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Short Interspersed Nuclear Elements (SINEs) are nonautonomous retrotransposons in the genome of most eukaryotic species. While SINEs have been intensively investigated in humans and other animal systems, SINE identification has been carried out only in a limited number of plant species. This lack of information is apparent especially in non-model plants whose genome has not been sequenced yet. The aim of this work was to produce a specific bioinformatics pipeline for analysing second generation sequence reads of a non-model species and identifying SINEs. We have identified, for the first time, 227 putative SINEs of the olive tree ( Olea europaea), that constitute one of the few sets of such sequences in dicotyledonous species. The identified SINEs ranged from 140 to 362 bp in length and were characterised with regard to the occurrence of the tRNA domain in their sequence. The majority of identified elements resulted in single copy or very lowly repeated, often in association with genic sequences. Analysis of sequence similarity allowed us to identify two major groups of SINEs showing different abundances in the olive tree genome, the former with sequence similarity to SINEs of Scrophulariaceae and Solanaceae and the latter to SINEs of Salicaceae. A comparison of sequence conservation between olive SINEs and LTR retrotransposon families suggested that SINE expansion in the genome occurred especially in very ancient times, before LTR retrotransposon expansion, and presumably before the separation of the rosids (to which Oleaceae belong) from the Asterids. Besides providing data on olive SINEs, our results demonstrate the suitability of the pipeline employed for SINE identification. Applying this pipeline will favour further structural and functional analyses on these relatively unknown elements to be performed also in other plant species, even in the absence of a reference genome, and will allow establishing general evolutionary patterns for this kind of repeats in plants. [ABSTRACT FROM AUTHOR]

In this study, we investigated by in silico analysis the possible correlation between microRNAs (miRNAs) and Anamnia V-SINEs (a superfamily of short interspersed nuclear elements), which belong to those retroposon families that have been preserved in vertebrate genomes for millions of years and are actively transcribed because they are embedded in the 3′ untranslated region (UTR) of several genes. We report the results of the analysis of the genomic distribution of these mobile elements in zebrafish ( Danio rerio) and discuss their involvement in generating miRNA gene loci. The computational study showed that the genes predicted to bear V-SINEs can be targeted by miRNAs with a very high hybridization E-value. Gene ontology analysis indicates that these genes are mainly involved in metabolic, membrane, and cytoplasmic signaling pathways. Nearly all the miRNAs that were predicted to target the V-SINEs of these genes, i.e., miR-338, miR-9, miR-181, miR-724, miR-735, and miR-204, have been validated in similar regulatory roles in mammals. The large number of genes bearing a V-SINE involved in metabolic and cellular processes suggests that V-SINEs may play a role in modulating cell responses to different stimuli and in preserving the metabolic balance during cell proliferation and differentiation. Although they need experimental validation, these preliminary results suggest that in the genome of D. rerio, as in other TE families in vertebrates, the preservation of V-SINE retroposons may also have been favored by their putative role in gene network modulation. [ABSTRACT FROM AUTHOR]

DNA methylation is presently accepted as a tentative regulatory parameter in splicing. Recently, we reported significant methylation differences among various exonic splicing-enhancing elements and alternative splicing events, based on CpG methylation data from the Human Epigenome Project for chromosomes 6, 20 and 22. Presently, using a different computational approach and the same database, we report: (a) significant increase of hypermethylation in intronic and exonic sequences close to acceptor sites, relative to overall introns and exons, respectively (1,973 CpGs examined); (b) frequent CpGs, mostly hypomethylated, in donors and infrequent CpGs mostly hypermethylated, in acceptors; and (c) hypermethylation in cassette exons which are occasionally spliced and have weaker average splicing potential, relative to constitutive exons ( p < 0.0001). CpGs are hypomethylated in non-coding exons (only 16 % hypermethylation). Single-exon genes, similarly to first exons, frequently contain hypomethylated CpGs, while in internal and last exons CpGs are more frequently hypermethylated. Methylation is also more frequent in strange introns and splice sites processed by the minor spliceosome, e.g., ATAC, ( p < 0.0001 in all cases), but not in sites of incomplete processing, e.g., retained introns or bleeding exons, ( p = 0.706 and p = 0.313, respectively). Most Alus, which are known to contribute to transcript presentation, are heavily methylated, in contrast with other Alus, e.g., AluJo and mammalian interspersed repetitive elements which have been previously associated with alternative expression. These results elucidate the role of intragenic methylation in association with alternative splicing and facilitate the evaluation of genomic variations/polymorphisms and the development of tools for the prediction of alternative splicing events. [ABSTRACT FROM AUTHOR]

