Decrease In Chromosome Number Research Articles

Cytotaxonomy and karyotype evolution in Neotropical Meliponini (Hymenoptera: Apidae) inferred by chromosomal mapping of 18S rDNA and five microsatellites

The Neotropical Meliponini bees, commonly known as stingless bees, are phylogenetically subdivided into three clades in which the chromosome numbers vary from n = 8 to n = 17. The goal of this study was to identify the major chromosomal rearrangements that occurred during the Neotropical Meliponini (Apidae) karyotypic evolution. In this way, we mapped 18S rDNA and five microsatellites in 33 stingless bee species collected from different Brazilian regions. The species belonged to 15 genera and showed six different chromosome numbers: n = 8, n = 9, n = 11, n = 14, n = 15, and n = 17. The 18S rDNA probe showed a variation from 2 to 12 marked chromosomes in different positions (terminal, subterminal, or centromeric), including 2 B chromosomes out of the 7 B found in Tetragonisca fiebrigi. The microsatellite (GA)15, (GAG)10, (CAA)10, and (TCAGG)6 probes formed clusters on the euchromatic regions of the chromosomes and were useful in the identification of putative Robertsonian fusion events that led to the decrease in the chromosome number during the evolution of the Neotropical Meliponini clade. (TTAGG)6 constituted the telomeric sequence of the analyzed species. The ancestral state of the three Neotropical Meliponini clades is difficult to infer, although, the putative ancestral karyotype probably had a single pair of 18S rDNA cistrons, and the decrease in chromosome number and increase in the 18S rDNA sites occurred independently between genera.

Robertsonian rearrangements in Neotropical Meliponini karyotype evolution (Hymenoptera: Apidae: Meliponini).

Genome changes, evidenced through karyotype or nuclear genome size data, can result in reproductive isolation, diversification and speciation. The aim of this study was to understand how changes in the karyotype such as chromosome number and nuclear genome size accompanied the evolution of neotropical stingless bees, and to discuss these data in a phylogenetic context focusing on the karyotype evolution of this clade. We sampled 38 species representing the three Neotropical Meliponini groups; 35 for karyotype analyses and 16 for 1C value measurement. The chromosome number varied from 2n=16 to 2n=34, with distinct karyotypic formulae and the presence of a few polymorphisms, such as B chromosomes in one species and arm size differences between homologous chromosomes in two species. The mean 1C value varied from 0.31 pg to 0.92 pg. We associated empirical data on chromosome number and mean 1C value to highlight the importance of Robertsonian fusion rearrangements, leading to a decrease in chromosome number during the Neotropical Meliponini evolution. These data also allowed us to infer the independent heterochromatin amplification in several genera. Although less frequent, Melipona species with 2n=22 represent evidence of Robertsonian fissions. We also pointed out the importance of chromosomal rearrangements that did not alter chromosome number, such as inversions and heterochromatin amplification.

Karyotypic Diversity and Evolutionary Trends in Neotropical Electric Fish of the Genus Gymnotus (Gymnotiformes: Gymnotidae).

Electric fish of the order Gymnotiformes are endemic to the Neotropical region, and their highest diversity is observed in the Amazon region. The family Gymnotidae, which consists of the genera Electrophorus and Gymnotus, is a natural group and is located at the base of the phylogeny of the order. Gymnotus is a widely distributed and specious genus with high karyotypic diversity, especially concerning to the diploid number and the locations of repetitive sequences. Our karyotyping results in five species of the family Gymnotidae (Gymnotus ucamara, Gymnotus cf. stenoleucus, Gymnotus cf. pedanopterus, Gymnotus mamiraua, and Gymnotus carapo "Maranhão") corroborate the proposal of plasticity of the diploid number in this group. Moreover, in this study, we propose that the 5S ribosomal DNA (rDNA) sequences were species-specific markers that act as a potential biogeographical marker for the genus. Besides, the sequence's location, particularly in G. mamiraua from Central Amazon, shows a close relationship with 5S of the Gymnotus species, with 54 chromosomes, from the Paraná-Paraguay basin in the Center-South of Brazil. Considering that the ancestral diploid number for Gymnotidae is 52 chromosomes, we also suggest that the trend in the family is toward a decrease in the chromosome number. However, the carapo clade stands out in this regard, with an increase and a decrease in chromosome number; this pattern may be reinforced with the ecologic behaviors and the geodispersal patterns of this clade.

