Evolution and recombination of bovine DNA repeats

Whole-Genome Sequencing and Characterization of Buffalo Genetic Resources: Recent Advances and Future Challenges

The buffalo was domesticated around 3000-6000 years ago and has substantial economic significance as a meat, dairy, and draught animal. The buffalo has remained underutilized in terms of the development of a well-annotated and assembled reference genome de novo. It is mandatory to explore the genetic architecture of a species to understand the biology that helps to manage its genetic variability, which is ultimately used for selective breeding and genomic selection. Morphological and molecular data have revealed that the swamp buffalo population has strong geographical genomic diversity with low gene flow but strong phenotypic consistency, while the river buffalo population has higher phenotypic diversity with a weak phylogeographic structure. The availability of recent high-quality reference genome and genotyping marker panels has invigorated many genome-based studies on evolutionary history, genetic diversity, functional elements, and performance traits. The increasing molecular knowledge syndicate with selective breeding should pave the way for genetic improvement in the climatic resilience, disease resistance, and production performance of water buffalo populations globally.

Satellite DNA in Neotropical Deer Species

The taxonomy and phylogenetics of Neotropical deer have been mostly based on morphological criteria and needs a critical revision on the basis of new molecular and cytogenetic markers. In this study, we used the variation in the sequence, copy number, and chromosome localization of satellite I-IV DNA to evaluate evolutionary relationships among eight Neotropical deer species. Using FISH with satI-IV probes derived from Mazama gouazoubira, we proved the presence of satellite DNA blocks in peri/centromeric regions of all analyzed deer. Satellite DNA was also detected in the interstitial chromosome regions of species of the genus Mazama with highly reduced chromosome numbers. In contrast to Blastocerus dichotomus, Ozotoceros bezoarticus, and Odocoileus virginianus, Mazama species showed high abundance of satIV DNA by FISH. The phylogenetic analysis of the satellite DNA showed close relationships between O. bezoarticus and B. dichotomus. Furthermore, the Neotropical and Nearctic populations of O. virginianus formed a single clade. However, the satellite DNA phylogeny did not allow resolving the relationships within the genus Mazama. The high abundance of the satellite DNA in centromeres probably contributes to the formation of chromosomal rearrangements, thus leading to a fast and ongoing speciation in this genus, which has not yet been reflected in the satellite DNA sequence diversification.

Identification and characterization of satellite DNAs in two-toed sloths of the genus Choloepus (Megalonychidae, Xenarthra)

Choloepus, the only extant genus of the Megalonychidae family, is composed of two living species of two-toed sloths: Choloepus didactylus and C. hoffmanni. In this work, we identified and characterized the main satellite DNAs (satDNAs) in the sequenced genomes of these two species. SATCHO1, the most abundant satDNA in both species, is composed of 117 bp tandem repeat sequences. The second most abundant satDNA, SATCHO2, is composed of ~ 2292 bp tandem repeats. Fluorescence in situ hybridization in C. hoffmanni revealed that both satDNAs are located in the centromeric regions of all chromosomes, except the X. In fact, these satDNAs present some centromeric characteristics in their sequences, such as dyad symmetries predicted to form secondary structures. PCR experiments indicated the presence of SATCHO1 sequences in two other Xenarthra species: the tree-toed sloth Bradypus variegatus and the anteater Myrmecophaga tridactyla. Nevertheless, SATCHO1 is present as large tandem arrays only in Choloepus species, thus likely representing a satDNA exclusively in this genus. Our results reveal interesting features of the satDNA landscape in Choloepus species with the potential to aid future phylogenetic studies in Xenarthra and mammalian genomes in general.

Sequence Analysis and FISH Mapping of Four Satellite DNA Families among Cervidae

Centromeric and pericentromeric chromosome regions are occupied by satellite DNA. Satellite DNAs play essential roles in chromosome segregation, and, thanks to their extensive sequence variability, to some extent, they can also be used as phylogenetic markers. In this paper, we isolated and sequenced satellite DNA I-IV in 11 species of Cervidae. The obtained satellite DNA sequences and their chromosomal distribution were compared among the analysed representatives of cervid subfamilies Cervinae and Capreolinae. Only satI and satII sequences are probably present in all analysed species with high abundance. On the other hand, fluorescence in situ hybridisation (FISH) with satIII and satIV probes showed signals only in a part of the analysed species, indicating interspecies copy number variations. Several indices, including FISH patterns, the high guanine and cytosine (GC) content, and the presence of centromere protein B (CENP-B) binding motif, suggest that the satII DNA may represent the most important satellite DNA family that might be involved in the centromeric function in Cervidae. The absence or low intensity of satellite DNA FISH signals on biarmed chromosomes probably reflects the evolutionary reduction of heterochromatin following the formation of chromosome fusions. The phylogenetic trees constructed on the basis of the satellite I-IV DNA relationships generally support the present cervid taxonomy.

Conservation, Divergence, and Functions of Centromeric Satellite DNA Families in the Bovidae

Repetitive satellite DNA (satDNA) sequences are abundant in eukaryote genomes, with a structural and functional role in centromeric function. We analyzed the nucleotide sequence and chromosomal location of the five known cattle (Bos taurus) satDNA families in seven species from the tribe Tragelaphini (Bovinae subfamily). One of the families (SAT1.723) was present at the chromosomes’ centromeres of the Tragelaphini species, as well in two more distantly related bovid species, Ovis aries and Capra hircus. Analysis of the interaction of SAT1.723 with centromeric proteins revealed that this satDNA sequence is involved in the centromeric activity in all the species analyzed and that it is preserved for at least 15–20 Myr across Bovidae species. The satDNA sequence similarity among the analyzed species reflected different stages of homogeneity/heterogeneity, revealing the evolutionary history of each satDNA family. The SAT1.723 monomer-flanking regions showed the presence of transposable elements, explaining the extensive shuffling of this satDNA between different genomic regions.

