The B-chromosome system of Allium schoenoprasum

Nine morphologically distinct euchromatic B-chromosomes have been identified in Allium schoenoprasum from the River Wye, South Wales. The most common type (89%) is telocentric (B^t–1) and it is likely that the non-standard Bs are derivatives of B^t–1 by deletion, centric shift and/or centric misdivision. New B-types have also been produced from standard Bs in controlled crosses. In general, the Bs are mitotically extremely stable, although occasional plants, particularly those carrying non-standard Bs, are conspicuously variable in their B-constitution between root-tip cells. In addition, B-chromosome number is enhanced in some anthers of about one third of plants. Behaviour of B-chromosomes during meiosis is described. Although there is little bivalent formation, less than 5% of the Bs are lost during meiosis in anthers. There is, however, no evidence of B-chromosome accumulation in the offspring of controlled crosses, usually a slight loss, and Bs have deleterious effects on aspects of vigour and fertility. Thus, no satisfactory explanation for populations with up to 65% B-containing individuals has yet been found.     

The B-chromosome system of Hypochoeris maculata

Populations of Hypochoeris maculata (Compositae) over a wide area of Europe contain a small proportion of individuals with mitoticallystable B-chromosomes. The telocentric Bs are about one sixth the length of the smallest A-chromosome and are heterochromatic. Offspring grown from seeds of natural populations have one or at most two B-chromosomes although plants with up to five Bs have been produced in experiment. The Bs rarely pair during meiotic prophase and the univalents usually undergo centric division at anaphase I. Despite lagging, the Bs are rarely lost during male meiosis. The inheritance of the B-chromosomes differs between the two sides with accumulation in the egg and random inheritance or loss on the pollen side. Although meiotic drive may play some part in the maintenance of the B-polymorphism there is a strict upper limit on the number of Bs in natural populations.     

Unstable B-chromosomes in a European population of Allium schoenoprasutn L. (Liliaceae)

In Allium schoenoprasum , small, dichromatic B-chromosomes occur in 74% of plants at Lac de Tracens, Pare National des Pyrénées. B-chromosomes from Lac de Tracens were compared with the previously well characterized Bs from British populations of A. schoenoprasum at the River Wye, Powys, and were found to differ in terms of both their morphology and the level of mitotic instability. At Lac de Tracens, two morphological types of B-chromosome were observed: a telocentric of about 1.15 μm in length and a metacentric of about 1.35 μm in length; the most frequent B-type in the River Wye populations (B^-1) was not found. B-chromosomes from the River Wye are essentially stable at root tip mitosis, whereas 25 out of 29 plants from Lac de Tracens (86%) showed between-cell variation in B-number, with non-modal numbers in 330 out of 794 cells (41.6%). Most of the variability occurred within individual roots, rather than between roots of single bulbs or of sister bulbs. No meiotic pairing was observed between the different Bs in hybrids from the geographically isolated populations, suggesting a lack of homology. An independent origin of B-chromosomes in the British and French populations is suggested.     

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.     

Origin and molecular organization of supernumerary chromosomes of Prochilodus lineatus (Characiformes, Prochilodontidae) obtained by DNA probes

In Prochilodus lineatus B-chromosomes are visualized as reduced size extra elements identified as microchromosomes and are variable in morphology and number. We describe the specific total probe (B-chromosome probe) in P. lineatus obtained by chromosome microdissection and a whole genomic probe (genomic probe) from an individual without B-chromosome. The specific B-chromosome was scraped and processed to obtain DNA with amplification by DOP-PCR, and so did the genomic probe DNA. Fluorescence in situ hybridization using the B-chromosome probe labeled with dUTP-Tetramethyl-rhodamine and the genomic probe labeled with digoxigenin-FITC permitted to establish that in this species supernumerary chromosomes with varying number and morphology had different structure of chromatin when compared to that of the regular chromosomes or A complement, since only these extra elements were labeled in the metaphases. The present findings suggest that modifications in the chromatin structure of B-chromosomes to differentiate them from the A chromosomes could occur along their dispersion in the individuals of the population.     