Complex meiotic configuration of the holocentric chromosomes: the intriguing case of the scorpion Tityus bahiensis

Mitotic and meiotic chromosomes of Tityus bahiensis were investigated using light (LM) and transmission electron microscopy (TEM) to determine the chromosomal characteristics and disclose the mechanisms responsible for intraspecific variability in chromosome number and for the presence of complex chromosome association during meiosis. This species is endemic to Brazilian fauna and belongs to the family Buthidae, which is considered phylogenetically basal within the order Scorpiones. In the sample examined, four sympatric and distinct diploid numbers were observed: 2n = 5, 2n = 6, 2n = 9, and 2 = 10. The origin of this remarkable chromosome variability was attributed to chromosome fissions and/or fusions, considering that the decrease in chromosome number was concomitant with the increase in chromosome size and vice versa. The LM and TEM analyses showed the presence of chromosomes without localised centromere, the lack of chiasmata and recombination nodules in male meiosis, and two nucleolar organiser regions carrier chromosomes. Furthermore, male prophase I cells revealed multivalent chromosome associations and/or unsynapsed or distinctly associated chromosome regions (gaps, less-condensed chromatin, or loop-like structure) that were continuous with synapsed chromosome segments. All these data permitted us to suggest that the chromosomal rearrangements of T. bahiensis occurred in a heterozygous state. A combination of various factors, such as correct disjunction and balanced segregation of the chromosomes involved in complex meiotic pairing, system of achiasmate meiosis, holocentric nature of the chromosomes, population structure, and species dispersion patterns, could have contributed to the high level of chromosome rearrangements present in T. bahiensis.

Rous sarcomas in mice: the chromosomal progression during early in vivo transplantation.

The chromosomal progression during early in vivo passages was studied in 26 different Rous mouse sarcomas. Several sarcomas, originally having a normal, diploid stemline, showed steps in their heteroploid transformation in accordance with observations in operation samples of the primary tumours, i.e. (1) formation of variant cells, (2) development of a heteroploid sideline from them, (3) increasing superiority of the sideline leading to an S-shift and (4) numerical and/or structural changes not changing the heteroploid S-category. Among the hypodiploids there was a tendency towards decrease in chromosome number; the pseudodiploids revealed signs of a further reshuffling of the genic material; the trends among the hyperdiploids were either elevation of the S-number or appearance of markers; the polyploids showed the “Ising” phenomenon and increasing structural deviations. Polyploidization usually represented a fifth step for heteroploid sarcomas in the diploid region and the following changes for them were in accordance with the modificative step for the polyploids. Thus, the general progressional trend was a development of hypotetraploid-near-triploid stemlines; they showed many similarities to stemlines observed in advanced murine tumours. The present study revealed a 6th case with the chromosomal aberration double-minutes. The histopathological picture was a parameter showing correlation to the stemline category and often to major progressional changes. The progressional pattern found in Rous mouse sarcomas differed from that seen in murine polyoma tumours, mammary carcinomas and lymphomas-leukemias and also from the pattern observed during spontaneous in vitro transformation of normal mouse cells.

Chromosomal evolution in the Ehrlich-Lettré complex of hyperdiploid mouse ascites tumors: results from seven laboratory strains.

The chromosomes of 7 strains of the hyperdiploid Ehrlich-Lettre mouse ascites tumor (ELD) were analyzed by conventional stainings and by G- and C-bandings. A tendency to slow decrease in chromosome number during a 20-year period was revealed in the 7 strains and in the ELD complex in general. A G-band idiogram of the normal mouse chromosomes was elaborated with numbering of regions and bands. By G-band analysis the following average composition of the ELD karyotypes was indicated: unchanged mouse chromosomes (30%,), marA, marB, minutes (11%), mar1—mar12, telocentric markers (10%), anonymous telocentric markers, often strongly rearranged (50%). Great differences were revealed among normal chromosomes and markers in their capacity to persist in the tumor stemline as structurally stable units. The presence of some unchanged chromosomes in most cells of all 7 strains indicated that they involved selective advantage. Evidently chromosomal evolution has operated by preferably maintaining certain elements unchanged and by submitting other elements to heavy rearrangement. The C-band analysis showed that, in addition to the centromeric C-heterochromatin present in all chromosomes, some had interstitial C-bands, as marA in all strains, mar5 and mar7 in specific single strains, and others in numbers varying among the strains.