Draft genome of the river water buffalo

Water buffalo (Bubalus bubalis), a large-sized member of the Bovidae family, is considered as an important livestock species throughout Southeast Asia. In order to better understand the molecular basis of buffalo improvement and breeding, we sequenced and assembled the genome (2n=50) of a river buffalo species Bubalus bubalis from Bangladesh. Its genome size is 2.77 Gb, with a contig N50 of 25 kb and the scaffold N50 of 6.9 Mbp. Based on the assembled genome, we annotated 24,613 genes for future functional genomics studies. Phylogenetic tree analysis of cattle and water buffalo lineages showed that they diverged about 5.8-9.8 million years ago. Our findings provide an insight into the water buffalo genome which will contribute in further research on buffalo such as molecular breeding, understanding complex traits, conservation, and biodiversity.

[STRUCTURE AND DISTRIBUTION OF THE RETROTRANSPOSON BOV-B LINE]

The classification of mobile elements was discussed. Special attention was devoted to the retroelement of the LINE group: retrotransposon Bov-B LINE. The history of its origin and distribution in the nature was considered. The results of the phenomenon of horizontal transition of the retrotransposon Bov-B LINE between evolutionally distant classes were discussed.

A genome survey sequencing of the Java mouse deer (Tragulus javanicus) adds new aspects to the evolution of lineage specific retrotransposons in Ruminantia (Cetartiodactyla)

Ruminantia, the ruminating, hoofed mammals (cow, deer, giraffe and allies) are an unranked artiodactylan clade. Around 50-60 million years ago the BovB retrotransposon entered the ancestral ruminantian genome through horizontal gene transfer. A survey genome screen using 454-pyrosequencing of the Java mouse deer (Tragulus javanicus) and the lesser kudu (Tragelaphus imberbis) was done to investigate and to compare the landscape of transposable elements within Ruminantia. The family Tragulidae (mouse deer) is the only representative of Tragulina and phylogenetically important, because it represents the earliest divergence in Ruminantia. The data analyses show that, relative to other ruminantian species, the lesser kudu genome has seen an expansion of BovB Long INterspersed Elements (LINEs) and BovB related Short INterspersed Elements (SINEs) like BOVA2. In comparison the genome of Java mouse deer has fewer BovB elements than other ruminants, especially Bovinae, and has in addition a novel CHR-3 SINE most likely propagated by LINE-1. By contrast the other ruminants have low amounts of CHR SINEs but high numbers of actively propagating BovB-derived and BovB-propagated SINEs. The survey sequencing data suggest that the transposable element landscape in mouse deer (Tragulina) is unique among Ruminantia, suggesting a lineage specific evolutionary trajectory that does not involve BovB mediated retrotransposition. This shows that the genomic landscape of mobile genetic elements can rapidly change in any lineage.

A New Class of SINEs with snRNA Gene-Derived Heads

Eukaryotic genomes are colonized by various transposons including short interspersed elements (SINEs). The 5' region (head) of the majority of SINEs is derived from one of the three types of RNA genes--7SL RNA, transfer RNA (tRNA), or 5S ribosomal RNA (rRNA)--and the internal promoter inside the head promotes the transcription of the entire SINEs. Here I report a new group of SINEs whose heads originate from either the U1 or U2 small nuclear RNA gene. These SINEs, named SINEU, are distributed among crocodilians and classified into three families. The structures of the SINEU-1 subfamilies indicate the recurrent addition of a U1- or U2-derived sequence onto the 5' end of SINEU-1 elements. SINEU-1 and SINEU-3 are ancient and shared among alligators, crocodiles, and gharials, while SINEU-2 is absent in the alligator genome. SINEU-2 is the only SINE family that was active after the split of crocodiles and gharials. All SINEU families, especially SINEU-3, are preferentially inserted into a family of Mariner DNA transposon, Mariner-N4_AMi. A group of Tx1 non-long terminal repeat retrotransposons designated Tx1-Mar also show target preference for Mariner-N4_AMi, indicating that SINEU was mobilized by Tx1-Mar.

Dispersal of an ancient retroposon in the TP53 promoter of Bovidae: phylogeny, novel mechanisms, and potential implications for cow milk persistency

BACKGROUND

In recent years, the perception of transposable genetic elements has changed from "junk DNA" to a focus of interest when appearing near or inside genes. Bov-A2 is a short interspersed nuclear element (SINE) that was first found in Bovidae and later in other ruminants. This retroposon is mostly used as a marker for phylogenetic analysis.

RESULTS

We describe insertions of Bov-A2 in the promoter region of TP53, a key tumor suppressor gene that is indispensable for diverse developmental processes, in Antilopinae and Tragelaphini (belonging to the Bovinae subfamily). In Tragelaphini two Bov-A2 elements were inserted sequentially, whereas in 5 tribes of Antilopinae only one Bov-A2 element was inserted, in a different site and reverse orientation. The entrance site in both cases employed short palindromes that can form hairpin secondary structures. Interestingly, mutations that create or disrupt base pairing in the palindrome sequence dictated the presence or absence of Bov-A2, such as in the domestic cow and buffalo, which lack Bov-A2. Transcription factor binding site analysis revealed unique binding sites for STAT3 and NFκB within the Bov-A2 sequence, which together with TP53 itself are known to play a crucial role in mammary involution.

CONCLUSIONS

This report demonstrates how short palindromes serve as hot spots for Bov-A2 retroposon insertion into the mammalian genome. The strict correlation between point mutation in the palindromes and the presence/absence of Bov-A2 retroposon insertions, questions the use of singular insertion events as valid phylogenetic markers inside families. Bov-A2 insertion into the TP53 promoter in Antilopinae and Tragelaphini may not only provide a genetic network that regulates mammary involution, but can also answer the need for rapid mammary involution in Savanna antelopes after weaning, partially in response to predation stress. The absence of Bov-A2 in domestic bovids may constitute the molecular background for greater lactation persistency.

