A repetitive DNA sequence common to the different B chromosomes of the genus Brachycome

Dot-like micro B chromosomes of Brachycome dichromosomatica were analysed for their sequence composition. Southern hybridization patterns of a total micro B probe to genomic DNA from plants with and without micro Bs demonstrated that the micro Bs shared sequences with the A chromosomes. In addition to telomere, rDNA and common A and B chromosome sequences, a new B-specific, highly methylated tandem repeat (Bdm29) was detected. After in situ hybridization with Bdm29 the entire micro B chromosome was labelled and clustering of the condensed micro Bs could be observed at interphase. A high number of Bdm29-like sequences were also found in the larger B chromosomes of B. dichromosomatica and in other Bs within the genus Brachycome. Received: 30 May 1997; in revised form: 20 August 1997 / Accepted: 20 August 1997     

Supernumerary chromosomes in the second outbreeding species of the wheat group

Meiosis was examined in PMC of 138 plants of the outbreeder Triticum speltoides (=Aegilops speltoides) and 175 plants of the closely related T. longissimum (= Ae. longissima), which is a selfer. Nine of the T. speltoides plants, originating fra two of the five Israeli populations under study, contained B chromosomes in addition to the normal complements (2n = 14). No supernumerary chromosomes were found in T. longissimum. — Plants were found carrying 1, 2 or 3 B chromosomes, which seem to be stable among PMC. Their size is 2/3 of the average length of the regular chromosomes, but otherwise they are similar in appearance. Pairing between Bs is common as bivalents and trivalents, but whether this association is ehiasmatic is not clear. There is a slight reduction in chiasma frequency of the regular complement in B-carriers compared to other plants from the same population. — The only other species in the wheat group reported to have B chromosomes in natural populations is also an outbreeder, namely T. tripsacoides (=Ae. mutica). The connection between outbreeding and B chromosomes might be significant.     

Organisation and origin of a B chromosome centromeric sequence from Brachycome dichromosomatica

Brachycome dichromosomatica is an Australian native daisy that has two pairs of A chromosomes and up to three B chromosomes in some populations. A putative B-specific tandem repeat DNA sequence (Bd49) was isolated previously. Here we describe further characterisation of this sequence and investigate its possible origin. Southern analysis showed that all individual B chromosomes examined have highly methylated tandem repeats of Bd49 but differences in banding pattern for distinct B isolates suggested that the sequence is in a state of flux. Using in situ hybridisation, the sequence was shown to be located at the centromeric region of the B chromosome. Southern analysis of genomic DNA with Bd49 demonstrated that multiple copies of the sequence exist in the genomes of B. eriogona, B. ciliaris, B. segmentosa and B. multifida (none of which have B chromosomes) whereas other species tested (including 0B plants of B. dichromosomatica and 0B B. curvicarpa and B. dentata) have few or no copies. Genomic clones and Bd49-like sequences derived by the polymerase chain reaction (PCR) were obtained from five species but determination of phylogenetic relationships within the genus and inference as to the possible origin of the B chromosome were problematic because of extensive intragenomic heterogeneity of the sequences.     

A monophyletic origin of the B chromosomes ofBrachycome dichromosomatica (Asteraceae)

The A and B chromosomes of different karyotype variants (cytodemes A1, A2, A3 and A4) ofBrachycome dichromosomatica were analysed by computer-aided chromosome image analysis and fluorescencein situ hybridisation (FISH). Ribosomal DNA and the B chromosome-specific sequence Bd49 were detected on all B chromosomes. In addition to minor size variation of the Bs, polymorphism of the rDNA and Bd49 position and copy number revealed two major types of B chromosomes. The B chromosomes of all the cytodemes were indistinguishable from each other in length, but that of A3 showed evidence of rearrangements consistent with its long-term geographic isolation. The results presented suggest a monophyletic origin of the B chromosomes ofB. dichromosomatica.     

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.     

