Chromosome banding in Amphibia. XXIV. The B chromosomes of Gastrotheca espeletia (Anura,Hylidae)

The mitotic chromosomes of an Ecuadorian population of the marsupial frog Gastrotheca espeletia were analyzed by means of banding techniques and fluorescence in situ hybridization. This species is characterized by unusual supernumerary (B) chromosomes. The maximum number of B chromosomes is 9 and they occur in three different morphological types. Banding analyses show that the B chromosomes are completely heterochromatic, consist of AT base pair-rich repeated DNA sequences, replicate their DNA in very late S-phase of the cell cycle, and are probably derived from a centromeric or paracentromeric region of a standard (A) chromosome. Exceptionally, the B chromosomes carry 18S + 28S ribosomal RNA genes and the conserved vertebrate telomeric DNA sequence appears to be underrepresented. Flow cytometric measurements of the nuclear DNA content differentiate between individuals with different numbers of B chromosomes. Significantly more B chromosomes are present in female than in male animals.     

Reproductive modes,ploidy distribution,and supernumerary chromosome frequencies of the flatworm Polycelis nigra (Platyhelminthes: Tricladida)

The hermaphroditic flatworm, Polycelis nigra, is characterized by two reproductive biotypes which differ with respect to ploidy; sexual individuals are diploid (n = 8, 2× = 16) and pseudogamous parthenogenetic individuals are polyploid (typically 3×). We have collected and karyotyped individuals from 15 sampling sites (13 in mid to northern Italy, one in Great Britain and one in The Netherlands). We found that biotypes can exist alone or in sympatry, and identified purely diploid, mixed diploid-polyploid, and purely polyploid populations. Karyotype data show that in addition to the normal autosome complement, B chromosomes of differing morphology as well as stable aneuploid chromosomes (extra-A) were found almost exclusively in polyploids (11 of 12 sites). We extensively sampled Lago di Toblino (northern Italy), a pure polyploid population characterized by a submetacentric to metacentric, mitotically stable B chromosome, as well as a stable extra-A chromosome. Here, individuals having 1–3 B chromosomes were more abundant (61%) than those having no B's, implying that B chromosome infection has little detrimental effect when occurring in low numbers. Furthermore, 66% of individuals from this population possessed extra-A chromosomes, although it is unclear whether these elements are aneuploid autosomes or B chromosomes of different morphology. The ubiquity of these chromosomes, within asexuals in particular, is suggestive of a correlation between the origination of the elements and the evolution of polyploidy, or may reflect increased tolerance of parthenogenetic genomes to aneuploidy. This revised version was published online in July 2006 with corrections to the Cover Date.     

The interaction of knobs and B chromosomes of maize in determining the level of recombination

Enhancement of recombination by B chromosomes is influenced by the kind of heterochromatic knob present in or near the tested region of the A chromosomes. In homomorphic chromosome 9 bivalents of K^s/K^s constitution, double exchanges were increased at the expense of singles, but in the K*/K^s heteromorphs there was a gain in both single and double exchanges at the expense of no-exchange tetrads. Modification of the B chromosome enhancement in different knob compounds was observed only in the megasporocytes.—Different frequencies of recombination are found in plants with odd and even numbers of B chromosomes; this effect is especially striking in the megasporocytes. The modification in recombination produced by an odd or even number of B chromosomes is a function of the interaction of a particular region and the knob constitution. Odd numbers of B chromosomes were more effective than even numbers in causing increased recombination.—It is concluded that heterochromatic knobs and the essentially heterochromatic supernumeraries may interact in the process of crossing over, with the level of recombination determined in part by knob constitution.     

