Telomeres and mechanisms of Robertsonian fusion

The Robertsonian (Rb) fusion, a chromosome rearrangement involving centric fusion of two acro-(telo)centric chromosomes to form a single metacentric, is one of the most frequent events in mammalian karyotype evolution. Since one of the functions of telomeres is to preserve chromosome integrity, a prerequisite for the formation of Rb fusions should be either telomere loss or telomere inactivation. Possible mechanisms underlying the formation of various types of Rb fusion are discussed here. For example, Rb fusion in wild mice involves complete loss of p-arm telomeres by chromosome breakage within minor satellite sequences. By contrast, interstitial telomeric sites are found in the pericentromeric regions of chromosomes originating from a number of vertebrate species, suggesting the occurrence of Rb-like fusion without loss of telomeres, a possibility consistent with some form of telomere inactivation. Finally, a recent study suggests that telomere shortening induced by the deletion of the telomerase RNA gene in the mouse germ-line leads to telomere loss and high frequencies of Rb fusion in mouse somatic cells. Thus, at least three mechanisms in mammalian cells lead to the formation of Rb fusions. Received: 11 November 1997 / Accepted: 21 December 1997     

On the continuity of chromosomal subunits: an analysis of induced ring chromosomes in Vicia faba

The proposition that subunits of a chromatid are continuous in a directional sense has been tested by observing the behaviour of induced ring chromosomes in Vicia faba. On the simplest hypothesis, that the subunits are the uninterrupted complementary strands of the DNA molecule, the polarity of rejoining should result in free separation of rings following replication in successive cell cycles. Centric and acentric ring chromosomes were separately assessed in both diploid and colchicine-accumulated tetraploid metaphase cells of primary root tips. Contrary to expectation large numbers of single and interlocked rings were observed in both cell cycles. Spontaneous sister chromatid exchanges and other breakage-reunion events can produce the configurations seen; with the postulated level of sister chromatid exchange equating that determined autoradiographically in rod chromosomes of V. faba. Unless the replication of ring chromosomes produces conditions unusual in rod chromosome replication, spontaneous breakage is probably common in replicating or post replication Vicia chromosomes. — A fundamental difference exists between the behaviour of centric and acentric ring chromosomes. Acentric ring chromosomes behave as if the chromatid arm were one DNA molecule, or a number of DNA molecules with identical directional sense. However, centric ring chromosomes behave as if there were a difference at the centromere in at least one (probably the metacentric) chromosome of the Vicia complement. That is, the two duplication-segregation subunits which extend the length of the chromosome, may contain a change in polarity at the centromere.     

The cytology of oxalis dispar (brown)

Summary The chromosome complement ofOxalis dispar 2n = 12 consists of a pair of submedian (SM) chromosomes, three subterminal (ST) and seven telocentric (TE) chromosomes. At mitosis and meiosis the TE chromosomes are as efficient and stable as the other chromosomes of the complement. Chromosome breakage at the centromere in the metacentric chromosomes may give rise to new TE chromosomes. It is suggested that all the TE chromosomes may have arisen from metacentric chromosomes in this way. They may have been favoured because of the deleterious nature of the discarded chromosome arms.     

Non-homologous synaptonemal complex formation in a heteromorphic bivalent in Keyacris scurra (Morabinae,Orthoptera)

In some populations of the grasshopper Keyacris scurra, there are many individuals heterozygous for centromere position polymorphisms. From a consideration of chiasma positions these are almost certainly due to pericentric inversions. In this species, as in other grasshoppers, the deleterious effects of chiasmata within these heterozygous regions are avoided by non-homologous, straight pairing. Reconstruction of synaptonemal complexes from two adjacent pachytene nuclei in an individual heterozygous for centromere position (telo/metacentric) on the second longest (CD) bivalent by electron microscopy of serial sections allowed the identification of all the bivalents. The centromeres were identified by characteristic densestaining material. The synaptonemal complex was found to form straight through the heteromorphic region, including both centromere positions. The pairing was clearly non-homologous at these asymmetrical centromere positions, and probably therefore, in the presumably inverted region between them. This regular non-homologous pairing explains why chiasmata never form in the heterozygous region, but does not conclusively prove that the rearrangement is an inversion rather than a centric transposition.     

A cytogenetic analysis of meiotic drive in the mosquito,Aedes aegypti (L.)

