* Dipartimento di Anatomia Patologica e Genetica, Sezione di Genetica, Bari, Italy
Dipartimento di Biologia e Genetica per le Scienze Mediche, Milano, Italy
Dipartimento di Medicina Sperimentale, Ambientale e Biotecnologie Mediche, Monza, Italy
Correspondence: E-mail: raffaella.meneveri{at}unimib.it.
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Abstract |
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Key Words: beta satellite duplication primate evolution
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Introduction |
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Duplicons can be grouped as intrachromosomal and interchromosomal, and the latter tends to be enriched in pericentrometic and subtelomeric regions. Pericentromeric duplications usually span the transition from euchromatic genes on the short/long arm to pericentromeric satellites/centromeric alpha satellites (Horvath et al. 2000). However, despite their location, the evolutionary history of alpha satellite appears distinct from that of the pericentromeric duplicons (Willard 1991).
Beta satellite repeats (BSR) represent another family of sequences that show a predominant heterochromatic distribution, which includes the short arm of acrocentric chromosomes and chromosomes 1q12, 3q12, 9q12, and Yq11 (Meneveri et al. 1985,1993; Agresti et al. 1987, 1989). In these regions, beta satellite arrays are in linkage with LSau repeats, which are part of complex repetition units of 3.3 kb in length (D4Z4-like repeats) (Agresti et al. 1989; Hewitt et al. 1994; Lyle et al. 1995; Ballarati et al. 2002). D4Z4 repetitions and BSRs are also localized on chromosomes 4q35 and 10q26 (Bakker et al. 1995; Lemmers et al. 2002).
Evolutionary studies suggest that 4q35 represents the ancestral locus for the D4Z4 sequence and that an extensive radiation of the region occurred after the divergence of Old World monkeys and hominoids (Clark et al. 1996; Winokur et al. 1996; Ballarati et al. 2002). In this respect, a few studies have been devoted to investigating the evolution of beta satellite sequences. The origin of this class of repeats can be tentatively traced back to the orangutan (PPY), where the beta satellite organization appears different from humans (HSA), chimpanzee (PTR), and gorilla (GGO) (Meneveri et al. 1995; Hirai, Taguchi, and Godwin 1999).
To further investigate this point, we screened two different orangutan genomic libraries in search of BSR-positive DNA sectors. All the gathered results indicate that BSR originated in an early hominoid ancestor and that the initial step of BSR amplification and spreading was mediated by the insertion of very short stretches of the repeat into a long genomic region, which underwent duplicative transpositions.
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Materials and Methods |
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Southern Blot Hybridization
Cosmid and BAC DNAs were digested with Sau3A restriction enzyme (Biolabs), fractionated by agarose gel electrophoresis, blotted onto nylon filters (Hybond N+; Amersham), and hybridized with 32P-labeled A17a probe. Molecular hybridizations were in 2xSSC at 60°C overnight, and filters were washed at 60°C in 1xSSC, twice for 20 min. Hybridization signals were quantified by phosphoimaging (Typhoon; Amersham).
Polymerase Chain Reaction and Sequencing
Genomic DNA was obtained from human placenta, lymphoblastoid, and fibroblast primate cell lines (chimpanzee, Pan troglodytes [PTR]; gorilla, Gorilla gorilla [GGO]; orangutan, Pongo pygmaeus [PPY]; lar gibbon, Hylobates lar [HLA]; rhesus monkey, Macaca mulatta [MMU]) (http://www.biologia.uniba.it/rmc/) and from a human monochromosomal somatic cell hybrid panel (http://www.biologia.uniba.it/rmc/) by standard methods.
Polymerase chain reaction (PCR) amplifications with the different DNA templates were carried out by the primer pairs BSR forw (5'-AGGGGCTTTATCCTCATTTCACAA-3') and rev (5'-GGCCTCCATATTCCCTAACTTCC-3'); PPY2 forw (5'-TGAATTTTAGCACCCACAA-3') and rev (5'-ATTCGATTCAACCCCAGTTA-3'); PPY4 forw (5'-AGTCACTGTGGCGATTCTTCTA-3') and rev (5'-GTGTGATGTCCCCTTTCCTG-3'); PPY14 forw (5'-ATGGGCCAGGAGCTATTCACAG-3') and rev (5'-CGCAACGCCCCCTTCAACC-3') and by Alu-PCR (Breen et al. 1992).