Tribe-specific satellite DNA in non-domestic Bovidae

Satellite sequences present in the centromeric and pericentric regions of chromosomes represent useful source of information. Changes in satellite DNA composition may coincide with the speciation and serve as valuable markers of phylogenetic relationships. Here, we examined satellite DNA clones isolated by laser microdissection of centromeric regions of 38 bovid species and categorized them into three types. Sat I sequences from members of Bovini/Tragelaphini/Boselaphini are similar to the well-documented 1.715 sat I DNA family. Sat I DNA from Caprini/Alcelaphini/Hippotragini/Reduncini/Aepycerotini/Cephalophini/Antilopini/Neotragini/Oreotragini form the second group homologous to the common 1.714 sat I DNA. The analysis of sat II DNAs isolated in our study confirmed conservativeness of these sequences within Bovidae. Newly described centromeric clones from Madoqua kirkii and Strepsiceros strepsiceros were similar in length and repetitive tandem arrangement but showed no similarity to any other satellite DNA in the GenBank database. Phylogenetic analysis of sat I sequences isolated in our study from 38 bovid species enabled the description of relationships at the subfamily and tribal levels. The maximum likelihood and Bayesian inference analyses showed a basal position of sequences from Oreotragini in the subfamily Antilopinae. According to the Bayesian inference analysis based on the indels in a partitioned mixed model, Antilopinae satellite DNA split into two groups with those from Neotragini as a basal tribe, followed by a stepwise, successive branching of Cephalophini, Aepycerotini and Antilopini sequences. In the second group, Reduncini sequences were basal followed by Caprini, Alcelaphini and Hippotragini.

Molecular cytogenetic insights to the phylogenetic affinities of the giraffe (Giraffa camelopardalis) and pronghorn (Antilocapra americana)

Five families are traditionally recognized within higher ruminants (Pecora): Bovidae, Moschidae, Cervidae, Giraffidae and Antilocapridae. The phylogenetic relationships of Antilocapridae and Giraffidae within Pecora are, however, uncertain. While numerous fusions (mostly Robertsonian) have accumulated in the giraffe's karyotype (Giraffa camelopardalis, Giraffidae, 2n = 30), that of the pronghorn (Antilocapra americana, Antilocapridae, 2n = 58) is very similar to the hypothesised pecoran ancestral state (2n = 58). We examined the chromosomal rearrangements of two species, the giraffe and pronghorn, using a combination of fluorescence in situ hybridization painting probes and BAC clones derived from cattle (Bos taurus, Bovidae). Our data place Moschus (Moschidae) closer to Bovidae than Cervidae. Although the alternative (i.e., Moschidae + Cervidae as sister groups) could not be discounted in recent sequence-based analyses, cytogenetics bolsters conclusions that the former is more likely. Additionally, DNA sequences were isolated from the centromeric regions of both species and compared. Analysis of cenDNA show that unlike the pronghorn, the centromeres of the giraffe are probably organized in a more complex fashion comprising different repetitive sequences specific to single chromosomal pairs or groups of chromosomes. The distribution of nucleolar organiser region (NOR) sites, often an effective phylogenetic marker, were also examined in the two species. In the giraffe, the position of NORs seems to be autapomorphic since similar localizations have not been found in other species within Pecora.

Isolation and comparison of tribe-specific centromeric repeats within Bovidae

A taxonomic division of the family Bovidae (Artiodactyla) is difficult and the evolutionary relationships among most bovid subfamilies remain uncertain. In this study, we isolated the cattle satellite I clone BTREP15 (1.715 satellite DNA family) and autosomal centromeric DNAs of members of ten bovid tribes. We wished to determine whether the analysis of fluorescence in situ hybridization patterns of the cattle satellite I clone (BTREP15) and tribe-specific centromeric repeats isolated by laser microdissection would help to reveal some of the ambiguities occurring in the systematic classification of the family Bovidae. The FISH study of the presence and distribution of the cattle satellite I clone BTREP15 (1.715 satellite DNA family) within members of ten bovid tribes was not informative. FISH analysis of autosomal centromeric DNA probes in several species within one tribe revealed similar hybridization patterns in autosomes confirming tribal homogeneity of these probes. Sex chromosomes showed considerable variation in sequence composition and arrangement not only between tribes but also between species of one tribe. According to our findings it seems that Oreotragus oreotragus developed its own specific satellite DNA which does not hybridize to any other bovid species analysed. Our results suggest O. oreotragus as well as Aepyceros melampus may be unique species not particularly closely related to any of the recognized bovid tribes. This study indicates the isolation of tribe-specific centromeric DNAs by laser microdissection and cloning the sequence representing the main motif of these repetitive DNAs could offer the perspectives for comparative phylogenetic studies.

Using species-specific repeat and PCR-RFLP in typing of DNA derived from blood of human and animal species

Species determination of tissue specimens, including blood, is an important component of forensic analysis to distinguish human from animal remains. DNA markers based on a method of species-specific PCR and amplifying the 359-base pair (bp) fragment of the mitochondrially encoded cytochrome-b gene and then digestion with the TaqI restriction enzyme were developed for detection and discrimination of human, cattle, buffalo, horse, sheep, pig, dog, cat and chicken blood samples. The results reveal that PCR-amplification of the gene encoding the species-specific repeat (SSR) region generated 603 bp in cattle and buffalo, 221 bp in horse, 374 bp in sheep, <or=100 bp in pig, 808 bp in dog, 672 bp in cat and 50 bp in chicken. Restriction analysis of the amplified 359-bp portion of the cytochrome-b gene using the TaqI restriction enzyme results in species-specific restriction fragment length polymorphism (RFLP) between buffalo, cattle and human. Two different bands were generated in buffalo (191 and 168 bp) and human (209 and 150 bp), with no digestion in cattle (359 bp). Cytochrome-b is a highly conserved region and consequently a good molecular marker for diagnostic studies. Therefore, the two complementary techniques, SSR-PCR and PCR-RFLP, could be used successfully as routine methods in forensics for sensitive, rapid, simple and inexpensive identification of the species in bloodstains.

