The distribution of B chromosomes across species

In this review we look at the broad picture of how B chromosomes are distributed across a wide range of species. We review recent studies of the factors associated with the presence of Bs across species, and provide new analyses with updated data and additional variables. The major obstacle facing comparative studies of B chromosome distribution is variation among species in the intensity of cytogenetic study. Because Bs are, by definition, not present in all individuals of a species, they may often be overlooked in species that are rarely studied. We give examples of corrections for differences in study effort, and show that after a variety of such corrections, strong correlations remain. Several major biological factors are associated with the presence of B chromosomes. Among flowering plants, Bs are more likely to occur in outcrossing than in inbred species, and their presence is also positively correlated with genome size and negatively with chromosome number. They are no more frequent in polyploids than in diploids, nor in species with multiple ploidies. Among mammals, Bs are more likely to occur in species with karyotypes consisting of mostly acrocentric chromosomes. We find no evidence for an association with chromosome number or genome size in mammals, although the sample for genome size is small. The associations with breeding system and acrocentric chromosomes were both predicted in advance, but those with genome size and chromosome number were discovered empirically and we can offer only tentative explanations for the very strong associations we have uncovered. Our understanding of why B chromosomes are present in some species and absent in others is still in its infancy, and we suggest several potential avenues for future research.     

A complex B chromosome system in the Korean field mouse, Apodemus peninsulae

Information on B chromosomes of six subspecies of A. peninsulae Thomas, 1906, from 79 local populations of Russia (Siberia, Altai, Buryatia and the Far East), Mongolia, China, Korea and Japan (Hokkaido) is reviewed. The frequency of animals with B chromosomes is higher in this taxon than in other mammals and ranges from 0.4 up to 1.0, excluding two insular populations (Sakhalin Island and Stenin Island, Primorye) where Bs were not found. The B chromosome polymorphism shows four levels of variation in number (intraindividual mosaicism, intrapopulational and interpopulational), as well as variability in size, morphology and differential staining. Geographic variation was found among populations for these cytogenetic characteristics and, in some cases, it coincided with subspecies distribution. Comparative chromosome banding of micro and macro Bs illuminates possible pathways for their origin.     

The B chromosomes in Brachycome

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 dichromosomatica, a species which has Bs of two different sizes. The larger Bs are somatically stable whereas the smaller, or micro, Bs are somatically unstable. Both B types contain clusters of ribosomal RNA genes that have been shown unequivocally to be inactive in the case of the larger Bs. The large Bs carry a family of tandem repeat sequences (Bd49) that are located mainly at the centromere. Multiple copies of sequences related to this repeat are present on the A chromosomes (As) of related species, whereas only a few copies exist in the A chromosomes of B. dichromosomatica. The micro Bs share DNA sequences with the As and the larger Bs, and they also have B-specific repeats (Bdm29 and Bdm54). In some cases repeat sequences on the micro Bs have been shown to occur as clusters on the A chromosomes in a proportion of individuals within a population. It is clear that none of these B types originated by simple excision of segments from the A chromosomes.     

Single-cell quantitative RT-PCR analysis of Cpt1b and Cpt2 gene expression in mouse antral oocytes and in preimplantation embryos

Among crustacean Decapoda numerical chromosome variability is frequent, and it has been hypothesized that the presence of supernumerary chromosomes accounts for this variability. Thanks to the improvement of cytogenetic analysis by chromosomal banding techniques, supernumerary B chromosomes (Bs) have been demonstrated in Nephrops norvegicus, Homarus americanus,Palinurus elephas and P. mauritanicus, belonging to different crustacean families. In all four species Bs were variable in number, mainly heterochromatic and undigested by various endonucleases, and in meiosis they showed non-Mendelian segregation. Compared to the other chromosomes of the complement, the Bs are very small in almost all species, but some of them were very large in N. norvegicus.     

Current knowledge on B chromosomes in natural populations of helminth parasites: a review