Interaction between rye B-chromosomes and wheat genetic systems controlling homoeologous pairing

The interactive effect on homoeologous pairing of rye B-chromosomes with the absence of both pairing suppressor (3A, 3D, 5B) and promotor (3B, 5A, 5D) chromosomes of common wheat (Triticum aestivum L.) is analyzed by comparison of pairing at Metaphase I of 27-, 27+2B, 28- and 28+2B-chromosome plants. These plants were obtained from crosses between the respective wheat monosomics (2n=41) and rye plants (Secale cereale L.) carrying or not carrying two B-chromosomes (2n=14 or 14+2Bs). —The effect of rye B-chromosomes on pairing depends on the function of the wheat chromosome which is absent in the appropriate hybrids, i.e., rye B-chromosomes have a suppressor effect on pairing when the pairing suppressing wheat chromosomes 3A, 3D or 5B are absent, while they behave as promotors when the pairing promoting chromosomes 3B, 5A or 5D are absent.     

B-chromosome condensation inPhleum pollen grains

Pollen grains were investigated in 11 specimens ofPhleum Boehmeri Wib. [P. phleoides (L.) Karst.] differing in the number of B-chromosomes (2n=14+0–3B). In some uninucleate pollen grains there were seen large chromocentres, not observed in other tissues. They resembled meiotically and mitotically condensed Bs both in size and shape. They occurred only in plants with 2 or 3 Bs. In the plants with 2 Bs, 2.5% pollen grains had a nucleus with 1 chromocentre; in the plants with 3 Bs, 41.4% of uninucleate pollen grains had 1 chromocentre and 0.2% of grains had 2 chromocentres. Comparison of the frequency of Bs in the metaphase of the first pollen mitosis and that of chromocentres in the nuclei of the uninucleate pollen grains was carried out. The author suggests that in pollen-grain nuclei one B chromosome per genome remains decondensed, and every next one (if it occurs) forms a chromocentre.     

The effect of B chromosomes on homoeologous pairing in species hybrids

Six diploid populations of Lolium rigidum were used in crosses with a standard homozygous line of Lolium temulentum in order to analyse the variation in the effect of B chromosomes on homoeologous pairing in the interspecific hybrid. — Large differences in chiasma frequency were found, both between the progeny of the different populations and, in one case, between the progeny of different plants from the same population. That this variation was due to differences in B chromosome genotype was ruled out by the use of appropriate 1B Lolium rigidum parents. It is concluded therefore that the effect of B's on homoeologous pairing in this interspecific hybrid can be modified quite substantially by the A chromosome genotype of the parents.     

Rye B chromosome behaviour at first and second pollen mitosis and its relationship with anther maturity

Summary Rye plants carrying B chromosomes with different nuclear and cytoplasmic constitutions have been analyzed at first pollen mitosis. No differences of B chromosome behaviour have been detected. It has been concluded that non-disjunction and preferential distribution are processes controlled by Bs themselves.At second pollen anaphase, B laggards have been observed. Both non-disjunction and B laggards occurred with higher frequency in younger anthers.     

Phyletic and evolutionary relationships ofBrachyscome lineariloba (Compositae)

A comparison of karyotypes ofBrachyscome breviscapis (2n = 8),B. lineariloba cytodemes E (2n = 10), B (2n = 12) and C (2n = 16) suggests that these species have a homoelogous basic set of four chromosome pairs, two large pairs and two small, and that theB. lineariloba cytodemes E, B and C are related toB. breviscapis by successive additions of small chromosomes. A pronounced asynchrony of chromosome condensation between these large and small chromosomes has been observed. In the artificial hybrids betweenB. dichromosomatica (2n = 4) ×B. breviscapis, and theB. lineariloba cytodemes, theB. dichromosomatica chromosomes are similar in size and condensation behaviour to the small chromosomes ofB. breviscapis and ofB. lineariloba cytodemes E, B and C. Meiotic pairing in these hybrids also demonstrates the strong affinities between these chromosomes. It is suggested thatB. breviscapis may be of amphidiploid origin between a species with two large early condensing chromosome pairs and another,B. dichromosomatica-like species with two small late condensing pairs. It seems most likely that the additional small and late condensing chromosomes inB. lineariloba cytodemes E, B and C are derived from theB. dichromosomatica-like parent, and that each addition increases vigour, fecundity and drought tolerance, allowing these cytodemes to colonize more open and arid environments. Transmission of the univalents in the quasidiploidB. lineariloba cytodeme E was verified as being via the pollen, and not via the embryo sacs.The cytology ofBrachyscome lineariloba (Compositae, Asteroidae), 10.     