Characterization and distribution of retrotransposons and simple sequence repeats in the bovine genome

Interspersed repeat composition and distribution in mammals have been best characterized in the human and mouse genomes. The bovine genome contains typical eutherian mammal repeats, but also has a significant number of long interspersed nuclear element RTE (BovB) elements proposed to have been horizontally transferred from squamata. Our analysis of the BovB repeats has indicated that only a few of them are currently likely to retrotranspose in cattle. However, bovine L1 repeats (L1 BT) have many likely active copies. Comparison of substitution rates for BovB and L1 BT indicates that L1 BT is a younger repeat family than BovB. In contrast to mouse and human, L1 occurrence is not negatively correlated with G+C content. However, BovB, Bov A2, ART2A, and Bov-tA are negatively correlated with G+C, although Bov-tAs correlation is weaker. Also, by performing genome wide correlation analysis of interspersed and simple sequence repeats, we have identified genome territories by repeat content that appear to define ancestral vs. ruminant-specific genomic regions. These ancestral regions, enriched with L2 and MIR repeats, are largely conserved between bovine and human.

Tracking the past: interspersed repeats in an extinct Afrotherian mammal, Mammuthus primigenius

The woolly mammoth (Mammuthus primigenius) died out about several thousand years ago, yet recent paleogenomic studies have successfully recovered genetic information from both the mitochondrial and nuclear genomes of this extinct species. Mammoths belong to Afrotheria, a group of mammals exhibiting extreme morphological diversity and large genome sizes. In this study, we found that the mammoth genome contains a larger proportion of interspersed repeats than any other mammalian genome reported so far, in which the proliferation of the RTE family of retrotransposons (covering 12% of the genome) may be the main reason for an increased genome size. Phylogenetic analysis showed that RTEs in mammoth are closely related to the family BovB/RTE. The incongruence of the reconstructed RTE phylogeny indicates that RTEs in mammoth may be acquired through an ancient lateral gene transfer event. A recent proliferation of SINEs was also found in the probocidean lineage, whereas the Afrotherian-wide SINEs in mammoth have undergone a rather flat and stepwise expansion. Comparisons of the transposable elements (TEs) between mammoth and other mammals may shed light on the evolutionary history of TEs in various mammalian lineages.

Hidden heterochromatin: Characterization in the Rodentia species Cricetus cricetus, Peromyscus eremicus (Cricetidae) and Praomys tullbergi (Muridae)

The use of in situ restriction endonuclease (RE) (which cleaves DNA at specific sequences) digestion has proven to be a useful technique in improving the dissection of constitutive heterochromatin (CH), and in the understanding of the CH evolution in different genomes. In the present work we describe in detail the CH of the three Rodentia species, Cricetus cricetus, Peromyscus eremicus (family Cricetidae) and Praomys tullbergi (family Muridae) using a panel of seven REs followed by C-banding. Comparison of the amount, distribution and molecular nature of C-positive heterochromatin revealed molecular heterogeneity in the heterochromatin of the three species. The large number of subclasses of CH identified in Praomys tullbergi chromosomes indicated that the karyotype of this species is the more derived when compared with the other two genomes analyzed, probably originated by a great number of complex chromosomal rearrangements. The high level of sequence heterogeneity identified in the CH of the three genomes suggests the coexistence of different satellite DNA families, or variants of these families in these genomes.

Different evolutionary trails in the related genomes Cricetus cricetus and Peromyscus eremicus (Rodentia, Cricetidae) uncovered by orthologous satellite DNA repositioning

Constitutive heterochromatin comprises a substantial fraction of the eukaryotic genomes and is mainly composed of tandemly arrayed satellite DNAs (satDNA). These repetitive sequences represent a very dynamic and fast evolving component of genomes. In the present work we report the isolation of Cricetus cricetus (CCR, Cricetidae, Rodentia) centromeric repetitive sequences from chromosome 4 (CCR4/10sat), using the laser microdissection and laser pressure catapulting procedure, followed by DOP-PCR amplification and labelling. Physical mapping by fluorescent in situ hybridisation of these sequences onto C. cricetus and another member of Cricetidae, Peromyscus eremicus, displayed quite interesting patterns. Namely, the centromeric sequences showed to be present in another C. cricetus chromosome (CCR10) besides CCR4. Moreover, these almost chromosome-specific sequences revealed to be present in the P. eremicus genome, and most interestingly, displaying a ubiquitous scattered distribution throughout this karyotype. Finally and in both species, a co-localisation of CCR4/10sat with constitutive heterochromatin was found, either by classical C-banding or C-banding sequential to in situ endonuclease restriction. The presence of these orthologous sequences in both genomes is suggestive of a phylogenetic proximity. Furthermore, the existence of common repetitive DNA sequences with a different chromosomal location foresees the occurrence of an extensive process of karyotype restructuring somehow related with intragenomic movements of these repetitive sequences during the evolutionary process of C. cricetus and P. eremicus species.

Conserved features of imprinted differentially methylated domains

Genomic imprinting is a conserved epigenetic phenomenon in eutherian mammals, with regards both to the genes that are imprinted and the mechanism underlying the expression of just one of the parental alleles. Epigenetic modifications of alleles of imprinted genes are established during oogenesis and spermatogenesis, and these modifications are then inherited. Differentially methylated domains (DMDs) of imprinted genes are the genomic sites of these inherited epigenetic imprints. We previously showed that CpG-rich imperfect tandem direct repeats within three different mouse DMDs (Snurf/Snrpn, Kcnq1 and Igf2r), each with a unique sequence, play a central role in maintaining the differential methylation. This finding implicates repeat-related DNA structure, not sequence, in the imprinting mechanism. To better define the important features of this signal, we compared sequences of these three DMD tandem repeats among mammalian species. All DMD repeats contain short indirect repeats, many of which are organized into larger unit repeats. Even though the larger repeat units undergo deletion and addition during evolution (most likely through unequal crossovers during meiosis), the size of DMD tandem repeated regions has remained remarkably stable during mammalian evolution. Moreover, all three DMD tandem repeats have a high-CpG content, an ordered arrangement of CpG dinucleotides, and similar predicted secondary structures. These observations suggest that a structural feature or features of these DMD tandem repeats is the conserved DMD imprinting signal.

