B-chromosome systems in flowering plants and animal species

Rye (Secale cereale) supernumerary (B) chromosomes associated with heat tolerance during early stages of male sporogenesis

BACKGROUND AND AIMS

Rye supernumerary (B) chromosomes have an accumulation mechanism involving the B subtelomeric domain highly enriched in D1100- and E3900-related sequences. In this work, the effects of heat stress during the early stages of male meiosis in 0B and +B plants were studied.

METHODS

In-depth cytological analyses of chromatin structure and behaviour were performed on staged rye meiocytes utilizing DAPI, fluorescence in situ hybridization and 5-methylcytosine immune labelling. Quantitative real-time PCR was used to measure heat effects on the expression of the Hsp101 gene as well as the 3·9- and 2·7-kb E3900 forms in various tissues and meiotic stages.

KEY RESULTS AND CONCLUSIONS

Quantitative real-time PCR established that heat induced equal up-regulation of the Hsp101 gene in 0B and 2B plants, with a marked peak in anthers with meiocytes staged at pachytene. Heat also resulted in significant up-regulation of E3900-related transcripts, especially at pachytene and for the truncated 2·7-kb form of E3900. Cytological heat-induced anomalies in prophase I, measured as the frequency of anomalous meiocytes, were significantly greater in 0B plants. Whereas telomeric sequences were widely distributed in a manner close to normal in the majority of 2B pachytene cells, most 0B meiocytes displayed abnormally clustered telomeres after chromosome pairing had occurred. Relevantly, bioinformatic analysis revealed a significantly high-density heat responsive cis regulatory sequence on E3900, clearly supporting stress-induced response of transcription for the truncated variant. Taken together, these results are the first indication that rye B chromosomes have implications on heat tolerance and may protect meiocytes against heat stress-induced damage.

The diversity of karyotypes and genomes within section Syllinum of the Genus Linum (Linaceae) revealed by molecular cytogenetic markers and RAPD analysis

The wide variation in chromosome number found in species of the genus Linum (2n = 16, 18, 20, 26, 28, 30, 32, 36, 42, 72, 84) indicates that chromosomal mutations have played an important role in the speciation of this taxon. To contribute to a better understanding of the genetic diversity and species relationships in this genus, comparative studies of karyotypes and genomes of species within section Syllinum Griseb. (2n = 26, 28) were carried out. Elongated with 9-aminoacridine chromosomes of 10 species of section Syllinum were investigated by C- and DAPI/С-banding, CMA and Ag-NOR-staining, FISH with probes of rDNA and of telomere repeats. RAPD analysis was also performed. All the chromosome pairs in karyotypes of the studied species were identified. Chromosome DAPI/C-banding patterns of 28-chromosomal species were highly similar. Two of the species differed from the others in chromosomal location of rDNA sites. B chromosomes were revealed in all the 28-chromosomal species. Chromosomes of Linum nodiflorum L. (2n = 26) and the 28-chromosomal species were similar in DAPI/C-banding pattern and localization of several rDNA sites, but they differed in chromosomal size and number. The karyotype of L. nodiflorum was characterized by an intercalary site of telomere repeat, one additional 26S rDNA site and also by the absence of B chromosomes. Structural similarities between different chromosome pairs in karyotypes of the studied species were found indicating their tetraploid origin. RAPD analysis did not distinguish the species except L. nodiflorum. The species of section Syllinum probably originated from a common tetraploid ancestor. The 28-chromosomal species were closely related, but L. nodiflorum diverged significantly from the rest of the species probably due to chromosomal rearrangements occurring during evolution.