Meiotic drive in Aedes aegypti (L.) is shown by a Giemsa C-banding technique to be associated with preferential isochromatid breakage of the X chromosome during male meiosis. These breaks remain open at least until anaphase-I and, since the range of cells affected is proportional to the sensitivity of the X chromosome to the Distorter gene, it is argued that they are directly related to the decreased number of spermatozoa found in distorting males. This reduction is considered to be attributable to the degeneration of more X- than Y-bearing spermatids but it is probable that some non-functional X-bearing spermatozoa are also produced. Chromosome breakage is almost completely confined to four sites, two adjacent to the centromere, one just proximal to the intercalary band and another about the centre of the unbanded arm. Although the first three of these lie within a region in which crossing-over does not take place, fragmentation occurs more frequently in a chiasmate arm than in one devoid of chromatid exchange.     

Centromere dynamics and chromosome evolution in marsupials

The eukaryotic centromere poses an interesting evolutionary paradox: it is a chromatin entity indispensable to precise chromosome segregation in all eukaryotes, yet the DNA at the heart of the centromere is remarkably variable. Its important role of spindle attachment to the kinetochore during meiosis and mitosis notwithstanding, recent studies implicate the centromere as an active player in chromosome evolution and the divergence of species. This is exemplified by centromeric involvement in translocations, fusions, inversions, and centric shifts. Often species are defined karyotypically simply by the position of the centromere on certain chromosomes. Little is known about how the centromere, either as a functioning unit of chromatin or as a specific block of repetitive DNA sequences, acts in the creation of these types of chromosome rearrangements in an evolutionary context. Macropodine marsupials (kangaroos and wallabies) offer unique insights into current theories expositing centromere emergence during karyotypic diversification and speciation.     

The pachytene chromomere map of the oocyte of the Turkish hamster (Mesocricetus brandti)

A study of the chromomere maps of the sex and twenty autosomal bivalents of Turkish hamster pachytene oocytes was carried out. The average total number of chromomeres in early/mid pachytene autosomes was 280 with 91 on the p (short arm) and 189 on the q (long arm). The submetacentric X1 chromosome had 20 chromomeres and the metacentric X2 had 27. Comparisons of the number and location of oocyte chromomeres are made with the pachytene spermatocyte chromomere maps of this species.     

High diversity of centric fusions with monobrachial homology in an area of chromosomal polymorphism of Mus musculus domesticus

One of the simplest models of chromosomal speciation is speciation by monobrachial centric fusion. This model is based on the assumption that a sterility barrier can develop between populations, in which fixed centric fusions show monobrachial homology, i.e. share only one chromosome arm. However, studies aimed at delineating intermediate stages of transition to reproductive isolation are lacking. In this paper, we describe a new area of chromosomal polymorphism in the house mouse, Mus musculus domesticus Schwarz and Schwarx, 1943, in Sicily (Italy). We trapped 79 mice at eighteen localities in an area of approximately 500 Km² surrounding the largest active European volcano, Mount Etna. Combining G‐banding and chromosome painting we identified twelve different Robertsonian (Rb) metacentrics. Considering the high number of Rb fusions, some of them shared with other documented areas, the presently studied area of chromosomal polymorphism is very likely to represent a mixture of allochthonous and autochthonous Rb fusions. The Rb(9.16) is the most widespread metacentric (overall frequency 0.80). Two Rb metacentrics, Rb(4.10) and Rb(5.6), have similar overall frequency, 0.29 and 0.37, respectively, and are narrowly co‐distributed in ten populations. Nine fusions – Rb(2.13), Rb(1.3), Rb(12.17), Rb(8.17), Rb(2.14), Rb(10.14), Rb(11.17), Rb(3.15), and Rb(11.14) – show a low frequency (0.04–0.01) and mostly non‐overlapping localization, but each of them shares monobrachial homology with at least one other metacentric. The overall geographical distribution of different Rb fusions seems to match an early stage of race formation. The eventual role of the presently studied hybrid zone in the context of chromosomal speciation by monobrachial centric fusions is discussed. © 2011 The Linnean Society of London, Biological Journal of the Linnean Society, 2011, 103 , 722–731.     