Genomic expansion of the Bov-A2 retroposon relating to phylogeny and breed management

Bov-A2 is a retroposon that is widely distributed among the genomes of ruminants (e.g., cow, deer, giraffe, pronghorn, musk deer, and chevrotain). This retroposon is composed of two monomers, called Bov-A units, which are joined by a linker sequence. The structure and origin of Bov-A2 has been well characterized but a genome-level exploration of this retroposon has not been implemented. In this study we performed an extensive search for Bov-A2 using all available genome sequence data on Bos taurus. We found unique Bov-A2-derived sequences that were longer than Bov-A2 due to amplification of three to six Bov-A units arranged in tandem. Detailed analysis of these elongated Bov-A2-derived sequences revealed that they originated through unequal crossing-over of Bov-A2. We found a large number of these elongated Bov-A2-derived sequences in cattle genomes, indicating that unequal crossing-over of Bov-A2 occurred very frequently. We found that this type of elongation is not observed in wild bovine and is therefore specific to the domesticated cattle genome. Furthermore, at specific loci, the number of Bov-A units was also polymorphic between alleles, implying that the elongation of Bov-A units might have occurred very recently. For these reasons, we speculate that genomic instability in bovine genomes can lead to extensive unequal crossing-over of Bov-A2 and levels of polymorphism might be generated in part by repeated outbreeding.

Physical organization of the 1.709 satellite IV DNA family in Bovini and Tragelaphini tribes of the Bovidae: sequence and chromosomal evolution

Repetitive DNA in the mammalian genome is a valuable record and marker for evolution, providing information about the order and driving forces related to evolutionary events. The evolutionarily young 1.709 satellite IV DNA family is present near the centromeres of many chromosomes in the Bovidae. Here, we isolated 1.709 satellite DNA sequences from five Bovidae species belonging to Bovini: Bos taurus (BTA, cattle), Bos indicus (BIN, zebu), Bubalus bubalis (BBU, water buffalo) and Tragelaphini tribes: Taurotragus oryx (TOR, eland) and Tragelaphus euryceros (TEU, bongo). Its presence in both tribes shows the sequence predates the evolutionary separation of the two tribes (more than 10 million years ago), and primary sequence shows increasing divergence with evolutionary distance. Genome organization (Southern hybridization) and physical distribution (in situ hybridization) revealed differences in the molecular organization of these satellite DNA sequences. The data suggest that the sequences on the sex chromosomes and the autosomes evolve as relatively independent groups, with the repetitive sequences suggesting that Bovini autosomes and the Tragelaphini sex chromosomes represent the more primitive chromosome forms.

DNA methylation in the preimplantation embryo: the differing stories of the mouse and sheep

In mammals, active demethylation of cytosine methylation in the sperm genome prior to forming a functional zygotic nucleus is thought to be a function of the oocyte cytoplasm important for subsequent normal development. Furthermore, a stepwise passive loss of DNA methylation in the embryonic nucleus has been observed as DNA replicates between two-cell and morula stages, with somatic cell levels of methylation being re-established by, or after the blastocyst stage when differentiated lineages are formed. The ability of oocyte cytoplasm to also reprogram the genome of a somatic cell by nuclear transfer (SCNT) has raised the possibility of directing reprogramming of a somatic nucleus ex ovo by mimicking the epigenetic events normally induced by maternal factors from the oocyte. Whilst examining DNA methylation changes in normal sheep fertilization, we were surprised to observe no demethylation of the sheep male pronucleus at any point in the first cell cycle. Furthermore, using quantitative image analysis, we observed limited demethylation of the sheep embryonic genome only between the two- and eight-cell stages and no evidence of remethylation by the blastocyst stage. We suggest that the dramatic differences in DNA methylation between the sheep and other mammalian species examined call in to question the requirement and role of DNA methylation in early mammalian embryonic development.

Real-time PCR detection of ruminant DNA

To control the spread of bovine spongiform encephalopathy, several DNA methods have been described for the detection of the species origin of meat and bone meal. Most of these methods are based on the amplification of a mitochondrial DNA segment. We have developed a semiquantitative method based on real-time PCR for detection of ruminant DNA, targeting an 88-bp segment of the ruminant short interspersed nuclear element Bov-A2. This method is specific for ruminants and is able to detect as little as 10 fg of bovine DNA. Autoclaving decreased the amount of detectable DNA, but positive signals were observed in feeding stuff containing 10% bovine material if this had not been rendered in accordance with the regulations, i.e., heated at 134 degrees C for 3 instead of 20 min.

Quantitative intra-short interspersed element PCR for species-specific DNA identification

We have designed and evaluated four assays based upon PCR amplification of short interspersed elements (SINEs) for species-specific detection and quantitation of bovine, porcine, chicken, and ruminant DNA. The need for these types of approaches has increased drastically in response to the bovine spongiform encephalopathy epidemic. Using SYBR Green-based detection, the minimum effective quantitation levels were 0.1, 0.01, 5, and 1 pg of starting DNA template using our bovine, porcine, chicken, and ruminant species-specific SINE-based PCR assays, respectively. Background cross-amplification with DNA templates derived from 14 other species was negligible. Species specificity of the PCR amplicons was further demonstrated by the ability of the assays to accurately detect trace quantities of species-specific DNA from mixed (complex) sources. Bovine DNA was detected at 0.005% (0.5 pg), porcine DNA was detected at 0.0005% (0.05 pg), and chicken DNA was detected at 0.05% (5 pg) in a 10-ng mixture of bovine, porcine, and chicken DNA templates. We also tested six commercially purchased meat products using these assays. The SINE-based PCR methods we report here are species-specific, are highly sensitive, and will improve the detection limits for DNA sequences derived from these species.

