Intraspecific variation in the distribution of the interstitial telomeric (TTAGGG)n sequences in Micoureus demerarae (Marsupialia: Didelphidae)

The distribution of the telomeric sequence (TTAGGG)_n was studied in chromosomes of Micoureus demerarae (2n=14), a South American marsupial, by fluorescence in-situ hybridization (FISH). The telomeric repeat sequence was present at both ends of all chromosomes, but also various interstitial telomeric sequences (ITS) were detected in the pericentromeric heterochromatic regions. Intraspecific differences in the number of ITS (2 to 8) were observed without intraindividual variation. The presence of telomere-like sequences in the same regions of constitutive heterochromatin suggest that these segments are not necessarily remnants of true telomeres resulting from chromosome rearrangements but could be part of the satellite DNA.     

Cytogenetic analyses of chromosomal rearrangements in Mus minutoides/musculoides from North-West Zambia through mapping of the telomeric sequence (TTAGGG)n and banding techniques

Three specimens of M. minutoides/musculoides from Zambia were cytogenetically studied through G- and C-banding, DAPI staining and fluorescence in-situ hybridization (FISH) with a (TTAGGG)_n telomeric sequence. Biarmed chromosomes were identified according to the current nomenclature as follows: Rb(2.7), Rb(3.12), Rb(4.5), Rb(6.8), Rb(9.16), and the sex chromosomes Rb(1.X), Rb(1.Y) and Rb(1.X_d), originated from the deleted X chromosome. One female showed the diploid number 2n=24; in the two other individuals, the Rb(9.16) occurred in a heteromorphic condition, and, accordingly, the diploid number was 2n=25. FISH showed the sites of telomeric sequences at telomeres of all the chromosomes, and in an interstitial position at the centromeres of all Robertsonian metacentrics, except one – the Rb(6.8), though the patterns of hybridization varied between chromosomes. Sex chromosome pairs, in the male and females, showed a similar C-banding pattern, but revealed clear differences after FISH. Traces of telomeric sequences were found dispersed in the whole-heterochromatic arm of the Rb(1.X_d). No visible bond between C-positive heterochromatin and telomeric sequences were detected in the other either bi- or uniarmed chromosomes, indicating that they may actually represent retained telomeres in the Robertsonian metacentrics. This revised version was published online in July 2006 with corrections to the Cover Date.     

Loss of telomeric sites in the chromosomes ofMus musculus domesticus (Rodentia: Muridae) during Robertsonian rearrangements

Mouse chromosomes possessing multiple Robertsonian rearrangements (Rb chromosomes) have been examined using fluorescencein situ hybridization with the telomeric consensus sequence (TTAGGG)_n. No hybridization signals were detected at the primary constriction of Rb chromosomes. This observation leads us to conclude that the formation of Rb chromosomes in the mouse is invariably associated with the loss of telomeric regions. More significantly, a further alteration in regions flanking the primary constrictions was observed after hybridizing with a minor satellite DNA probe to Rb chromosomes. It seems likely that the breakpoints required for a Robertsonian process do not include telomeric sites exclusively but extend to the adjacent pericentromeric regions of the original acrocentric chromosomes. In contrast to previous reports, these observations demonstrate the elimination of substantial amounts of chromosomal DNA during the formation of mouse Rb chromosomes.     

Intrachromosomal distribution of telomeric repeats in Eumops glaucinus and Eumops perotis (Molossidae, Chiroptera)

The location of chromosomal telomeric repeats (TTAGGG)_n was investigated in two species of the Molossidae family, Eumops glaucinus and Eumops perotis. The diploid chromosome number (2n) is 40 in E. glaucinus and 48 in E. perotis and the fundamental numbers (FN) are 64 and 58, respectively. It has been suggested that the E. glaucinus karyotype has evolved from the E. perotis karyotype through Robertsonian fusion events. In the present study, the telomeric sequences were detected at the termini of chromosomes in both species. In addition, E. glaucinus also displayed telomeric repeats in centromeric and pericentromeric regions in almost all biarmed chromosomes. Conversely, in E. perotis pericentromeric signals were only observed in two biarmed chromosomes. In both E. glaucinus and E. perotis, such telomeric sequences were observed as part of the heterochromatin. The interstitial sites of telomeric sequences suggest that they are remnants of telomeres of ancestral chromosomes that participated in the fusion event.     

