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This review presents a historical account of studies of B chromosomes in the genus Brachycome Cass. (synonym: Brachyscome) from the earliest cytological investigations carried out in the late 1960s though to the most recent molecular analyses. Molecular analyses provide insights into the origin and evolution of the B chromosomes (Bs) of Brachycome...
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... was further analyzed considering both its sequence variation and occurrence in the genomes of other Brachycome species with an aim of gaining an insight into its possible origin and, by inference, that of the B chromosome itself. FISH (Fig. 5) showed that high copy-number clusters of Bd49 were constant features of B chromosomes in B. dichromosomatica. Initially the predominantly centromeric location of the Bd49 cluster suggested a possible role for this sequence in the drive processes that cause greater than expected transmission of the B. However a distal cluster in ...
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... of B chromosomes in B. dichromosomatica. Initially the predominantly centromeric location of the Bd49 cluster suggested a possible role for this sequence in the drive processes that cause greater than expected transmission of the B. However a distal cluster in cytodeme A 3 and signal size vari- ation between all the Bs of different cytodemes (Fig. 5 against this and also indicated that the sequence was in a state of flux. Southern analysis of genomic DNA with Bd49 demon- strated that multiple copies of related sequences were also present in the genomes of B. eriogona, B. ciliaris, B. segmentosa and B. multifida (none of which have B chromosomes), whereas other species tested ...
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Although the monomer size, nucleotide sequence, abundance and species distribution of tandemly organized DNA families are well characterized, little is known about the internal structure of tandem arrays, including total arrays size and the pattern of monomers distribution. Using our rye specific probes, pSc200 and pSc250, we addressed these issues...
Citations
... A variant of B. lineariloba C possesses a chromosome that is morphologically similar to the large B of B. dichromosomatica. A Bd49-like repeat is found in B. lineariloba C, and the ITS1 and ITS2 regions of rRNA genes on the large B from B. dichromosomatica are nearly as similar to B. lineariloba C as they are to B. dichromosomatica A chromosome loci (Leach et al. 2004). ...
Supernumerary B chromosomes (Bs) have been observed in over 2000 plant, animal, and fungal species. Bs are nonessential and may be deleterious to the host genome at high copy numbers. In order to maintain their presence in a population, they frequently display non-Mendelian inheritance via mitotic or meiotic drive mechanisms. Advances in sequencing technology have recently enabled researchers to confirm earlier assumptions that Bs often arise as amalgamations of normal chromosomes. Because Bs are nonessential, they tend to accumulate transposons, repetitive DNA, and organellar DNA. Some Bs have also been shown to harbor transcribed genes and noncoding loci. Not surprisingly, the presence and/or transcription of Bs may affect transcription of the host genome, and much effort has been expended investigating this possibility. The dispensable nature of Bs makes them excellent tools for studying centromere biology, as well as making them amenable to serving as platforms for genetic engineering. Here, we review the origin, composition, inheritance mechanisms, transcription, and potential uses of Bs from multiple kingdoms.
... According to Lim and Simmons (1994) and Dimitri et al. (1997), the accumulation of repetitive DNA sequences, including transposable elements in specific areas of the chromosomes can render such sites prone to chromosomal rearrangements. Additionally, the molecular structure analysis shows that these B chromosomes may be subject to gene silencing, accumulation of repetitive DNA and also to heterochromatinization processes (Leach et al. 2004). Therefore, the presence of large heterochromatic segments in the A and in some B chromosomes of M. sanctaefilomenae, and the reduced number of active ribosomal genes in these chromosomes may corroborate the above hypothesis about the gene silencing due to their close relationship with heterochromatin. ...
B chromosomes are dispensable genomic elements found in different groups of animals and plants. In the present study, a whole chromosome probe was generated from a specific heterochromatic B chromosome occurring in cells of the characidae fish Moenkhausia sanctaefilomenae (Steindachner, 1907). The chromosome painting probes were used in fluorescence in situ hybridization (FISH) experiments for the assessment of metaphase chromosomes obtained from individuals from three populations of Moenkhausia sanctaefilomenae. The results revealed that DNA sequences were shared between a specific B chromosome and many chromosomes of the A complement in all populations analyzed, suggesting a possible intra-specific origin of these B chromosomes. However, no hybridization signals were observed in other B chromosomes found in the same individuals, implying a possible independent origin of B chromosome variants in this species. FISH experiments using 18S rDNA probes revealed the presence of non-active ribosomal genes in some B chromosomes and in some chromosomes of the A complement, suggesting that at least two types of B chromosomes had an independent origin. The role of heterochromatic segments and ribosomal sequences in the origin of B chromosomes were discussed.
... This study reported for the first time the presence of B chromosomes in the Hyphessobrycon genus. According to Leach et al. (2004) analyses of the molecular structure have shown that B chromosomes are subject to gene silencing, repetitive DNA accumulation and heterochromatinization. Thus, most of the heterochromatic of B chromosomes are due to the presence of chromatin characterized by a high degree of condensation during the cell cycle, and this natural condensation results from the high content of the repetitive DNA of many types, especially satellite and ribosomal DNAs (Camacho 2005). ...
