B chromosome selection in rye

Summary As a consequence of selection the distribution and frequency of B chromosomes varies among survivors of rye seedlings planted in different environments. The mean B frequency per plant decreased under conditions of high as compared with low density of planting. Differences in weather and soil type between two sites also caused a significant shift in the distribution and frequency of B chromosomes.We are grateful for the opportunity, in this commemorative issue, to pay tribute to Professor Rhoades as a scientist who has contributed massively to knowledge and who, on a more personal level, has inspired respect and affection among his many friends and acquaintances throughout the world.     

Aurora B kinases restrict chromosome decondensation to telophase of mitosis

The Aurora kinases comprise a family of evolutionary conserved serine/threonine kinases that have important functions in centrosome duplication, mitotic spindle assembly, chromosome condensation, chromosome biorientation on the spindle and chromosome segregation. Vertebrates have three Aurora kinases, Aurora-A, -B and -C, while invertebrates have only Aurora-A and -B and yeasts have a single Aurora kinase, Ipl1 in S. cerevisiae and Ark1 in S. pombe. Recently, the role of Aurora kinases in chromosome condensation has been defined; Aurora B plays a crucial role in the axial shortening of chromosomes during anaphase, presumably in order to prevent chromosome arms from becoming trapped within the cytokinetic plate.     

The Tabby cat locus maps to feline chromosome B1

The Tabby markings of the domestic cat are unique coat patterns for which no causative candidate gene has been inferred from other mammals. In this study, a genome scan was performed on a large pedigree of cats that segregated for Tabby coat markings, specifically for the Abyssinian (Ta-) and blotched (tbtb) phenotypes. There was linkage between the Tabby locus and eight markers on cat chromosome B1. The most significant linkage was between marker FCA700 and Tabby (Z = 7.56, theta = 0.03). Two additional markers in the region supported linkage, although not with significant LOD scores. Pairwise analysis of the markers supported the published genetic map of the cat, although additional meioses are required to refine the region. The linked markers cover a 17-cM region and flank an evolutionary breakpoint, suggesting that the Tabby gene has a homologue on either human chromosome 4 or 8. Alternatively, Tabby could be a unique locus in cats.     

Mps1 phosphorylates Borealin to control Aurora B activity and chromosome alignment

Maintenance of chromosomal stability relies on coordination between various processes that are critical for proper chromosome segregation in mitosis. Here we show that monopolar spindle 1 (Mps1) kinase, which is essential for the mitotic checkpoint, also controls correction of improper chromosome attachments. We report that Borealin/DasraB, a member of the complex that regulates the Aurora B kinase, is directly phosphorylated by Mps1 on residues that are crucial for Aurora B activity and chromosome alignment. As a result, cells lacking Mps1 kinase activity fail to efficiently align chromosomes due to impaired Aurora B function at centromeres, leaving improper attachments uncorrected. Strikingly, Borealin/DasraB bearing phosphomimetic mutations restores Aurora B activity and alignment in Mps1-depleted cells. Mps1 thus coordinates attachment error correction and checkpoint signaling, two crucial responses to unproductive chromosome attachments.     

Assignment of the human gene for hexosaminidase B to chromosome 5

Mouse-human somatic cell hybrids between different mouse and human cells were studied for the expression of human hexosaminidases A and B activities. The expression of human hexosaminidase B in the hybrids was found to segregate concordantly with the presence of the human chromosome 5. Mouse-human hybrid clones containing either the human chromosomes 5 and 7 only or the human chromosome 7 only were also included in this study. Expression of human hexosaminidase B activity was detected only in those clones containing human chromosome 5. These results indicate that the gene(s) for human hexosaminidase B is located on chromosome 5. No hexosaminidase A activity was detected in clones which contained either human chromosomes 5 and 7 or chromosome 7.     