GEOGRAPHICAL DISTRIBUTION OF B CHROMOSOMES OF XANTHISMA TEXANUM (COMPOSITAE: ASTEREAE). II. LOCAL VARIATION WITHIN AND BETWEEN POPULATIONS AND FREQUENCY VARIATION THROUGH TIME

Variation in the frequency of individuals of Xanthisma texanum with B chromosomes among populations at the local level and variation in the frequency and distribution through time of individuals with B chromosomes within populations was investigated in a small area of southwestern Oklahoma and adjacent Texas between 1974 and 1987. The chromosome numbers of 1,224 individuals were determined from meiotic material; 15.8% of these had B chromosomes, less than 1% had three or four B chromosomes; none had more than four B chromosomes. The frequency of plants with B chromosomes among all sites sampled ranged from 0 to 33% and from 0 to 25% in sites separated by only a hundred meters; large differences in sample size occurred. Even at the subsite level large differences in frequencies in plants with B chromosomes were found. Within single sites, B chromosome frequency was not constant through time: the pattern of change differed among subsites within populations. In some cases the frequency went to zero. No correlations between environmental parameters and B chromosome frequency were documented.     

Unstable B chromosomes in Silene maritima With. (Caryophyllaceae)

Unstable B chromosomes in Silene maritima With. (Carophyllaceae). B chromosomes have been found in 7 out of 39 populations of Silene maritima With. (Carophyllaceae) studied. These 7 populations are all from the Norfolk coast. The B chromosome is small, 0.8 μm long, telocentric and appears euchromatic. Within plants the B chromosome is highly unstable with different numbers in cells of the same root or pollen mother cells (pmcs) of an anther. The transmission of the Bs is efficient and in controlled crosses progeny plants usually have higher mean B frequencies than the B-containing parents. At metaphase I the pairing behaviour of the Bs is variable and they often appear as univalents. In general the presence of Bs in pmcs results in an increase in chiasma frequency/cell and populations with Bs have higher mean chiasma frequencies than those without Bs. Cloned plants grown under stress conditions induced by the addition of NaCl to a nutrient solution showed no difference in mean B number/cell or distribution when compared with controls grown in nutrient solution only.     

A fascinating island: 2n = 4

Abstract

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.     

B Chromosome Polymorphism in the Fish, Astyanax scabripinnis

Four populations of Astyanax scabripinnis (Pisces, Characidae) were analyzed for B chromosome features. This species contains a metacentric macrochromosome (B_M), similar in size to the first chromosome of the karyotype, besides two variant forms, a large submetacentric (B_SM) and a small metacentric (B_m), both showing a reduced frequency. These variant forms were observed only in the populations from the Campos do Jordão region (São Paulo State, Brazil), although not present in all the populations sampled. The three B chromosome forms are heterochromatic and at least the B_M and B_SM are also GC-rich, as evidenced by C-banding and chromomycin A₃ staining. In spite of the morphological similarity between the B_M form and the first chromosome pair, they differ in the G-banding pattern, which does not favor a possible relationship among these chromosomes. FISH with a telomeric DNA probe evidenced signals on the terminal region of all chromosomes, including the Bs. Interstitial telomeric bands, indicative of some chromosomal rearrangements such as translocation or tandem fusion in the origin of the B chromosomes, were not observed. B_SM and B_m are probable derivative B chromosome forms from an ancestral B_M chromosome, showing a restricted occurrence and low frequency in the populations.     

Cloning and characterisation of polymorphic heterochromatic segments of Brachycome dichromosomatica

After selective enrichment and differential hybridisation of C_ot-1 DNA fractions of plants with and without polymorphic heterochromatic segments, a repetitive sequence (called Bds1) specific to the polymorphic chromosome segments of Brachycome dichromosomatica (Brachyscome dichromosomatica) was isolated. A single repeat unit of Bds1 is 92 bp long and is organised in tandem arrays at three different polymorphic segment sites on the chromosomes of cytodeme A2. Although all three sites showed extensive polymorphism between plants, the karyotypes of all analysed mitotic root cells were stable within a single plant. Electron microscopy revealed heavily condensed chromatin structures at the most obvious polymorphic site. The mechanisms that generate and maintain the observed chromosome structure polymorphisms are discussed. Received: 16 March 1999; in revised form: 28 September 1999 / Accepted: 11 November 1999     