Helminths, traditionally classified into three phyla Platyhelminthes, Nemathelminthes and Acanthocephala, are a phylogenetically broadly diversified group of invertebrates, characterised by a parasitic life style. Current estimates of the helminth species diversity are at least 23-40,000 platyhelminthes, 10-26,000 nematodes and 1,200 acanthocephalans. Recent information on helminth karyotypes is fragmentary, and basic karyological data are known from approximately 1.1% of known species. Supernumerary chromosomes have been reported in selected populations of only 11 digenean flukes (Platyhelminthes), 1 thorny-headed worm (Acanthocephala) and 4 roundworms (Nematoda), which represent 3.6, 7.7 and 1.3% of the total number of species cytogenetically analysed to date within respective helminth groups. B chromosome presence was not generally associated with heteromorphic sex chromosomes as they occurred both in hermaphroditic flukes and dioecious helminths, and in species having male or female heterogametic sex chromosomes (ZW of schistosomes, XO of acanthocephalans and XY of nematodes). Numbers of B chromosomes varied from 1 to 10. Most often, Bs represented one or two of the smallest elements of the complement but they could be much bigger in some digenean flukes. B chromosomes showed a diverse morphology, including telocentric to metacentric structure. There is no detailed banding or ultrastructural study of Bs in the majority of helminth carriers. Assumptions on the possible relation between the occurrence of Bs in endoparasitic helminths and extreme environments are discussed.     

B chromosomes and genome size in flowering plants.

B chromosomes are extra chromosomes found in some, but not all, individuals within a species, often maintained by giving themselves an advantage in transmission, i.e. they drive. Here we show that the presence of B chromosomes correlates to and varies strongly and positively with total genome size (excluding the Bs and corrected for ploidy) both at a global level and via a comparison of independent taxonomic contrasts. B chromosomes are largely absent from species with small genomes; however, species with large genomes are studied more frequently than species with small genomes and Bs are more likely to be reported in well-studied species. We controlled for intensity of study using logistic regression. This regression analysis also included effects of degree of outbreeding, which is positively associated with Bs and genome size, and chromosome number, which is negatively associated with Bs and genome size, as well as variable ploidy (more than one ploidy level in a species). Genome size, breeding system and chromosome number all contribute independently to the distribution of B chromosomes, while variable ploidy does not have a significant effect. The genome size correlates are consistent with reduced selection against extra DNA in species with large genomes and with increased generation of B sequences from large A genomes.     

B染色体起源的研究进展

B染色体(简写为Bs)起源的传统观点是认为它起源于携带者所在物种的基因组.目前又发现了许多新的证据来推测Bs的起源.它可能具有两种起源:种内起源和种间起源(起源于另一物种的基因组),且有证据表明同一物种的Bs可能是多次起源,同时对Bs的起源机制也作了总结.认为B8起源的研究已取得了一定的进展,但要给Bs起源下一个确切的结论仍需要大量的实验证据.     

The mammalian model for population studies of B chromosomes: the wood mouse (Apodemus)

The presence of B chromosomes was reported in six species of the genus Apodemus (A. peninsulae, A. agrarius, A. sylvaticus, A. flavicollis, A. mystacinus, A. argenteus). High frequencies of Bs were recorded particularly in A. peninsulae and A. flavicollis. The origin of Bs in Apodemus seems to be rather ancient, and it is possible that the supernumerary elements, and/or a tendency for their appearance, were inherited from the common ancestor of the extant species. We have not found any correlated changes between frequencies of Bs and the level of protein polymorphism and/or heterozygosity assessed in electrophoretic studies. No measurable effect of Bs on overall genetic variability was thus revealed in studied populations. The pattern of evolutionary dynamics of Bs can be distinctly different between geographical populations, and both the parasitic and the heterotic models can be applied to explain the maintenance of Bs in different populations. Further studies are desirable to improve our understanding of the complicated evolutionary dynamics of Bs in the Apodemus species. An essential condition for success in this respect is much more detailed information on inheritance and the molecular structure of Bs.     

Selfish DNA and breeding system in flowering plants

In many species, some individuals carry one or more B chromosomes: extra, or supernumerary chromosomes not part of the normal complement. In most well-studied cases, Bs lower the fitness of their carrier and persist in populations only because of accumulation mechanisms analogous to meiotic drive. It has been suggested that such genomic parasites are expected to persist only in outcrossed sexual species, in which uninfected lines of descent can be continuously reinfected; in inbred or asexual species, all selection is between lines of descent, and the genomic parasites are either lost or must evolve into commensals or mutualists. Here we present a simple population genetic model of the effect of outcrossing rate on the frequency of B chromosomes, and find that outcrossing facilitates the spread of parasitic Bs, but inhibits the spread of mutualists. Data compiled from the literature on breeding system and B chromosomes of British plants indicate that Bs are much more likely to be reported from obligately outcrossed species than inbred species. These results support the ideas that most B chromosomes are parasitic, and that breeding systems play a central role in the biology of selfish genes.     

