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.     

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.     

Diversity and individuality of variants of the system of B chromosomes in mice <Emphasis Type="Italic">Apodemus peninsulae</Emphasis>

A complicated population system of B chromosomes has been revealed in the karyotypes of the East-Asian mouse Apodemus peninsulae over the whole range. Based on our previous data for mice from Altai, Siberia, Pribaikal’e, and Mongolia and on the results of the present work, we infer that most of animals studied (196 out of 312, or 63%) have an individual variant of the B chromosome system. Only three mice lacked B chromosomes. New data for 44 A. peninsulae mice caught in 2000–2005 in 16 localities of seven regions of Altai, Siberia, and Pribaikal’e showed that all of them had individual variants of the system of B chromosomes due to different combinations (from 2 to 18 B chromosomes) of five classes of B macro- and microchromosomes.     

Multiregional origin of B chromosomes in the grasshopper<Emphasis Type="Italic"> Eyprepocnemis plorans</Emphasis>

Analysis of chromosome localization of three molecular markers, 18S-5.8S-28S rDNA, 5S rDNA and a 180 bp satDNA, showed that B chromosomes in the grasshopper Eyprepocnemis plorans originated independently in Eastern (Caucasus) and Western (Spain and Morocco) populations. Eastern B chromosomes are most likely derived from the smallest autosome, which is the only A chromosome carrying the three markers, in coincidence with Caucasian B chromosomes. Western B chromosomes, however, lack 5S rDNA and are most likely derived from the X chromosome, which is the only A chromosome carrying the two remaining markers, always in the same order with respect to the centromere, as the B chromosome.Communicated by E.R. Schmidt     

Variability of Minisatellite Loci and mtDNA in Individuals with and without B Chromosomes from Populations of the Grasshopper <Emphasis Type="Italic">Dichroplus elongatus</Emphasis>

Dichroplus elongatus is an extensively distributed South American grasshopper considered a pest of major crops. Argentinean populations show a widespread B-chromosome polymorphism which could be maintained as the result trade-offs among opposite selective effects and interactions with their mitotic instability. The main objective of this study was to evaluate the relationships between B chromosomes and mtDNA sequences coupled with minisatellites loci, and verify the genotype/karyotype covariation in 12 populations located at both sides of Paraná River (Eastern and Western Regions). B carrier individuals showed significantly higher genetic diversity (H_E and X) respect to standard individuals. AMOVAs based on nuclear loci and mtDNA sequence datasets showed statistically significant levels of differentiation among karyotypes in the Eastern Region. Cluster analysis through Bayesian procedure considering nuclear loci splits B carriers and standard individuals into different genetic clusters in some Eastern populations. The Bayesian phylogenetic analysis showed two divergent mtDNA clades. Haplogroup 1 is composed exclusively of standard individuals, however all B chromosome carriers are included in haplogroup 2. There is an association between some haplotypes and B chromosomes and a strong effect of phylogenetic signal on B chromosome population structure. Genetic differentiation between karyotypes at Eastern Region revealed by AMOVA, Bayesian approaches and clustering analysis based on uniparental and biparental inherited markers may be due to the inherent nature of the B chromosome, to karyotype biased dispersal or to difference tolerance of B chromosomes on different genetic background. The combination of molecular and chromosome analysis performed in this study indicated that B chromosomes in D. elongatus is an important factor in explaining the genetic population structure at minisatellite and mitochondrial DNA levels.     

The B chromosome system in the varying lemming <Emphasis Type="Italic">Dicrostonyx torquatus</Emphasis> Pall., 1779 from natural and laboratory populations

In varying lemmings from seven natural populations (from Bolshezemelskaya tundra to Chukotka Peninsula), the number of B chromosomes ranged from 0 to 15. In populations surveyed for several years, B chromosome frequencies were stable. Two laboratory colonies (founded by Dicrostonyx torquatus torquatus from the Polar Urals and D. t. chionopaes from Yakutia) produced more than 3000 animals, of which 1699 were karyotyped. A small excess of B chromosomes in the progeny over that in their parents was observed in each generation. Coefficients of transmission k ₁ of additional chromosomes were on average 0.519 in D. t. torquatus and 0.511 in D. t. chionopaes. In oocytes I and II, an accumulation of B chromosomes was observed as compared to somatic cells ( k₂ = 0.66). The reproductive output of animals from both laboratory colonies did not depend on size of the B genome. The reduction of body and scull sizes observed in animals carrying numerous B chromosomes may confer negative selective value in the conditions of the Extreme North. In general, the system of B chromosomes in D. Torquatus is well balanced and very stable, being apparently under strong genetic control.Translated from Genetika, Vol. 40, No. 12, 2004, pp. 1686–1694.Original Russian Text Copyright © 2004 by Gileva.     