The heterogeneity of B-chromosome DNA: no evidence for a B-chromosome specific repetitive DNA correlated with B-chromosome effects on meiotic pairing in the triticinae

The compositional heterogeneity of DNAs of A (normal) and B (supernumerary) chromosomes of Aegilops speltoides, Ae. mutica and Triticum aestivum has been compared in order to elucidate the mechanism of B-chromosome disruption of meiotic pairing in interspecific hybrids. Comparisons of % heterologous association after DNA/DNA hybridation at C₀t 10^?2 (highly repetitious DNA) and C₀t 100 (moderately repetitious DNA), and comparisons of nucleotide base divergence (ΔT_ms) and thermal elution profiles of homologous and heterologous duplexes, show that genotypes of Aegilops spp., having large numbers of Bs, do not carry additional families of repetitious DNA exclusive to B-chromosomes. Neither the presence of Bs nor the direction of DNA/DNA hybridisation affect the above parameters. No cryptic DNA satellites were revealed in A- and B-chromosome DNA after sedimentation in actinomycin D-CsCl gradients; and there were no significant differences in buoyant densities of main-band DNA. Mean melting temperatures (T_m); transition temperatures (ΔT) and numbers and positions of peaks of dissociating DNA fractions in profiles of differentiated melting curves of native DNAs were similar in strictly comparable denaturation conditions. One small AT-rich (< 5%) DNA fraction correlated with speltoides Bs was revealed; however, no corresponding fraction is associated with mutica Bs. The overall similarity in numbers and base composition of families of DNA (repetitious and unique) of As and Bs is discussed in relation to the origin of Bs and the origin of the meiotic diploidising system in haploid T. aestivum.     

Instability of B-chromosomes in somatic and germline cells of Apodemus peninsulae

Instability of B-chromosomes was estimated in somatic and germline cells of samples Apodemus peninsulae from different localities of the species range. In 84 out of 188 animals (45%), in cells assessed for B-chromosome mosaicism, bone marrow cells with different B-chromosome number were observed. The numbers of B-chromosomes in spermatocytes at the pachytene stage were estimated in ten males. It was shown that the average number of B-chromosomes and the number of cell clones in germline cells was higher than the corresponding numbers in bone marrow cells. The higher number of B-chromosomes and their higher variability in germline cells than in somatic cells suggest the existence of a mechanism of premeiotic accumulation of B-chromosomes in spermatogenesis of A. peninsulae     

Geographical distribution of cytotypes and genomic structures in natural populations of the<Emphasis Type="Italic">Scilla scilloides</Emphasis> complex in Korea

Cytotype distribution, B-chromosome frequency, and genomic constitution in the natural populations of theScilla scilloides complex in Korea were analyzed. Plants with various cytotypes were found: AA (2n = 16), BB (18), AAB (25), ABB (26), ABB (34), ABBB (35), BBBB (36), AABBB (43), AAABBB (51) and AAAABBBB (68). Allotetraploid AABB plants predominated with a frequency of 68.3%, and were found to distribute all over the Korean peninsula and Cheju-do. In diploids, the type AA plants distributed throughout the Korean midwest, while the type BB plants were limited to Cheju-do. Two other cytotypes, ABBB and AABBB, were found only in the southern part of the Korean peninsula including Cheju-do. Chromosomal variations, aneuploidy, and centromeric shifts were also found in the natural populations. The cytotypes AAB and AAAABBBB are reported here for the first time. B-chromosomes were found in 149 (85.6%) of 174 populations, the highest frequency being 81.8% in BB populations. The number of B-chromosomes per plant ranged from 1 to 31, and IB plants predominated (21.0%). Subtypes, with respect to the number and composition of B-chromosomes, indicated that sexual reproduction is still prevalent in AABB populations.     

