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.     

Differences in ribosomal DNA distribution on A and B chromosomes between eastern and western populations of the grasshopper Eyprepocnemis plorans plorans

Distribution of ribosomal DNA (rDNA) on standard (A) and supernumerary (B) chromosomes of the grasshopper Eyprepocnemis plorans was analysed in specimens collected in Turkey and Armenia, belonging to the E. p. plorans subspecies, and in South Africa, belonging to the E. p. meridionalis subspecies. The latter individuals showed rDNA loci in chromosomes 9 and 11 only, whereas those from Armenia carried it in chromosomes 9 and 11 or else in chromosomes 9-11, depending on the population. The specimens from Turkey carried it in chromosomes 1, 9-11 and X. A comparison of this pattern with those previously observed in populations from Spain, Morocco, and Greece (belonging to E. p. plorans) suggests the existence of two evolutionary patterns in rDNA chromosome location in A chromosomes of this subspecies: eastern populations showing rDNA restricted to the small (9-11) chromosomes (as in E. p. meridionalis and other closely related taxa within the Eyprepocneminae subfamily) and western populations carrying rDNA in most A chromosomes (Spain) or all of them (Morocco). The intermediate pattern discerned in geographically intermediate populations (in Greece and Turkey), with rDNA also being located on the X chromosome, suggests a possible east-west cline. Additional support for east-west differentiation in the rDNA location pattern comes from the analysis of B chromosomes. In eastern populations, including Daghestan, Armenia, Turkey, and Greece, B chromosomes are composed mostly of rDNA, whereas in western populations (Spain and Morocco) they contain roughly similar amounts of rDNA and a 180-bp tandem repeat (satDNA), the latter being scarce in eastern Bs.     

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.     

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.     

The B chromosome polymorphism of the grasshopper Eyprepocnemis plorans in North Africa: III. mutation rate of B chromosomes

B chromosome variation in nine Moroccan populations of the grasshopper Eyprepocnemis plorans was analysed for 3 consecutive years. In addition to B1, which was the predominant B chromosome in all nine populations, we found 15 other B variants, albeit at very low frequency. Eight variants were found in adults caught in the wild, four appeared in adults reared in the laboratory and seven were found in embryo progeny of controlled crosses between a 0B male and a B-carrying female. Some variants were found in more than one kind of material. At least the seven B variants that appeared in embryo progeny of females carrying a different B type arose de novo through mutation of the maternal B chromosome. The mutation rate of B chromosomes was 0.73%, on average, which explains the high variety of morphs and banding patterns found. The most frequent de novo mutations observed in these chromosomes were centromere misdivision with or without chromatid nondisjunction, which generates iso-B-chromosomes or telocentric Bs, respectively, as well as translocations with A and B chromosomes and deletions. But the whole variation observed, including that found in adult individuals, suggests that other mutations such as duplications, inversions and centric fusions do usually affect B chromosomes. Finally, B chromosome mutation rate was remarkably similar in both Moroccan and Spanish populations, which suggests that it might be dependent on B chromosome intrinsic factors.     

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.     

Evolutionary dynamics of a B chromosome invasion in island populations of the grasshopper Eyprepocnemis plorans

Four natural populations of the grasshopper Eyprepocnemis plorans in the Mallorca island were analysed for several years revealing the recent invasion of the B₁ chromosome from the south‐west part of the island (Palma region) towards the north and to the east. In only 10 years, the mean number of Bs in the northern population at Pollença increased from 0.053 to 0.692. Therefore, B chromosome invasion seems to be very rapid and has recently arrived to the north of the island. The south‐west (close to Palma) is the most likely point at which B invasion started in the Mallorca Island. Finally, the number of B chromosomes was significantly associated to an increase in chiasma frequency (and thus recombination) in A chromosomes.     