Banding pattern of A and B chromosomes of Prochilodus lineatus (Characiformes, Prochilodontidae), with comments on B chromosomes evolution

B chromosomes in Prochilodus lineatus, a migratory neotropical fish, were analyzed in a comparative study among populations from the Dourada lagoon (State of Paraná, Brazil) and from Mogi-Guaçu river (State of São Paulo, Brazil). The data on C-banding and fluorescent in situ hybridization with a satellite DNA probe (SATH1), indicate that the small metacentric B chromosome might correspond to an isochromosome. On the other hand, both populations presented a distinct set of B chromosomes, differentiated either by their number and by the presence of variant B types in the population from Mogi-Guaçu river. The present results indicate that the B chromosomes of P. lineatus should have an ancient origin, and have undergone a differential evolutionary pathway among distinct populations.     

Supernumerary (B) chromosomes in populations of Picea abies (L.) H. Karst. from Western Rhodopes (Bulgaria)

Investigations on B chromosomes found for the first time for Picea abies (L.) H. Karst. have been conducted. Seeds of Picea abies from two populations of Western Rhodopes (Bulgaria) located at the southern border of species range, and protected according to Bern Convention and EC Habitat Directive were collected for this study. Mixoploidy was detected in some germinating seeds of Picea abies. It was found that metaphase cells of germinating seeds contain 0–4 B chromosomes of both metacentric and submetacentric types. The variability of B chromosomes number and their occurrence was observed. Along with B chromosomes, some chromosome aberrations such as fragments and ring chromosomes were revealed in metaphase cells of Picea abies from studied populations. The possible adaptive role of B chromosomes presence for Picea spp. is discussed.     

Low-pass single-chromosome sequencing of human small supernumerary marker chromosomes (sSMCs) and <Emphasis Type="Italic">Apodemus</Emphasis> B chromosomes

Supernumerary chromosomes sporadically arise in many eukaryotic species as a result of genomic rearrangements. If present in a substantial part of species population, those are called B chromosomes, or Bs. This is the case for 70 mammalian species, most of which are rodents. In humans, the most common types of extra chromosomes, sSMCs (small supernumerary marker chromosomes), are diagnosed in approximately 1 of 2000 postnatal cases. Due to low frequency in population, human sSMCs are not considered B chromosomes. Genetic content of both B-chromosomes and sSMCs in most cases remains understudied. Here, we apply microdissection of single chromosomes with subsequent low-pass sequencing on Ion Torrent PGM and Illumina MiSeq to identify unique and repetitive DNA sequences present in a single human sSMC and several B chromosomes in mice Apodemus flavicollis and Apodemus peninsulae. The pipeline for sequencing data analysis was made available in Galaxy interface as an addition to previously published command-line version. Human sSMC was attributed to the proximal part of chromosome 15 long arm, and breakpoints leading to its formation were located into satellite DNA arrays. Genetic content of Apodemus B chromosomes was species-specific, and minor alterations were observed in both species. Common features of Bs in these Apodemus species were satellite DNA and ERV enrichment, as well as the presence of the vaccinia-related kinase gene Vrk1. Understanding of the non-essential genome elements content provides important insights into genome evolution in general.     

The genetic control of the α-amylase isozymes of the durum wheat (<Emphasis Type="Italic">Triticum durum</Emphasis> Desf.)

The hybridoiogical analysis was provided on several durum wheat genotypes with utilizing three F₂ populations developed from the crossing between parental forms that differed in the invariable malt-zone triplet on elecirophoretic spectrum of α-amylase. Three components of this zone are controlled by three genes with an independent way of inheritance: one of them is located on the 6B or 5B chromosome, and two genes are located on the chromosomes of A subgenome. The article is published in the original.     

B chromosomes of the Podisma sapporensis Shir. (Orthoptera, Acrididae) analysed by chromosome microdissection and FISH

The analysis of the distribution of repetitive DNA of the B chromosomes of Podisma sapporensis in the A and B chromosomes of the natural populations and in A chromosomes of three other species of the Podismini grasshoppers were made. DNA-libraries of the B chromosome and the euchromatic segment of the A chromosome of P. sapporensis were generated by meiotic chromosome microdissection followed by degenerated oligonucleotide primed polymerase chain reaction (DOP-PCR). Paints based on these DNA-libraries were used for FISH analysis to detect localization of homologous sequences in A and B chromosomes of P. sapporensis from different natural populations. On the basis of the FISH analysis the authors suggest that evolution of the B chromosomes in Podisma sapporensis was associated mainly with the insertions of "alien DNA sequences" into ancestral A chromosome and their further amplification. The number of initial sites of amplifications differed in the different Bs, the distance between these sites also varying. Karyotype evolution in P. sapporensis was associated partly with the insertion of "alien DNA sequences" into pericentromeric chromosomal regions. Insertion into the small short arms of the acrocentric chromosomes followed, with the DNA amplification leading to the formation of the additional C-heterochromatic arms or euchromatic-like regions of different size.     