Canine cytogenetics--from band to basepair

Humans and dogs have coexisted for thousands of years, during which time we have developed a unique bond, centered on companionship. Along the way, we have developed purebred dog breeds in a manner that has resulted unfortunately in many of them being affected by serious genetic disorders, including cancers. With serendipity and irony the unique genetic architecture of the 21st century genome of Man's best friend may ultimately provide many of the keys to unlock some of nature's most intriguing biological puzzles. Canine cytogenetics has advanced significantly over the past 10 years, spurred on largely by the surge of interest in the dog as a biomedical model for genetic disease and the availability of advanced genomics resources. As such the role of canine cytogenetics has moved rapidly from one that served initially to define the gross genomic organization of the canine genome and provide a reliable means to determine the chromosomal location of individual genes, to one that enabled the assembled sequence of the canine genome to be anchored to the karyotype. Canine cytogenetics now presents the biomedical research community with a means to assist in our search for a greater understanding of how genome architectures altered during speciation and in our search for genes associated with cancers that affect both dogs and humans. The cytogenetics 'toolbox' for the dog is now loaded. This review aims to provide a summary of some of the recent advancements in canine cytogenetics.

Karyotypic characterization of Apatemon gracilis

The cytotaxonomical characteristics of parthenitae of Apatemon gracilis (Rudolphi, 1819) Szidat, 1928 were studied using karyometric analysis to extend our knowledge of chromosome morphology and karyosystematics among trematodes. The karyotype, reported here for the first time, consists of ten pairs (2n = 20) of chromosomes divided into two size groups: five pairs of comparatively large and five pairs of small chromosomes. Biarmed chromosomes prevail in the chromosome set. According to centromere index values, chromosome 1 is submetacentric to metacentric, 2 is subtelocentric, 3 and 5 are acrocentric, 4, 9 and 10 are metacentric, 6 is submetacentric and 7 and 8 are submeta-subtelocentric. The small uniarmed B-chromosome was found in the chromosome set of parthenitae of A. gracilis from one snail. Data are discussed with reference to the karyotypes previously described within the Strigeidae.

B chromosomes of B. dichromosomatica show a reduced level of euchromatic histone H3 methylation marks

B chromosomes (Bs) are dispensable, less-transcriptionally active components of the genomes of numerous species. Little information is available on the chromatin composition of Bs and whether it differs in any way from that of the A chromosomes. Methylated isoforms of histone H3 are of particular interest because of their role in eu/heterochromatin formation. Immunofluorescence using site-specific antibodies demonstrates that the chromatin in A and both types of Bs of B. dichromosomatica differs markedly in euchromatic histone H3 methylation marks. While A chromosomes are labelled brightly, the micro B and large B chromosomes are faintly labelled with antibodies against H3K4me2/3, H3K9me3 and H3K27me2/3. The heteropycnotic, tandem-repeat enriched micro Bs were even less labelled with euchromatic histone H3 methylation marks than large Bs, most probably due to different DNA composition. No differences in immunolabelling intensity between A and B chromosomes were found as to the heterochromatic marks H3K9me1/2 and H3K27me1, indicating that Bs are not additionally labelled by heterochromatin typical histone H3 modifications. Analysis of DNA replication timing suggests that micro Bs are replicating throughout S-phase.

Nuclear DNA content and nuclear and cell volume are positively correlated in angiosperms

Volumes of flow sorted nuclei were analyzed from two highly endopolyploid (diploids with endopolyploid tissues) species (Arabidopsis thaliana and Barbarea stricta), from a less endopolyploid species (Allium cepa) and from two non-endopolyploid species (Chrysanthemum multicolor and Fritillaria uva-vulpis). Intraspecific as well as interspecific comparisons revealed a highly positive correlation (r > 0.99) between DNA content and nuclear volume. No significant differences between expected and measured nuclear volumes were noted indicating that chromatin packing is not increased with increasing DNA content in the tested plant species. In epidermis cells of A. thaliana, A. cepa and Ch. multicolor, a lower (r between 0.6 and 0.7) but significant positive correlation between nuclear volume and cell volume was found. This correlation is compatible with the hypothesis that endopolyploidization (EP = consecutive replication cycles not separated by nuclear and cell divisions) might speed up the growth of endopolyploid species and compensate for small genome size.

