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.     

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.     

The cytogenetic structure of Tasmanian populations of Phaulacridium vittatum

In 8 out of 20 Tasmanian populations of Phaulacridium vittatum from 0.3-11.0 percent of the males carried a single supernumerary chromosome. In such males the X and B univalents are both heteropycnotic at first prophase of male meiosis and associate with another in a non-homolgous manner in about two-thirds of the diplotene cells examined. In all 56 B-containing individuals studied, however, these associations lapse by first metaphase and the X and the B move at random with respect to one another at first anaphase. The supernumerary in this species is stable and only 5 of the 56 individuals with supernumeraries showed evidence of B-chromosome non-disjunction in the pre-meiotic mitoses. Since there was no other evidence for the loss or gain of supernumeraries in the male line it is clear that B-transmission is regular in the males of this species. There were significant differences between population with respect to mean cell chiasma frequency but there was no significant effect of the B-chromosome on this metric. Additionally a comparison of mean cell chiasma frequency in follicles with and without supernumeraries from a mosaic individual shows no significant difference.     

B chromosome variation in Euphydryas colon (Lepidoptera: Nymphalidae)

Cytogenetic analysis of an Idaho population of the checkerspot butterfly, Euphydryas colon, has revealed considerable inter- and intra-individual variation in chromosome number which turns out to be a classic case of B chromosome variation. The basic chromosome complement of the species is n (, )=31. The A chromosomes were aligned equatorially at mitotic metaphase and metaphase II, and axially at metaphase I, indicating a restriction of centric activity at the first meiotic division. No failure of pairing between homologous A chromosomes was observed and, although a marked asynchrony of chromatid separation was found to be characteristic of mitotic telophase and telophase II, the frequency of macrospermatid formation was low. The B chromosomes were at least partly heterochromatic but exhibited some variation in both pycnosity and size. Mitotically stable B-containing individuals showed a preponderance of unpaired Bs at first metaphase and these divided at either first or second anaphase. The presence of Bs was associated with a heightened production of abnormal spermatids particularly in individuals with high numbers of B chromosomes. Among the 25 individuals sampled, 21 carried from 1–6 B chromosomes, and of these 14 were mitotically stable. In all 7 unstable individuals the mean number of B chromosomes per cell exceeded the modal number. Assuming that the modal number represents the zygotic number, these results suggest that a mechanism to boost the number of B chromosomes exists in males of E. colon.     

The B-chromosomes of two species ofCylindrotettix (Leptysminae,Acrididae)

Twenty-four males ofCylindrotettix obscurus and ten ofC. santarosae (Leptysminae, Acrididae) from Rio Claro, Brazil, were cytologically analysed. Both species are 2n = 23 ♂ (22+XO), all chromosomes being telocentric except for 4 acrocentric pairs ofC. obscurus. Two different B-chromosomes were found inC. obscurus: (1) a small, mitotically unstable telocentric (B^T) and (2) a medium-sized, mitotically stable submetacentric (B¹). B¹ has a large pericentromeric heterochromatic portion and two distal euchromatic segments where both terminal associations and interstitial chiasmata are regularly formed producing a ring univalent at prophase I. B¹ is almost certainly an iso-chromosome which has undergone a centromeric shift through pericentric inversion or centric transposition (heterochromatin duplication or deletion could also be the cause of arm inequality). InC. santarosae, a single male carried a small mitotically stable telocentric B-chromosome. Both B types ofC. obscurus significantly increased the production of abnormal sperm (diploid, tetraploid and micro-spermatids) when compared to 0B individuals although their effects were differential: B^T induced higher frequencies of microspermatids especially in those follicles with odd B^T numbers, while in B¹ carriers the increase was chiefly due to polyploid spermatids. None of the Bs ofC. obscurus affected cell mean chiasma frequency nor the between-cell variance. The B ofC. santarosae had no effect on abnormal sperm production but it probably increased cell mean chiasma frequency.     

The B-chromosome system of Allium schoenoprasum

Populations of Allium schoenoprasum, 2n=16, from six of the eight British localities have been examined. A B-chromosome polymorphism has been found which is limited to the River Wye, South Wales, where A. schoenoprasum grows along a 15 kilometre stretch. Four interrelated types of small, euchromatic, mitotically-stable B-chromosomes occur. Twelve different B-karyotypes, including an individual with 18 Bs, have been recorded. Populations contain up to 39% of plants with Bs. The B-chromosomes are not uniformly distributed along the river but are limited to populations in the downstream part of the range. An abrupt change from 34% to 0% B-contaiuing plants takes place over only 1.2 kilometres. The unique combination of riverside populations in linear order and the sudden discontinuity in the cytological structure of these populations may enable the factors influencing B-chromosome distribution to be established.     

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.     

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.     