Internal transcribed spacer sequence analyses indicate cytoevolutionary patterns within Brachycome Cass. (Asteraceae)

The sequences of the Internal Transcribed Spacer regions (ITS1 and ITS2) within the genes coding for cytoplasmic ribosomal (r) RNAs on the A chromosome complement of 34 members of the higher plant genus Brachycome (synonym Brachyscome) have been compared. The ITS1 sequence of species within the B. lineariloba complex contains a 56 bp tract that is absent from at least 12 Brachycome species but is present in other species within Brachycome as well as other Asteraceae. Phylogenetic data support the suggestion that the number of chromosomes reduced in several independent Brachycome lineages during speciation. Comparisons with the B chromosome ITS2 of B. dichromosomatica cytodeme A1 suggests an origin of the B chromosome at a time prior to the divergence of the four cytodemes of B. dichromosomatica.     

Comportement des chromosomes surnuméraires et relation avec le taux de mortalité dans une population africaine de Locusta migratoria migratorioides

Transmission of supernumerary chromosomes is studied in adults and embryos of an African population of Locusta migratoria migratorioides; 34% of the animals show one or two B chromosomes (24% one, 10% two). This percentage is the same in both sexes. During mitosis, B chromosomes are very stable. At meiosis, in some cases they show pairing, in other cases they enter as univalents. The eventuality of synapsis or association is discussed. These two kinds of behaviour could not be explained by the presence of two different B chromosomes as is shown from the study of parthenogenetic progeny. Repartition of B chromosomes in the progeny is different depending on whether the male or the female parent supplies them; so the behaviour of these supernumerary should be conditioned, not by their own structure, but by a connection with the cell. — One B chromosome apparently is neutral. When there are two B chromosomes, in case of synapsis, the number of animals with two B in the progeny is slightly lower than previous; in case of asynapsis, there is a higher lethality of individuals and oocytes with two B chromosomes.     

Genetic differentiation in populations of the yellow-necked mouse,Apodemus flavicollis,harbouring B chromosomes in different frequencies

Two alternative models are used to explain maintenance of polymorphism of B chromosomes (Bs) in populations of a great number of species. The parasitic model suggests deleterious effects of Bs on fitness of carriers, while the heterotic model assumes that, in the absence of drive, equilibrium is produced by beneficial effects of Bs at low numbers. In order to determine the potential contribution of Bs to genetic differentiation and diversity, four populations of Apodemus flavicollis, differing in frequency of Bs (from 0.23 to 0.38) and settled in ecologically different habitats, were analyzed by 471 AFLP markers. Although numerous loci were demonstrated to be population specific, none of them was associated with individuals with Bs. AMOVA showed that the presence of Bs does not affect population differentiation, pointing to greater genetic similarity of Bs to A chromosomes. The greatest genetic diversity (0.241) was found in the population settled in optimal conditions for this species featured by the lowest frequency of animals with Bs (0.23). We found that the majority of loci marked as loci under directional selection, are characteristic of either a population with lower or one with a higher frequency of Bs. Several loci detected as outliers were associated with environmental variables that could directly and/or indirectly influence population dynamics of A. flavicollis. Thus, we suggest that the different frequency of Bs carriers in populations is related to adaptive differentiation to diverse habitats, which is in accordance with the heterotic model of Bs maintenance.     

B chromosome polymorphism in populations of Apodemus flavicollis in Yugoslavia

Four populations of yellow necked mice Apodemus flavicollis were karyologically studied for the presence of B chromosomes. One or two B chromosomes were found in all populations except one, in frequencies which differ significantly. The Bs cannot be accurately distinguished from chromosomes of normal complement even after application of differential staining.     

Induced translocation of an A and a B Chromosome in Lolium perenne

A reciprocal translocation between an A and a B chromosome was identified in the progeny of X-irradiated heads of Lolium perenne. The resulting centric B^A fragment was found to have lost the capacity to undergo non-disjunction at pollen grain mitosis and consequently was transmitted in the normal mendelian manner. The synthesis of a stable population of 16 chromosome L. perenne plants is therefore a possibility. —Analysis of the effect of both the translocated B segment and the centric B fragment on meiosis in diploid L. temulentum x L. perenne hybrids showed that both parts are capable, to a certain extent at least, of suppressing homoeologous chromosome association at first metaphase.