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 evolution of hypervariable sex and supernumerary (B) chromosomes in the relict New Zealand frog,Leiopelma hochstetteri

Chromosomes exhibiting elevated levels of differentiation are termed hypervariable but no proposed mechanisms are sufficient to account for such enhanced evolutionary divergence. Both hypervariable sex and supernumerary (B) chromosomes were investigated in the endemic New Zealand frog, Leiopelma hochstetteri, which is chromosomally polymorphic both within and between populations and has sufficiently elevated variation that different populations can be identified solely by their C-banded karyotypes. This frog is further distinguished by the univalent, female-specific W-chromosome (0W/00 sex determination) uniquely possessed by North Island populations. This sex chromosome exhibited variation in morphology, size, and heterochromatin distribution, sufficient to resolve 11 different types, including isochromosomes. Five of the 12 populations examined also had supernumerary chromosomes that varied in number (up to 15 per individual) and morphology. Specific variations seen among the hypervariable chromosomes could have resulted from heterochromatinisation, chromosome fusions, loss-of-function mutations, deletions, and/or duplications. Frogs of the same species from Great Barrier Island, however, had neither supernumeraries nor the female-specific chromosome. The 0W/00 sex chromosome system must have been derived after the isolation of Great Barrier Island from North Island populations by raised sea levels between 14 000 and 8000 years ago. Furthermore, biochemical divergence between populations is minor and therefore the chromosomal variation seen is comparatively recent in origin. The one characteristic common to all known hypervariable chromosomes is curtailment or lack of recombination. Their accelerated evolution therefore is possible via the mechanism of Muller's ratchet, either alone or in concert with other factors.     

大林姬鼠的核型与B染色体研究

采用骨髓染色体制片法 ,对分布于吉林长白山、山东泰山和陕西秦岭的大林姬鼠的染色体组型、C -带、G -带和减数分裂的染色体行为进行了观察分析。发现 3个地区标本的染色体数目存在着显著差异。东北标本的 2n =48～ 51 ,A组染色体为 48条 ,均由端着丝粒染色体组成 ,同时具有 1～ 3条B染色体 ,其形态为中着丝粒染色体 ;山东标本的 2n =53～ 62 ,A染色体同样为 48条端着丝粒染色体组成 ,具 5～ 1 4条B染色体 ,其中 1条为较大的中着丝粒染色体 ,其余为小的中着丝粒和点状染色体。秦岭标本 2n =48～ 49,A染色体为 48条端着丝粒染色体 ,具 1条形态很小的端着丝粒B染色体。3地标本的B染色体均存在个体间和个体内变异。长白山标本B染色体的细胞克隆数目为 1～ 2 ,泰山标本为 1～ 3。在 3地标本中 ,中着丝粒B染色体呈现C -带阴性 ,点状B染色体呈中度深染。通过对减数分裂的观察 ,多数B染色体是以单价体的形式存在。中国长白山种群的B染色体数目和形态与朝鲜种群相似。与欧洲种群存在着显著差异。泰山种群的B染色体数目和形态与朝鲜种群及欧洲种群均存在显著差异。泰山种群与秦岭标本同属华北亚种 ,但它们的B染色体形态和数目差别很大。     

Mitotically unstable B chromosome polymorphism in the grasshopper Dichroplus elongatus

Dichroplus elongatus, a widespread South American phytophagous grasshopper, exhibits polymorphisms for supernumerary chromosomes and segments (SS) in natural populations in Argentina. In this paper we review the available information on B chromosome polymorphism in D. elongatus related to geographic distribution, patterns of chromosome variation and influence on sperm formation. In D. elongatus the different forms of supernumerary variants are not independent. The proportion of B-carrying individuals (B prevalence) is negatively correlated with SS10 and positively with SS6 frequencies. The analysis of population structure considering the different supernumerary variants would suggest that the patterns of chromosome variation can not be explained only by random factors. Geographic distribution was analyzed scoring the prevalence of B chromosomes in 13 natural populations collected in three different biogeographical provinces from Northwest (Las Yungas province) and East (Espinal and Pampeana provinces) of Argentina. The detected heterogeneity may be explained by significant differentiation between Northwest and East regions and among populations within Las Yungas and Pampeana provinces. Correlation analysis suggested that B chromosome prevalence is associated with maximum temperature and with latitude. Additional information about the nature of the patterns of B chromosome variation was obtained comparing them with those obtained at the mitochondrial DNA level. The hierarchical analysis of molecular differentiation revealed discrepancy with respect to chromosome differentiation and also suggested that the pattern of B chromosomes may not be explained by historical factors. We also discussed the probable influence on fertility of carriers considering the production of abnormal sperm formation (macro and microspermatids) in relation to the number of Bs per follicle.     