Interaction among different heterocromatic variants in the grasshopper Dichroplus elongatus

Simultaneous chromosome polymorphisms for supernumerary elements allow us to analyse the relationships among different forms of heterochromatic variation in nature. We report simultaneous variation patterns for supernumerary segments in chromosomes S10 (SS10), S9 (SS9) and S6 (SS6) and B chromosomes in nine populations of the grasshopper Dichroplus elongatus from two biogeographic provinces from east Argentina. Our results show spatial chromosome differentiation for three out of four supernumerary heterochromatic variants (B chromosomes, SS6 and SS10). The incidence of B chromosomes was negatively correlated with the SS10 frequency. The distribution pattern analysis shows different degree of differentiation among populations for each supernumerary heterochromatic variant suggesting that the detected chromosome variation cannot be explained by interaction between migration and genetic drift. Moreover, the observed population chromosome differentiation was not in agreement with the hierarchical analysis of molecular of heterogeneity at mitochondrial DNA level (mtDNA). The present results point out the importance of the interaction among heterochromatic variants in the chromosome intraspecific variation in east Argentina natural populations of the grasshopper D. elongatus.     

Cytogenetic mapping of the retroelements Rex1, Rex3 and Rex6 among cichlid fish: new insights on the chromosomal distribution of transposable elements

To enhance our understanding of the organization of the genome and chromosome evolution of cichlid fish species, we have isolated and physically mapped onto the chromosomes the transposable elements (TEs) Rex1, Rex3 and Rex6, which are conserved in teleost fish, in the chromosomes of African and South American cichlid species. The physical mapping of different Rex elements showed that they are primarily compartmentalized in the pericentromeric heterochromatic regions, although dispersed or clustered signals in euchromatic regions were also observed. The presence of TEs in heterochromatin can be correlated with their role in the structure and organization of heterochromatic areas (such as centromeres) or with the lower selective pressure that act on these gene-poor regions. The Rex elements were also concentrated in the largest chromosome pair of the Nile tilapia, Oreochromis niloticus. This chromosome pair is supposed to have originated by fusions, demonstrating the possible involvement of TEs with chromosome rearrangements. Besides general patterns of chromosomal distribution, comparative analysis suggests that Rex elements could differ in their chromosomal distribution among different fish groups or species and that intrinsic aspects of the genomes could influence the spread, accumulation or elimination of TEs.     

Is the aneuploid chromosome in an apomictic Boechera holboellii a genuine B chromosome?

The Boechera holboellii complex comprises B. holboellii and B. drummondii, both of which can reproduce through sex or apomixis. Sexuality is associated with diploidy, whereas apomictic individuals can either be diploid, aneuploid or triploid. Aneuploid individuals are found in geographically and genetically distinct populations and contain a single extra chromosome. It is unknown whether the supernumerary chromosomes are shared by common descent (single origin) or have originated via introgressive hybridizations associated with the repeated transition from diploidy to triploidy. Diploid plants containing the extra chromosome(s) reproduce apomictically, suggesting that the supernumerary elements are associated with apomixis. In this study we compared flow cytometry data, chromosome morphology, and DNA sequences of sexual diploid and apomictic aneuploids in order to establish whether the extra chromosome fits the classical concept of a B chromosome. Karyotype analyses revealed that the supernumerary chromosome in the metaphase complement is heterochromatic and often smaller than the A chromosomes, and differs in length between apomictic plants from different populations. DNA sequence analyses furthermore demonstrated elevated levels of non-synonymous substitutions in one of the alleles, likely that on the aneuploid chromosome. Although the extra chromosome in apomictic Boechera does not go through normal reductional meiosis, in which it may get eliminated or accumulated by a B-chromosome-specific process, its variable size and heterochromatic nature does meet the remaining criteria for a genuine B chromosome in other species. Its prevalence and conserved genetic composition nonetheless implies that this chromosome, if truly a B, may be atypical with respect to its influence on its carriers.     

Polyploidy and karyotype variation in Turkish Bellevalia (Hyacinthaceae)

There are 20 Bellevalia species in Turkey, half of which are endemic. Chromosome numbers are known for 15 species. A chromosome survey of 145 Bellevalia individuals showed that the karyotype is remarkably stable. All are based on x  = 4. The majority are diploid with 2 n  = 8, but there is also a polyploid series of 2 n  = 16, 24 and 32. Aneuploidy occurs only at the octoploid level. Eleven individuals had metacentric B chromosomes, one had acrocentric Bs and one had telocentric Bs. Bellevalia pycnantha and B. paradoxa are morphologically similar, with B. pycnantha reduced to a synonom of B. paradoxa .  © 2003 The Linnean Society of London, Botanical Journal of the Linnean Society , 2003, 143 , 87–98.     