Instability of the maize B chromosome

Summary The B ⁹ chromosome of maize exhibits a very ordered type of instability at the second pollen mitosis, when nondisjunction may reach a level of 95%. Much less commonly the chromosome is unstable during early development of the kernel. Instability in the kernel produces recessive sectors in either the endosperm or the sporophyte, reflecting the absence of dominant markers carried by the B ⁹. The causes of B ⁹ loss in the endosperm and the sporophyte were investigated for the two observable classes of sectoring: fractional loss (single event) and multiple loss (mosaic pattern). The fractional class represents isochromosome formation by the B ⁹ (Carlson, 1970, 1971). Data presented here suggest that the isochromosome is a by-product of telocentric formation at the second pollen mitosis, and does not arise directly from the B ⁹ chromosome. The chromosomal basis for the mosaic pattern of B ⁹ loss is not completely known. However, one class of mosaic kernels displays a heritable instability of the B ⁹ chromosome which apparently results from ring chromosome formation by the B ⁹. The time of origin of the ring B ⁹ chromosome is prior to the second pollen mitosis, since the unstable chromosome generated in the male parent is transmitted to both the endosperm and the sporophyte. Finally, a genetic factor controlling B ⁹ stability in the developing endosperm has been found. A single plant (1818-1), crossed as a female parent to a B ⁹-containing stock, induced a mosaic pattern of B ⁹ loss in the endosperm at a very high rate. The characteristics of this plant are being investigated.Dedicated with much appreciation and respect to Dr. M. M. Rhoades on the occasion of his 70th birthday.     

Linkage of loci affecting a murine liver protein and arylsulfatase B to chromosome 13

As-1 is the putative structural locus for murine arylsulfatase B, and Lth-1 determines the presence or absence of a 35 000 dalton acidic liver protein. As-1 and Lth-1 were found to be closely linked using recombinant inbred (RI) strains. Both loci were found to have been cotransferred with the pearl (pe) coat color mutation (chromosome 13) in the B6.C3H pe/pe congenic strain. The linkage relationships between pe, Lth-1, and As-1 were further defined in a backcross. On the basis of the RI data, the congenic strain result, and the backcross data, the following genetic distances were estimated: pe--As-1, 7.1 +/- 4.0 cM; As-1--Lth-1, 2.5 +/- 1.0 cM; and pe--Lth-1, less than 6.9 cM. As-1 and Lth-1 are the first biochemically defined loci to be added to the chromosome 13 linkage map.     

B chromosome frequency in yugoslav rye populations

Cytologická analysa 25 populací ?ita z Jugoslavie, které byly vybrány ve Slovinsku, Srbsku a Makedonii, ukázala, ?e ve v?ech analysovaných populacích (100 %) se vyskytovaly B-chromosomy. Po?et p?ítomných B-chromosom? byl nízký, od 1 do 3, p?i ?em? pr?mě rný po?et na jednu rostlinu se pohyboval v rozmezí od 0,13 do 0,97. Pr?měrná ?etnost ?itných rostlin s B-chromosomy byla pro Jugoslavii 27, 10%. ?etnost rostlin s B-chromosomy a pr?měrný po?et chromosom? této kategorie, vzta?ený na jednu rostlinu, byly zna?ně vět?í v aridní oblasti Makedonie ne? v humidní oblasti Slovinska. Rozdíly byly reálné a významné, jak to bylo proká záno biometricky. P?i hodnocení údaj? jiných autor? o populacíoh primitivních asijských ?it bylo zji?těno, ?e frekvence B-chromosom? je ni??í v populacích z oblastí, ozna?ovanýeh jako centra p?vodu tohoto rostlinného druhu (Blízký Východ, Malá Asie) a postupně se jejich frekvence zna?ně zvy?uje s postupem ve směru na vychod (Zabajkali, Korea) i ve směru na západ (Balkánský poloostrov, Jugoslavie). Diskutují se mo?né p?í?iny zvy?ování ?etnosti B-chromosom? v populacich vně center p?vodu ?ita a je zd?razněn význam jejich studia v západoevropských populaeí eh primitivních forem ?ita.     