We construct a wide class of families of exponentials, sines, and cosines generating Riesz bases of the corresponding Hilbert spaces with uniformly bounded upper bounds and uniformly positive lower bounds. [ABSTRACT FROM AUTHOR]

The review is dedicated to one of the most common classes of repetitive elements in eukaryotic genomes: short interspersed elements. Their structure, origin, and functioning in the genome are discussed. The variation and abundance of these neutral genomic markers make them a convenient and reliable tool for phylogenetic analysis. The main methods of such analysis are outlined, and the potential and limitations of this approach are illustrated by examples. [ABSTRACT FROM AUTHOR]

Genes of the major histocompatibility complex (MHC) are exceptionally polymorphic due to the combined effects of natural and sexual selection. Most research in wild populations has focused on the second exon of a single class II locus ( DRB), but complete gene sequences can provide an illuminating backdrop for studies of intragenic selection, recombination, and organization. To this end, we characterized class II loci in the banner-tailed kangaroo rat ( Dipodomys spectabilis). Seven DRB-like sequences (provisionally named MhcDisp-DRB*01 through *07) were isolated from spleen cDNA and most likely comprise ≥5 loci; this multiformity is quite unlike the situation in muroid rodents such as Mus, Rattus, and Peromyscus. In silico translation revealed the presence of important structural residues for glycosylation sites, salt bonds, and CD4+ T-cell recognition. Amino-acid distances varied widely among the seven sequences (2–34%). Nuclear DNA sequences from the Disp- DRB*07 locus (∼10 kb) revealed a conventional exon/intron structure as well as a number of microsatellites and short interspersed nuclear elements (B4, Alu, and IDL-Geo subfamilies). Rates of nucleotide substitution at Disp- DRB*07 are similar in both exons and introns ( π = 0.015 and 0.012, respectively), which suggests relaxed selection and may indicate that this locus is an expressed pseudogene. Finally, we performed BLASTn searches against Dipodomys ordii genomic sequences (unassembled reads) and find 90–97% nucleotide similarity between the two kangaroo rat species. Collectively, these data suggest that class II diversity in heteromyid rodents is based on polylocism and departs from the muroid architecture. [ABSTRACT FROM AUTHOR]

Small untranslated RNA genes transcribed by RNA polymerase III (class III genes) account for a substantial part (about 10%) of the mammalian genome. In addition to the well-known 5S rRNA, tRNA, and 7SL RNA genes and short interspersed elements (SINEs), class III includes several new genes, whose function is poorly understood. The expression regulation mechanisms substantially differ among class III genes on recent evidence, suggesting their gene, tissue, and cell state (resting, stress, apoptosis) specificity. The review considers the known genes for small untranslated RNAs, possible mechanisms regulating their expression (with the example of some SINEs), and the role of their transcripts in regulating cell processes. [ABSTRACT FROM AUTHOR]

Using computer-based methods, we determined the global distribution of short interspersed nuclear elements ( SINEs) on human and mouse X chromosomes. It was shown that this distribution was similar to the distribution of CpG islands and genes, but was different from the distribution of LINE1 elements. Since SINEs (human Alu and mouse B2) may have binding sites for Polycomb protein YY1, we suggest that these repeats can serve as additional signals (“boosters”) in Polycomb-dependent silencing of gene-rich segments during X inactivation. [ABSTRACT FROM AUTHOR]

Genomic DNA libraries were prepared from two endemic species of Lake Victoria haplochromine (cichlid) fish and used to isolate and characterize a set of short interspersed elements (SINEs). The distribution and sequences of the SINEs were used to infer phylogenetic relationships among East African haplochromines. The SINE-based classification divides the fish into four groups, which, in order of their divergence from a stem lineage, are the endemic Lake Tanganyika flock (group 1); fish of the nonendemic, monotypic, widely distributed genus Astatoreochromis (group 2); the endemic Lake Malawi flock (group 3); and group 4, which contains fish from widely dispersed East African localities including Lakes Victoria, Edward, George, Albert, and Rukwa, as well as many rivers. The group 4 haplochromines are characterized by a subset of polymorphic SINEs, each of which is present in some individuals and absent in others of the same population at a given locality, the same morphologically defined species, and the same mtDNA-defined haplogroup. SINE-defined group 4 contains six of the seven previously described mtDNA haplogroups. One of the polymorphic SINEs appears to be fixed in the endemic Lake Victoria flock; four others display the presence-or-absence polymorphism within the species of this flock. These findings have implications for the origin of Lake Victoria cichlids and for their founding population sizes. [ABSTRACT FROM AUTHOR]