Chromosome banding and genome compartmentalization in fishes

The diploid chromosome number of the Chinese raccoon dog varies from 54 (no B chromosomes) to 58 (4 B chromosomes). The B chromosomes are totally heterochromatic. An electron microscopic study was made of the synaptonemal complexes (SC) in spermatocytes of these animals. The SC karyotype consists of 27 regular chromosome pairs (autosomes and the sex chromosomes) plus the B chromosomes. The Bs pair effectively with one another at pachytene, but the SC axes of the B chromosomes are much denser than those of the A chromosomes. Depending on the number of Bs, both bivalents and multivalents have been observed. When three B chromosomes are present in a cell, parallel alignment of all three SCs can be seen. Formation of multivalents indicates high homology among these supernumerary heterochromatic chromosomes. Fusiform bulges are found along unpaired regions of all chromosomes which are particularly pronounced in diplotene.     

B chromosomes and developmental homeostasis in the yellow-necked mouse, Apodemus flavicollis (Rodentia, Mammalia): effects on nonmetric traits

B chromosomes are found in almost all populations of the yellow-necked mouse, Apodemus flavicollis (Rodentia, Mammalia). Their effects on developmental homeostasis in this species were analyzed using morphological nonmetric traits (number of foramina) in a sample of 218 animals from locality Mt Jastrebac in the former Yugoslavia. Variations of the parameters of developmental homeostasis (the degree of fluctuating asymmetry--FA, the number of asymmetrical characters per individual--NA, and the total phenotypic variability--PV) were examined in three groups: in animals without Bs, with one B chromosome, and with more than one B chromosome. Significant differences in the level of FA between groups of animals were found for two characters. Carriers of one B chromosome displayed the highest level of phenotypic variability. Parameters of developmental stability (DS) were monitored in the population in which significant variations in the frequency of animals with Bs (fB) were established during the season earlier. The FA levels for four foramina out of a total of 12 examined followed the changes of frequencies of animals with Bs. Furthermore, a significant seasonal correlation between NA and fB was found. The presence of B does not cause a disturbance of homeostasis in a way that allows changes in homeostasis to be directly related to B chromosome's presence. However, carriers of B react differently to environmental changes than do noncarriers.     

Prevalence of B chromosomes in Orthoptera is associated with shape and number of A chromosomes

We analyze the prevalence of B chromosomes in 1,601 species of orthopteran insects where chromosome number and shape are known. B chromosomes have been reported in 191 of these species. Bs are not uniformly distributed among orthopteran superfamilies, with evident hotspots in the Pyrgomorphoidea (32.3% of species carrying Bs), Grylloidea (14.9%), Acridoidea (14.6%) and Tetrigoidea (14.3%). As expected under the theory of centromeric drive, we found a correlation between B chromosome presence and A chromosome shape-Bs are more frequent in karyotypes with more acrocentric A chromosomes. We also found that Bs are less common in species with high chromosome numbers and appear to be most common at the modal chromosome number (2n = 24). Study effort, measured for each genus, was not associated with B prevalence, A chromosome shape or A chromosome number. Our results thus provide support for centromeric drive as an important and prevalent force in the karyotypic evolution of Orthoptera, just as it appears to be in mammals. We suggest that centromeric drive may provide a mechanistic explanation for White's principle of karyotypic orthoselection.     

The odd-even effect in mitotically unstable B chromosomes in grasshoppers

The odd-even effect, by which B chromosomes are more detrimental in odd numbers, has been reported in plants and animals. In grasshoppers, there are only a few reports of this effect and all were referred to as traits related to the formation of aberrant meiotic products (AMPs). Here we review the existing information about B chromosome effects on AMPs, chiasma frequency and the number of active nucleolus organizer regions (NORs) per cell. Polysomy for A chromosomes and B chromosomes are two kinds of chromosome polymorphism frequently found in grasshoppers. In some aspects, e.g. meiotic behaviour and mitotic instability leading to individual mosaicism (in the case of mitotically unstable Bs), polysomic As show similar characteristics to B chromosomes. In fact, polysomy is regarded as one of the main mechanisms for B chromosome origin. Here we review some features of meiotic behaviour in known cases of polysomy and mitotically unstable Bs in grasshoppers, in looking for possible causes for the odd-even effect. In all these traits, the odd-even effect was apparent, although its appearance was not universal in any case, with variation among species or populations within the same species. The equational division and lagging of the extra chromosomes, when univalents, could favour the appearance of abnormal meiotic products, and the formation of bivalents, when there are two or more extra chromosomes, inhibits this process. Therefore, the odd-even effect might be a consequence of the concomitant operation of both aspects of extra chromosome meiotic behaviour. The possibility that the odd-even effect might result from an increase in cell stress generated by odd numbers is suggested.     