Buffering effect of B-chromosome system of Trigonella foenum-graecum against different soil types

Summary Accessory (B–) chromosomes have been reported in many plants in various regions of the world. Studies on natural populations of certain plants like Phleum phleoides revealed that B-chromosomes were present in different frequencies in different habitats indicating a correlation between B-frequency and environmental conditions. The present experiment was performed to determine the response of B-chromosome carrier and non-carrier plants of Trigonella to different soil conditions. When the morphological data was subjected to analysis of variance, it was found that the carrier plants exhibited uniformity of characters in the different soil types, while the non-carrier plants exhibited considerable variability. The conclusion that can be drawn from this experiment is that B-chromosomes act as a buffering agent, neutralising the variability effect of different soil types.     

B-Chromosomes and E-1 isozyme activity in mosaic bulbs of Scilla autumnalis (Liliaceae)

The electrophoretic patterns of esterase (E-1), alcohol dehydrogenase (ADH), and glutamate oxaloacetate transaminase (GOT) isozymes were studied in two Spanish populations of the lily Scilla autumnalis with B-chromosome carrying individuals. The E-1 isozyme activity appears only in those individuals with B-chromosomes. None of the bulbs free of B's show it. Five bulbs, mosaic for B-content, were identified. Electrophoretic analysis shows that these bulbs are characterised by mosaicism for E-1 isozyme activity. An analysis of individual roots by both electrophoretic and cytological methods shows that tissue mosaicism for B-content correlates with tissue mosaicism for E-1 isozyme activity. The electrophoretic analysis of different roots from bulbs heterozygous for the Est-1 locus indicates that the structural gene for E-1 is not located on the B-chromosome itself. Rather there is a derepressor effect of Bs on E-1 isozyme activity. Since ADH and GOT patterns are unaffected by the presence of B-chromosomes it is clear that they do not exhibit a generalised derepressor effect.     

The B-chromosome system of myrmeleotettix maculatus (thunb.)

It is established that in populations of Myrmeleotettix maculatus with B-chromosomes these chromosomes occur at stable frequencies and are present to the same extent in both the males and the females of the same population. It is also established that the B-chromosome content of a population is positively correlated with its chiasma frequency and that, within a population, individuals with single B-chromosomes tend to have higher chiasma frequencies than individuals lacking B-chromosomes. Since this effect is not increased by the addition of further supernumeraries it is argued that selection operates in favour of individuals with single B-chromosomes. Finally it is shown that the level of B-chromosomes in a population is related to temperature and especially to rainfall so that B-chromosomes are absent from populations in climatically stringent environments.     

THE GEOGRAPHICAL DISTRIBUTION OF B-CHROMOSOMES OF XANTHISMA TEXANUM DC. (ASTERACEAE). I. SURVEY OF THE RANGE

The distribution of B-chromosomes throughout the range of Xanthisma texanum has been determined from approximately 750 individuals from 80 wild populations in Texas, Oklahoma, and New Mexico. Counts of meiotic chromosome figures of n = 4_II + 1 B_I and n = 5_II (4_II + IB_II) are reported for the first time for subsp. texanum var. texanum and for subsp. texanum var. orientalis. The wide occurrence of B-chromosomes in natural populations of subsp. drummondii is also documented. B-chromosomes are present in 22 (27.2%) randomly distributed populations whose characteristics are in no way distinct from populations without B-chromosomes. In populations with B-chromosomes present, 19.8% of the plants contained one or two B-chromosomes, but in total only 5.8 % of all plants contained one or two B-chromosomes. All populations in one isolated population group of subsp. drummondii lack B-chromosomes.     

Population cytology of the genus Phaulacridium

Samples were taken from twenty three Australian mainland populations of the shorthorned grasshopper, Phaulacridium vittatum. Five of these populations were found to be polymorphic for B-chromosomes. The anaphase I segregation of the B-chromosome appeared to be random with respect to the X (contra Jackson and Cheung, 1967). The B-chromosomes did not appear to influence chiasma frequency in the population in which they were found.     