Alu elements have inserted in the human genome throughout primate evolution. A small number of Alu insertions have occurred after the divergence of humans from nonhuman primates and therefore should not be present in nonhuman primate genomes. Most of these recently integrated Alu elements are contained with a series of discrete Alu subfamilies that are related to each other based upon diagnostic nucleotide substitutions. We have extracted members of the Alu Yd subfamily that are derivatives of the Alu Y subfamily that share a common 12-bp deletion that defines the Yd lineage from the draft sequence of the human genome. Analysis of the Yd Alu elements resulted in the recovery of two new Alu subfamilies, Yd3 and Yd6, which contain a total of 295 members (198 Yd3 and 97 Yd6). DNA sequence analysis of each of the Alu Yd subfamilies yielded age estimates of 8.02 and 1.20 million years old for the Alu Yd3 and Yd6 subfamilies, respectively. Two hundred Alu Yd3 and Yd6 loci were screened using polymerase chain reaction (PCR) assays to determine their phylogenetic origin and associated levels of human genomic diversity. The Alu Yd3 subfamily appears to have started amplifying relatively early in primate evolution and continued propagating albeit at a low level as many of its members are found in a variety of hominoid (humans, greater and lesser ape) genomes. Only two of the elements are polymorphic in the human genome and absent from the genomes of nonhuman primates. By contrast all of the members of the Alu Yd6 subfamily are restricted to the human genome, with 12% of the elements representing insertion polymorphisms in human populations. A single Alu Yd6 locus contained an independent parallel forward insertion of a paralogous Alu Sq sequence in the owl monkey. These Alu subfamilies are a source of genomic fossil relics for the study of primate phylogenetics and human population genetics. [ABSTRACT FROM AUTHOR]

The family Vespertilionidae comprises the largest number of species in the order Chiroptera, and its members are distributed over most of the world. We collected sequences of the mitochondrial NDI (NADH dehydrogenase subunit 1) gene from 38 species as well as 20 sequences of exon28 of the vWF (von Willebrand Factor) gene and analyzed phylogenetic relationships in Vespertilionidae using maximum-likelihood analyses and SINE (short interspersed elements) insertions. Our data strongly suggest the following: (1) Murininae and Myotis, each of which is classified into different subfamilies, are monophyletic; (2) the monophyly of Murininae, Myotis, Pipistrellini, and Plecotini is supported by our analyses of the NDI and nuclear vWF as well as data from SINE insertions, while Miniopterinae was not included in this group; and (3) Rhinolophoidea (microchiroptera) is separated from all the other microchiropteran species. Our study represents the most comprehensive phylogenetic model of Vespertilionidae to date. [ABSTRACT FROM AUTHOR]

The two alleles of the rat T-cell differentiation alloantigen RT6 are highly divergent and their expression is distinctively regulated. While the majority of T cells of RT6a/RT6b heterozygous laboratory rats (Rattus norvegicus) expresses both alleles, a subpopulation expresses only RT6b. To identify cis-regulatory elements that potentially control monoallelic expression, we compared the sequences of both alleles. A striking difference is the presence or absence of a rodent identifier (ID) sequence in intron 7. All investigated inbred RT6a rat strains (n=7) had this integration, while it was absent in all investigated RT6b rats (n=9). An ID element was also identified at precisely the same integration site in one RT6 allele of the closely related species Rattus rattus. The ID elements of both species showed nucleotide substitutions characteristic of different subfamilies, and their flanking repeats differed in length, indicating that two independent integration events had occurred into the same site adjacent to a mammalianwide interspersed repeat. Analysis of the surrounding sequences did not disclose any motifs to explain preferential integration into one allele. Our data indicate that the RT6 alleles diverged about 1 myr ago and that the ID element integrated into the RT6a locus soon after this. We have previously shown that DNA methylation plays an important role in regulating monoallelic RT6 expression. The possibility that the ID element in the RT6a allele interferes with the required demethylation process and thus accounts for monoallelic expression is discussed. [ABSTRACT FROM AUTHOR]