Telomere Disruption Results in Non-Random Formation of De Novo Dicentric Chromosomes Involving Acrocentric Human Chromosomes

Genome rearrangement often produces chromosomes with two centromeres (dicentrics) that are inherently unstable because of bridge formation and breakage during cell division. However, mammalian dicentrics, and particularly those in humans, can be quite stable, usually because one centromere is functionally silenced. Molecular mechanisms of centromere inactivation are poorly understood since there are few systems to experimentally create dicentric human chromosomes. Here, we describe a human cell culture model that enriches for de novo dicentrics. We demonstrate that transient disruption of human telomere structure non-randomly produces dicentric fusions involving acrocentric chromosomes. The induced dicentrics vary in structure near fusion breakpoints and like naturally-occurring dicentrics, exhibit various inter-centromeric distances. Many functional dicentrics persist for months after formation. Even those with distantly spaced centromeres remain functionally dicentric for 20 cell generations. Other dicentrics within the population reflect centromere inactivation. In some cases, centromere inactivation occurs by an apparently epigenetic mechanism. In other dicentrics, the size of the α-satellite DNA array associated with CENP-A is reduced compared to the same array before dicentric formation. Extra-chromosomal fragments that contained CENP-A often appear in the same cells as dicentrics. Some of these fragments are derived from the same α-satellite DNA array as inactivated centromeres. Our results indicate that dicentric human chromosomes undergo alternative fates after formation. Many retain two active centromeres and are stable through multiple cell divisions. Others undergo centromere inactivation. This event occurs within a broad temporal window and can involve deletion of chromatin that marks the locus as a site for CENP-A maintenance/replenishment.     

The recurrence of chromosome fusion in inter-population hybrids of the grasshopper Atractomorpha similis

Single-pair matings of the grasshopper Atractomorpha similis (2n=19 ; 20 ) were set up in the laboratory. Among the progeny, three families out of eleven exhibited a tendency toward chromosome fusion. From eight to ten different fusions were observed involving one interstitial and 15 to 19 procentric exchange events. At least seven of the fusions were not transmitted from the parent generation but arose spontaneously within the individuals in which they were observed. In one family, more than one fusion type arose within each of two individuals. It is concluded that the three families with the fusion syndrome have a transmissible tendency towards a particular type of chromosome rearrangement, namely centric fusion. The fusions observed involve at least six of the ten chromosome pairs. Thus the tendency to procentric exchange is non-specific in respect of the linkage groups involved. This phenomenon is discussed in relation to the process of karyotypic orthoselection and the data presented are used to model this process. The relatively low frequency of chromosome rearrangement in natural populations of this species suggests that the high frequency observed in laboratory populations is a consequence of crossing geographically distant populations which would not normally interbreed.     

A new assay capturing chromosome fusions shows a protection trade-off at telomeres and NHEJ vulnerability to low-density ionizing radiation

Chromosome fusions threaten genome integrity and promote cancer by engaging catastrophic mutational processes, namely chromosome breakage–fusion–bridge cycles and chromothripsis. Chromosome fusions are frequent in cells incurring telomere dysfunctions or those exposed to DNA breakage. Their occurrence and therefore their contribution to genome instability in unchallenged cells is unknown. To address this issue, we constructed a genetic assay able to capture and quantify rare chromosome fusions in budding yeast. This chromosome fusion capture (CFC) assay relies on the controlled inactivation of one centromere to rescue unstable dicentric chromosome fusions. It is sensitive enough to quantify the basal rate of end-to-end chromosome fusions occurring in wild-type cells. These fusions depend on canonical nonhomologous end joining (NHEJ). Our results show that chromosome end protection results from a trade-off at telomeres between positive effectors (Rif2, Sir4, telomerase) and a negative effector partially antagonizing them (Rif1). The CFC assay also captures NHEJ-dependent chromosome fusions induced by ionizing radiation. It provides evidence for chromosomal rearrangements stemming from a single photon–matter interaction.     

Chromosomes of sixteen European species ofLongitarsus flea-beetles (Coleoptera,Chrysomelidae)

Sixteen species ofLongitarsus have been chromosomally surveyed, showing a continuous range of even numbers from 2n=26 to 2n=32 chromosomes. Among the total of twenty-three known species, about 40% display a 14+Xy male karyotypic formula, the possible modal and most primitive one for the genus. The current taxonomy of species groupings is in good agreement with the chromosome numbers in some cases, but not in others. Also, there is no interrelationship between chromosome numbers and foodplant selection. The number of large bivalents at metaphase I is generally negatively correlated with the diploid value, suggesting the possible role of centric fusions coupled to shifts in the amount of chromatin as the main chromosomal changes in the evolution ofLongitarsus. The karyotypes of a few studied species are composed of metacentric chromosomes, some of them of rather large size, and a minute y-chromosome. A possible example of polymorphism for the chromosome number inL. nigrofasciatus is reported and briefly discussed.     

