Centric Fusion Research Articles

An X1X1X2X2/X1X2Y multiple sex chromosome system in a new speciesof the genus Gymnotus (Pisces, Gymnotiformes)

We characterize the karyotype of Gymnotus sp., an undescribed fish species from the Parana River. The diploid number is 2n = 40 chromosomes for females and 2n = 39 chromosomes for males, showing a multiple sex chromosome system of the X1X1X2X2/X1X2Y type, a first description in the genus Gymnotus. The nucleolar organizing regions sites are located in only one chromosome pair, a common feature among Gymnotiformes, and heterochromatin is present in the centromeric region of all chromosomes. Although the metacentric neo-Y chromosome appears to have arisen by centric fusion in three species from different Gymnotiformes families, the presence of the X1X1X2X2/X1X2Y sex chromosome system seems to be a homoplasic event in this group.

Sperm segregation analysis of a (13;22) Robertsonian translocation carrier by FISH: a comparison of locus-specific probe and whole chromosome painting

The t(13;22) Robertsonian translocation constitutes a rare form of rearrangement between acrocentric human chromosomes. Most of the meiotic segregation studies of human Robertsonian translocations have been performed on common t(13;14) and t(14;21) translocations. Analysis of the chromosomal constitution in sperm of Robertsonian translocation carriers is of great interest for assessing the risk of unbalanced forms and adapting genetic counselling. In the present study, we present the first meiotic segregation study of a t(13;22) Robertsonian translocation in human sperm. A total of 11 787 sperm nuclei were scored using two distinct FISH labelling techniques, i.e. the locus-specific probes (LSI) method and the whole chromosome painting (WCP) technique. The frequency of normal or balanced sperm resulting from alternate meiotic segregation was 86%. Incidences of unbalanced complements resulting from adjacent segregation modes were 12.79% and 14.36% in LSI and WCP assays, respectively. No significant excess of nullisomy or disomy for the affected chromosomes was observed. Similar results in segregation were obtained with the two techniques, demonstrating the efficiency of the two strategies for the direct segregation analysis of Roberstsonian translocations. The results obtained indicated a moderate meiotic production of imbalance. This study shows that the rare Robertsonian translocation (13;22) displays a similar distribution of balanced and unbalanced sperm patterns as the common Robertsonian translocations previously studied. This suggests that the behaviour of acrocentric chromosomes was similar in all cases of centric fusion.

Variability along a latitudinal gradient in the chiasma frequency and morphological characters of Dichroplus pratensis (Orthoptera: Acrididae)

The grasshopper Dichroplus pratensis Bruner is polymorphic and polytypic for a complex Robertsonian system. In this species, centric fusions induce changes in number and position of chiasmata, and thus potentially affect intrachromosomal genetic recombination and genetic variability. Males and females, from 23 populations covering most of the geographic range of the species and spanning 22 degrees of latitude, were studied. We analyzed chiasma frequency in relation to variability in six exomorphological characters. The chromosomal polymorphisms of D. pratensis are widely geographically distributed, and show a central-marginal pattern, in which the central populations (those occupying the ecologically optimal habitats) have high mean frequencies of different fusions per individual (F) of up to F = 3.00 and total chiasma frequencies as low as XT = 8.98 per cell, while those near the margins of the distribution (central Patagonia and the Andes) have very low levels of chromosomal polymorphisms [down to F = 0.00 in most geographically marginal locations), monomorphic karyotypes and high chiasma frequencies (XT = 11.66, in the southernmost (Rada Tilly, 45°57´S) and XT = 12.01 in the northernmost population (Volcan, 23°55´S)]. Increasing chiasma frequencies towards the margins of the range are positively and significantly correlated with increasing levels of morphological variability. The decrease in fusion polymorphism and the consequent increase in genetic recombination (both inter- and intrachromosomal) in the marginal areas, is a result of natural selection favouring higher levels of variability, which could be adaptive in ecologically harsher and changing environments.