B chromosome morphotypes of Apodemus peninsulae (Rodentia) from the Russian Far East

Polymorphism of B chromosome morphotypes in the natural populations of the Korean field mouse Apodemus peninsulae (n = 367) has been studied in five regions of the Russian Far East: Magadan and Amur regions, the Jewish Autonomous region, Khabarovsk and Primorskii regions. Six groups of B chromosome morphotypes have been described on the size and morphological parameters. On the basis of their combinations 78 cell clones have been revealed, 23 of which are found in the animals with a stable karyotype and 76--in mosaics. The mosaics have also more diverse and unique combinations of B chromosome morphotypes. No differences between the geographic and local populations of mice on the studied characteristics have been found. Homogeneous structure of DNA in the B chromosomes of this species at the territory of the Russian Far East, as shown previously, allowed us to combine the numerical and dimensional data for each clone by introduction conditional "mass quantity" of B chromosomes (mB index). The leading role of natural selection in production of "critical" mass of supernumerary chromosomes in individuals with a stable karyotype and the weakening of its role in mosaics has been suggested.     

Multiplicity of B microchromosomes in a siberian population of mice <Emphasis Type="Italic">Apodemus peninsulae</Emphasis> (2<Emphasis Type="Italic">n</Emphasis> = 48 + 4–30 B chromosomes)

Differentiation of four Siberian populations of East-Asian (Korean) field mice (Apodemus peninsulae) inhabiting the basin of the mid-stream of the Yenisei River was carried out according to the variants of the B chromosome system. A multiplicity of B microchromosomes (from 4 to 30) was found for the first time in all 26 mice from the left shore of the Yenisei River in the mid-stream area. All of them probably belong to a population with B microchromosomes. It is likely that in this population further reorganization of B microchromosomes into B macrochromosomes typical of this species does not occur. Two mice from this population had a large number of B chromosomes (26) earlier not observed in this species. In one mouse, the modal number of B microchromosomes was 30. This is a new maximum number of B chromosomes in this mouse species.     

B-chromosome evolution

B chromosomes are extra chromosomes to the standard complement that occur in many organisms. They can originate in a number of ways including derivation from autosomes and sex chromosomes in intra- and interspecies crosses. Their subsequent molecular evolution resembles that of univalent sex chromosomes, which involves gene silencing, heterochromatinization and the accumulation of repetitive DNA and transposons. B-chromosome frequencies in populations result from a balance between their transmission rates and their effects on host fitness. Their long-term evolution is considered to be the outcome of selection on the host genome to eliminate B chromosomes or suppress their effects and on the B chromosome's ability to escape through the generation of new variants. Because B chromosomes interact with the standard chromosomes, they can play an important role in genome evolution and may be useful for studying molecular evolutionary processes.     

Polymorphism of the Sphaerium corneum (Bivalvia, Veneroida, Sphaeriidae) revealed by cytogenetic and sequence comparison

Two populations of Sphaerium corneum were sampled from River Vilnelė and small pond in Vilnius, Lithuania. The chromosomes were studied using conventional Giemsa staining and karyometric analysis. Inter- and intra-individual variation in the diploid chromosome numbers was revealed and two different sources of chromosome variability were identified: B chromosomes and the structural changes of chromosomes of the basic (A) set. The chromosome set of the more common karyotypic form, 2 n  = 30, found in both populations, consists of all biarmed metacentric and meta-submetacentric chromosomes of gradually decreasing size. Small, biarmed, mitotically unstable B chromosomes were found in the cells of this karyotypic form. Specimens with 2 n  = 36 were found only in pond. No B chromosomes were detected in their cells. The karyotype is characterized by presence of two pairs of medium telocentrics and four pairs of small subtelocentrics. The remaining chromosomes are biarmed. Robertsonian fusions appear to be involved in formation of two karyotypic forms of S. corneum . DNA sequence analyses showed that ITS1 is identical in both karyotypic forms. On the other hand, differences in 16S sequence were revealed and two haplotypes, corresponding to two karyotypic forms, were identified. The present study opens new perspectives in establishing species-specific characters for confident identification of Sphaerium species and provides insights to the genetic intraspecific variability and possible mechanisms of speciation.  © 2006 The Linnean Society of London, Biological Journal of the Linnean Society , 2006, 89 , 53–64.     