The origin of tertiary monosomics in the tomato

Six aneuploid tomato plants with 2n–1=23 chromosomes were observed in populations grown from the seedlings treated with thermal neutrons and from seeds treated with X-rays. Four of the aneuploids were tertiary monosomics in which, as a result of centromeric interchanges between two different chromosomes, two whole arms were missing from the complement and two arms connected at the centromere. In one aneuploid, as a result of centromeric breakage, the two short arms of a homologous pair were missing from the complement and the two long arms connected to the long arm and the short arm respectively of another chromosome in which breakage had occurred also at the centromere. In one aneuploid, the interchange has occurred in the arms, and not in the centromere. Here the aneuploid condition is due to the loss of an arm with a centromere and a short piece of the other arm.In most of the tertiary monosomics the missing arms were either the short arms of sub-metacentric chromosomes or any of the arms of metacentric chromosomes. However, in one case the long arms of two submetacentric chromosomes were lost from the complement. That in spite of such large chromosomal deletions the sporophyte can survive, may be due to the fact that the aberrant plants are mostly chimeras.This study was part of a project resulting from a contract between the Association Euratom-I.T.A.L., and the Agricultural University of Wageningen.     

NOR-Chromosome morphology and evidence for rDNA selection in phylactolaemates

Silver-stained chromosome spreads from the phylactolaemate ectoprocts Fredericella indica, Fredericella browni, Plumatella repens, Pectinatella magnifica and Asajirella gelatinosa show that each of these species has a single pair of NORs. In the first four species each NOR is immediately adjacent to the centromere of a large metacentric chromosome. Giemsa-stained preparations from Cristatella mucedo show centromeric gapping in a similar large metacentric, indicating the same type of NOR-chromosome. In Asajirella gelatinosa the two NORs are on a pair of smaller chromosomes, and are not immediately adjacent to a centromere. A close NOR-centromere association would inhibit meiotic crossing over in the NOR region. Since in phylactolaemates reproduction is primarily asexual, often by formation of statoblasts, there is potential intraclonal selection for favorable ribosomal DNA (rDNA) mutations, including from simultaneous germination of otherwise genetically identical statoblasts within a limited area and competition between cell lines within statoblasts. Inhibition of meiotic crossing over within the NOR would maintain its integrity during periods when intraclonal selection would not be occurring. This suggests that a more rapid rate of rDNA evolution is possible in phylactolaemates than in most other organisms, and is consistent with a derivation of this group from the morphologically similar phoronids, despite phylogenetic interpretations of rDNA sequence data that suggest the two groups are not closely related.     

Evolution of a new chromosomal lineage in a laboratory population ofDrosophila through centric fission

Structural rearrangements of chromosomes have played a decisive role in the karyotypic evolution of species. It is also known that inversions, translocations, fusions, fissions, heterochromatin variations and other chromosomal changes occur as transient events in natural populations. Herein we report the occurrence of a rare event of centric fission of a metacentric chromosome in a laboratory population ofDrosophila, called Cytorace 1. This centric fission has been fixed in a sub-population of Cytorace 1, resulting in a new chromosomal lineage called Fissioncytorace-1.     

Variation of the chromosome phenotype in zea,solanum and salvia

Summary InSolanum lycopersicum pachytene chromosomes the gradient in chromomere size, originating on both sides of the kinetochore, reveals the following characteristics: 1. a relatively abrupt decrease in size of the large chromomeres, 2. the gradient is related to arm length in 9 of the 12 chromosomes, 3. the gradient is particularly irregular in the short arm of the nucleolar chromosome and in the long arm is not conspicuous, 4. chromosome 6 shows an abrupt interruption in the gradient close to the kinetochore. Salvia viridis andZea mays chromosomes represent intermediate conditions between species with well defined and species without gradients. InSalvia the intermediate condition is manifested by the presence of a very large chromomere on each side of the kinetochore followed by very small chromomeres. In two chromosomes the intermediate condition is particularly apparent. In these chromosomes two chromomeres of intermediate size are present in the proximal region of the long arm. The nucleolar organizing arm has also an irregular pattern in this species.Maize has a less distinct gradient than tomato in all its chromosomes. Chromosomes 3, 4, 5 and 8 are those where the gradient is the least sharp. The nucleolar organizing arm of chromosome 6 has also an irregular pattern.In a translocation between chromosomes 5 and 6 of maize, a segment composed of very small chromomeres from the distal region of 5 which was moved to the right of the kinetochore of chromosome 6, did not change appreciably its phenotype after ten years of cultivation. During the period of cultivation a selection was made for plants where the original phenotype was preserved so that this result cannot be considered as demonstrating an absence of change in chromomere phenotype with changed position.InDrosophila andChironomus salivary gland chromosomes where chromomeres are large, and no selection has been carried out with such a purpose, the pattern and nucleic acid content of the bands is known to change when rearrangements occur within the chromosome.Supported by a grant from the Swedish Natural Science Research Council toA. Lima-De-Faria. This work was partly carried out at the Department of Botany, University of Illinois, U.S.A. during a visit to this department byA. Lima-De-Faria.P. Sarvella's collaboration in this work was done during her stay at the University of Lund.     