B chromosome in a rice aneuploid variation

An awned rice plant was discovered among the progeny of the triploid Zhongxian 3037. Cytological investigation showed that this awned plant was a variation with extra chromosomes. Based on the properties of the extra chromosomes during both meiosis and mitosis – e.g., short stature, darkly staining, instability in chromosome number, and lack of synapsis with A chromosomes – they could be considered to be B chromosomes in rice. The B chromosome(s) in both asexual and sexual progenies exhibited a unique segregation that is different from that of telotrisomics and other aneuploids. Moreover molecular marker analysis detected no dosage effects between the B-chromosome plant and the normal diploid of Zhongxian 3037, indicating that the B chromosomes might not be directly derived from any A-chromosome fragments in rice. Received: 8 June 1999 / Accepted: 30 December 1999     

Sequences associated with A chromosome centromeres are present throughout the maize B chromosome

Maize chromosome spreads containing the supernumerary B chromosome were hybridized with probes from various repetitive elements including CentC, CRM, and CentA, which have been localized to centromeric regions on the A chromosomes. Repetitive elements that are enriched or found exclusively near the centromeres of A chromosomes hybridized to many sites distinct from the centromere on the B chromosome. To examine whether these elements recruit kinetochore proteins at locations other than the canonical B centromere, cells were labeled with antibodies against CENH3, a key kinetochore protein. No labeling was detected outside the normal centromere and no evidence of B chromosome holocentromeric activity was observed. This finding suggests that, as in other higher eukaryotes, DNA sequence alone is insufficient to dictate kinetochore location in plants. Additionally, examination of the B centromere region in pachytene chromosomes revealed that the B-specific element ZmBs hybridizes to a much larger region than the site of hybridization of CentC, CRM, and CentA and the labeling by anti-CENH3 antibodies.This revised version was published online in December 2004 with corrections to Table 1.     

Utilization of deletion bins to anchor and order sequences along the wheat 7B chromosome

Key message

A total of 3,671 sequence contigs and scaffolds were mapped to deletion bins on wheat chromosome 7B providing a foundation for developing high-resolution integrated physical map for this chromosome.

Abstract

Bread wheat (Triticum aestivum L.) has a large, complex and highly repetitive genome which is challenging to assemble into high quality pseudo-chromosomes. As part of the international effort to sequence the hexaploid bread wheat genome by the international wheat genome sequencing consortium (IWGSC) we are focused on assembling a reference sequence for chromosome 7B. The successful completion of the reference chromosome sequence is highly dependent on the integration of genetic and physical maps. To aid the integration of these two types of maps, we have constructed a high-density deletion bin map of chromosome 7B. Using the 270 K Nimblegen comparative genomic hybridization (CGH) array on a set of cv. Chinese spring deletion lines, a total of 3,671 sequence contigs and scaffolds (~7.8 % of chromosome 7B physical length) were mapped into nine deletion bins. Our method of genotyping deletions on chromosome 7B relied on a model-based clustering algorithm (Mclust) to accurately predict the presence or absence of a given genomic sequence in a deletion line. The bin mapping results were validated using three different approaches, viz. (a) PCR-based amplification of randomly selected bin mapped sequences (b) comparison with previously mapped ESTs and (c) comparison with a 7B genetic map developed in the present study. Validation of the bin mapping results suggested a high accuracy of the assignment of 7B sequence contigs and scaffolds to the 7B deletion bins.     

Origin of an apparent B chromosome by mutation,chromosome fragmentation and specific DNA sequence amplification

The present study documents the de novo origin of an apparent B chromosome in Plantago lagopus. The origin was associated with mutation (aneuploidy), chromosome fragmentation, specific DNA sequence amplification, addition of telomeric repeats, and centromeric misdivision. It originated in the progeny of trisome 2, from the excision of 5S rDNA and 18S, 5.8S, 25S rDNA sequences located on chromosome 2, and within a few generations acquired many characteristics of an apparent B chromosome. The B chromosome has preferential transmission through the male (41%, P<0.025) and female gametes (42%, P<0.01) but does not affect plant phenotype. The B chromosome is completely heterochromatic, has a functional centromere and does not pair at meiosis with any A chromosomes of the standard complement. Fluorescence in situ hybridization analysis showed that it arose from massive amplification of 5S rDNA sequences, has 18S, 5.8S, 25S rDNA sequences at the ends of both arms and telomeric repeats at both termini. Ag-NOR-banding and determination of the maximum number of nucleoli in interphase cells indicate that the nucleolar organizer regions at the ends of both arms of the B chromosome are active in organizing nucleoli. RNA blot analysis showed that the 5S rDNA sequences are not transcribed. To our knowledge, this is the first report that fully documents one of the mechanisms by which B chromosomes may arise in nature.     