Interpopulation variation in the satellite DNA from grasshoppers with B-chromosomes

When run on a CsCl gradient the DNA from individuals containing B-chromosomes reveals a satellite peak in addition to the main DNA peak found in individuals without B-chromosomes. This was shown in several populations of grasshoppers. This B-chromosome DNA contains 28% repeated and 72% unique sequences as determined by hydroxyapatite chromatography. This was shown to be the case in two of the populations. The really surprising observation was that the repeated nucleotide sequences of the B-chromosome DNA have no apparent homology in this single species of grasshopper. This was demonstrated by the lack of hybridisation between labelled C-RNA transcribed from one B-chromosome DNA and the DNA from the B-chromosome peak from another population. This lack of homology was also suggested by density differences between B-chromosome satellites in CsCl gradients. Furthermore, there was no sequence homology between the satellite (B-chromosome) DNA and the main peak (nuclear) DNA.     

THE CYTOGENETICS OF XANTHISMA TEXANUM DC. (ASTERACEAE) AND ITS B-CHROMOSOMES

The karyotype of A-set and B-chromosomes of Xanthisma texanum DC. are described in detail and measurements are given. The distribution of B-chromosomes in stem tissue is described and the loss of B-chromosomes from primary roots confirmed. The number of B-chromosomes in pollen mother cells was constant within single inflorescence buds and in all buds throughout the flowering period. The behavior of one and two B-chromosomes during meiosis is described in detail. During meiosis, B₁'s divided in 71% of the cases during anaphase II and in 29% of the cases during anaphase I. First anaphase division resulted in some lagging and elimination of B-chromatids during second division. During anaphase I, B₁₁'s divided into chromosomes and in anaphase II these divided into chromatids in 95% of the cases. In the other 5 % non-disjunction of the B-chromosomes or chromatids and A-set chromosome abnormalities occurred. Post meiotic preferential distribution during pollen mitosis resulting in a doubling of the number of B-chromosomes passed to progeny via pollen occurred, but no doubling occurred in the female line. Crosses involving plants with two B-chromosomes each yielded fewer plants with three B-chromosomes than expected. Polyploidy is extremely rare in the species. The evolutionary significance of B-chromosome behavior is discussed in relation to its origin and survival.     

CYTOLOGY OF SOME WESTERN CANADIAN ALLIUM SPECIES

Cytology of four species of Allium from western Canada was studied. Three species viz: A. acuminatum 2n = 14, A. cernuum 2n = 14, and A. schoenoprasum 2n = 16 were found to be diploids with normal meiosis. Some populations of A. cernuum and A. schoenoprasum contained a high number of B-chromosomes. Diploid (2n = 14) and tetraploid (2n = 28) populations were encountered in A. textile. While the diploid meiosis was regular, the tetraploids showed 1–5 quadrivalents at metaphase I, suggesting their possible autoploid origin.     

Trisomics inArabidopsis thaliana and the location of linkage groups

Different populations of the grasshopper Arcyptera fusca located through a small valley of the Pyrenees present an unstable B-chromosome system. Frequencies of individuals carrying Bs ranged from 11% to 50%. In the testes of these males the number of Bs varied among the different follicles ranging from 0 to 4 with 2 being the number most commonly found. The variation in the number of supernumeraries probably resulted from their preferential non-disjunction in the carly mitosis prior to the differentiation of the follicles. The meiotic behaviour of Bs depends on their number within cach follicle. When two or more Bs are present they usually pair and segregate regularly; B univalents divide in anaphase I and segregate without further division in anaphase II in 75% of the cells observed. The presence of Bs does not affect the chiasma frequency, however, the males with Bs had fewer follicles in their testes; this event could be related with the non-existence of follicles with more than 4 Bs.     