Here we describe a new short retroposon family of rodents. Like the primate Alu element consisting of two similar monomers, it is dimeric, but the left and right monomers are different and descend from B1 and ID short retroposons, respectively. Such elements (B1-dID) were found in the genomes of Gliridae, Sciuridae, Castoridae, Caviidae, and Hystricidae. Nucleotide sequences of this retroposon can be assigned to several structural variants. Phylogenetic analysis of B1-dID and related sequences suggests a possible scenario of B1-dID evolution in the context of rodent evolution. [ABSTRACT FROM AUTHOR]

We conducted comprehensive sequence analysis of 5′ flanking regions of primate Alu elements. Information contents were computed and frequencies of 1024 pentanucleotides were measured to approximate the location of a characteristic sequence and to specify its pattern(s), which may be involved in the integration of Alu elements into their host genomes. A large number of samples was used, the wide region of the 5′ end of Alu elements was analyzed, and comparisons were made among different subfamilies. Through our analyses, ``TTTTAAAAA'' or ``(T)m(A)n'' can be stated as a candidate for the characteristic sequence pattern, which resides around the region 5 to 20 base pairs upstream of the 5′ end of Alu elements. This characteristic sequence pattern was more prominent in the sequences of younger Alus, which is a strong indication that the sequence pattern has a role at the time of Alu integration. [ABSTRACT FROM AUTHOR]

Aluinsertional elements, the most abundant class of SINEs in humans are dimeric sequences approximately 300 bp in length derived from the 7SL RNA gene. These sequences contain a bipartite RNA pol III promoter, a central poly A tract, a 3′ poly A tail, numerous CpG islands and are bracketed by short direct repeats. An estimated 500,000 to 1 × 106units are dispersed throughout the human haploid genome primarily in AT rich neighborhoods located within larger GC dense chromosomal regions via a mechanism known as retroposition. Retroposition activity of Aluelements is determined by both internal and flanking regulatory elements as well as distant genes affecting transcription or transcript stability. Aluelements impact the organization and expression of the human genome at many levels including the processes of recombination, transcription and translation. Twelve subfamilies of Aluare defined by distinct patterns of diagnostic base substitutions. Subfamilies may be classified as young, intermediate or old reflecting the time since the start of retroposition by their members. Some insertions of the youngest subfamilies are not yet fixed in the human species and represent polymorphic loci. Alus are excellent molecular markers for a variety of reasons. They aid in tracing the complex pattern of duplication and rearrangements that occurred during the evolution of primate genome. Unlike other mutations, Alusequences are rarely lost completely once retroposed, have a defined ancestral state and are free from homoplasy since independent and identical insertions are highly unlikely. Because of these characteristics, Alus are literally molecular fossils. Polymorphic Aluloci are especially useful in studies of human genetic diversity and in pedigree and forensicanalysis. [ABSTRACT FROM AUTHOR]

Our group demonstrated recently that the 3 ′ ends of several families of tRNA-derived SINEs (short interspersed repetitive elements) originated from the 3′ ends of LINEs (long interspersed repetitive elements) [Ohshima et al. (1996) Mol. Cell. Biol. 16:3756-3764]. Two fully characterized examples of such organization were provided by the tortoise Pol III/SINE and the salmonid Hpal family of SINEs, and two probable examples were provided by the tobacco TS family of SINEs and the salmon Smal family of SINEs. This organization of SINEs can explain their potential to retropose in the genome since it appears reasonable that the sites for recognition of LINEs by reverse transcriptase should be located within the 3′-end sequences of LINEs. We now add another example to this category of SINEs. In the bovine genome, there are Bov-tA SINEs, which belong to the superfamily of tRNA-derived families of SINEs, and Bov-B LINEs, which were recently demonstrated to belong to a LINE family. Moreover, Bov-tA and Bov-B share the same 3′-end tail. We propose a possible scenario whereby the composite structure of the bovine Bov-tA family of SINEs might have been generated from the Bov-B family of LINEs during evolution. [ABSTRACT FROM AUTHOR]

Alu elements are a class of repetitive DNA sequences found throughout the human genome that are thought to be duplicated via an RNA intermediate in a process termed retroposition. Recently inserted Alu elements are closely related, suggesting that they are derived from a single source gene or closely related source genes. Analysis of the type III collagen gene (COL3A1) revealed a polymorphic Alu insertion in intron 8 of the gene. The Alu insertion in the COL3A1 gene had a high degree of nucleotide identity to the Sb family of Alu elements, a family of older Alu elements. The Alu sequence was less similar to the consensus sequence for the PV or Sb2 subfamilies, subfamilies of recently inserted Alu elements. These data support the observations that at least three source genes are active in the human genome, one of which is distinct from the PV and Sb2 subfamilies and predates either of these two subfamilies. Appearance of the Alu insertion in different ethnic populations suggests that the insertion may have occurred in the last 100,000 years. This Alu insert should be a useful marker for population studies and for marking COL3A1 alleles. [ABSTRACT FROM AUTHOR]