Establishment and partial characterization of an ovine synovial membrane cell line obtained by transformation with Simian Virus 40 T antigen

The small ruminant lentiviruses, namely caprine arthritis encephalitis virus (CAEV) and Maedi Visna virus (MVV) are grown currently in secondary synovial membrane cells. Primary and secondary cell cultures are sometimes difficult to obtain and support a low number of passages and, therefore, permissive cell lines are needed. A transformed cell line was obtained by transfection of ovine synovial membrane secondary cell culture with a plasmid containing the SV40 large T antigen gene. The transformed cell culture described in this paper showed a higher growth rate and a more homogenous population of fibroblast-like cells when compared to the original ovine synovial membrane secondary cell cultures. Karyotype analysis has indicated the induction of many random chromosome changes, leading to a decrease in chromosome number. The SV40 DNA was detected in the nucleus and in the cytoplasm of transformed cells. The putative expression of large T antigen was presumed by the detection of the corresponding mRNA by PCR. Finally, the transformed ovine synovial membrane cells were shown to be permissive to small ruminant lentiviruses, and these are suggested as a cell line for in vitro isolation and propagation of these viruses.

DNA Loss and Related Alterations in Long-term Cultures of Diploid, Tetraploid and Octaploid Meth-A Cells

The DNA content of polyploid cells sometimes decreases during subculturing. The mechanism of DNA reduction is not yet known. Precise measurements of DNA decay and related alterations in polyploid cells will be required to understand the mechanism. Diploid, tetraploid and octaploid Meth-A cells were continuously cultured for 244 d and the cellular DNA content was measured from the DNA histograms. The DNA content decays gradually with day t as expressed by f(t)=Ip exp{−G(t)}, where Ip is the initial ploidy, G(t)=at/{exp(−bt)+ct}, and a, b and c are the following parameters: a=0.026, b=0.01 and c=0.0175 (for tetraploid cells) or c=0.01 (for octaploid cells). The DNA content of diploid Meth-A cells was constant within the experimental errors. The DNA loss of polyploid cells was confirmed by a decrease in chromosome number. The cellular morphology changed in diploid and octaploid Meth-A cells, but not in tetraploid cells, suggesting that DNA loss and morphology alteration are independent. The doubling time shortened with culture time in the tetraploid and octaploid cells. The findings suggested that chromosomes are not independent in polyploid cells.

Karyomorphology of Taiwanese Begonia (Begoniaceae): taxonomic implications.

The karyomorphology of all 14 species of Taiwanese Begonia was investigated to elucidate their chromosome features and chromosomal evolution. Among all species investigated, differences in chromosome features are found in: (1) chromosome number 2 n = 22, 26, 36, 38, 52, 60, 64, 82, and (2) frequencies of chromosomes with secondary, tertiary, and/or small constrictions of polyploids, ranging from 23% to 63%, which is higher than the expected value of about 9%. It is suggested that after polyploidization from the diploid species (i.e., 2 n = 22 and frequencies of chromosomes with secondary, tertiary, and/or small constrictions of polyploids of about 9%), chromosome translocations occurred, followed by a decrease in chromosome number, and subsequently stabilized genomes were formed in various species in Taiwan. The karyomorphological evidence also suggested that the chromosome morphology has evolved in parallel in the begonias belonging to different sections in Taiwan. The variation in chromosomal features is more complex than the variation in floral and fruit morphologies. Karyomorphological data also supports the recognition of five new species in Taiwan: Begonia bouffordii, B. chuyunshanensis, B. pinglinensis, B. tengchiana, and B. wutaiana. Based on detailed karyomorphological analyses, the taxonomic implications, speciation, and chromosomal evolution in Taiwanese Begoniaare discussed.