Morphology and Meiotic/Mitotic Behavior of B Cromosomes in a Japanese Harvestman, Metagagrella tenuipes (Arachnida: Opiliones): No Evidence for B Accumulation Mechanisms

Earlier, it has been demonstrated that wild populations of a Japanese harvestman Metagagrella tenuipes (Arachnida: Opiliones) are polymorphic for B chromosomes. In this paper, we present results of a study of the morphology and mitotic and meiotic behavior of the Bs. The B chromosomes varied considerably in size and proportion of eu- and heterochromatin. The single nucleolus organizing region, found in males, was located on a chromosome of the A complement. Some intercell variation in number of Bs may be explained by accidental chromosome losses during chromosome preparation. We also found no intertissue variation in number of Bs. There were also no differences in mean number of B chromosomes per individual among males and females, adult and subadult harvestmen. Segregation of Bs in mitotic and meiotic divisions was nonrandom; B chromosomes tended to segregate equally between daughter cells. The results obtained provide no support for the hypothesis of existence of B accumulation mechanism in this species.     

The effect of B chromosomes on fitness components inAegilops speltoides Tausch

The effect of B chromosomes (Bs) on various components of vigour and fertility inAegilops speltoides was studied. We designed three types of synthetic populations. In the first type we compared plants from the localities Haifa (which has Bs in nature) and Ashkelon (which has not). Non significant differences were found between the plants from both populations, and between B carriers and non carriers among Haifa individuals. In the second synthetic population we compared 0B, 1B, 2B, 3B and 5B plants from Haifa. We found that vigour variables were not affected by the presence of Bs, while fertility variables were significantly correlated with B number, in such a way that 1B plants produce more grains per plant than any other constitution, while 5Bs produce a remarkable deleterious effect. In the third type of synthetic populations we compared plants with 0B and 1B, 0B and 2B, 0B and 3B, and 0B and 5B. In this case we also found that 1B plants were the most fertile, but the variation of reproductive fitness was higher from plot to plot within the same B class than among B classes. We conclude that the effect of B chromosomes on the fitness variables that we have estimated is close to neutrality in this species. We discuss the relation between the strength of the mechanism of accumulation and the deleterious effects of B chromosomes.     

Nature and evolution of B chromosomes in plants: A non-coding but information-rich part of plant genomes

This work reviews recent advances providing insights on the origin and evolution of B chromosomes (Bs) in representative plant species. Brachyome dichromosomatica has large and micro Bs. Both carry an inactive ribosomal gene cluster. The large Bs contain the B-specific Bd49 family, mainly located at the centromere. Multiple copies are present in the A chromosomes (As) of related species, whereas only a few copies exist in B. dichromosomatica As. The micro Bs share sequences with the As, the large Bs and have the B-specific repeats Bdm29 and Bdm54. It seems that the large and micro Bs are related in origin. It is very unlikely that the Bs originated by simple excision from the As. Rye Bs are composed of sequences predominantly shared with the As. B-specific sequences are located at the heterochromatic end of the long arm. Probably, they originated from the As after many rearrangements, with a tendency for duplication. The E3900 family derives from a Ty3 gypsy retrotransposon, but the D1100 family shows no evidence of genic origin. The overall composition of maize As and Bs is similar suggesting a common origin. Several B-specific sequences have been found, the most studied being pZmBs, which is located at the B centromere. It shows partial homology to the centromere of chromosome 4 and to the knobs. It is not known whether the B centromere derives from centromere 4, or whether both have a more distant common origin. The dynamics of Bs in populations depends on their non-Mendelian mechanisms of transmission, their effects on carrier fitness and on A genes modulating their parasitic properties. Three representative examples are reviewed. The Bs of Allium schoenoprassum are transmitted at a mean lower than Mendelian and adversely affect vigour and fertility. However, there is a differential selection operating in favour of B-containing seedlings. Rye Bs undergo strong drive, which is counteracted by harmful effects on fertility and instabilities at meiosis. Both nondisjunction and meiotic behaviour, and consequently the establishment of B polymorphisms, mainly depend on the Bs themselves. B nondisjunction in maize is controlled by the B, but the As control preferential fertilisation. Considering the non-equilibrium model, the Bs of Allium seem to have been neutralised by the A genome, the As of maize provide defence against B attack, whereas the Bs of rye are only slightly neutralized.     