Effects of supernumerary heterochromatin on chiasma condition in two species of Acrididae (Orthoptera)

Amblytropidia australis and Dichroplus elongatus were found to be polymorphic for supernumerary heterochromatin. In both, basic karyotypes are 2n=22+XO in males.Mitotically unstable extra chromosomes were detected in a population of A. australis. The Bs are telocentric and their number varies from O to 2 within individuals. Mean frequencies of interstitial and total chiasmata at diplotene were compared between individuals with and without Bs. The mean frequency of interstitial chiasmata increases with the number of Bs per cell.A supernumerary terminal segment in S₁₀ pair was observed in a heterozygous condition in several individuals of D. elongatus. The localization and frequency of chiasmata at diplotene were studied. The segment has an intrachromosomal effect since it modifies the location of chiasmata in the bivalent involved.Fellow of the Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET).     

The cytology of Brachycome

Over 1,000 plants of B. dichromosomatica have been counted. 10% of these carried one, two or three B chromosomes. The B chromosome is large, and though it is not heteropycnotic it condenses precociously at mitosis. It behaves regularly at mitosis, and when two are present they pair regularly at meiosis. Non-disjunction and preferential distribution of the B to the generative nucleus occurs at the first pollen grain mitosis, with very high frequency. This is corroborated by data from crosses, which also indicate that the transmission of the B through the female gamete is normal. — The frequency of B chromosomes in marginal populations of var. dichromosomatica is significantly higher than in central populations. In one population of var. alba the frequency of Bs increased significantly after two very dry seasons. It is suggested that both these cases of increased frequency were in response to a selective advantage of plants with Bs under arid conditions. — Plants with one B chromosome appear to be less fit than plants with 2 Bs. The combination of the calculated effects of the nondisjunction mechanism and the inferred relative fitness of the 0B, 1B and 2B plants, provides a reasonable explanation of the observed frequencies of the 0B, 1B and 2B plants in the populations studied.     

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.     

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.     

Analyse de la transmission des chromosomes surnuméraires chez Locusta migratoria migratorioides R. et F. (Orthoptera,Acrididae)

In a African grasshopper population of Locusta migratoria migratorioides reared in the laboratory, B chromosome transmission is analysed in progeny of pairs taken at random from the population. An accumulation mechanism of Bs exists in the females which presumably results from preferentiel segregation of the Bs in the female pronucleus. The analysis also shows B chromosome elimination during embryonic and larval development by means of mitotic non-disjunction: these non-disjunctions induce interfollicular variations in the number of Bs but all the cells of a follicle have the same number of Bs. The frequency of animals with B chromosome rises from 34% to 75% as the diurnal temperature rises from 27° C to 35° C. Thus frequency is controlled by genetic and environmental factors.     

Stability of the population variants of the B-chromosome system in the East-Asian mouse Apodemus peninsulae from the Baikal region and Northern Mongolia

Analysis of the B-chromosome frequency and morphotypes in 160 mice Apodemus peninsulae from 17 localities of the Baikal region, Northern Mongolia, Trans-Baikalia, and the Russian Far East showed that the mice were from five geographical populations. The interpopulation difference was determined by variations of 0?C4 macro B chromosomes and 0?C11 micro B chromosomes. The B-chromosome number and morphotypes proved to be stable over the past 30 years in the geographical populations under study.     

B-chromosome systems in the pocket mouse,Perognathus baileyi: Meiosis and C-band studies

The heterochromatin characteristics and meiotic behavior of the B-chromosome system of the pocket mouse, Perognathus baileyi, are described. B-chromosomes are associated both with a meiotic accumulation mechanism and with an increase in average chiasma frequency in the A-chromosome set in males. Three morphological classes of B-chromosomes are recognizable, and the mechanisms of origin of each are discussed.     

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     

Genome size and numerical polymorphism for the B chromosome in races of maize (Zea mays ssp. mays, Poaceae)

Twenty-one native populations (1120 individuals) of maize from Northern Argentina were studied. These populations, which belong to 13 native races, were cultivated at different altitudes (80-3620 m). Nineteen of the populations analyzed showed B chromosome (Bs) numerical polymorphism. The frequency of individuals with Bs varied from 0 to 94%. The number of Bs per plant varied from 0 to 8 Bs, with the predominant doses being 0, 1, 2, and 3. Those populations with varying number of Bs showed a positive and statistically significant correlation of mean number of Bs with altitude. The DNA content, in plants without Bs (A-DNA)(2n = 20), of 17 populations of the 21 studied was determined. A 36% variation (5.0-6.8 pg) in A-DNA content was found. A significant negative correlation between A-DNA content and altitude of cultivation and between A-DNA content and mean number of Bs was found. This indicates that there is a close interrelationship between the DNA content of A chromosomes and doses of Bs. These results suggest that there is a maximum limit to the mass of nuclear DNA so that Bs are tolerated as long as this maximum limit is not exceeded.     

中国银额果蝇自然群体B染色体的分布

经对我国银额果蝇自然群体Ｂ染色体（Ｂｓ）的分布进行多年广泛地调查,共作过２４个群体,５７２个单雌系,６７３８只个体的观察。从细胞和群体水平研究Ｂｓ的地理分布。结果发现,中国（除台湾省外）银额果蝇自然群体中普遍存在Ｂｓ。结合前人报道得出。Ｂｓ的地理分布出现了由东向西和由南向北的规律性的依次升高趋势。这种跨越不同地理环境的区域性梯度变异与其宿主向大陆内地扩散的推论相符。Ｂｓ频率最高（８７．７％）的是海     

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.