Hybrid plant regeneration from interfamilial somatic hybridization between grapevine (vitia vinifera L.) and Red Thorowax (Bupleurum scorzonerifolium willd)

The protoplasts of Red Thorowax (Bupleurum scorzonerifolium) irradiated by ultraviolet light (UV) at an intensity of 260 μW/cm2 for 0, 1,2 and 3 min respectively were fused with that of grapevine (Vitia vinifera). The regenerated 19 clones, every one derived from a single fused cell, were identified as hybrids by phenotype, isozyme, chromosome and 5S rDNA spacer region analysis. The results reveal that all of them are somatic hybrids. 11 hybrid calli including asymmetric and symmetric products regenerated somatic embryos and young leaves after 5 months of culture, of which 4 hybrid cell lines derived from asymmetric fusion regenerated plants with roots after 8–10 months of culture. Inspection of chromosome showed that regeneration of whole plant was related to the decrease of chromosome number. Identification of 5S rDNA spacer region of the plants confirmed that they were interfamilial hybrid plants.

Telomeric signals in Robertsonian fusion and fission chromosomes: implications for the origin of pseudoaneuploidy

In situ hybridization with synthetic plant telomeric sequences resulted in labeling of all broad bean (Vicia faba) chromosomes at their ends only. Telocentric chromosomes derived by fission of the metacentric satellite chromosome of V. faba also showed signals at both of their ends, whereas the ancestral metacentric did not display signals at its primary constriction, the point of fission. As in V. faba, all acrocentric mouse chromosomes were labeled by in situ hybridization with a vertebrate telomeric probe at both ends of each chromatid exclusively. However, different metacentric Robertsonian chromosomes derived by fusion of defined acrocentrics did not show signals at their primary constrictions. The mechanism of Robertsonian rearrangement leading to a pseudoaneuploid increase or decrease in chromosome number therefore cannot consist solely of a simple fission or fusion of chromosomes without a concomitant gain or loss of chromatin material. The additional assumption of a subdetectable deletion of telomeric sequences after fusion and amplification of these sequences following fission is necessary to explain the present observations.

Karyotypic Evolution in Ctenomys (Rodentia, Ctenomyidae)

Cytogenetic relationships among Andean species of Ctenomys were examined by use of nondifferentially stained chromosomes, C-band patterns, and sperm morphology. Chromosomal diversity shows evolutionary karyotypic pathways that are remarkably varied. C-band analyses indicate that tandem fusions and other non-Robertsonian mechanisms account for the variation observed in the genus. Evolution of heterochromatin has proceeded through whole-arm additions and euchromatin transformation. Evolutionary trends toward an increase or decrease in chromosome number have been independent of heterochromatic changes. Heterochromatic short arms do not account for the large number of metacentric chromosomes. Accordingly, neither high nor low fundamental numbers per se represent derived conditions in this taxon.

On the karyotypes of the Neorhabdocoela species and karyological evolution of Turbellaria

The karyotypes of 10 species belonging to the Neorhabdocoela order (Turbellaria) are described: Proxenetes flabelliger, 2n=6 (Trigonostomidae), Promesostoma rostratum, 2n=12 (Promesostomatidae), Castrada sp., 2n=6, Rhynchomesostoma rostratum, 2n=6, Bothromesostoma esseni, 2n=10, Mesostoma lingua, 2n=8 (Typhloplanidae), Provortex karlingi, 2n=8 (Provorticidae), Halamovortex macropharynx, 2n=16 (Dalyellidae), Macrorhynchus crocea, 2n=16, and Gyratrix hermaphroditus, 2n=4 (Polycystidae). It is proposed that the karyotypes of the ancestral marine neorhabdocoel forms, as well as those of the other groups of turbellarians, must have consisted of 16–20 biarmed chromosomes. The processes of adaptation and speciation in each group seem to be accompanied by a gradual decrease in chromosome number to 2n=8−4, this being due to Robertsonian fusions and translocations. In some freshwater turbellarians the diploid number increased by polyploidisation. The same processes evidently took place in different groups of parasitic worms.

