Karyotype, centric fusion polymorphism and chromosomal aberrations in captive-born mountain reedbuck (Redunca fulvorufula)

Chromosomes of fourteen captive-born mountain reedbucks (Redunca fulvorufula) have been investigated. The diploid chromosome number was 2n = 56 (FN = 60). The mountain reedbuck karyotype consists of 26 acrocentric and two biarmed chromosome pairs resulting from two centric fusions involving chromosomes 2 and 25, and 6 and 10, respectively. In some animals, 57 chromosomes were detected. Variation in the diploid number was found to be due to polymorphism for the centric fusion 6;10. Both X and Y chromosomes are large and acrocentric. The entire Y chromosome and the proximal part of the X chromosome consist of heterochromatin. The chromosomes X, 9 and 14 appeared to be of caprine type. Chromosome aberrations have been detected in two of the 14 animals investigated. A de novo formed Robertsonian translocation rob(6;13) was found in one female heterozygous for the fusion 6;10. CBG-banding revealed one block of centromeric heterochromatin in the de novo formed translocation rob(6;13) and also in the evolutionarily fixed centric fusions 6;10 and 2;25. One examined male homozygous for fusion 6;10, had a mosaic 56,XY/57,XYY karyotype, with 11% of analyzed cells containing two Y chromosomes. The findings were confirmed by cross-species fluorescence in situ hybridization (FISH) with bovine (Bos taurus L.) chromosome painting probes. The study demonstrates the relevance of cytogenetic screening in captive animals from zoological gardens.     

Translocation (14;21)(q11;q22) in a woman with history of abortions and a child with Down's syndrome

A translocation (14;21)(q11;q22) was observed in a woman with history of abortions and in her child with Down's syndrome. This appears to be the first report of such a translocation with no centric fusion between the acrocentric chromosomes leading to a count of 45 chromosomes in the carrier and giving birth to a Down's syndrome child.     

Centric fusion differences among Oryx dammah, O. gazella, and O. leucoryx (Artiodactyla, Bovidae)

G- and C-banded karyotypes of the genus Oryx were compared using the standard karyotype of Bos taurus. Chromosomal complements were 2n = 56 in O. gazella gazella, 2n = 58 in O. g. beisa and O. g. callotis, 2n = 56-58 in O. dammah, and 2n = 57-58 in O. leucoryx. The number of autosomal arms in all karyotypes was 58. Nearly all variation in diploid number was the result of three independent centric fusions, but one 2n = 57 specimen of O. g. gazella deviated from the normal complement of 2n = 56 due to XXY aneuploidy. A 2;17 centric fusion was fixed in O. g. gazella, whereas O. g. beisa and O. g. callotis lacked this fusion and had indistinguishable karyotypes. Oryx dammah was polymorphic for a 2;15 centric fusion, and O. leucoryx was polymorphic for an 18;19 centric fusion. The five Oryx taxa shared a fixed 1;25 centric fusion; the small acrocentric element involved in the 1;25 fusion was identified by fluorescence in situ hybridization using a cosmid specific to Bos chromosome 25. The X and Y chromosomes were also conserved among the five taxa. Oryx g. gazella differed from the other Oryx species because of the fixed 2;17 centric fusion. This difference reflects an apparently longer period of geographic isolation between O. g. gazella and other populations of Oryx, and it is consistent with the classification of O. gazella and O. beisa as distinct species (see Kingdon, 1997). The lack of monobrachial relationships among the Oryx taxa indicates that sterility barriers between species have not developed. Viability of hybrid offspring constitutes a threat to captive breeding programs designed for endangered species conservation; in the case of Oryx, the 2;15, 2;17, and 18;19 metacentrics could serve as marker chromosomes for assessing hybridization between certain Oryx taxa.     

A complex sex-chromosome system in the hare-wallaby Lagorchestes conspicillatus Gould

Among specimens of the spectacled hare-wallaby Lagorchestes conspicillatus Gould (Marsupialia, family Macropodidae) 4 males had 15 chromosomes and 2 females 16 chromosomes. The sex chromosomes are X₁X₁X₂X₂ in the female and X₁X₂Y in the male, the Y being metacentric and both X chromosomes are acrocentric. In about 96% of sperm mother cells at meiosis the sex chromosomes form a chain trivalent and in more than 99% of these this orients convergently so that the X₁ and X₂ move to the same pole. Evidence is presented that L. conspicillatus has evolved from a form with 22 chromosomes including a small X and a minute Y. Autoradiographic studies show that the proximal fifth of the X₁ chromosome replicates late. This is probably the ancestral X chromosome which has been translocated to an autosome. The fate of the original Y is obscure but an hypothesis is proposed that it forms the centromeric region of the Y. A single male had 14 chromosomes and was heterozygous for a translocation involving the centric fusion of two acrocentric autosomes. In about 30% of sperm mother cells the autosomal trivalent did not disjoin regularly but, despite this, all secondary spermatocytes observed at metaphase 2 had balanced complements of chromosomes. It is assumed that unbalanced secondary spermatocytes died before reaching metaphase.     