Cybernetic formulation of the definition of life

A definition of life (a living individual) in cybernetic terms is proposed. In this formulation, life (a living individual) is defined as a network of inferior negative feedbacks (regulatory mechanisms) subordinated to (being at service of) a superior positive feedback (potential of expansion). It is suggested that this definition is the minimal definition, necessary and sufficient, for life to be distinguished from inanimate phenomena and, as such, it describes the essence of life. Subsequently, a quantitative expression for the amount of the biologically relevant ("purposeful") information (as opposed to the amount of information in the thermodynamic sense) is proposed. This is followed by the application of the formulated approach to different phenomena of a dubious status existing presently on the Earth as well as to the process of origination of life on our planet.

B chromosomes in plants

This review of supernumerary B chromosome systems in flowering plants deals mainly with work published in the last 15 yr, hut also includes older material which has not hitherto been presented systematically. The term B chromosome (B) is defined, and there is an introductory overview dealing with general characteristics and the significance of Bs as a widespread chromosome polymorphism. Detailed sections are then presented covering the occurrence of Bs in different taxa, their structure and molecular organization, their irregular modes of inheritance, their phenotypic effects, population dynamics and origin. Particular attention is paid to the research growth points in molecular analysis of the structure and genome organization of Bs, and to transmission genotypes in the context of their adaptive versus their selfish properties. Where appropriate, reference is also made to likely future lines of research, and also to the usefulness of B chromosomes in genetic analysis and as model systems to study general phenomena of genome organization and evolution, nuclear physiology and architecture, chromosome polymorphism and selfish DNA. CONTENTS Summary 411 I. Introduction 411 II. Occurrence 411 III. Structure and organization 415 IV. Inheritance 422 V. Phenotypic effects 426 VI. Populations 429 VII. Applications 430 VIII. Origin 430 Acknowiedgements 430 References 431.

B-chromosomes: germ-line parasites which induced changes in host recombination

The object of this paper is to suggest that there may be an unexpected connexion between parasites and the evolution of sex, using for illustration an unfamiliar type of parasite, the selfish chromosome. The major intellectual challenge of sexuality is to an environment which is continually getting worse. The elegant solution given by the Red Queen theory (Levin, 1975; Hamilton, 1980; Bell, 1982; Bell & Maynard Smith, 1988) is that the relevant aspect of the environment is provided by antagonists—pathogens, predators and competitors—which, because they can respond adaptively so as to negate any improvement that has been made, provide a constant stimulus for continued evolution. Sexuality and recombination are favoured because some of the new combinations of genes which they create are resistant to the current population of antagonists. In other respects, sex and recombination are probably highly disadvantageous: outcrossed sex is expensive because it halves the rate of transmission of genes, while recombination breaks up successful combinations of genes. It is only in certain circumstances that the necessity for continual counter-adaptation will overcome these disadvantages: in particular, the damage (reduction in fitness) caused by an antagonist must be substantial, and the amount of damage must depend on a genetic interaction between the antagonistic species. These requirements are often satisfied by host—parasite systems, where both the ecological and genetic interactions between the antagonists may be very severe and highly specific (see reviews by Day, 1974 and Burdon, 1987). It is possible, therefore, that sex and recombination are maintained in natural populations largely through the dynamics of the coevolution of hosts and their parasites. This is certainly compatible with the major ecological patterns shown by sexual systems, with outcrossed sex being more common in the sea than in freshwater, more common at low than at high latitudes, and generally more common in stable, complex, climax environments where interactions between species are expected to be more frequent and intense (Bell, 1982). However, there is as yet no evidence which conclusively supports a direct causal link between the incidence of parasitism and the rate of recombination. In particular, it has never been demonstrated that a particular parasite has the effect of eliciting, directly or indirectly, a greater rate of genetic recombination in its host. We suggest that such a parasite exists; both the parasite and its effects are well known, but have never been interpreted in the context of the evolution of recombination through host—parasite coevolution. It is in many respects a rather unusual parasite. We shall argue thatB-chromosomes represent highly evolved parasitic DNA, transmitted through the germ line and often eliciting greater rates of recombination in the host genome.