Meiotic self-pairing of the Psalidodon (Characiformes,Characidae) iso-B chromosome: A successful perpetuation mechanism

B chromosomes are non-essential additional genomic elements present in several animal and plant species. In fishes, species of the genus Psalidodon (Characiformes, Characidae) harbor great karyotype diversity, and multiple populations carry different types of non-essential B chromosomes. This study analyzed how the dispensable supernumerary B chromosome of Psalidodon paranae behaves during meiosis to overcome checkpoints and express its own meiosis-specific genes. We visualized the synaptonemal complexes of P. paranae individuals with zero, one, or two B chromosomes using immunodetection with anti-medaka SYCP3 antibody and fluorescence in situ hybridization with a (CA)₁₅ microsatellite probe. Our results showed that B chromosomes self-pair in cells containing only one B chromosome. In cells with two identical B chromosomes, these elements remain as separate synaptonemal complexes or close self-paired elements in the nucleus territory. Overall, we reveal that B chromosomes can escape meiotic silencing of unsynapsed chromatin through a self-pairing process, allowing expression of their own genes to facilitate regular meiosis resulting in fertile individuals. This behavior, also seen in other congeneric species, might be related to their maintenance throughout the evolutionary history of Psalidodon.     

Hybridisation and introgression between Brassica napus and B. rapa in the Netherlands

We used flow cytometry, chromosome counting and AFLP markers to investigate gene flow from the crop plant oilseed rape, Brassica napus (AACC) to wild B. rapa (AA) in the Netherlands. From 89 B. napus source populations investigated, all near cropping fields or at transhipment sites, only 19 contained a B. rapa population within a 2.5‐km radius. During our survey we found only three populations with F1 hybrids (AAC), as recognized by their nine extra chromosomes and by flow cytometry. These hybrids were all collected in mixed populations where the two species grew in close proximity. Populations with F1 hybrids were not close to crops, but instead were located on road verges with highly disturbed soils, in which both species were probably recruited from the soil seed bank. Many plants in the F2, BC1 or higher backcrosses are expected to carry one to eight C chromosomes. However, these plants were not observed among the hybrids. We further investigated introgression with molecular markers (AFLP) and compared sympatric B. rapa populations (near populations of B. napus) with control populations of B. rapa (no B. napus within at least 7 km). We found no difference between sympatric and control populations in the number of C markers in B. rapa, nor did we find that these sympatric populations closely resembled B. napus. Our data show that hybrids occur but also suggest no recent introgression of alleles from the crop plant B. napus into wild B. rapa in the Dutch populations studied.     

B chromosomes in Stylidium crossocephalum (Angiospermae:Stylidiaceae)

Populations of Stylidium crossocephalum contain two common types of B chromosomes, macro B chromosomes and micro B chromosomes. The macro B chromosomes are telocentric, slightly smaller than the smallest A chromosomes and mitotically unstable. They have so far been found associated with 6 of the 16 stable genomes known in S. crossocephalum, occurring in populations covering a substantial portion of the species range. Micro B chromosomes are about one third the length of the smallest A chromosome, acrocentric and show some mitotic instability. They occur associated with 3 stable genomes in populations found in the more medial regions of the species range. Both types of B chromosomes generally show regular behaviour during meiosis, although when two or more are present their pairing efficiency is reduced when compared to the A chromosomes. In addition a single very large mega chromosome was found in a single cell of one heterokaryotypic plant. Its origin although conjectural at this stage may be of relevance in understanding the origin of macro and micro B chromosomes in this species.     

Robertsonian chromosomal variation and identification of metacentric chromosomes in feral mice

Cytogenetic studies of feral mice (M. musculus) from various but predominantly Alpine areas of Switzerland, carried out on random samples collected by spot-checks, established the widespread existence of metacentric chromosomes in the somatic karyotype. Despite the finding of the common occurrence of some of the metacentrics in different places, the examination of the possible homology or heterology by breeding procedures revealed the surprising fact that independence, partial or heterobrachial homology of the metacentric chromosomes prevail among mice from different geographical areas. Thus, the general picture is that of an array of different metacentric chromosomes derived from independent events of Robertsonian variation in the process of evolution. — While heterozygosity with independent metacentrics within a Robertsonian system may have a bearing on the fertility rate of a given mouse population, a more severe impairment of the reproductive capacity must be taken into account in mouse populations which possess different metacentrics with mono- or heterobrachial homologies. These conditions favour the assumption of the existence of a selective system of reproductive barriers further subdividing the species in many, more or less stable, micro-populations. — The chromosomal arms (telocentrics) involved in the formation of the metacentric chromosomes could be identified by Q- and G-banding techniques in combination with the results of crossbreeding, and were assigned to the corresponding telocentric autosomes of the mouse (Comm. Standard. Genet. Nomenclat. for Mice, 1972). Most of the telocentric autosomes of the mouse are included in one or more of the metacentrics found in the feral populations. By means of their isolation in separate lines, these metacentrics may be useful in experimental biology as marker chromosomes of defined identity carrying known linkage groups.