Different rate of intrapopulation chromosome polymorphism in two species of Black Sea Blennies (Blenniidae,Pisces)

Somatic karyotypes in seven specimens of Blennius sanguinolentus include 22 subtelocentric and 26 acrocentric chromosomes, whereas one male has 2n=47=1M+22ST+24A: polymorphism is evidently a result of centric fusion of two acrocentrics. Blennius tentacularis is characterized by the availability of four karyomorphs out of which three coincide with karyotypes described earlier (Carbone et al., 1987). Karyttype-I consists of a 48 small uni-armed chromosome, but both karyotypes II and III with 2n=48 and 2n=47 respectively include one large acrocentric chromosome, and karyotype-IV has one large submetacentric out of the 47 chromosomes. Karyotypic variability of B. tentacularis is attributed either to polymorphism by 1–3 chromosome rearrangements or to availability of sex-determining mechanism, including the Y-autosome translocation. This diverse series of male karyomorphs may reflect the complicated behavioural structure.     

Frequency of satellite association in individuals with structural abnormalities of nucleolus organiser region

Summary The frequency of involvement of acrocentric chromosomes into satellite association has been extensively studied in recent years. These studies have aimed to show the existence of a relationship between satellite association, centric fusion and non-disjunction. In the present paper, evidence is given that different D-group chromosomes all bearing structural abnormalities in the nucleolus organizer region are preferentially involved in satellite association. Some interpretations of this finding are discussed.

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Zusammenfassung Die Häufigkeit der Beteiligung akrozentrischer Chromosomen bei Satellitenassoziation ist in den letzten Jahren eingehend untersucht worden. Diese Untersuchungen hatten zum Ziel, die Beziehungen zwischen Satellitenassoziation, zentrischer Fusion und Non-disjunction aufzuzeigen. Hier könnte dargelegt werden, daß verschiedene D-Chromosomen, die einen Defekt in der Region des Nucleolus-Organisators haben, bevorzugt an der Satellitenassoziation beteiligt sind. Erklärungen dieser Befunde werden diskutiert.

     

Karyotype analysis of mithun (Bos frontalis) and mithun bull x Brahman cow hybrids

We examined the cytogenetics of mithun (Bos frontalis), a domesticated version of the Asian gaur, and hybrids (F(1) generation) produced by artificial insemination of Brahman cows (Bos indicus) with mithun semen. Reproductive potential was also examined in the F(1) generation and a backcrossed heifer for utilization of heterosis. Metaphase chromosome spreads were examined by conventional staining and fluorescence in situ hybridization hybridized with the entire chromosome 1 of mithun as a specific probe. Chromosome 1 of mithun was found to be equivalent to Bos taurus chromosomes 2 and 28. The karyotype of the female mithun (N = 4) comprised 58 chromosomes, including 54 acrocentric and four large submetacentric chromosomes, without the four acrocentric chromosomes found in the domesticated species B. indicus. However, one of the four female mithuns with a normal mithun phenotype had an abnormal karyotype (2n = 59), indicating introgression from B. taurus or B. indicus. The F(1) karyotypes (N = 6, 3♂3♀) of the mithun bull × Brahman cow cross had 2n = 59, intermediate between their parents; they were consistent heterozygous carriers with a centric fusion involving rob(2;28), as expected. Two pronounced red signals were seen in the mithun karyotypes, three red signals in the mithun × Brahman hybrids, and four red signals in the Brahman cattle, in good agreement with centric fusion of bovine rob(2;28). The female backcross hybrid (N = 1) with 2n = 59 had a similar chromosome configuration to the F(1) karyotypes and had rob(2;28). Such female backcross hybrids normally reproduce; however, the F(1) bulls (N = 3) had not yet generated normal sperm at 24 months.     