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.     

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.     

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.     

Intranuclear destruction of heterochromatin in two species of armored scale insects

Spermatogenesis is described in two species of armored scale insects,Parlatoria proteus andParlatoria ziziphus. In the males of both species, a haploid set of four chromosomes becomes heterochromatic during early embryogeny. The heterochromatic chromosomes are lost later by two different mechanisms during spermatogenesis. Just before meiosis begins one or more heterochromatic chromosomes disappear from each primary spermatocyte as a consequence of a rapid intranuclear chromosome destruction. Meiosis consists of a single achiasmatic division. At prophase four euchromatic and from one to three heterochromatic chromosomes are present in each cell. Although both the euchromatic and remaining heterochromatic chromosomes divide, the heterochromatic chromosomes are later eliminated by posttelophase ejection; the eliminated chromosomes then disintegrate slowly in the cytoplasm. Each of the two species displays a species specific level of heterochromatin retention and both differ in this regard from the previously describedParlatoria oleae. The evolution of a chromosome system involving intranuclear chromosome destruction is discussed.     

The genomic complexity of micro B chromosomes of Brachycome dichromosomatica

A major sequence component of the micro B chromosome of Brachycome dichromosomatica (2 n=4) is the tandem repeat Bdm29, which was found by in situ hybridisation to be distributed along the entire length of the chromosome. A high copy number of this sequence does not occur as a regular feature of the A chromosomes in this species but it was found in infrequent individuals in two wild populations that were analysed. In these instances Bdm29 is localised within heterochromatic, polymorphic segments on the long arm of chromosome 1. The origin of the micro B chromosomes was investigated by determining whether they are related to this A chromosome polymorphism by simple excision and/or integration. Results obtained by using Bdm29, together with a newly isolated repeat sequence, Bdm54, and a number of other sequences known to occur on the micro B chromosome, as probes in in situ hybridisation and Southern analysis demonstrated that the formation of micro B chromosomes is a complex multistep process. The observation that the genomic organisation of the micro B chromosome is unlike anything found on the A chromosomes precludes their origin by simple excision and also indicates that micro Bs do not integrate directly into the A complement to form polymorphic heterochomatic segments.     

A note on the presence of B chromosome in the small Japanese field mouse,Apodemus argenteus,in central Honshu,Japan

Previously, many studies have revealed the presence of B chromosomes in wild mouse taxa of the genus Apodemus (Rodentia, Muridae). In one of the Apodemus species, A. argenteus, which is endemic to Japan, it is known that B chromosomes were confirmed only in individuals (2n = 46 + B chromosome) from Hokkaido, Japan. There is no report of the presence of B chromosomes from other localities in the Japanese Islands. In this study, we analyzed the chromosomal constitutions of 43 individuals of A. argenteus from three localities in Honshu, Japan. A total of three individuals from central Honshu showed 2n = 47, and each individual carried a dot-like B chromosome. In addition, these B chromosome features were analyzed by differential staining methods, and the C- and QM-banding patterns of the B chromosomes were identical to those of the X chromosomal heterochromatic region showing the delayed-fluorescent response. Thus, it is considered that these B chromosomes would be derived from the heterochromatin of the X chromosomes, as reported in previously published papers.     