Ribosomal RNA genes in B chromosomes of Crepis capillaris detected by non-radioactive in situ hybridization

A non-radioactive in situ hybridization method using biotin-labelled rDNA has made it possible to localize rRNA genes not only at the secondary constriction in both homologous chromosomes No. 3 of Crepis capillaris but also in the B chromosomes occurring in the plants employed. Very clear dot-like rDNA signals at the telomeres of both arms were observed in all B chromosomes. Histochemical silver staining, which is indicative of transcriptional activity of rRNA gene clusters, resulted in both darkly-staining nucleolar constrictions of chromosomes No. 3 and silver deposits at the telomeres of Bs. We conclude that the B chromosomes of C. capillaris are isochromosomes with active rRNA genes located near both telomeres.

Elimination of Solanum phureja nucleolar chromosomes in S. tuberosum + S. phureja somatic hybrids

The karyotype of the dihaploid SVP1 line of S. tuberosum (2n=2x=24) showed two nucleolar chromosomes with differently sized satellites. The diploid SVP5 line (2n=2x=24) and tetraploid regenerants of S. phureja had larger but similar satellites. Somatic hybrids between the diploid lines of these potato species with genome combinations 4 tub + 2 ph (plants 1-3), 2 tub + 4 ph (plants 4-7) and 4 tub + 4 ph (plant 8) had lost 2 phureja nucleolar chromosomes if 4 phureja genomes were present. One phureja nucleolar chromosome of plants 1-3 and both of plants 5 and 7 had rearranged satellites. Elimination of the two nucleolar chromosomes occurred preferentially, was under genetic control, and probably took place during early callus development. NOR activity resulting in rear-rangements between NORs may have caused the elimination.

Quantitative analysis of diploid translocation heterozygotes: test of models and equations

Equations have been derived for two different models of chromosome pairing and chiasmata distribution. The first model represents the normal condition and assumes complete synapsis of homologous bivalents and the arms of interchange quadrivalents. This is followed by a nonrandom distribution of chiasmata among bivalents and multivalents such that each bivalent or bivalent-equivalent always has at least one chiasma. Univalents occur only as part of a III, I configuration at diakinesis or metaphase I. The second model assumes that a hologenomic mutation is present in which all chromosomes of a genome are equally affected. Two different assumptions can be made for such a mutation, and both give the same results: (1) homologous or homoeologous chromosome arms may be randomly paired or unpaired, but synapsis always leads to a crossover; (2) homologous or homoeologous arms always pair, but chiasmata are randomly distributed among the arms. The meiotic configurations at diakinesis or metaphase I are the same for both assumptions. Meiotic configurations of normal diploid interchange heterozygotes show good agreement with numbers predicted by the equations for nonrandom chiasmata distribution among configurations. Inter-specific hybrids with supernumerary chromosomes produced meiotic configurations frequencies in agreement with predictions of equations for random chiasmata distribution, but a hybrid without supernumeraries fitted the nonrandom expectations.

Forty four probands with an additional "marker" chromosome

Information is presented which has been obtained from an exhaustive examination of 44 probands with a supernumerary marker chromosome (mar) and their families. The data include the derivation of the mar, frequency in various populations, inheritance and possible effect on fertility, congenital abnormality, and mental ability. The practical problems in assessing the risk of abnormality in a foetus discovered during prenatal diagnosis to be carrying a mar, are discussed.