Deoxycytidine reverses the suppression of nucleolar organizer regions' activity caused by BrdU

In this article we report that 5'-bromodeoxyuridine (BrdU) significantly suppresses the nucleolar organizer regions (NORs) activity of Chinese hamster cells (both dipliod cells and cell line Wg3-h) (P<0.001). One of the most obvious characteristics of the suppression is a significant decrease in the total number of the Ag-NORs per cell rather than in a frequency variation of the associated Ag-NORs. The decrease in the Ag-NORs number is mainly because of the decrease in number of chromosomes bearing 2 Ag-NORs. The degree of the suppression increases with increase in BrdU concentration in the culture medium. There is a close relationship between the suppression and the BrdU-treatment time, i.e. for a given concentration of the BrdU, the longer the BrdU-treatment time, the stronger the suppression. When the BrdU-treated cells are transferred into BrdU-free medium and allowed to grow in it for another 30 h, NORs activity can be restored. Therefore, the suppression of NORs activity may be due to BrdU toxicity. When deoxycytidine (dC) is added into medium containing 30 μg/ml of BrdU, the total number of both the Ag-NORs and the chromosomes bearing Ag-NORs per cell increases to the level of untreated cells. Our results thus indicate that the addition of dC reverses the suppression of the NORs activity caused by BrdU.

Stability of the “two active X” phenotype in triploid somatic cells

We examined triploid cells of XXY karyotype heterozygous for glucose 6 phosphate dehydrogenase (G6PD) electrophoretic variants with regard to the stability of their X chromosome phenotype. Clonal populations of cells derived from these human fibroblasts maintained a precise 1:2:1 ratio of A:heteropolymer:B isozymes throughout their life span, indicating stability of the two active X chromosomes in these cells. To determine the influence of the autosomal complement on X chromosome expression, we attempted to perturb the relationship. Fusion of these triploid cells with human diploid fibroblasts carrying a novel G6PD variant (B') resulted in heterokaryons expressing a novel heteropolymer, presumably indicating that all three parental X chromosomes were active. However, no derepression of the inactive X chromosome was observed. Analysis of interspecific hybrids derived from triploid cells and mouse fibroblasts confirmed that activity of parental X chromosomes is maintained. Some human mouse hybrid clones, however, expressed only a single human G6PD isozyme, probably attributable to segregation of the pertinent X chromosome, but elimination of a relevant autosome cannot be excluded. The triploid cells transformed by SV40 showed alterations in LDH pattern and an approximately 10–20% decrease in chromosome number, but maintained the precise G6PD phenotype of the untransformed cell. These studies provide evidence for the stability of the X chromosome phenotype in triploid cells.

Revertants of mouse cells transformed by murine sarcoma virus: flat variants without a rescuable sarcoma virus from a clone of BALB/3T3 transformed by Kirsten MSV.

SUMMARY Clonal subline of Kirsten murine sarcoma virus (Ki-MSV)-transformed non-producer BALB/3T3 (Ki-BALB) cells spontaneously produced flat variants with some properties of non-transformed cells at frequencies of 0.01 to 0.001. Such variants were epithelioid, contact-inhibited, grew to low saturation density and exhibited variable cloning efficiences in soft agar. They demonstrated no murine leukaemia group-specific antigen(s), no reverse transcriptase activity, and no infectious virus, but were agglutinable by concanavalin A. Murine leukaemia virus (MuLV) was induced after 5-iododeoxyuridine (IUDR) treatment from both the flat variants and from the parental Ki-BALB cells. However, Ki-MSV could not be rescued from these flat variants by superinfection with MuLV, nor induced by treatment with IUDR. The state of reversion was stable, revealing no back-transformation during 30 to 40 subcultures. The tumourigenicity of the flat variants in BALB/c mice was markedly reduced (actually undetectable) compared to that of the Ki-BALB cells. All flat variants were susceptible to MSV and MuLV infection. The loss of the transformed phenotype in spontaneous flat variants was associated with a decrease in chromosome number to a level similar to BALB/3T3 cells. These observations suggest that these revertants may have lost some or all of the Ki-MSV genome.

Chromosome number within the class Ascidiacea

Additional evidence has been obtained that demonstrates the low capacity of ascidians to polyploidize. The evolution of ascidians appears to have been accompanied by a decrease of chromosome number within the Phlebobranchiata, and by both a decrease and increase of chromosome number in the other two suborders, Aplousobranchiata and Stolidobranchiata. There are indications that the basic chromosome number, 2n=32, of Stolidobranchiata is of very early origin within the class Ascidiacea.

The biology and systematics of Geranium, sections Anemonifolia Knuth and Ruberta Dum.