Molecular and functional dissection of the maize B chromosome centromere

The centromere of the maize (Zea mays) B chromosome contains several megabases of a B-specific repeat (ZmBs), a 156-bp satellite repeat (CentC), and centromere-specific retrotransposons (CRM elements). Here, we demonstrate that only a small fraction of the ZmBs repeats interacts with CENH3, the histone H3 variant specific to centromeres. CentC, which marks the CENH3-associated chromatin in maize A centromeres, is restricted to an approximately 700-kb domain within the larger context of the ZmBs repeats. The breakpoints of five B centromere misdivision derivatives are mapped within this domain. In addition, the fraction of this domain remaining after misdivision correlates well with the quantity of CENH3 on the centromere. Thus, the functional boundaries of the B centromere are mapped to a relatively small CentC- and CRM-rich region that is embedded within multimegabase arrays of the ZmBs repeat. Our results demonstrate that the amount of CENH3 at the B centromere can be varied, but with decreasing amounts, the function of the centromere becomes impaired.     

Nondisjunction and isochromosome formation in the B chromosome of maize

Bianchi et al. (1961) found that sectored losses of B-translocation chromosomes occur at a significant rate during early development of the endosperm and sporophyte. The losses were attributed to nondisjunction of the chromomosome, since B type chromosomes are known to undergo nondisjunction at the second pollen mitosis. Sector formation was further analyzed in the present paper, using the translocation, TB-9b. It was found that losses of the B⁹ chromosome during early endosperm mitoses occur only if the 9^B chromosome is present. In addition, sectors are produced in the sporophyte only if the 9^B and B⁹ chromosomes are inherited from the male parent. Both of these findings suggest that nondisjunction is indeed responsible for the B⁹ losses (see text). However, cytological observation of sectored plants demonstrates that isochromosome formation, rather than nondisjunction, produces most B⁹ losses in the sporophyte. The conflicting results can be reconciled by assuming that the same basic event, perhaps stickiness of the B⁹ chromosome, produces nondisjunction at the second pollen mitosis and isochromosome formation in the developing sporophyte. Observation of the isochromosome in pachytene reveals that a heterochromatic region corresponding to the short arm of the normal B⁹ is missing. The normal B⁹ chromosome is, therefore, an acrocentric chromosome.     

Regulation of mitotic chromosome cohesion by Haspin and Aurora B

In vertebrate mitosis, cohesion between sister chromatids is lost in two stages. In prophase and prometaphase, cohesin release from chromosome arms occurs under the control of Polo-like kinase 1 and Aurora B, while Shugoshin is thought to prevent removal of centromeric cohesin until anaphase. The regulatory enzymes that act to sustain centromeric cohesion are incompletely described, however. Haspin/Gsg2 is a histone H3 threonine-3 kinase required for normal mitosis. We report here that both H3 threonine-3 phosphorylation and cohesin are located at inner centromeres. Haspin depletion disrupts cohesin binding and sister chromatid association in mitosis, preventing normal chromosome alignment and activating the spindle assembly checkpoint, leading to arrest in a prometaphase-like state. Overexpression of Haspin hinders cohesin release and stabilizes arm cohesion. We conclude that Haspin is required to maintain centromeric cohesion during mitosis. We also suggest that Aurora B regulates cohesin removal through its effect on the localization of Shugoshin.     

Minichromosomes derived from the B chromosome of maize

Fourteen minichromosomes derived from the B chromosome of maize are described. The centromeric region of the B chromosome contains a specific repetitive DNA element called the B repeat. This sequence was used to determine the transmission frequency of the different types of minichromosomes over several generations via Southern blot analysis at each generation. In general, the minichromosomes have transmission rates below the theoretical 50% frequency of a univalent chromosome. The gross structure of each minichromosome was determined using fluorescence in situ hybridization (FISH) on root tip chromosome spreads. The presence of the B centromeric repeat and of the adjacent heterochromatic knob sequences was determined for each minichromosome. In two cases, the amount of the centromeric knob repeat is increased relative to the progenitor chromosome. Other isolates have reduced or undetectable levels of the knob sequence. Potential uses of the minichromosomes are discussed.