Adriamycin-induced centric fusions in mouse chromosomes: in vivo and in vitro analysis

The nature of metacentric chromosomes induced by the anthracycline antibiotic adriamycin (ADR) in mice was studied. The analysis of bone marrow cells of mice injected intraperitoneally with a single dose of the chemical and sacrificed at different posttreatment lapses demonstrated the persistence of biarmed chromosomes even after 30 days. Observations of BUdR-substituted chromosomes from primary cell cultures after one replication cycle revealed the maintenance of DNA polarity through the centromere in most of the metacentric chromosomes. These facts could be considered as a good indication that ADR induces Robert-sonian fusions in mouse chromosomes.     

Mechanisms of chromosome number evolution in yeast

The whole-genome duplication (WGD) that occurred during yeast evolution changed the basal number of chromosomes from 8 to 16. However, the number of chromosomes in post-WGD species now ranges between 10 and 16, and the number in non-WGD species (Zygosaccharomyces, Kluyveromyces, Lachancea, and Ashbya) ranges between 6 and 8. To study the mechanism by which chromosome number changes, we traced the ancestry of centromeres and telomeres in each species. We observe only two mechanisms by which the number of chromosomes has decreased, as indicated by the loss of a centromere. The most frequent mechanism, seen 8 times, is telomere-to-telomere fusion between two chromosomes with the concomitant death of one centromere. The other mechanism, seen once, involves the breakage of a chromosome at its centromere, followed by the fusion of the two arms to the telomeres of two other chromosomes. The only mechanism by which chromosome number has increased in these species is WGD. Translocations and inversions have cycled telomere locations, internalizing some previously telomeric genes and creating novel telomeric locations. Comparison of centromere structures shows that the length of the CDEII region is variable between species but uniform within species. We trace the complete rearrangement history of the Lachancea kluyveri genome since its common ancestor with Saccharomyces and propose that its exceptionally low level of rearrangement is a consequence of the loss of the non-homologous end joining (NHEJ) DNA repair pathway in this species.     

Kinetochores of grasshoppers with Robertsonian chromosome fusions

The pachytene karyotypes of three grasshopper species with 2 and 3 Robertsonian fusions were constructed from electron micrographs of serially sectioned spermatocyte nuclei. Tracings of the synaptonemal complexes permitted identification of each bivalent and its centromeric region. Chromosomes with the centromere in a terminal position have a knob of centric heterochromatin on the synaptonemal complex where it ends at the nuclear envelope. In Chorthippus and in Chloealtis the submetacentric Robertsonian fusion chromosomes each have a single centric knob in the appropriate place. In Neopodismopsis each of the 2 submetacentric chromosomes have a centric knob which is double in size and structure. In spermatogonial metaphases the submetacentric chromosomes of Neopodismopsis have 70–80 microtubules per kinetochore while the telocentric chromosomes have 30–40 tubules per kinetochore. These observations are correlated with evidence from light microscopy that Robertsonian fusions may produce mono- or dicentric chromosomes.     

Evidence for eight tandem and five centric fusions in the evolution of the karyotype ofAethomys namaquensis A. Smith (Rodentia: Muridae)

G- and C-banded chromosomes ofAethomys namaquensis (2n=24),A. chrysophilus (2n=44), andPraomys coucha (2n=36) are compared and contrasted with publised material on Australian Muridae and North American Sigmodontidae. Direction and types of chromosomal rearrangements are established using cladistic methodology. An acrocentric morphology for chromosomes 5, 14, 15 and 20 (numbering system fromPeromyscus) are proposed as primitive for the common ancestor of the Muridae and Sigmodontidae rodent lineages. Reduced diploid number ofAethomys namaquensis is derived by eight tandem and five centric fusions since divergence from the common ancestor withA. chrysophilus. The two species ofAethomys share one derived metacentric chromosome that distinguishes them fromPraomys. Praomys has unique chromosomes which can be derived from the proposed primitive condition by five centric fusions and five pericentric inversions. It is concluded that karyotypic orthoselection for tandem and centric fusions is best explained by cellular or biochemical mechanisms rather than variation in population characteristics.