A Comparative Karyotypic Study of Three Species in Fritillaria

The present paper deals with a comparative karyotypic study of three species in Fritillaria-F. thuncergii Miq., F. anhuiensis S. . Chen et S. F. Yin and F. hupehensis Hsiao et K. C. Hsia. The karyotype of F. anhuiensis S. C. Chen et S. F. Yin is first reported. The karyotypes of the three species of Fritillaria are rather similar, all with K(2n)=24= 2m+2sm+12t+4st+4m (SAT), showing a close interspecific relationship. They all have two pairs of st chromosomes, one of which is the third chromosome in all the three species studied, but the other is the seventh in F. thunbergii Miq, the eighth in F. anhuiensis S. C. Chen et S. F. Yin, and the fifth in F. hupehensis Hsiao et K. C. Hsia. It tells us that there are some differences in their karyotypes. All of the three species possess two pairs of satellite chromosomes with the satellites located on the long arms. A heterochromatic zone is found sometimes on long arms of No. IX chromosome in each species of Fritillaria and on one of No. I chromosomes in both F. thunbergii Miq. and F. anhuiensis S. C. Chen et S. F. Yin, a chromosome polymorphism occurring between populations of Fritillaria. In addition, three B chromosomes are always found in most root-tip cells of F. hupehensis Hsiao et K. C. Hsiao.     

Chromosome arrangement and nuclear architecture but not centromeric sequences are conserved between Arabidopsis thaliana and Arabidopsis lyrata

In contrast to the situation described for mammals and Drosophila, chromosome territory (CT) arrangement and somatic homologous pairing in interphase nuclei of Arabidopsis thaliana (n = 5) are predominantly random except for a more frequent association of the chromosomes bearing a homologous nucleolus organizer region. To find out whether this chromosome arrangement is also characteristic for other species of the genus Arabidopsis, we investigated Arabidopsis lyrata ssp. lyrata (n = 8), one of the closest relatives of A. thaliana. First, we determined the size of each chromosome and chromosome arm, the sequence type of centromeric repeats and their distribution between individual centromeres and the position of the 5S/45S rDNA arrays in A. lyrata. Then we demonstrated that CT arrangement, homologous pairing and sister chromatid alignment of distinct euchromatic and/or heterochromatic regions within A. lyrata interphase nuclei are similar to that in A. thaliana nuclei. Thus, the arrangement of interphase chromosomes appears to be conserved between both taxa that diverged about 5 million years ago. Since the chromosomes of A. lyrata resemble those of the presumed ancestral karyotype, a similar arrangement of interphase chromosomes is also to be expected for other closely related diploid species of the Brassicaceae family.     

Polymorphism and mosaicism of B chromosome number in Korean field mouse <Emphasis Type="Italic">Apodemus peninsulae</Emphasis> (Rodentia) in the Russian Far East

A karyotype analysis of the Apodemus peninsulae (n = 355) from 41 trapping points from the Russian Far East has allowed us to identify B chromosomes in 87.9% of the animals, 61.7% of which are mosaics. Different levels of variability in the B chromosome numbers have been studied, including both the inter- and intrapopulational, as well as intraindividual variability (mosaicism). It was found that the frequencies of the occurrence of individuals with B chromosomes and mosaicism between different population samples were not constant. The range of the modal B chromosome number variability and variation of the xB index (zero to four; on average, the xB amounted to 1.67) were studied for the first time in different samples and populations of this species. Individuals with the predominant numbers of B chromosome (as a rule, zero to two) were revealed in both groups of animals (with stable and mosaic karyotypes), but the frequency was different in geographical regions. The spectra of B chromosome variability were wider in mosaics (zero to seven) compared to animals with stable karyotypes (zero to four). The importance of this for species of the high frequency of individuals with B chromosomes and with mosaicism has been discussed. The adaptive role of the low number of B chromosomes (one to two), as well as the imbalance of the B chromosome system for the species as a whole is assumed.     

Human supernumeraries: are they B chromosomes?