Primer pairs BSR and PPY2, PPY4, and PPY14 were derived, respectively, from the human genomic DNA sequence AC006987 (Yp11.2) and from sequences obtained by Alu-PCR (cosmids O05112Q2 and H1653Q2) and T3 end (cosmid N1981Q2) of BSR-positive PPY cosmids. For inter-Alu and T3/T7 ends, only one strand was sequenced.
PCR was carried out in a final volume of 25 µl that contained 0.2 mM dNTPs, 1 µM each primer, and 1U Taq polymerase in standard reaction buffer (Sigma). For amplification denaturation was at 95°C for 1 min, and annealing was as follows: BSR (60°C), PPY2 (46°C), PPY4 (54°C), and PPY18 (54°C), for 1 min, with extension 72°C for 1 min. The total number of cycles was of 30 to 35. PCR products were analyzed by 1% agarose gel electrophoresis, subcloned into the pGEM-T easy vector (Promega), and sequenced on both strands by the Big Dye terminator system (Perkin-Elmer Applied Biosystems) following the instructions of the vendor.
All fluorescent traces were analyzed on the Applied Biosystem Model 3100 DNA Sequencing System (PerkinElmer Applied Biosystems). DNA sequence analysis was performed by DNASTAR software and by NCBI facilities (www.ncbi.nlm.nih.gov). New sequence data from this article are available at http://www.ncbi.nlm.nih.gov/, with accession numbers AY546195 to AY546245.
Fluorescence in situ Hybridization
Metaphase spreads were obtained by standard methods from peripheral blood lymphocytes of normal human donors and from lymphoblastoid or fibroblast primate cell lines. The probes were directly labeled with Cyanine 3-dUTP by nick-translation, according to the protocol of the vendor (Perkin-Elmer), and hybridized overnight to chromosomal preparations. Hybridizations were in 50% formamide (v/v), 10% dextran sulfate, SSC at 37°C, in the presence of Cot1 human DNA (Gibco-BRL). Posthybridization washing was in 50% formamide, SSC at 42°C, followed by three washes in SSC at 60°C (HSA), and in 50% formamide, SSC at 37 C, followed by three washes in SSC at 42°C (PTR, GGO, PPY, HLA, and MMU). Chromosomes were stained with DAPI (4',6-diamidino-2-phenylindole). Digital images were captured by a Leica DMRXA epifluorescence microscope equipped with a cooled CCD camera (Princeton Instruments). Cy3 (red) and DAPI (blue) fluorescence signals were recorded separately as gray-scale images. Pseudocoloring and merging of images were performed by use of Adobe Photoshop software.
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Results |
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A primer pair (BSR forw and rev) from outside both repeat regions and defining 520 and 450 bp of DNA on chromosomes Yp11.2 and 15, respectively (fig. 1B), was then used to analyze a total of 51 BSR-positive orangutan genomic clones (27 cosmids and 24 BACs) and orangutan genomic DNA. Only two clones gave a band of 520 bp (long BSR [fig. 1C]), whereas the great majority of the analyzed clones were positive for the 450-bp band (short BSR [fig. 1C]). The same PCR reaction on orangutan genomic DNA showed the simultaneous presence of both PCR bands, with a ratio in favor of the light band (fig. 1C).
The alignment of one long BSR sequence with the consensus derived from nine short BSR DNA sequences (figure 2A and Supplementary Materials online) revealed an organization comparable to that derived from chromosomes Yp11.2 and 15: 170 and 100 bp of BSR were embedded within unrelated DNA. Furthermore, only within the 170-bp BSR region, two Sau3A sites define a 68-bp beta satellite unit. The phylogenetic analysis of the nine short BSR sequences clearly indicated the occurrence of sequence divergence, which suggests their derivation from different orangutan genomic regions (figure 2B and Supplementary Materials online). The 100-bp and 170-bp orangutan BSR regions showed a similarity of approximately 94% and 92% with a human BSR consensus sequence (not shown).