The Hyphessobrycon are allocated in the incertae sedis group of the Characidae family, one of the genera with more species of the group. The chromosomes of some species of Hyphessobrycon are known, and the diploid number most common for genus is 2n = 50 chromosomes. The aims of this study were to examine the karyotype macrostructure in the Hyphessobrycon
eques Steindachner, 1882, and show a new origin hypothesis for B chromosomes. The diploid number observed for Hyphessobrycon
eques was 2n = 52 chromosomes, and a karyotype formulae of 12m + 18sm + 8st + 14a, with FN (fundamental number) = 90 for both sexes. Only two females showed one B chromosome. The heterochromatin was observed mainly on centromeric regions, and in the long arm of the B chromosome. In this paper, the relationship of the B chromosome of Hyphessobrycon
eques with an occasional chromosome rearrangement was discussed.
... The de novo origin of B chromosomes has also been shown in experimental crosses between N. vitripennis and N. giraulti [Perfectti and Werren, 2001]. Leach et al. [2004] claimed that most B-specific DNA sequences hitherto reported are actually present in a few copies in the A chromosomes. In the fish Alburnus alburnus , however, it has been shown that the presence of B chromosomes composed mainly of a specific retrotransposable DNA element showing homology with the Drosophila Gypsy/Ty3 [Ziegler et al., 2003;Schmid et al., 2006]. ...
The origin of supernumerary (B) chromosomes is still a debated topic, with intra- and interspecific origins being the most plausible options. In the bee Partamona helleri, a sequence-characterized amplified region (SCAR) marker being specific to B chromosomes suggested the possibility of interspecific origin. Here, we search for this marker in 3 close relative species and perform DNA sequence comparison between species. The SCAR sequence does not show homology with other sequences in the databases, but does contain an open reading frame with sequence homology with a reverse transcriptase. Dot-blot hybridization using the SCAR marker as a probe confirmed that it is present in B-carrying, but not B-lacking larvae of P. helleri, and indicated its presence in adult individuals of P. cupira and P. criptica. Additionally, PCR amplification of the SCAR marker was successful on genomic DNA obtained from P. helleri and P. rustica larvae carrying B chromosomes, and on genomic DNA obtained from adult individuals of P. cupira, P. criptica and P. rustica. Finally, a comparison of the DNA sequence of the SCAR markers amplified from these 4 species showed very few nucleotide differences between the species. The complete association between B chromosome and SCAR presence and the scarce divergence observed for this DNA sequence between the 4 species analyzed suggest the possibility that this B chromosome has recently been transferred between species through several episodes of interspecific hybridization. © 2015 S. Karger AG, Basel.
... Donald et al. (1997) localized rRNA genes in the B chromosomes and Houben et al. (1999) suggested a monophyletic origin of the B chromosomes by computer-aided chromosomes image analysis and fluorescence in situ hybridization of ribosomal DNA and B chromosome-specific sequence . In this contest it is also necessary to recall two recent reviews by Jones and Houben (2003) and Leach et al. (2004). Ornithogalum tenuifolium -Very few are the reports in the literature regarding Ornitogalum tenuifolium. ...
Plant with very low chromosome number are of interest for the analysis of the structure of chromosome and chromatin organization. Many studies have been carried out on the evolution of the karyotype in the An-giosperms with only two chromosomes in their haploid complement. The results of these analyses in the five An-giosperms with 2n=4 have been reported with the aim to provide further insight into the origin and the organization of their chromosomes.
... Precise karyomorphometric data, as well as the location of centromeres, telomeres, 45S and 5S rDNA for the whole haploid standard chromosome set, including B chromosomes , were obtained (Houben et al. 1999). Among all the Angiosperms with 2n ¼ 4 B. dichromosomatica is the only one possessing supernumerary B chromosomes (Leach et al. 2004; Jones 2012). The B chromosomes are present in many wild populations of animal and plant species and represent one of the many causes of numerical chromosome variations. ...
... Several studies on B chromosomes of B. dichromosomatica have been carried out with the aim of explaining their origin and function. It emerges that they possess such an intrinsic complexity that it is difficult to interpret their sequence composition and arrangements as well as their derivation (Leach et al. 2004). However, both standard and micro B chromosomes appear to be transcriptionally inactive and contain silent rRNA genes, derived from those of the A chromosomes (Marschner et al. 2007b); this is a common feature for B chromosomes also in other plant species (Jones 1995). ...