Incorporation of bovine bone marrow stromal cells into porcine foetal tissues after xenotransplantation

Bovine bone marrow stromal cells (BMSCs) were injected into the liver of foetal pigs at about 40 days of gestation to test whether these cells could populate developing tissue, and if so, which ones. Approximately 40 days after injection, the foetuses were harvested and tissue sections from many areas of the body were analysed for the presence of bovine cells using two different methods. First, using PCR, bovine repetitive DNA was found to be present in DNA extracted from foetal pig tissues. Secondly, using oligonucleotide primed in situ synthesis (PRINS), the in situ presence of bovine cells was found within porcine tissue sections. PRINS-labelled cells were found within cartilage, perichondrium, connective tissue and smooth muscle. These data suggest that bovine BMSCs integrate throughout the foetal pig.

A patchwork interspersed sequence is present in a high copy number in the sheep genome

We have isolated a new interspersed sequence present in a high copy number in the ovine genome. This patchwork sequence, named 3.79 AS1, is part of a larger element encompassing similarities to constant region of reverse transcriptase and to art2 shared with the Bovine Dimer Driven Family (BDDF). The 3.79 AS1 sequence includes homologies to amplification promoting sequences (APS), to a potential origin of bidirectional DNA replication (OBR), to the Alu core sequence motif GGAGGC required for RNA polymerase III promoter function and to the ATGGCTGCCAT sequence that has been shown to be able to induce amplification-dependent transformation in murine cells. Fluorescent in situ hybridization experiments using probes derived from both ends of the 3.79 AS1 sequence showed a widespread signal over all sheep chromosomes, except the Y chromosome. We propose that the structural features of the 3.79 AS1 patchwork sequence, that is likely to be a subfamily of Bov B LINE that invaded the Artiodactyl genome prior to the separation of the Bovidae species, facilitated its massive amplification and dispersion in the ovine genome.

Hybridization of banteng (Bos javanicus) and zebu (Bos indicus) revealed by mitochondrial DNA, satellite DNA, AFLP and microsatellites

Hybridization between wild and domestic bovine species occurs worldwide either spontaneously or by organized crossing. We have analysed hybridization of banteng (Bos javanicus) and zebu (Bos indicus) in south-east Asian cattle using mitochondrial DNA (PCR-RFLP and sequencing), AFLP, satellite fragment length polymorphisms (SFLP or PCR-RFLP of satellite DNA) and microsatellite genotyping. The Indonesian Madura zebu breed is reputed to be of hybrid zebu-banteng origin, but this has never been documented and Bali cattle are considered to be a domesticated form of banteng. The banteng mitochondrial type was found in all animals sampled on the isle of Bali, Indonesia, but only in 35% of the animals from a Malaysian Bali-cattle population. The Madura animals also carried mitochondrial DNA of either zebu and banteng origin. In both populations, zebu introgression was confirmed by AFLP and SFLP. Microsatellite analysis of the Malaysian Bali population revealed for 12 out of 15 loci screened, Bali-cattle-specific alleles, several of which were also found in wild banteng animals. The tools we have described are suitable for the detection of species in introgression studies, which are essential for the genetic description of local breeds and the preservation of their economic and cultural value.

Variation in satellite DNA profiles--causes and effects

Heterochromatic regions of the eukaryotic genome harbour DNA sequences that are repeated many times in tandem, collectively known as satellite DNAs. Different satellite sequences co-exist in the genome, thus forming a set called a satellite DNA library. Within a library, satellite DNAs represent independent evolutionary units. Their evolution can be explained as a result of change in two parameters: copy number and nucleotide sequence, both of them ruled by the same mechanisms of concerted evolution. Individual change in either of these two parameters as well as their simultaneous evolution can lead to the genesis of species-specific satellite profiles. In some cases, changes in satellite DNA profiles can be correlated with chromosomal evolution and could possibly influence the evolution of species.

Pstl repeat: a family of short interspersed nucleotide element (SINE)-like sequences in the genomes of cattle, goat, and buffalo

The PstI family of elements are short, highly repetitive DNA sequences interspersed throughout the genome of the Bovidae. We have cloned and sequenced some members of the PstI family from cattle, goat, and buffalo. These elements are approximately 500 bp, have a copy number of 2 x 10(5) - 4 x 10(5), and comprise about 4% of the haploid genome. Studies of nucleotide sequence homology indicate that the buffalo and goat PstI repeats (type II) are similar types of short interspersed nucleotide element (SINE) sequences, but the cattle PstI repeat (type I) is considerably more divergent. Additionally, the goat PstI sequence showed significant sequence homology with bovine serine tRNA, and is therefore likely derived from serine tRNA. Interestingly, Southern hybridization suggests that both types of SINEs (I and II) are present in all the species of Bovidae. Dendrogram analysis indicates that cattle PstI SINE is similar to bovine Alu-like SINEs. Goat and buffalo SINEs formed a separate cluster, suggesting that these two types of SINEs evolved separately in the genome of the Bovidae.

Xenotransplantation of bovine bone marrow stromal cells into pig fetuses: incorporation into skeletal muscle

Bone marrow stromal cells are multipotent and have been shown to differentiate into a wide variety of mesenchymal cell types in vivo. In this study we tested the ability of bovine bone marrow stromal cells to contribute to the development of porcine skeletal muscle tissue. Fetal pigs were injected early in gestation with bone marrow stem cells originating from slaughtered steers. After approximately forty days of development the fetuses were harvested and sections of their skeletal muscle were analyzed for the presence of bovine cells. PCR was used to detect bovine DNA present in DNA extracted from the fetal pig skeletal muscle. We also used a PRINS (Oligonucleotide Primed In- Situ Synthesis) protocol to confirm the presence of bovine cells within the porcine skeletal muscle tissue sections. The results of both assays indicate that bovine bone marrow stromal cells can participate in the development of porcine skeletal muscle. This study helps to demonstrate the potential that bone marrow stromal cells have to contribute to advances in animal biotechnology and medicine.