Chromosome evolution of the blue sheep/bharal (Pseudois nayaur)

A male dwarf blue sheep was collected 60 km south of Batang east to the Jinsha Jiang river, and a male Subei blue sheep (Greater form) was collected from Gansu, China, representing two geographically separated blue sheep forms. Chromosome preparations were prepared from fibroblast cultures. The dwarf blue sheep has a 2n = 54 and a karyotype with three biarmed formations that resulted from acrocentric chromosome fusions (based on the 2n = 60 Capra autosomal equivalents) 14p/5q, 27p/1q, and 29p/2q from the largest to the smallest biarmed chromosome, respectively. The 14p/5q fusion is metacentric, whereas the 27p/1q and 29p/2q are submetacentric. The Subei blue sheep had a 2n = 56, with only the 27p/1q and 29p/2q biarmed chromosome fusions. The remainder of the chromosomes in both blue sheep are acrocentric; the X is the largest acrocentric chromosome and the Y is a minute biarmed chromosome. Our observation is one evidence showing that chromosome evolution within blue sheep has followed a series of centric fusions resulting in the reduction of chromosome number, which is typical of all extant genera within the tribe Caprini.     

Synaptonemal complex analysis of heteromorphic trivalents in Lemur hybrids

Synaptonemal complexes (SCs) in surface spread pachytene spermatocytes of Lemur resemble those in other mammals and are of two types: metacentric (or submetacentric) and acrocentric, with a very short second arm. In autosomal SC and mitotic karyotypes of Lemur fulvus (2n=60) a 11 proportionality in relative length is observed as in other mammals. In an intraspecific lemur hybrid (2n=55) obtained by mating L. fulvus rufus (2n=60) x L. fulvus collaris (2n=51), G-band patterns show that 10 single acrocentric mitotic chromosomes correspond to the arms of 5 single metacentrics, implying homology. It is inferred that the metacentrics have evolved by centric (Robertsonian) fusion of the acrocentrics. In the SC karyotype of the hybrid all SCs are normal except for five which have the configurations expected of metacentric-acrocentric trivalents. Similarly, in L. f. collaris (2n= 51), with one unpaired metacentric and two unpaired acrocentrics, one such SC trivalent is present in the complement. In an SC trivalent, each of the acrocentric long axes is synapsed with an arm of the metacentric axis, confirming the homology predicted from banding similarities. At late zygotene, the acrocentric short arms, which are non-homologous, are the last to pair, demonstrating that synapsis of the homologous arms occurs first. At later pachytene the acrocentric short arms are fully synapsed, producing a short SC side arm. This subsequent non-homologous synapsis is taken to be an instance of the synaptic adjustment phenomenon which has been shown to lead to non-homologous synapsis in a duplication and several inversions in the mouse. The kinetochore of the metacentric is the same size as those of the acrocentrics, and thus is unlikely to have arisen by true centromeric fusion, but rather by a translocation. The kinetochores of the acrocentrics always lie together on the same side of the metacentric kinetochore (cis configuration), implying a single pairing face on the metacentric axis. The observed trivalent configuration may well constitute a prerequisite for proper meiotic disjunction in metacentric-acrocentric heterozygotes. Such a mechanism is consistent with fertility regularly observed in such hybrid lemurs.     

Chromosomal polymorphism in Equus hemionus.

The first cytogenetic studies of the Turkmenian kulan, Equus hemionus kulan, are reported, and a polymorphism in diploid chromosome number is described. Chromosome fusion is apparently involved in the alterations of the karyotype of E. hemionus kulan (2n = 55, 54) when compared to the karyotype of the onager, E. hemionus onager (2n = 56). Additionally, the rearrangement involved has been identified in animals unrelated through captive breeding; inheritance of the fusion chromosomes has also been observed.     

Acentric chromosome ends are prone to fusion with functional chromosome ends through a homology-directed rearrangement

The centromeres of many eukaryotic chromosomes are established epigenetically on potentially variable tandem repeats; hence, these chromosomes are at risk of being acentric. We reported previously that artificially created acentric chromosomes in the fission yeast Schizosaccharomyces pombe can be rescued by end-to-end fusion with functional chromosomes. Here, we show that most acentric/functional chromosome fusion events in S. pombe cells harbouring an acentric chromosome I differed from the non-homologous end-joining-mediated rearrangements that result in deleterious dicentric fusions in normal cells, and were elicited by a previously unidentified homologous recombination (HR) event between chromosome end-associated sequences. The subtelomere repeats associated with the non-fusogenic ends were also destabilized in the surviving cells, suggesting a causal link between general subtelomere destabilization and acentric/functional chromosome fusion. A mutational analysis indicated that a non-canonical HR pathway was involved in the rearrangement. These findings are indicative of a latent mechanism that conditionally induces general subtelomere instability, presumably in the face of accidental centromere loss events, resulting in rescue of the fatal acentric chromosomes by interchromosomal HR.     