A recently identified Alu element (Leeflang et al. J. Mol. Evol. 1993, 37:559-565), referred to as the 'putative founder of the HS (PV) subfamily,' was found to be present at orthologous loci in the human, chimpanzee, gorilla, and gibbon lineages. The evolution of this Alu suggested that it is a source gene in the evolution of Alu family repeats for one of the most recent subfamilies, HS. We have determined that this putative founder of the HS subfamily was not present at the orthologous loci in older primates, including old world and new world monkeys. Thus, this particular Alu locus has only been responsible for the establishment of a very small subfamily of Alu sequences. We have further demonstrated that this putative founder Alu was not responsible for the de novo Alu insertion into the neurofibromatosis-1 gene of an individual causing neurofibromatosis. Our data demonstrate that although the putative founder of the HS subfamily found by Leeflang et al. (1993) probably gave rise to one of the most recent subfamilies of Alu sequences, it has not been very active in retroposition. [ABSTRACT FROM AUTHOR]

ID sequences are members of a short interspersed element (SINE) repetitive DNA family within the rodent genome. The copy number of individual ID elements varies by up to three orders of magnitude between species. This amplification has been highly sporadic in the order Rodentia and does not follow any phylogenetic trend. Using library screening and dot-blot analysis, we estimate there are 25,000 copies of ID elements in the deer mouse, 1,500 copies in the gerbil (both cricetid rodents), and 60,000 copies of either ID or ID-like elements in a sciurid rodent (squirrel). By dot-blot analysis, we estimate there are 150,000, 4,000, 1,000, and 200 copies of ID elements in the rat, mouse, hamster, and guinea pig, respectively (which is consistent with previous reports) and 200 copies in the hystricognath rodent, nutria. Therefore, a rapid amplification took place not only after the divergence of rat and mouse but also following the deer mouse ( Peromyscus) and hamster split, with no evidence of increased amplifications in hystricognath rodents. No notable variations of sequences from the BC1 genes of several myomorphic rodents were observed that would possibly explain the varied levels of ID amplification. We did observe subgenera and species-group-specific variation in the ID core sequence of the BC1 gene within the genus Peromyscus. Sequence analysis of cloned ID elements in Peromyscus show most ID elements in this genus arose prior to Peromyscus subgenus divergence. Correspondence of the consensus sequence of individual ID elements in gerbil and deer mouse further confirms BC1 as a master gene in ID amplification. Several possible mechanisms responsible for the quantitative variations are explored. [ABSTRACT FROM AUTHOR]

We have determined sequences of PCR-amplified B 1 elements from hamster and rat (Myomorpha), chipmunk (Sciuromorpha), and guinea pig (Caviomorpha). Between three and six B 1 subfamilies were found in these species. In the phylogenetic analysis Bl sequences of hamster, mouse, and rat clustered separately from those of chipmunk and those of guinea pig. This is consistent with an independent evolution of B1 elements in separate rodent lineages. We exclude the possibility of convergent mutations to explain certain diagnostic characters within the modern B1 quasi-dimers and view these elements as mosaic structures assembling preexisting mutations. Furthermore, the presence of Alu-like structural motifs supports the hypothesis of the monophyletic origin of Alu and B1 repeats, i.e., from a common 7SL RNA-derived retroposing monomeric element. [ABSTRACT FROM AUTHOR]

We compared the exon/intron organization of vertebrate genes belonging to different isochore classes, as predicted by their GC content at third codon position. Two main features have emerged from the analysis of sequences published in GenBank: (1) genes coding for long proteins (i.e., ≥500 aa) are almost two times more frequent in GC-poor than in GC-rich isochores; (2) intervening sequences (=sum of introns) are on average three times longer in GC-poor than in GC-rich isochores. These patterns are observed among human, mouse, rat, cow, and even chicken genes and are therefore likely to be common to all warm-blooded vertebrates. Analysis of Xenopus sequences suggests that the same patterns exist in cold-blooded vertebrates. It could be argued that such results do not reflect the reality because sequence databases are not representative of entire genomes. However, analysis of biases in GenBank revealed that the observed discrepancies between GC-rich and GC-poor isochores are not artifactual, and are probably largely underestimated. We investigated the distribution of microsatellites and interspersed repeats in introns of human and mouse genes from different isochores. This analysis confirmed previous studies showing that Ll repeats are almost absent from GC-rich isochores. Microsatellites and SINES (Alu, B1, B2) are found at roughly equal frequencies in introns from all isochore classes. Globally, the presence of repeated sequences does not account for the increased intron length in GC-poor isochores. The relationships between gene structure and global genome organization and evolution are discussed. [ABSTRACT FROM AUTHOR]