CYTOGENETIC VARIATION IN THE BRITISH FLORA: ORIGINS AND SIGNIFICANCE

Cytogenetic variation in the British flora is discussed in relation to the taxonomy and evolution of species and to their distribution and past history. In polyploids there may be two or more cytotypes in Britain with different distributions and ecology, e.g. Empetrum nigrum and Hippocrepis comosa; alternatively only one cytotype is present in Britain, but it forms part of a wider polyploid pattern, the study of which may clarify past patterns of migration, e.g. Potentilla fruticosa and Sisyrinchium bermudiana. The study of polyploids may also have taxonomic significance, e.g. Cochlearia, Symphytum and Mentha. In some situations gene flow between polyploid levels takes place and may be in both directions. In other polyploid groups sexually sterile cytotypes may be maintained by vegetative reproduction, e.g. Ranunculus ficaria and Holcus mollis or by gametophytic apomixis. In some cases a polyploid and aneuploid series may be maintained in which all the components are fertile; this situation is discussed in relation to the genus Erophila. There has been considerable emphasis in the past in attempting to identify the genomic ancestors of allopolyploids; two British species, Spartina anglica and Senecio cambrensis are among the few examples of recent allopolyploids of known origin; in both cases formation has depended on the establishment of an introduced species as one parent. The significance of B chromosomes is discussed; although a range of British species contain B chromosomes it has only been possible to investigate their adaptive significance in one case. Interchanges exist in British species either in the heterozygous form, e.g. in Alopecurus species, where they can be identified by their meiotic consequences, or in the homozygous form, e.g. Epilobium sect. Epilobium species, where they can be identified by meiotic analysis of hybrids. The use of modern cytogenetic techniques including chromosome banding and in-situ hybridization and the investigation of characteristics such as infra-species variation in DNA amounts are discussed in relation to future cytogenetic investigations of the British flora.

Gene amplification in a mouse embryo? Double minutes in cell lines independently derived from a Mus musculus X M. caroli fetus

Double minutes (DM) were found to be present in six of seven clones derived from a 16-day female Mus musculus X M. caroli fetus. The DM-positive clones derived from three primary populations independently set up from the fetus, and included clones with an active M. caroli X chromosome as well as clones with an active M. musculus X. The simplest explanation of these findings is that DM were already present in cells of the M. musculus X M. caroli embryo at the time of X chromosome inactivation and persisted during in vivo development and in vitro culture. This suggests that gene amplification occurred in the early embryo, or even the fertilized egg, perhaps because of interactions between components of germ cells contributed by the M. musculus and M. caroli parents. Alternatively, induction may have occurred independently in these lines, requiring that amplification is an unusually common occurrence in cells from interspecific hybrids.

Maintenance of the 2 microns circle plasmid in populations of Saccharomyces cerevisiae

The 2 microns circle plasmid is maintained at high frequencies in populations of yeast cells. To find out how the plasmid is maintained, three forces were measured: the selective advantage or disadvantage conferred by 2 microns circles, the rate of generation of [Cir0] cells, and the rate of illegitimate transfer of 2 microns circles from cell to cell. It was found that under the conditions used, 2 microns circles confer a selective disadvantage of about 1%, that [Cir0] cells are generated at the rate of 7.6 x 10(-5) per [Cir+] cell per generation, and that illegitimate transfer of 2 microns circles occurs at a rate less than 10(-7) per recipient cell per generation. The most likely explanation of 2 microns circle maintenance is that the plasmid is sexually transmitted at such a rate that it spreads through populations despite selection against it.

TRANSMISSION AND SIGNIFICANCE OF B CHROMOSOMES IN ANTHURIUM WAROCQUEANUM

Somatic and meiotic chromosomes of one plant of Anthurium warocqueanum J. Moore and its selfed offspring were analyzed. The parent showed 2n = 30 + 3B in both somatic cells and pollen mother cells. The B chromosomes divided normally in somatic cells, but meiotic associations of Bs varied. Three configurations of three B chromosomes were observed at metaphase I of parent meiosis: one trivalent, one bivalent and one univalent, or three univalents. The number of B chromosomes in offspring ranged from 0 to 6, indicating their transmission from both male and female gametes. Offspring with two B chromosomes appeared in greatest frequency. It was hypothesized that both male and female gametes of the 3 B parent frequently contained one B chromosome through the normal distribution of the bivalent Bs at meiosis and the elimination of the univalent B chromosome due to lagging. Examination of pollen mother cells of offspring also revealed irregular behavior of B chromosomes. With a high number of B chromosomes, normal A chromosome bivalent formation seemed to be reduced. No phenotypic effects of B chromosomes were observed.