Localization of Repetitive DNAs to Zebrafish (Danio rerio) Chromosomes by Fluorescence in situ Hybridization (FISH)

The genome of the zebrafish, Danio rerio, contains two major classes of tandem repetitive elements (AT-rich and GC-rich). The AT-rich repeats can be further subdivided into two subgroups which differ by about 10% of 185 bp in the repeating unit. The chromosomal location of these sequences and the moderately repetitive 5S rDNA sequences was determined in two diploid zebrafish cell lines using in-situ hybridization with fluorochrome-labeled probes. The AT-rich sequences were found at the centromeres of all chromosome pairs and the GC-rich sequences were found in paracentromeric location on over half of the chromosomal pairs. Different patterns of hybridization were found for the two subgroups of the AT-rich family. One type hybridized primarily to centromeres of one half to two thirds of the chromosomal pairs and the other type to centromeres of about three fourths of the chromosomal pairs. The pattern of hybridization with the GC-rich sequences varied somewhat between the cell lines consistent with interindividual variation in the location of paracentromeric heterochromatin. The 5S rRNA genes are found on the long arm of chromosome 3. Most of this chromosome arm is late replicating, but apparently does not contain either the AT-rich or GC-rich repetitive sequences.     

Non-telomeric chromosome localization of (TTAGGG) n repeats in the genus Eulemur

The chromosomal distribution of the (TTAGGG)_n telomeric repetitive sequences was studied in the Malagasy species Eulemur fulvus fulvus (2n = 60), Eulemur rubriventer (2n = 50), Eulemur coronatus (2n = 46) and Eulemur macaco (2n = 44). These sequences hybridize to the telomeres of all chromosomes of the four species and also to the pericentromeres of all chromosomes of E. fulvus, E. coronatus and E. macaco, with the exception of the pericentromeres of E. coronatus and E. macaco chromosomes 9, the homeologous E. fulvus chromosomes 2 and E. macaco chromosomes 1. In E. rubriventer only a very weak signal was detected at the pericentromeres of a few chromosomes. In E. fulvus, E. coronatus and E. macaco, non-telomeric (TTAGGG)_n sequences collocalize with constitutive heterochromatin. The interspecific differences of the hybridization pattern of (TTAGGG)_n sequences at the pericentromeres suggest that E. rubriventer branched off the common trunk before amplification of endogenous (TTAGGG)_n sequences occurred in pericentromeric regions.This revised version was published online in November 2005 with corrections to the Cover Date.     

PRINS Analysis of the Telomeric Sequence in Seven Lemurs

We examined the chromosomal localization of the telomeric sequence, (TTAGGG)_n, in seven species of the lemurs and one greater galago, as an outgroup, using the primed in-situ labeling (PRINS) technique. As expected, the telomeric sequence was identified at both ends of all chromosomes of the eight prosimians. However, six species showed a signal at some pericentromeric regions involving constitutive heterochromatin as well. The pericentromeric region of chromosome 1 of Verreaux's sifaka (Propithecus verreauxi verreauxi) was labeled with a large and intense signal. The range of the signal considerably exceeded the area of DAPI positive heterochromatin. On the other hand, in the five lemurs, a large signal was detected also in the short arm of acrocentric chromosomes. Acquisition of the large block of the telomeric sequence into the acrocentric short arm might be interpretable in terms of the tandem growth of the heterochromatic short arm and the reciprocal translocation between heterochromatic short arms involving the telomeric sequence. Subsequently, it was postulated that meta- or submetacentric chromosomes possessing the telomeric sequence at the pericentromeric region could be formed by centric fusion between such acrocentric chromosomes.     

Rapid,independent, and extensive amplification of telomeric repeats in pericentromeric regions in karyotypes of arvicoline rodents

The distribution of telomeric repeats was analyzed by fluorescence in situ hybridization in 15 species of arvicoline rodents, included in three different genera: Chionomys, Arvicola, and Microtus. The results demonstrated that in most or the analyzed species, telomeric sequences are present, in addition to normal telomeres localization, as large blocks in pericentromeric regions. The number, localization, and degree of amplification of telomeric sequences blocks varied with the karyotype and the morphology of the chromosomes. Also, in some cases telomeric amplification at non-pericentromeric regions is described. The interstitial telomeric sequences are evolutionary modern and have rapidly colonized and spread in pericentromeric regions of chromosomes by different mechanisms and probably independently in each species. Additionally, we colocalized telomeric repeats and the satellite DNA Msat-160 (also located in pericentromeric regions) in three species and cloned telomeric repeats in one of them. Finally, we discuss about the possible origin and implication of telomeric repeats in the high rate of karyotypic evolution reported for this rodent group.     