Identification of species in animal feedstuffs by polymerase chain reaction--restriction fragment length polymorphism analysis of mitochondrial DNA

Restriction site analysis of Polymerase Chain Reaction (PCR) products of cytochrome b mitochondrial DNA was applied to identify species in meat meal and animal feedstuffs. PCR was used to amplify a variable region of cytochrome b mitochondrial DNA gene. Species differentiation was determined by digestion of the obtained 359 bp amplicon with restriction enzymes, which generated species-specific electrophoresis patterns; the sequencing of PCR products was used as confirming analysis. PCR-RFLP analysis revealed the presence of meat meal in animal feedstuffs and distinguished species of interest. The results supported the application of the method in control measures which should be adopted for meat-meal-based animal feed, as suggested by EU law. As a technical improvement, to simplify the analysis, the number of enzymes presented in this study for the detection of different species was smaller than others described in the literature; discrimination between ruminant and nonruminant species and between mammalian and poultry species was possible with few digestions.

Single nucleotide polymorphisms (SNPs) within Bov-A2 SINE in the second intron of bovine and buffalo k-casein (CSN3) gene

The genetic polymorphism of an entire Bov-A2 element located in the second intron of the buffalo and bovine k-casein (CSN3) gene was investigated by amplification and sequencing of PCR products. Single nucleotide polymorphisms were detected. A PCR-RFLP method was developed to detect an A or G mutation at position 605 of bovine Bov-A2 element which creates a BfaI polymorphic site. The frequencies of the B allele, with the BfaI site, were for 0.275, 0.775, 0.750, 0.975, respectively, for Italian Holstein Friesian, Grey Alpine, Friuli Red Pied and Reggio bovine breeds. The mutation rate (substitutions and deletions/insertions per nucleotide site per year) was 2.5 x 10(-9) for Bov-A2 sequences in the second intron of CSN3. The comparison with other Bov-A2 elements suggests that this retroelement might be an important source of single nucleotide polymorphism for analysis of Bovidae genomes.

Species origin of milk in Italian mozzarella and Greek feta cheese

Several animal species such as cattle, goats, sheep, and water buffalo provide milk for dairy products. We describe a simple procedure for detecting the species origin of milk used for cheese production. DNA was isolated from Italian mozzarella or Greek feta by sequential organic extractions and resin purification. This DNA was analyzed by polymerase chain reaction-restriction fragment length polymorphism as described previously for meat samples. This procedure differentiated mozzarella made from water buffalo milk and from less expensive bovine milk and also feta cheeses made from bovine, ovine, and caprine milk.

Phylogenetic information in inter-SINE and inter-SSR fingerprints of the artiodactyla and evolution of the bov-tA SINE

Various interspersed repeated sequences and elements (IRSs) can be utilized to generate PCR-based multilocus fingerprint profiles by amplifying the interelement segments, using primers matching the elements themselves. We assessed the utility of inter-IRS fingerprinting in phylogenetic comparisons among six artiodactyl species using several primers derived from two abundant genomic components: the Bov-tA short interspersed nuclear elements (SINEs) and simple sequence repeats or microsatellites (SSRs). Character- and distance-based analyses of the fingerprint data produced trees conforming to the established phylogenetic relationships of species. The strength of phylogenetic signal from different primers varied; combining data from different experiments resulted in robust trees. Within the Cervidae, the hierarchical relationship [(Odocoileus, Rangifer) Alces] was strongly supported. Both methods appear useful tools for systematic studies at time scales <30 Myr. To elucidate the material basis of inter-SINE fingerprints, we obtained the first sequences of the 'bovid' Bov-tA element also from two cervids (reindeer and white-tailed deer) and analysed their relationship to a number of paralogous bovid elements. The differences among sequences, both intra- and interspecific, were relatively high (mean 18.5%); the sequences showed no clear clustering with the species from which they had been isolated. Most individual elements probably date back to the cervid-bovid ancestor >25 Myr ago, which is in line with the observed fingerprint distributions.

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.

Satellite DNA polymorphisms and AFLP correlate with Bos indicus-taurus hybridization

We describe satellite DNA variation that detects hybridization of Bos indicus (zebu or indicine cattle) and Bos taurus (taurine cattle) in African cattle populations. On Southern blots hybridized to a satellite III probe, relative intensities of Hinfl fragments correlated with the taurine-zebu composition in hybrid animals as deduced from AFLP genotyping of the same animals and previous data on microsatellite allele frequencies. Similar results were obtained by PCR-RFLP analysis of a zebu-specific mutation in the repeat unit of satellite 1.711b. Analysis of individuals from 20 African cattle breeds indicate that the centromeric satellites of the sanga breeds are of the taurine type and that several East-African zebu breeds are hybrids between taurine and zebu. These satellite RFLP, or SFLP, markers provide a fast method to screen the genetic makeup of African cattle.

The Bov-B lines found in Vipera ammodytes toxic PLA2 genes are widespread in snake genomes

In the fourth intron of two toxic Vipera ammodytes PLA2 genes a Ruminantia specific 5'-truncated Bov-B LINE element was identified. Southern blot analysis of Bov-B LINE distribution in vertebrates shows that, apart from the Ruminantia, it is limited to Viperidae snakes (V. ammodytes, Vipera palaestinae, Echis coloratus, Bothrops alternatus, Trimeresurus flavoviridis and Trimeresurus gramineus). The copy number of the 3' end of Bov-B LINE in the V. ammodytes genome is between 62,000 and 75,000. At orthologous positions in other snake PLA2 genes the Bov-B LINE element is absent, indicating that its retrotransposition in the V. ammodytes PLA2 gene locus has occurred quite recently, about 5 Myr ago. The amplification of Bov-B LINEs in snakes may have occurred before the divergence of the Viperinae and Crotalinae subfamilies. Due to its wide distribution in Viperidae snakes it should be a valuable phylogenetic marker. The neighbour-joining phylogenetic tree shows two clusters of truncated Bov-B LINE, a Bovidae and a snake cluster, indicating an early horizontal transfer of this transposable element.

