The ORF1p RNPs are distinct from endogenous RNPs that contain stored oocyte mRNAs and two specific endogenous mRNAs do not become associated with ORF1p. ORF1p appears to be capable of associating with its own mRNA and with other injected RNAs, independent of specific recognition sequences. Although nuclear localization of ORF1p was anticipated, based both on the supposed mechanism of transposition and on the presence of a potential nuclear localization signal, no significant fraction of the protein was found in the oocyte nucleus. Nonetheless, the RNA binding capability of ORF1p is consistent with the proposed model for transposition of non-LTR retrotransposons.