Telomeric and interstitial telomeric sequences in holokinetic chromosomes of Lepidoptera: Telomeric DNA mediates association between postpachytene bivalents in achiasmatic meiosis of females

Telomeres, besides their main role in the protection and maintenance of chromosome ends, have several other vital functions in the cell cycle. We studied their role in the achiasmatic meiosis of female Lepidoptera, insects with holokinetic chromosomes. By fluorescence in-situ hybridization (FISH) with the insect telomeric probe, (TTAGG) _n , we mapped the distribution of telomeric and interstitial telomeric sequences (ITS) in female meiotic chromosomes of two species, Orgyia antiqua with a reduced chromosome number (2n=28) and Ephestia kuehniella mutants, possessing a radiation-induced chromosome fusion in the genome (2n=59). In addition to the strong typical telomeric signals, O. antiqua displayed weaker hybridization signals in interstitial sites of pachytene bivalents. The observed ITS most probably reflect remnants of chromosomal rearrangements and support the hypothesis that the Orgyia karyotype had arisen by multiple fusions of ancestral chromosomes. On the other hand, the absence of ITS in the chromosome fusion of Ephestia indicated the loss of telomeres before the two original chromosomes fused. When the telomeric probe was amplified by enzymatic reaction with tyramid, the number of ITS observed increased in Orgyia, and a few ITS were also observed in several chromosomes of Ephestia but not in the fused chromosome. This suggests that the genomes of both species also contain ITS other than those originating from chromosome fusions. The analysis of female meiotic prophase I revealed non-homologous associations of postpachytene bivalents mediated by telomeric DNA, which were not observed in the pachytene stage. Surprisingly, in early postpachytene nuclei the telomeric associations also involved ITS, whereas later postpachytene nuclei displayed chains of bivalents interconnected only by true telomeres. This finding favours a hypothesis that telomeric associations between bivalents play a role in chromosome segregation in the achiasmatic meiosis of female Lepidoptera. This revised version was published online in July 2006 with corrections to the Cover Date.     

Unusual distribution pattern of telomeric repeats in the shrews <Emphasis Type="Italic">Sorex araneus</Emphasis> and <Emphasis Type="Italic">Sorex granarius</Emphasis>

Sorex araneus and Sorex granarius are sibling species within the Sorex araneus group with karyotypes composed of almost identical chromosome arms. S. granarius has a largely acrocentric karyotype, while, in S. araneus, various of these acrocentrics have combined together by Robertsonian (Rb) fusions to form metacentrics, with the numbers and types of metacentrics differing between chromosomal races. Our studies on telomeric sequences in S. araneus and S. granarius revealed differences between chromosomes and between species. In S. araneus (the Novosibirsk race), hybridization signals were present on the telomeres of all the chromosomes after FISH with a PCR-generated telomeric probe. In addition, hybridization signals were observed at high frequencies in the pericentric regions of some but not all metacentrics formed by Rb fusion. There were fewer signals on those metacentrics formed earlier in the evolution of S. araneus. This suggests that S. araneus chromosomes retain at least some telomeric repeats during Rb fusion, but that these repeats are lost or modified over time. These results are critical for the interpretation of the well-studied hybrid zones between chromosomal races of S. araneus, given that Rb fission has been postulated in such hybrid zones and that the likelihood of Rb fission will relate to presence/absence of telomeric sequences at the centromeres of metacentrics. In S. granarius, there were strong signals at the proximal (centromeric) telomeres of the acrocentrics after FISH with a DNA telomeric probe. FISH with a PNA telomeric probe on S. granarius acrocentrics showed that the proximal telomeres were 213 kb on average, while the length of the distal telomeres was 3.8 kb on average. Two-colour FISH, using a telomeric DNA probe and a microdissected probe generated from the pericentric regions of the S. granarius chromosomes a and b, revealed regions on distinct chromatin fibres where telomeric and microdissected probes were colocalized or localized sequentially. The proximal telomeres of S. granarius are highly unusual both in their large size and their heterogeneous structure relative to the telomeres of other mammals.     