Orientation and segregation of Robertsonian trivalents in Dichroplus pratensis (Acrididae)

Pairing behavior, metaphase I orientation, and anaphase I segregation of centric fusion trivalents were studied in 26 single, 15 double, and 2 triple male fusion heterozygotes of the polymorphic South American melanopline grasshopper Dichroplus pratensis. They represent the seven different fusions and their combinations already described in different populations of the species. Our analysis showed the following: (1) pairing behavior is very regular in all trivalents; (2) frequencies of linear orientation was very low irrespective of the trivalent involved; (3) reorientation seems to occur frequently since frequencies of abnormal segregation and aneuploid second division cells were invariably lower than those of nonconvergent orientation; (4) aneuploidy and abnormal sperm production increases with increasing number of fusions; (5) chiasma frequency and localisation is relevant to trivalent orientation since trivalents with nonconvergent orientations showed proximal and interstitial chiasmata more frequently than convergently oriented ones. The results are in agreement with the hypothesis that these polymorphisms are old and stable, and confirm that for the maintenance of a balanced polymorphism, if this polymorphism is adaptive because of its consequences on recombination, position effects, etc., changes tending to stabilise trivalent orientation and segregation are central.     

Incomplete translocations induced in mature sperm of Drosophila

Frequencies of incomplete translocations in subcentromeric heterochromatin induced by gamma-rays in mature sperm were estimated in the "two right acrocentrics and left compound" system of Drosophila chromosome 2. The value obtained at 3000R was (2.5-2.9) X 10(-3). Apart from incomplete translocations leading to formation of new metacentrics, acrocentrics were also formed. It is suggested that they appear via reunion of fragments after replication of paternal chromosomes. Incomplete translocation were shown to be formed as a result of loss of centric heterochromatin fragment of acrocentric and acentric arm of the compound, as opposed to formation, due to loss of acentric heterochromatin fragment of acrocentric and centric fragment of the compound. Analysis of the dose-effect curve for half-translocations show that the latter are two hit events.     

Genetic and cytological characterisation of fusion chromosomes of Dictyostelium discoideum

Abnormally large chromosomes which appear to result from the fusion of 2 chromosomes of the normal karyotype have been found in diploids of Dictyostelium discoideum formed by parasexual fusion of haploid strains HU483 (n=7) and HU245 (n=7). These fusion chromosomes appear to be the products of the tandem translocation of most, if not all, of one acrocentric chromosome to the telomere of a second acrocentric. Thus the chromosome number of the diploids is reduced from the normal 2n=14 to 2n=13 with the formation of an abnormally large acrocentric fusion chromosome. Experimental haploidisation of such diploids results in two types of products, those with a normal 7 chromosome karyotype and those with an abnormal 6 chromosome karyotype which contains the fusion chromosome. Genetic analysis of haploid segregants indicates that linkage groups II and VII are involved in this fusion. Phenotypes of recombinant diploids obtained following mitotic crossing-over establishes that linkage group II is proximal to linkage group VII. Cytological examination of the karyotypes of haploid strains bearing the fusion chromosome suggest that chromosome 2 may correspond to linkage group II and chromosome 3 to linkage group VII. Haploid strains bearing the fusion chromosome grow and develop normally so little or no genetic information can have been lost in the fusion event. While the nature of this event is unknown it may have involved aberrant recombinational DNA repair since the parental haploid strain HU483 bears the radB13 DNA repair mutation.     

Synapsis, recombination, and meiotic segregation in the mesquite lizard, Sceloporus grammicus, complex. II. Fission heteromorphism of the FM2 cytotype and evolution of chromosome 2.

Somatic and meiotic chromosomal and synaptonemal complex techniques were used to characterize the chromosomal complement and to study the fission heteromorphism of chromosome 4 in the FM2 cytotype of Sceloporus grammicus. Analysis of silver-stained somatic metaphases revealed that the nucleolar organizer region in this cytotype is located at the distal end of a pair of medium-sized acrocentric chromosomes, rather than on the largest acrocentric chromosomal pair, as previously reported. This condition is hypothesized to be the result of at least two sequential rearrangements. Analysis of surface-spread zygotene and pachytene nuclei indicated that the components of the chromosome 4 trivalent initiated synapsis at their distal telomeric regions. Although synapsis of the fission trivalent was synchronous with that of the homomorphic autosomal pairs, completion of synapsis was delayed in the trivalent. Associations between the fission trivalent and other autosomal or sex-chromosomal elements occurred in approximately one third of the pachytene nuclei examined. Analysis of secondary spermatocytes (metaphase II configurations) revealed low levels of nondisjunction in fission heterozygotes. These analyses indicate that FM2 individuals heterozygous for the fission rearrangement of chromosome 4 suffer no meiotic deficit.     