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Evolution of the Duplication Carrying the BSR Region
Five BACs (162K8, 1O23, 25F18, 60F17, and 98A13) were used as probes on primate chromosome spreads (HSA, PTR, GGO, PPY, HLA, and MMU). An example of comparative FISH experiments is shown in figure 4, and a summary of the chromosome location obtained by all clones is reported in table 1. On the macaque and the gibbon, the probes highlighted only the "marker chromosome," at the level of the secondary constriction bearing the rDNA array (fig. 4A [MMU and HLA]). Furthermore, the intensity of the hybridization signals was constantly higher in the gibbon than in the macaque. Conversely, the other analyzed species (HSA, PTR, GGO, and PPY) showed a multiple chromosome location that included pericentromeric regions and the short arm of acrocentric chromosomes (fig. 4A and table 1). In addition to the macaque and the gibbon, the other analyzed primates also showed almost all the chromosomal regions bearing the rDNA labeled by the duplication. The above FISH experiments, however, although clearly demonstrating the spreading of the duplication, are not proof that BSR sequences were also involved. A human clone that contained only BSR sequences was then used in FISH experiments on orangutan, gibbon, and macaque chromosome spreads. The negative results (not shown) suggested that in these species, BSR sequences were either absent or below FISH resolution. To further investigate this point, we performed a PCR assay with the BSR primer pair on primate genomic DNAs (fig. 4B). A band of 450 bp was detected in great apes and in the gibbon but not in the macaque. In addition, as already observed for the orangutan (see figure 1C), humans, chimpanzee, and gorilla also showed an additional band of approximately 520 bp. The ratio of the two bands varied in the different species. In the orangutan, the shorter band was predominant, whereas the opposite was found in the gorilla. Conversely, chimpanzee and humans showed comparable amounts of the 450-bp and 520-bp bands. The 450-bp and 520-bp bands yielded positive results when hybridized with a human BSR probe (not shown). Furthermore, we performed with the human BSR probe at low-stringency conditions, the Southern blot analysis of macaque genomic DNA and the screening of a macaque BAC library (not shown). In both cases, no hybridization signals were detected, which further supports the absence of BSR sequences in the macaque genome.
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Discussion |
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The ancestral locus of the duplicon, found in a chromosomal region at the boundary of the rDNA on the marker chromosome (chromosome 13) of the macaque, does not carry any BSR subsequently found in the gibbon and orangutan. Indeed, Southern analysis and BAC library screening of the macaque were negative for the BSR sequence. Conversely, beta satellite sequences were detected within the duplicon in the genome of the gibbon. Similarly to the macaque, also in the gibbon, the duplicon mapped to a single chromosomal location, at the boundary of the secondary constriction bearing rDNA on chromosome 12 (marker chromosome). Furthermore, the comparison between the FISH signal intensity in the macaque and the gibbon suggested the occurrence in the latter species of the initial duplication of the sequence.
Thus, the ancestral copy of the duplicon appeared in OWMs (<35 MYA), whereas the prototype of beta satellite repeats and their initial amplification took place in a gibbon ancestor, after apes/OWM divergence (25 MYA). Subsequently, a burst in spreading of the duplicon carrying the beta satellite was observed in the orangutan, after lesser apes divergence from the great apeshumans lineage (