Telomeric (TTAGGG)n Sequences Are Associated With Nucleolus Organizer Regions (NORs) in the Wood Lemming

The distribution of the (TTAGGG)_n telomeric sequence was studied in chromosomes of the wood lemming, Myopus schisticolor, by fluorescence in-situ hybridization. As expected, the hybridization signals were observed at telomeres of all chromosomes. However, quite a number of interstitial telomeric sites were present in the pericentric heterochromatic regions. Consistent strong hybridization signals were also seen at one terminus of chromosomes 5, 7 and 12–15. By post-hybridization G-banding and silver-staining, the large blocks of the telomeric sequences on chromosomes 5 and 12 were localized to nucleolus organizer regions (NORs). This revised version was published online in August 2006 with corrections to the Cover Date.     

Chromosomal study of a lamprey (<Emphasis Type="Italic">Lampetra zanandreai</Emphasis> Vladykov, 1955) (Petromyzonida: Petromyzontiformes): conventional and FISH analysis

Karyotype and other chromosomal characteristics in the Adriatic brook lamprey Lampetra zanandreai, representative of one of the most ancestral group of vertebrates, were examined using conventional (Ag-staining, C-banding as well as CMA₃ and DAPI fluorescence) and molecular (FISH with 18/28S rDNA and EcoRI satDNA as probes) protocols with metaphase chromosomes derived from whole blood cultures. The chromosome complement had a modal diploid chromosome number of 2n = 164, as in other petromyzontid lamprey species. Ag-staining and CMA₃ fluorescence, as well as FISH with 18/28S rDNA probes, detected nucleolar organizer regions (NORs) close to the centromeres of the biarmed chromosomes of pairs 1 and 2, the largest chromosome pairs of the complement. In addition to NORs, CMA₃ fluorescence revealed positive signals in approximately 40 other chromosomes. DAPI stained mostly centromeric regions of many chromosomes as well as conspicuously massive blocks overlapping NOR sites. C-banding evidenced a large amount of constitutive heterochromatin in somatic chromosomes, with approximately 40 C-positive acrocentric elements completely heterochromatic, corresponding with the 40 CMA₃+ chromosomes and positive heterochromatic blocks in pericentromeric regions of chromosome pairs 1 and 2. Polymerase chain reaction (PCR)-based cloning of satellite DNA with primers derived from Petromyzon marinus centromeric sequences was successful for L. zanandreai genomic DNA. The sequence was AT-rich (59%) and characterized by short consensus motifs similar to other centromeric satellite motifs. FISH using satDNA clones as a probe produced a fluorescent signal on a single pair of small chromosomes. This sequence was PCR-amplified also in L. planeri and P. marinus genomic DNA, and the evolution of this repetitive element in the above species was analysed.     

Molecular structures of centromeric heterochromatin and karyotypic evolution in the Siamese crocodile (Crocodylus siamensis) (Crocodylidae, Crocodylia)

Crocodilians have several unique karyotypic features, such as small diploid chromosome numbers (30–42) and the absence of dot-shaped microchromosomes. Of the extant crocodilian species, the Siamese crocodile (Crocodylus siamensis) has no more than 2n = 30, comprising mostly bi-armed chromosomes with large centromeric heterochromatin blocks. To investigate the molecular structures of C-heterochromatin and genomic compartmentalization in the karyotype, characterized by the disappearance of tiny microchromosomes and reduced chromosome number, we performed molecular cloning of centromeric repetitive sequences and chromosome mapping of the 18S-28S rDNA and telomeric (TTAGGG) _n sequences. The centromeric heterochromatin was composed mainly of two repetitive sequence families whose characteristics were quite different. Two types of GC-rich CSI-HindIII family sequences, the 305 bp CSI-HindIII-S (G+C content, 61.3%) and 424 bp CSI-HindIII-M (63.1%), were localized to the intensely PI-stained centric regions of all chromosomes, except for chromosome 2 with PI-negative heterochromatin. The 94 bp CSI-DraI (G+C content, 48.9%) was tandem-arrayed satellite DNA and localized to chromosome 2 and four pairs of small-sized chromosomes. The chromosomal size-dependent genomic compartmentalization that is supposedly unique to the Archosauromorpha was probably lost in the crocodilian lineage with the disappearance of microchromosomes followed by the homogenization of centromeric repetitive sequences between chromosomes, except for chromosome 2.     