Contributions to the cytotaxonomy of the Commelinaceae. Chromosome evolution in Tradescantia section Cymbispatha

The five species of Tradescantia section Cymbispatha studied, including one species T. poelliae D. R. Hunt, have chromosome numbers of In = 12, 14, 16, 22, 28, 30 and 36 and karyotypes of acrocentric, metacentric or telocentric chromosomes, or mixtures of both acrocentric and metacentric chromosomes. The numbers of major chromosome arms of these cytotypes give a nombre fondamentaP series of 14, 28, 42 and 56 which, in combination with meiotic analyses, indicates plants which, in genetical terms at least, are diploid, tetraploid, hexaploid and octoploid. This series has evolved from a 2 n = 14 acrocentric or telocentric karyotype by a combination of Robertsonian fusion and polyploidy. Pseudo-iso-chromosomes are sometimes formed in this evolutionary development and can persist as stable members of normal complements.     

Contributions to the cytotaxonomy of the Commelinaceae. Chromosome evolution in Tradescantia section Cymbispatha

The five species of Tradescantia section Cymbispatha studied, including one species T. poelliae D. R. Hunt, have chromosome numbers of In = 12, 14, 16, 22, 28, 30 and 36 and karyotypes of acrocentric, metacentric or telocentric chromosomes, or mixtures of both acrocentric and metacentric chromosomes. The numbers of major chromosome arms of these cytotypes give a nombre fondamentaP series of 14, 28, 42 and 56 which, in combination with meiotic analyses, indicates plants which, in genetical terms at least, are diploid, tetraploid, hexaploid and octoploid. This series has evolved from a 2 n = 14 acrocentric or telocentric karyotype by a combination of Robertsonian fusion and polyploidy. Pseudo-iso-chromosomes are sometimes formed in this evolutionary development and can persist as stable members of normal complements.     

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.     

A comparative chromosome study of twenty killifish species of the genus Fundulus (Teleostei: Cyprinodontidae)

Female karyotypes from ovarian cell cultures of 20 species of killifish (Fundulus) ranged in diploid number from 32 to 48, but in arm number (NF) from 48 to 52. The small F chromosomes, which constituted the fundamental elements in the karyotype, were evenly graded in length. The large biarmed chromosomes (L), which were about twice the length of the average Fs, characterized only those species with 2N less than 48 chromosomes. And among these species, an increase in complement by a pair of L's was always accompanied by a decrease of two pairs of A's, indicating Robertsonian changes by the centric fusion of two A's to form one L chromosome. Other diagnostic chromosome characters included: the number and structure of biarmed and satellited F chromosomes and the percentage of F's with relatively short short-arms (SSA). Besides centric fusion, mechanisms of chromosomal evolution in Fundulus probably included pericentric inversion, producing biarmed F chromosomes from acrocentric F's and partial loss of a chromosome segment producing smaller biarmed F chromosomes from larger ones. The percentage of SSA chromosomes generally decreases from relatively primitive to specialized species. The presumably most primitive species have only SSA type acrocentric F chromosomes. The 20 Fundulus species were classified into 2 major groups according to the percentage of SSA chromosomes: the SSA group, including 3 subgroups, had more than 50% SSA's; the LSA group, including 2 subgroups, had fewer than 50% SSA's. This classification based only on karyotypic characters generally agreed with others based on gross morphological characters. A possible evolutionary scheme is proposed to account for the derived killifish karyotypes.     

大熊猫与黑熊显带染色体的比较研究

以体外培养的大熊猫（Ａｉｌｕｒｏｐｏｄａｍｅｌａｎｏｌｅｕｃａ）与黑熊（Ｓｅｌｅｎａｒｃｔｏｓｔｈｉｂｅｔａｎｕｓ）外周血淋巴细胞为实验材料,应用ＢｒｄＵ复制带显示技术,研究了大熊猫和黑熊染色体晚复制带带型。通过对大熊猫与黑熊显带染色体带型的比较,发现黑熊部分具端着丝粒的染色体与大熊猫部分具中,亚中,或亚端着丝粒的染色体的整个短臂或整个长臂有明显的带型相似性,在黑熊具中,亚中着丝粒染色体中,仅３３