In humans, the presence of supernumerary chromosomes is an unusual phenomenon, which is often associated with developmental abnormalities and malformations. In contrast to most animal and plant species, the extensive knowledge of the human genome and the ample set of molecular and cytogenetic tools available have permitted to ascertain not only that most human supernumerary chromosomes (HSCs) derive from the A chromosome set, but also the specific A chromosome from which most of them arose. These extra chromosomes are classified into six types on the basis of morphology and size. There are both heterochromatic and euchromatic HSCs, the latter being more detrimental. Most are mitotically stable, except some producing individual mosaicism. No information is available on the HSC transmission rate since extensive familial studies are not usually performed generally because of death of the relatives or lack of cooperation. The main B chromosome property failing in HSCs seems to be their population spread as polymorphisms, since most HSCs seem to correspond to extra A chromosomes or centric fragments spontaneously arisen in the analysed individual or one of his/her parents. However, we cannot rule out at this moment, that more intensive studies on population distribution and frequency of those HSCs most closely resembling B chromosomes (i.e. those heterochromatic and thus less detrimental) would reveal possible HSCs polymorphisms. Although HSCs cannot be considered B chromosomes, some of them might be a source for future B chromosomes. The best candidates would be heterochromatic HSCs, which might manage to drive in either sex. To ascertain this possibility, research on inheritance and population studies would be very helpful in combination with the powerful cytogenetic and molecular tools available for our species.     

Cytogenetic studies of Hynobiidae (Urodela)XVIII. A ZZ/ZW sex-determining mechanism in a hynobiid salamander species,Hynobius tokyoensis Tago

The karyotype of Hynobius tokyoensis (2n = 56) was analyzed using three kinds of banding methods to determine the morphological differentiation of the sex chromosomes of this species. Salamanders and egg sacs were collected from seven localities around Tokyo, Japan. Of 28 chromosome pairs, microchromosome No. 21 was identified as a ZZ/ZW-type sex chromosome. The Z chromosome was acrocentric, whereas the W chromosome was submetacentric, with a heterochromatic, elongated short arm. Interestingly, the W chromosome is of three distinct types, W(A), W(B), and W(C), based on R-banding and Ag-NOR patterns. W(A) was detected in five populations from southern habitats, whereas W(B) and W(C) were detected in one population each from northern habitats. W(A), W(B), and W(C) were all found to carry Ag-NORs on their heterochromatic short arms. Considering the karyotypes of other species belonging to the same genus, we discuss the evolution of the sex chromosomes of H. tokyoensis.     

濒危植物三棱栎四个居群的核型

报道了濒危植物三棱栎 4个居群的染色体数目和核型 ,4个居群的核型公式均为 2n =14 =10m 2sm 2st 2bs,核型类型均为 2A。间期核为复杂染色中心型 (complexchromocentertype) ,细胞有丝分裂前期染色体为中间型 (interstitialtype)。 4个居群在前期染色体中 ,可观察到2个B染色体 ,但在中期染色体中较少发现。根据核型分析结果 ,4个居群间核型变异不明显     

Chromosome plasticity in Ctenomys (Rodentia Octodontidae): chromosome 1 evolution and heterochromatin variation

A chromosome 1 (Cr1) pericentric inversion is described in six of seven species in the genus Ctenomys (tuco-tucos) from Uruguay. The inversion was inferred from G-band analyses of subtelocentric Cr1 hypothesised to be derived from the ancestral metacentric condition. Cr1 varies across species in heterochromatin amount and localisation including a metacentric chromosome without positive C-bands in C. torquatus, a subtelocentric chromosome with heterochromatic short arms in C. rionegrensis, and a subtelocentric chromosome negative after C-banding in five of the species analysed here. Pachytene chromosomes from C. rionegrensis, a species with the highest heterochromatin content, and C. torquatus, one of the species with the lowest heterochromatin content, were analysed in order to assess possible mechanisms of heterochromatin evolution. This analysis revealed the presence of three heterochromatic chromocenters in C. rionegrensis where bivalents converge, while in C. torquatus only one chromocenter was observed. In both species, highly repetitive DNA was observed, localised in chromocenters after “in situ” hybridisation. Heterochromatin associated protein M31 was localised in chromocenters of both species after immuno-detection. The spread of heterochromatin in Ctenomys chromosomes could be produced by chromatin exchanges at the chromocenter level. We propose the exchange of this DNA associated proteins between non-homologous chromosomes in pachytene to be the responsible for the spread of heterochromatin through the karyotypes of species like C. rionegrensis