B chromosome polymorphism of blackflies (Diptera, Simuliidae) from the north-western region of Russia

We have studied karyofonds of natural populations and B-chromosome morphology of 8 species of blackflies from the North-Western region of Russia: Odagmia ornata Mg., Hellichiella crassa Rubz., Simulium morsitans Edw., Simulium argyreatum Mg., Shoenbaueria pusilla Fries., Cnetha fontinalis Radzv., Stegopterna duo-decimata Rubz., and Archesimulium tuberosum Lundstr. For this purpose we made slides of squashed blackflies larvae with salivary gland polytene chromosomes stained by aceto-orcein, in addition to similarly stained slides with mitotic chromosomes from gonads and ganglia. Morphology of polytene B-chromosomes of Shoenbaueria pusilla Fries., Cnetha fontinalis Radzv., Stegopterna duodecimata Rubz., and Archesimulium tuberosum Lundstr. has been first described. B-chromosome polymorphism was found in all species, but the number of B chromosomes was conserved within each differences in polytene individual. Stable and distinct interspecific differences in the morphology of polytene B-chromosomes were demonstrated, and these characters are advisable to use to distinguish the species. We have investigated for the first time karyofonds of Od. ornata populations from Arkhangelsk Region (Solovetskie Islands) and Leningrad Region (railway station Sablino), and those of S. argyreatum populations from Murmansk Region (Kandalaksha environs) and Karelia (railway station Chupa). A long term study of Od. ornata and S. argyrestum population from North-Western Russia revealed interspecific and interpopulation dynamics of the occurrence of specimens with B-chromosomes. Some populations showed an increased percentage of individuals with B-chromosomes. It is suggested that B-chromosomes may play a role in adaptation of polulations to severe environmental conditions.     

Variable transmission rates of a B-chromosome in Myrmeleotettix maculatus (Thunb.) (Acrididae: Orthoptera)

Karyotype comparisons of both parents and progeny from single pair matings in the grasshopper Myrmeleotettix maculatus have shown that there is an accumulation of the large mitotically stable B-chromosome when transmitted through the female. This is presumed to result from a preferential segregation of univalent B-chromosomes at the first division of female meiosis and occurs irrespective of whether the B's are odd or even in number. In the male there is a loss of B-chromosomes. This loss does not appear to be due simply to the lagging and elimination of B-chromosomes in meiosis but probably involves sperm formation or function. When the balance of the gain and loss after one generation is calculated, it shows large overall accumulation in crosses involving individuals from a population in Wales, and a slight loss in individuals from a population in East Anglia. Such differences in transmission rates may be responsible for differences in B-frequency between populations. Since the B-chromosome frequency of these two populations has remained stable over five years, possible forces in the maintenance of the equilibria are examined. Females with B-chromosomes produce more aneuploid embryos than 0B females, but neither this cause of inviability nor general embryo mortality seem sufficient to produce an equilibrium situation. It is necessary to postulate that progeny with more than 2B chromosomes are inviable in order to approach equilibria. The presence of B-chromosomes in females has also led to the formation of polyploid embryos. The possible involvement of repetitive DNA in the formation of unreduced egg nuclei and preferential segregation is discussed.     

B-Chromosomen beiChironomus

In the population “Herrenmühle” ofChironomus plumosus 11% of the individuals contain one supernumerary chromosome. This B-chromosome is present both in germ-line and somatic cells. — InChironomus melanotus 6% of the larvae of the population “Falkau” carry supernumerary chromosomes. These B-chromosomes cannot be found in all nuclei of testis and soma, their number varies between cells within the individual. In both species the B-chromosomes represent centromeric fragments of chromosome IV as can be shown by their structure and pairing behaviour. — The polytene B-chromosome ofCh. plumosus exhibits a banding pattern in the salivary gland nuclei. Furthermore it is able to form an additional nucleolus in the nuclei of the malpighian tubules. InCh. melanotus band structures can be seen only in the B-chromosome of malpighian tubules. The larvae ofCh. melanotus, carrying B-chromosomes, show heterochromatic bodies in the salivary gland nuclei, varying in number and size in the nuclei of the same gland. These bodies are interpreted to be polytenic B-chromosomes divided into subunits.

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