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.     

Robertsonian translocations and B chromosomes in the Wellington tree weta, Hemideina crassidens (Orthoptera: Anostostomatidae)

Two karyotypes within the species Hemideina crassidens are described, 2n = 15 (XO) and 2n = 19 (XO). These two karyotypes have a NF of 28. The 19-karyotype was found exclusively in the southern part of the species range and the 15-karyotype was found in the north. The differences between the two karyotypes are interpreted as arising from two Robertsonian translocations (fission/fusion). Laboratory matings between weta with the two karyotypes produced viable offspring. During meiosis in F1 intraspecific hybrids metacentric and acrocentric autosomes aligned to form two trivalents, confirming homologies predicted by Robertsonian translocations. The subspecies H. c. crassicruris, (confined to Stephens Island) was found to be polymorphic for a metacentric B chromosome. An unusual association of sex and presence of B chromosome was observed in this island population with Bs found only in male weta.     

Intra individual variation in the number of B chromosomes in the yellow-necked mouse, Apodemus flavicollis (Mammalia, Rodentia)

The yellow-necked mouse, Apodemus flavicollis, is characterized by a frequent occurrence of B chromosomes. The frequency of intra individual mosaicism of Bs was studied in 995 animals collected at six localities in Serbia. It was found that 329 (33.06%) possessed B chromosomes. Among these, 87 animals (26.44%) were mosaics. A total of 32 mosaic animals with more than one B chromosome were analyzed for distribution of Bs which was found to be quite different between groups of animals with different numbers of Bs and increases with their number. The frequency of mosaics differs between localities and ranges from 0.22 to 0.55.     

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.     

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.     

Localization of the genes controlling B chromosome transmission rate in maize (Zea mays ssp. mays, Poaceae)

In previous papers we found that the frequency of B chromosomes in native races of maize varies considerably in different populations. Moreover, we found genotypes that control high and low transmission rates (TR) of B chromosomes in the Pisingallo race. In the present work crosses were made to determine whether the genes controlling B-TR are located on the normal chromosome set (As) or on the B chromosomes (Bs). We made female f.0B × male m.2B crosses between and within high (H) and low (L) B-TR groups. The Bs were transmitted on the male side in all cases. The mean B-TR from the progeny of f.0B (H) × m.2B (H) and f.0B (H) × m.2B (L) crosses was significantly higher than that from f.0B (L) × m.2B (L) and f.0B (L) × m.2B (H) crosses. The results show that the B-TR of the crosses corresponds to the H or L B-TR of the 0B female parents irrespective of the Bs of the male parent. This indicates that B-TR is genetically controlled by the 0B female parent and that these genes are located on the A chromosomes.     

B chromosomes in the fish Astyanax scabripinnis (Characidae, Tetragonopterinae): an overview in natural populations

Astyanax scabripinnis, a small neotropical freshwater fish, is a headwater species living in small tributaries of many Brazilian rivers, where they form isolated populations. This species harbors a B chromosome system in several populations. Among the several kinds of Bs reported in this species, the B(M) variant, a large metacentric of a similar size to the largest A chromosome, is the most widespread in natural populations. It probably corresponds to the ancestral B type in this species and a very similar B chromosome is also found in other Astyanax species. Strong evidence suggests that this B is an isochromosome showing structural and functional homology between its two arms, as shown by satellite DNA localization and the formation of a ring B univalent during meiosis. The B(SM) and B(m) variants, a large submetacentric and a small metacentric, respectively, represent rare variants and may be derived from structural rearrangements of the B(M) chromosome. In addition, B microchromosomes (B(micro)) were found in some populations. Frequency analyses in mountain populations have shown that B chromosomes are found in populations located at high altitude, but are absent in populations at low altitude, which is consistent with their parasitic nature, given the ecological peculiarities of both kinds of populations.     

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