Among Angiosperms, only six species are known to possess the lowest identified chromosome number, i.e., 2n=4. These plants are the monocotyledons Zingeria biebersteiniana, Colpodium versicolor, Ornithogalum tenuifolium and Rhynchospora tenuis, and the dicotyledons Haplopappus gracilis, and Brachyscome dichromosomatica. The low chromosome number may be cytogenetically derived from a different ancestral basic number, characteristic of each genus, by different processes, including tandem fusion, or unequal reciprocal translocations with loss of centric fragments. All these plants possess low nuclear 4C DNA contents, ranging from 1.56 pg (R. tenuis) to 8.20 pg (H. gracilis), and they generally display a similar chromosome size and a similar position of the nucleolus organising region (NOR), that is often located in the terminal or subterminal region of the small chromosome pair.All these characteristics could be a consequence of common adaptative mechanisms. Peculiar characteristics within these karyotypes are the holocentric chromosomes of R. tenuis and the presence of B chromosomes in B. dichromosomatica. Plants with a very low chromosome number may be considered to constitute a fascinating “island of interest”; moreover, they represent simple systems helpful for the examination of the structural organisation and evolution of Angiosperm chromosomes.
... In B. dichromosomatica, a set of tandemly repeated sequences were identified that reside in the A chromosomes and/or in the B chromosomes (Leach et al., 2004). For example, Bds1 is a tandemly repeated 91-bp element that is a main component of a supernumerary A-chromosome segment (Houben et al., 2000). ...
... An Arabidopsis-type telomere probe and the Arabidopsisspecific centromeric 180-bp repeat probe were synthesized by thermal cycling according to the method described by Ijdo et al. (1991) and Kawabe and Nasuda (2005), respectively. The inserts of the clones Bdm29 and Bds1 were used as Brachycome-specific probes (Leach et al., 2004). ...
Extrachromosomal circular DNA (eccDNA) is one characteristic of the plasticity of the eukaryotic genome. It was found in various non-plant organisms from yeast to humans. EccDNA is heterogeneous in size and contains sequences derived primarily from repetitive chromosomal DNA. Here, we report the occurrence of eccDNA in small and large genome plant species, as identified using two-dimensional gel electrophoresis. We show that eccDNA is readily detected in both Arabidopsis thaliana and Brachycome dichromosomatica, reflecting a normal phenomenon that occurs in wild-type plants. The size of plant eccDNA ranges from > 2 kb to < 20 kb, which is similar to the sizes found in other organisms. These DNA molecules correspond to 5S ribosomal DNA (rDNA), non-coding chromosomal high-copy tandem repeats and telomeric DNA of both species. Circular multimers of the repeating unit of 5S rDNA were identified in both species. In addition, similar multimers were also demonstrated with the B. dichromosomatica repetitive element Bdm29. Such circular multimers of tandem repeats were found in animal models, suggesting a common mechanism for eccDNA formation among eukaryotes. This mechanism may involve looping-out via intrachromosomal homologous recombination. The implications of these results on genome plasticity and evolutionary processes are discussed.
... Although factors required for B chromosome survival have been mapped to chromosome regions, no genes on the maize B chromosome or any other B chromosome have been characterized to date that play a role in accumulation mechanisms. High copy number repetitive elements specific to B chromosomes have been identified in several species including maize (Alfenito & Birchler 1993, Cheng & Lin 2004, rye (Wilkes et al. 1995), and others (Ziegler et al. 2003, Leach et al. 2004. Two of the elements that are specific to the maize B chromosome are organized in long tandem arrays with repeat units of similar size. ...
Dispensable chromosomes in addition to the normal complement in diverse taxa are called B chromosomes. The maize B chromosome is discernible in mitotic chromosome spreads as a small compact chromosome composed mainly of heterochromatin. Although much of this chromosome consists of repetitive elements common to the A chromosomes, several sequences specific to the B chromosome have been identified. In the work described here we used the sequence from a B-specific RAPD (random amplification of polymorphic DNA) marker, pBGBM18.2, to isolate another DNA element, StarkB, present in many copies on the B chromosome. StarkB was mapped to the third and fourth blocks of distal heterochromatin using translocation breakpoints and fluorescent in-situ hybridization (FISH). Sequence analysis revealed that StarkB is composed of fragments from the A genome as well as B-specific sequences. The StarkB element is much larger than the other B-specific elements and is not present in large tandem arrays. Different copies of StarkB varied by small insertions, deletions, and duplications as well as single-nucleotide polymorphisms. Reverse transcriptase PCR showed that portions of the StarkB element are expressed. Using the LTR divergence of retroelements interrupting the B-specific sequences, the minimum age of the StarkB repeat array and, by inference, of the B chromosome, was estimated to be 2 million years.
A survey of new species with B chromosomes (Bs), based on a collection of 900 publications, gives an estimate of 406 +B species discovered since they were last surveyed in 1980. There are 13 species of fungi, 136 plants and 257 animals with newly discovered Bs. The fungi, the single bird and the 100 species of fish are all new entries which were unknown in 1980. New species with B chromosomes continued to be found on a regular basis, and there is opportunity for trying to further our knowledge of the genomic properties and evolutionary significance of these enigmatic chromosomes.
B chromosomes are dispensable elements that do not recombine with the A chromosomes of the regular complement and that follow their own evolutionary pathway. Here, we survey current knowledge on the DNA/chromatin composition, origin, and drive mechanisms of B chromosomes and discuss the potential research applications of supernumerary chromosomes.