Unusual horizontal transfer of a long interspersed nuclear element between distant vertebrate classes

We have shown previously by Southern blot analysis that Bov-B long interspersed nuclear elements (LINEs) are present in different Viperidae snake species. To address the question as to whether Bov-B LINEs really have been transmitted horizontally between vertebrate classes, the analysis has been extended to a larger number of vertebrate, invertebrate, and plant species. In this paper, the evolutionary origin of Bov-B LINEs is shown unequivocally to be in Squamata. The previously proposed horizontal transfer of Bov-B LINEs 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 horizontal transfer of Bov-B LINEs from Squamata to the ancestor of Ruminantia is evident from the genetic distances and discontinuous phylogenetic distribution. The ancestor of Colubroidea snakes is a possible donor of Bov-B LINEs to Ruminantia. The timing of horizontal transfer has been estimated from the distribution of Bov-B LINEs in Ruminantia and the fossil data of Ruminantia to be 40-50 My ago. The phylogenetic relationships of Bov-B LINEs from the various Squamata species agrees with that of the species phylogeny, suggesting that Bov-B LINEs have been maintained stably by vertical transmission since the origin of Squamata in the Mesozoic era.

Self-amplification of satellite DNA in vitro

We describe a PCR-like reaction in which genomic DNA acts as a template as well as a primer. Interaction between genomic tandem repeat units leads to self-amplification of satellite DNA. This genomic self-priming PCR (GSP-PCR) allowed the rapid amplification of species-specific tandem repeats of horse, cattle, dolphin, and chicken. A novel specific satellite of ostrich with a repeat unit of 60 bp was isolated using this method.Key words: satellite DNA, amplification, isolation, species-specific probes.

Artiodactyl interspersed DNA repeats in cetacean genomes

Interspersed repeats that emerged at different evolutionary times are informative in mammalian phylogeny. Here we show that the ancient short interspersed elements (SINEs) ARE1 and ARE2 are abundantly present in the genomes of artiodactyls and cetaceans but not in other mammalian genomes. This supports the classification of the cetaceans with the artiodactyls by a shared character that is unlikely to be the result of convergence.

Bov-B long interspersed repeated DNA (LINE) sequences are present in Vipera ammodytes phospholipase A2 genes and in genomes of Viperidae snakes

Ammodytin L is a myotoxic Ser49 phospholipase A2 (PLA2) homologue, which is tissue-specifically expressed in the venom glands of Vipera ammodytes. The complete DNA sequence of the gene and its 5' and 3' flanking regions has been determined. The gene consists of five exons separated by four introns. Comparative analysis of the ammodytin L and ammodytoxin C genes shows that all intron and flanking sequences are considerably more conserved (93-97%) than the mature protein-coding exons. The pattern of nucleotide substitutions in protein-coding exons is not random but occurs preferentially on the first and the second positions of codons, which suggests positive Darwinian evolution for a new function. An Ruminantia specific ART-2 retroposon, recently recognised as a 5'-truncated Bov-B long interspersed repeated DNA (LINE) sequence, was identified in the fourth intron of both genes. This result suggests that ammodytin L and ammodytoxin C genes are derived by duplication of a common ancestral gene. The phylogenetic distribution of Bov-B LINE among vertebrate classes shows that, besides the Ruminantia, it is limited to Viperidae snakes (Vipera ammodytes, Vipera palaestinae, Echis coloratus, Bothrops alternatus, Trimeresurus flavoviridis and Trimeresurus gramineus). The copy number of the 3' end of Bov-B LINE in the Vipera ammodytes genome is between 62,000 and 75,000. The absence of Bov-B LINE at orthologous positions in other snake PLA2 genes indicates that its retrotransposition in the V. ammodytes PLA2 gene locus has occurred quite recently, about 5 My ago. The amplification of Bov-B LINEs in snakes may have occurred before the divergence of the Viperinae and Crotalinae subfamilies. Due to its wide distribution in Viperidae snakes it may be a valuable phylogenetic marker. The neighbor-joining phylogenetic tree shows two clusters of truncated Bov-B LINE, a Bovidae and a snake cluster, indicating an early horizontal transfer of this transposable element.

The origin of interspersed repeats in the human genome

Over a third of the human genome consists of interspersed repetitive sequences which are primarily degenerate copies of transposable elements. In the past year, the identities of many of these transposable elements were revealed. The emerging concept is that only three mechanisms of amplification are responsible for the vast majority of interspersed repeats and that with each autonomous element a number of dependent non-autonomous sequences have co-amplified.

Conservation of a 31-bp bovine subrepeat in centromeric satellite DNA monomers of Cervus elaphus and other cervid species

A centromeric satellite DNA clone was isolated from the genome of the European red deer (Cervus elaphus hippelaphus) and designated Ce-Pst1. This clone was localized to the centromeric region of all red deer chromosomes with the exception of a single pair of metacentric autosomes and the Y chromosome. DNA sequence analysis of the 806-bp Ce-Pst1 clone showed 73.0-78.9% sequence homology to four previously isolated cervid centromeric satellite DNA clones, suggesting that the Ce-Pst1 clone is yet another member of the major cervid centromeric satellite DNA family. Using a DNA sequence comparison system, internal 31-bp tandem subrepeats were found in the Ce-Pst1 clone as well as in the other previously reported cervid centromeric satellite DNA monomer sequences. A 31-bp consensus sequence was constructed for each cervid monomer clone and shown to be highly homologous to the 31-bp subrepeat consensus sequence found in bovine 1.715 centromeric satellite DNA. The identification of internal subrepeats in the satellite monomers studied could suggest that amplification of an ancestral 31-bp DNA sequence may have contributed to the genesis of major cervid centromeric satellite DNA. The homology between the 31-bp subrepeats found in cervid and bovid centromeric satellite DNAs substantiates the theory that amplification of this 31-bp DNA sequence may have occurred before the evolutionary separation of these two families 20-25 million years ago.