The external morphology of the six species is considered in detail, special attention being given to the branching System, flower form and the type of seed dispersal, which has often been misunderstood and has parallels among basidiomycete Fungi. In addition petiole anatomy is described, all species being different, and unpublished chromosome counts by other workers are reported (including 2n = 128 for G. canariense, a number erroneously reported earlier for G. palmatum). Crossing experiments (partly carried out by Mr W. Jackson) are described; the only successful new interspecific cross was G. maderense x G. palmatum. It is concluded that the six species, some of which are large pachycaul herbs endemic to Madeira or the Canary Islands, form an advanced group, and their affinities are discussed. A survey of other species with the same seed-discharge mechanism, and of annual or biennial species, suggests that among the latter decrease in chromosome number has taken place along several different lines, and it is suggested that thèse lowered numbers have been combined by amphidiploidy to create new basic numbers in the genus. A formal taxonomic treatment of sections Anemonifolia and Ruberta includes observations on infraspecific variation. An appendix reports the chromosome number of G. cataractarum as 2n = 36.

Chromosome multiformity in the genus Ctenomys (Rodentia, Octodontidae). A progress report.

The karotypes of eleven species of the South American burrowing rodents, genus Ctenomys are described, and information on the somatic number of two other species of the same genus is given. The studied species are: C. torquatus (2n ♂=68), C. tuconax (2n ♂=61), C. minutus (2n=50), C. talarum (2n= 48), C. porteousi (2n=48), C. cf. minutus (2n=48), C. australis (2n=46), C. azarae (2n=48), C. latro (2n=42), C. magellanicus fueguinus (2n=36), C. tucumanus (2n=28), C. opimus luteolus (2n=26), and C. occultus (2n ♀=22). This extreme intrageneric variation in somatic number is also reflected by a great amount of diversity in chromosome structure. Karyotypes seem to be rather constant at the species level. Autosomal polymorphism has been found in two of the species, namely C. talarum and C. latro. The hypothesis of the superimposition of Robertsoman rearrangements, pericentric inversions and translocations in the evolution of the karyotype of Ctenomys is advanced. The direction of chromosome change, either toward increase or decrease in chromosome number, is discussed. It is emphasized that high chromosome multiformity is correlated in Ctenomys with a rapid and explosive pattern of species diversification; the meaning of the small size of populations in enhancing the role of chromosome rearrangements in the evolution of Ctenomys is discussed.

BIOCHEMICAL AND GENETICAL STUDIES ON RADIORESISTANT MOUSE CELLS IN CULTURE

A cell line resistant to γ-irradiation was isolated from the original mouse strain L cells after seven successive irradiations with total dose of 14, 000R and was designated Lγ7. The various biochemical and genetical properties of these resistant cells were investigated.The Lγ7 cells manifested a resistance to radiation, although the degree of resistance was not so high. The mean generation time of the original L cells was 27 hours during the logarithmic phase of growth, whereas that of the Lγ7 cells was 84 to 90 hours. Their cell morphology also differed from that of the original L cells; the Lγ7 cells were spherical, whereas the original L cells were fibrous.The lower metabolic activity of oxidative phosphorylation in the Lγ7 cells seems to have a relation to their reduced multiplication rate and radioresistance. Furthermore, the content of SH groups per cell in both protein and non-protein SH reactions were significantly greater in the radioresistant Lγ7 cells than in the original L cells. The high content of SH groups in the radioresistant cells also may have some relation to their radioresistance.The original L cells had a mode of chromosome number at 63. The Lγ7 cells had a mode of 44 chromosomes. Thus there was a shift in chromosome number. This decrease in chromosome number in the latter cells was gradually derived by seven repeated γ-irradiations of 2, 000R each. Similar phenomenon was observed in the changes in number of chromosomes in irradiated Ehrlich ascites tumor cells. The original Ehrlich ascites tumor cells had a mode around 58 chromosomes, whereas EX4 cells, isolated after four successive irradiation with total dose of 8, 000R of X-rays, had a mode of 51 chromosomes.In the present investigation it was indicated that a correlation may exist between chromosome number and radioresistance in mammalian cells. In addition, the gradual shift in the chromosome number of the radioresistant cells (Lγ7 cells and EX4 cells) from the original L cells and Ehrlich ascites tumor cells seems to suggest that radioresistant cells may be derived from the pre-existing resistant cells in the original unirradiated population.