The genomic complexity of micro B chromosomes of Brachycome dichromosomatica

A major sequence component of the micro B chromosome of Brachycome dichromosomatica (2 n=4) is the tandem repeat Bdm29, which was found by in situ hybridisation to be distributed along the entire length of the chromosome. A high copy number of this sequence does not occur as a regular feature of the A chromosomes in this species but it was found in infrequent individuals in two wild populations that were analysed. In these instances Bdm29 is localised within heterochromatic, polymorphic segments on the long arm of chromosome 1. The origin of the micro B chromosomes was investigated by determining whether they are related to this A chromosome polymorphism by simple excision and/or integration. Results obtained by using Bdm29, together with a newly isolated repeat sequence, Bdm54, and a number of other sequences known to occur on the micro B chromosome, as probes in in situ hybridisation and Southern analysis demonstrated that the formation of micro B chromosomes is a complex multistep process. The observation that the genomic organisation of the micro B chromosome is unlike anything found on the A chromosomes precludes their origin by simple excision and also indicates that micro Bs do not integrate directly into the A complement to form polymorphic heterochomatic segments.     

Study on Genetic Behaviour of B Chromosomes at Meiosis in Agropyron Gaertn

The chromosomal behaviour at meiosis in pollen mother cells of 33 sample materials from 4 species in Agropyron Gaerm. distributed over various regions in China were observed. We. found that B chromosomes were mainly-present in A. mongolicum Keng (2n=2x=14) and were absent in the tetraploid ,A. cristarum (L.) Gaertn. (2n=4x=28) and ,A. michnoi Roshev. (2n=4x=28). Of all A. desertorum (Fisch.) Schult. (2n=4x=28) populations, they were present in 40%. The pairing between even numbers of B chromosomes usually occured at diakinesis where as uneven numbers of B chromosomes at metaphase 1. B chromosomes at anaphase I separated at random. The Fagging Bs tended to reach two poles by chromatids through precocious division of Bs. The pairing and chiasma frequencies between A chromosomes were increased by the larger numbers of Bs. The relationship between B chromosomes and species ploidy, the homeology among Bs, etc were discussed.     

Characterisation of repeated sequences from microdissected B chromosomes of Crepis capillaris

The B chromosome of Crepis capillaris was isolated from the standard chromosomes by microdissection, and the chromosomal DNA amplified using the degenerate oligonucleotide-primed polymerase chain reaction (DOP-PCR). The PCR product was cloned and a Bspecific library created and characterised. Southern and in situ hybridisation analyses of the DOP-PCR product from microdissected B chromosomes confirmed that the B chromosome is composed mainly of sequences also present in the A chromosomes but lacks the main repeated DNA families located in the A-chromosomal heterochromatin. From 100 clones analysed, 12% of the generated B-chromosomal library was shown to be composed of dispersed repeats located in both the A and B chromosomes. No B-specific repeated sequence was detected. One of the most abundant repeated DNAs within the library, the family B134, was further characterised. Repeating units show a sequence similarity range from 69% to 90% and are characterised by their richness in (CA)n repeats. In situ hybridisation revealed that members of this family are dispersed throughout the A and B chromosomes but are more concentrated in the pericentromeric heterochromatin of the B, indicating that the molecular organization of B heterochromatin is different from that of the A chromosomes. Compared with the A chromosomes, the Bs contain about 20,000 copies per micron more of the B134 sequence. This indicates that B134 was amplified on the B chromosome after its origin. The B134 sequences in the B chromosomes have also diverged from those on the A chromosomes. Although the DNA composition of A and B chromosomes is similar, Bs are evolving separately from A chromosomes at the molecular level.     

B chromosomes in plants

Abstract

B chromosomes (Bs) can be described as “selfish chromosomes”, a term that has been used for the repetitive DNA which comprises the bulk of the genome in large genome species, except that Bs have a life of their own as independent chromosomes. They can accumulate in number by various processes of mitotic or meiotic drive, especially in the gametophyte phase of the life cycle of flowering plants. This parasitic property of drive ensures their survival and spread in natural populations, even against a gradient of harmful effects on the host plant phenotype. B chromosomes are inhabitants of the nucleus and they are subject to control by “genes” in the A chromosome (As) complement. This interaction with the As, together with the balance between drive and harmful effects makes a dynamic system in the life of a Bs. In this review, we concentrate mainly on recent developments in the Bs of rye and maize, two of the species currently receiving most attention. We focus on their population dynamics and on the molecular basis of their structural organisation and mechanisms of drive, as well as on their mode of origin and potential applications in plant biotechnology.