Reverse transcription has been an important mediator of genomic change. This influence dates back more than three billion years, when the RNA genome was converted into the DNA genome. While the current cellular role(s) of reverse transcriptase are not yet completely understood, it has become clear over the last few years that this enzyme is still responsible for generating significant genomic change and that its activities are one of the driving forces of evolution. Reverse transcriptase generates, for example, extra gene copies (retrogenes), using as a template mature messenger RNAs. Such retrogenes do not always end up as nonfunctional pseudogenes but form, after reinsertion into the genome, new unions with resident promoter elements that may alter the gene's temporal and/or spatial expression levels. More frequently, reverse transcriptase produces copies of nonmessenger RNAs, such as small nuclear or cytoplasmic RNAs. Extremely high copy numbers can be generated by this process. The resulting reinserted DNA copies are therefore referred to as short interspersed repetitive elements (SINEs). SINEs have long been considered selfish DNA, littering the genome via exponential propagation but not contributing to the host's fitness. Many SINEs, however, can give rise to novel genes encoding small RNAs, and are the migrant carriers of numerous control elements and sequence motifs that can equip resident genes with novel regulatory elements [Brosius J. and Gould S.J., Proc Natl Acad Sci USA 89, 10706-10710, 1992]. Retrosequences, such as SINEs and portions of retroelements (e.g., long terminal repeats, LTRs), are capable of donating sequence motifs for nucleosome positioning, DNA methylation, transcriptional enhancers and silencers, poly(A) addition sequences, determinants of RNA stability or transport, splice sites, and even amino acid codons for incorporation into open reading frames as novel protein domains. Retroposition can therefore be considered as a major pacemaker for evolution (including speciation). Retroposons, with their unique properties and actions, form the molecular basis of important evolutionary concepts, such as exaptation [Gould S.J. and Vrba E., Paleobiology 8, 4-15, 1982] and punctuated equilibrium [Elredge N. and Gould S.J. in Schopf T.J.M. (ed). Models in Paleobiology. Freeman, Cooper, San Francisco, 1972, pp. 82-115]. [ABSTRACT FROM AUTHOR]

B2 repeats are a group of short interspersed elements (SINEs) specific for rodent genomes. Copy numbers were determined for different rodent genera. All the Muroid (rat, mouse, deer mouse, hamster, gerbil) rodent genomes analyzed exhibited 80,000–100,000 copies per haploid genome, whereas the squirrel genome contains only 2,500 copies, and fewer than 100 (if any) copies were observed for the Hystricognath rodents (guinea pig and nutria). These findings demonstrate that there was an ‘explosion’ of amplification of B2 elements within muroid rodents. The similar copy number of B2 elements within the different muroid species could be explained by formation of a high proportion of the B2 elements prior to the divergence of the different muroid species. However, the 3′-end of the B2 sequence is unique between murid and cricetid rodents suggesting that the majority of elements amplified after the divergence of these species. Also consistent with recent amplification of these elements in parallel within the muroid genomes is the finding that within mouse and rat there are distinct subfamilies of B2 repeats. The pattern of consistent parallel amplification of B2 elements in muroid species contrasts with the sporadic nature of ID repeat amplification in the same genomes. The consensus of the young mouse subfamily of elements corresponds to the B2 RNA that is preferentially transcribed in embryonic, tumor, and normal liver cells. The subfamily is young based on both its low divergence from the subfamily consensus sequence and the finding that the most recent B2 element insertions in the mouse genome are members of this subfamily. [ABSTRACT FROM AUTHOR]

Continued insertion into the genome of functional Alu sequences is expected to compensate for the functional eclipse of older sequences attributable to structural adulteration and can be presumed to establish a renewable store of functional sequences at a relatively elevated numerical level. This store of functional sequences could be maintained at almost no selective cost. A strategy of maintaining function in multiple sequence copies with selection limited to a very few master (source) sequences may be resorted to also by other types of DNA sequences that are generated repeatedly during evolution and that are spread over many sectors of the genome. [ABSTRACT FROM AUTHOR]