Integration of cytogenetic and genetic linkage maps of <Emphasis Type="Italic">Lotus japonicus</Emphasis>, a model plant for legumes

To construct a high-resolution pachytene chromosome map, we used the chromosome image analyzing system version 3 and fluorescence in situ hybridization. Two ribosomal RNA genes (45S rDNA and 5S rDNA), two major tandem repeat DNAs (LjTR1 and LjTR2), two major retroelements (LjRE1 and LjRE2), and 27 transformation-competent artificial chromosome clones were physically localized on Lotus japonicus (Miyakojima MG-20, 2n = 12) chromosomes. The distributions of heterochromatin and euchromatin along six chromosomes were compared based on the linkage map. Distortion between the recombination frequencies and physical chromosomal distance was recognized where the centromeric heterochromatic regions and constitutive heterochromatin are composed of the highest copy tandem repeat LjTR1 on the interstitial specific regions. Our study shows that the heterochromatin are composed of the specific repeated sequences, and the discrepancy between the recombination frequency and cytological information detected in L. japonicus chromosomes is due to the heterochromatin.     

Characterization of the atypical karyotype of the black-winged kite Elanus caeruleus (Falconiformes: Accipitridae) by means of classical and molecular cytogenetic techniques

The karyotype of the black-winged kite (Elanus caeruleus), a small diurnal raptor living in Africa, Asia and southern Europe, was studied with classical (G-, C-, R-banding, and Ag-NOR staining) and molecular cytogenetic methods, including primed in-situ labelling (PRINS) and fluorescence in-situ hybridization (FISH) with telomeric (TTAGGG) and centromeric DNA repeats. The study revealed that the genome size, measured by flow cytometry (3.1pg), is in the normal avian range. However, the black-winged kite karyotype is particularly unusual among birds in having a moderate diploid number of 68 chromosomes, and containing only one pair of dot-shaped microchromosomes. Moreover, the macrochromosomes are medium-sized, with the Z and W gonosomes being clearly the largest in the set. C-banding shows that constitutive heterochromatin is located at the centromeric regions of all chromosomes, and that two pairs of small acrocentrics and the pair of microchromosomes are almost entirely heterochromatic and G-band negative. The distribution pattern of a centromeric repeated DNA sequence, as demonstrated by PRINS, follows that of C-heterochromatin. The localization of telomeric sequences by FISH and PRINS reveals many strong telomeric signals but no extratelomeric signal was observed. The atypical organization of the karyotype of the black-winged kite is considered in the context of the modes of karyotypic evolution in birds.     

Multiple repetitive DNA sequences in the paracentromeric regions of Arabidopsis thaliana L.

Nine repetitive DNA sequences, present in the haploid Arabidopsis thaliana genome in 7–300 copies, were hybridized in situ to metaphase and interphase chromosomes. Every sequence was detected on all five chromosome pairs, but was not evenly dispersed over the genome. Clusters of signals were found in particular regions of the centromeric heterochromatin, and each sequence showed a characteristic distribution pattern. Some sequences hybridized more strongly on different chromosomes, reflecting chromosome-specific amplification or the presence of homologous sequences. No hybridization signals could be detected on euchromatic regions. In situ hybridization on extended chromatin fibres showed that the pAL1 repeats are interrupted by another repetitive DNA sequence. A cosmid subclone (74A) contained a (GA)₃₈ microsatellite motif, and hybridization with a (GA) oligonucleotide revealed that most of the hybridization sites of 74A correspond to the distribution of this microsatellite motif. The results show that the paracentromeric heterochromatin of A. thaliana chromosomes is composed not only of the tandemly arranged 180-bp repeat family pAL1/pAtMr, but also of some other repetitive sequences, thus giving a better understanding of the organization of sequences at the centromeres of A. thaliana.This revised version was published online in November 2005 with corrections to the Cover Date.     