18 MYA).
The analysis of the orangutan genome also indicated the existence of two variants of the duplication, differing for the length of inserted beta satellite repeats. The shorter repeat organization is 100 bp, whereas the longer one is 170 bp. The latter organization was probably generated by nonhomologous recombination between two 100-bp repeated regions. Likely, this event led to the duplication of the single Sau3A site present in the 100-bp variant, generating, in this way, the Sau3A 68-bp repetition unit of beta satellite sequences. The longer beta satellite organization seems to be absent in the gibbon, but it characterizes the hominoid genomes from orangutan to humans. In the orangutan, the ratio between the 100-bp and 170-bp repeated stretches is in great favor of the short one. Conversely, this ratio is reversed in the gorilla, which is also characterized by two large arrays of beta satellite repeats on chromosomes IV and Y (Hirai, Taguchi, and Godwin 1999). The large arrays of beta satellite originated, therefore, in the African apeshumans ancestor, after the divergence from the orangutan (14 MYA). The generation of the beta satellite clustered organization can be hypothesized to have occurred by unequal crossing-over during meiosis or by distinct mechanisms acting internally to the duplicons carrying the initial BSR seeds. If this hypothesis is correct, one would expect in African great apes a certain extent of colocalization of clustered BSR and these duplicons. Data from the gorilla, however, does not support this scenario. The analysis of DNA sequences flanking the beta satellite clusters in the gorilla could allow the definition of the more plausible evolutionary hypothesis. Conversely, the comparison of chimpanzee/human location of BSR showed a consistent distribution of both clustered and interspersed repeats, which further supporting their greater closeness in respect to the gorilla (Caccone and Powell 1989; Ruvolo 1997).
During primate evolution, the distribution of BSR duplicons involved the marker chromosome in the macaque and the gibbon, and preferentially the acrocentrics in the hominoid species. This distribution strongly paralleled the evolutionary dispersal of rDNA clusters (Hirai, Taguchi, and Godwin 1999). Conversely, human chromosomes 20 and Y acquired the BSR duplication, respectively, before gorilla and chimpanzee divergence. Location consistency between duplications bearing BSR and rDNA clusters, with potential functionality implications, is intriguing (Horvath et al. 2001). Evolutionary comparison among sequences (when available) (Eichler and DeJong 2002) of these chromosomal regions will probably elucidate the possible role of these duplications.
As derived by the comparison between FISH and PCR data, human chromosomes 1 and X carry a very reduced length of the duplication, characterized by the occurrence of 100 and 170 bp of BSR, respectively. The two BSR variants showed also distinct patterns of distribution in human acrocentric chromosomes. Chromosomes 13, 15, and 21 contain a very similar 100-bp stretch of beta satellite, whereas chromosomes 14 and 22 carry the same 170-bp version. Acrocentric short arm sequence homogenization has been hypothesized to be a consequence of their physical association at meiotic prophase and somatic interphase (Schweizer and Loidl 1987). However, this does not seem to occur for the identified duplication, as well as for other subclasses of repetitive sequences (i.e., satIII), proposed to be at the basis of preferential dicentric Robertsonian translocations among chromosomes 13, 14, and 21 (Sullivan et al. 1996; Bandyopadhyay et al. 2001). These results suggest the existence of constrains against a full homogenization of the DNA sequences within the short arm of acrocentrics. In this respect, chromosomes 13 and 21, as well as 14 and 22 share high homologous subsets of alphoid sequences (Choo, Vissel, and Earle 1989). In addition, large paralogous regions are shared between these same couples of chromosomes (13/21 and 14/22) (Bailey et al. 2002b; Golfier et al. 2003).
In conclusion, we have delineated the evolutionary history of beta satellite sequences, which began in a lesser apes ancestor as a low-copy, or nonduplicated, BSR sequence. After lesser apes/great apes divergence, we observed a burst of BSR spreading as part of a duplicon. In a second burst dispersal, which occurred after orangutan/gorilla, chimpanzee, and human divergence, the BSR acquired the basic features of classical satellite DNA. The specific causes that triggered the two kinds of events are not clear. The only hypothesized mechanism of duplicon spreading points to Alu sequences because they have been seen significantly over-represented at duplicon junctions (Bailey et al. 2003). The preferential location of BSR in pericentromeric regions suggests that centromere-specific mechanisms are involved. This conclusion is indirectly supported by the fact that inactivated centromeres tend to rapidly loose all satellite DNAs (Eder et al. 2003; Ventura et al. 2003). On the contrary, newly seeded centromeres have rapidly acquired the complex organization of normal centromeres that feature large blocks of satellite DNA (Ventura et al. 2003). At moment, however, we have a very poor understanding of forces that drive these processes.
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Acknowledgements |
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Footnotes |
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