Spontaneous occurrence of a Robertsonian fusion involving chromosome 19 by single whole-arm reciprocal translocation (WART) in wild-derived house mice

Chromosomal races of the house mouse (Mus musculus domesticus) bear Robertsonian (Rb) fusions, which consist of centric translocations between two non-homologous acrocentric chromosomes. The high level of diversity of these fusions in house mice is generated by de-novo formation of Rb fusions and subsequent whole-arm reciprocal exchanges (WARTs). This paper describes the spontaneous occurrence of a new Rb fusion, Rb(4.19), in progeny of wild-derived house mice segregating for Rb(4.12). The chromosomal mutation was traced to a female which exhibited germline and somatic mosaicism indicating an early embryonic origin of the mutation. FISH analysis of centromerically-located ribosomal genes suggested that no modification was observed on chromosomes 12 and 19 prior to or following the occurrence of Rb(4.19). Distribution of telomeric sequences showed that both Rb fusions lacked telomeres in their centromeric regions. It is argued that this spontaneous mutation most likely originated by single whole-arm reciprocal translocation (WART) between Rb(4.12) and an acrocentric chromosome 19, resulting in Rb(4.19) and a neo-acrocentric chromosome 12. Sequences required for centromeric function and proximal telomeres would have been transferred to the neo-chromosome 12 from chromosome 19 during the translocation. The existence of such WARTs which generate derived acrocentric chromosomes has several implications for chromosomal evolution in house mice.     

A Karyotypic Analysis of Nilgai, Boselaphus Tragocamelus (Artiodactyla: Bovidae)

A combination of chromosomal banding and fluorescence in situ hybridization (FISH) was used to characterize the karyotype of Boselaphus tragocamelus (nilgai) relative to the domestic cattle standard karyotype. G-, Q- and C-band karyotypes of nilgai are presented, and the chromosomal complement of nilgai is determined to be 2n = 46 (female FN = 60, male FN = 59; NAA = 56), consistent with previous reports for the species. Comparisons with cattle identified extensive monobrachial homologies with some noteworthy exceptions. Chromosome 25 is centrically fused to 24, and chromosome 16 is acrocentric. Both appear to have additional pericentromeric material not seen in the equivalent cattle acrocentrics. This pericentromeric chromatin may be the result of de novo additions or translocation of pericentromeric material from chromosome 6, which is shown to be centrically fused to 13 but is only about two-thirds the length of cattle 6. Comparisons with cattle demonstrated that nilgai chromosome 17 has undergone a paracentric inversion and that chromosome 20 has two blocks of interstitial constitutive heterochromatin. The identities of both chromosomes were confirmed by chromosomal FISH. Furthermore, chromosomal banding and FISH were used to determine that autosome 14 has been fused to the ancestral X and Y of nilgai to form compound neo-X and -Y chromosomes. Additional FISH analyses were conducted to confirm other proposed chromosome homologies and to identify nucleolar organizing regions within the nilgai complement.     

Colocalization of (TTAGGG)n telomeric sequences and ribosomal genes in Atlantic eels

The distribution of (TTAGGG)_n telomeric repeats was studied in chromosomes of two Atlantic eels,Anguilla anguilla andA. rostrata. We found that these sequences hybridize to all the telomeres but also to the entire nucleolar organizer region (NOR) localized in both species at the short arm of chromosome 8. This was considered to be due to the interspersion of telomeric sequences within the NOR ones. Whatever the significance of this interspersion may be, it seems to be limited toA. anguilla andA. rostrata since inMuraena helena (family muraenidae), which also belongs to the Anguilliformes, no telomeric hybridization signals were found along the NOR regions.     

Selection against Robertsonian fusions involving housekeeping genes in the house mouse: integrating data from gene expression arrays and chromosome evolution

Monobrachial homology resulting from Robertsonian (Rb) fusions is thought to contribute to chromosomal speciation through underdominance. Given the karyotypic diversity characterizing wild house mouse populations [Mus musculus domesticus, (MMU)], variation that results almost exclusively from Rb fusions (diploid numbers range from 22 to 40) and possibly whole arm reciprocal translocations (WARTs), this organism represents an excellent model for testing hypotheses of chromosomal evolution. Previous studies of chromosome size and recombination rates have failed to explain the bias for certain chromosomes to be involved more frequently than others in these rearrangements. Here, we show that the pericentromeric region of one such chromosome, MMU19, which is infrequently encountered as a fusion partner in wild populations, is significantly enriched for housekeeping genes when compared to other chromosomes in the genome. These data suggest that there is selection against breakpoints in the pericentromeric region and provide new insights into factors that constrain chromosomal reorganizations in house mice. Given the anticipated increase in vertebrate whole genome sequences, the examination of gene content and expression profiles of the pericentromeric regions of other mammalian lineages characterized by Rb fusions (i.e., other rodents, bats, and bovids, among others) is both achievable and crucial to developing broadly applicable models of chromosome evolution.