Karyotype diversification in fishes of the Balistidae, Diodontidae and Tetraodontidae (Tetraodontiformes)

Among the great marine fish diversity, the Order Tetraodontiformes is remarkable by presenting post-Perciformes modern features, representing one of the major branches of the teleosteans radiation. Patterns of chromosomal evolution in this group are not fully understood. In the present work, cytogenetical analyses were carried out in the species Balistes vetula (2n=44; FN=44) and Melichthys niger (2n=40 and FN=40) (Balistidae), Chilomycterus antennatus (2n=52; 6M+46st/a, FN=58) (Diodontidae) and Sphoeroides testudineus (2n=46; 18m+4sm+6st+18a, FN=74) (Tetraodontidae), collected along the Brazilian coast and Saint Pauls Rocks. All spe- cies presented Ag-NORs sites on a single chromosomal pair. Heterochromatic regions in this group are reduced and located at centromeric position over most of chromosomal pairs. The evolutionary patterns of chromosomal changes were diverse in the distinct Tetraodontiformes families. In Balistidae, the evolution process seems to be determined by in tandem or centric fusions, followed by pericentric inversions. The higher chromosomal number in Diodontidae indicates that centric fissions and pericentric inversions played an important role in the karyotypi- cal definition of this group. The Tetraodontidae S. testudineus displayed small chromosomes with a modal number shared with other species previously analyzed from this family. Such great karyotypical diversity is com- patible with a scenario of several modifications established by the adaptative irradiation of this group.

Chromosomal polymorphism and speciation in Laetacara cf. dorsigera (Teleostei, Perciformes, Cichlidae) from the river Paraná PR Brazil

Abstract Four sympatric and syntopic karyotype forms of Laetacara cf. dorsigera, centric fusion-originated were analyzed. Specimens with 2n=46, 2n=45, 2n=44 and 2n=43 chromosomes and differences with regard to the number of metacentric chromosomes, inversely proportional to their diploid number, were reported. Karyotypic formulae of the four cytotypes were: cytotype A=2M + 44A; cytotype B=3M + 42A; cytotype C=4M + 40A and cytotype D=5M + 38A and fundamental number constant (NF=48). Two centric fusion events are suggested: a) Robertsonian trans-location, which originated the first and second metacentric chromosomal pairs; b) isochromosome formed by the fusion of acrocentric homologous chromosomes in cytotypes with odd chromosomes. Genetic balance in the four karyotype forms of Laetacara cf. dorsigera seemed to have been determinant to their reproductive isolation. The presence of individuals with different karyotypic structures living in sympatry and syntopy and the absence of “intermediate individuals” (r...

B chromosomes: the troubles of integration

Starting with a spontaneous B-A centric fusion found in a natural population of the grasshopper Eyprepocnemis plorans, we have obtained different strains carrying the rearrangement in various conditions and doses. Using this material, we have analyzed the meiotic behavior of the translocated chromosome in living cultured spermatocytes, simulating the successive steps of a hypothetical process of integration of a B chromosome into the standard genome via B-A centric fusion. Remarkably, the behavior of fusion heterozygotes, the initial step of the integration process, is much more regular than that of any other configuration, including homozygotes. The reasons for the failure of B chromosome integration into the normal complement by translocation are discussed.

Historical Perspectives

Fifty years ago, Professor Paul Polani and colleagues described young women who had Turner syndrome and were found to have apparently male “chromosomal sex.” (1) At that time, the generally accepted number of chromosomes in humans was 48. Five years earlier, Barr and Bertram described the presence of a prominent mass of chromatin lying close to the nucleolar membrane in neurons of some female cats. (2) This “sex-chromatin” was not found in the males. These “Barr bodies” later were detected in cells grown from skin biopsies. (3) Then, in 1956, Tjio and Levan reported that the number of diploid chromosomes was 46, (4) which rapidly was confirmed by others. (5) From this ripple, a wave soon developed. Three years later, the first association of a clinical syndrome with an abnormal number of chromosomes was reported from France (6) and quickly confirmed in Scotland. (7) This was the association of Down syndrome (mongolism) with an extra chromosome number 21 (trisomy 21), although this was called chromosome number 22 at the time. Not long afterward, translocation was noted in a girl who had Down syndrome born to a 21-year-old mother, (8) and translocation was described as a cause of familial mongolism. (9) Other trisomies soon were described. (10)(11) Based on the realization that the number of sex-chromatin bodies was one less than the number of X chromosomes present, the chromosomes of women who had Turner syndrome and appeared to be chromosomally “male” were investigated. They were found to have 45 chromosomes, with the sex chromosomes being XO. (12) Many of these exciting chromosomal discoveries were made by Professor Polani and his colleagues. Elsewhere, Professor Polani (now 90 years old) has described in detail the origins and evolution of clinical cytogenetics. (13) In this issue of NeoReviews , …

The evolutionary history of human chromosome 7

We report on a comparative molecular cytogenetic and in silico study on evolutionary changes in human chromosome 7 homologs in all major primate lineages. The ancestral mammalian homologs comprise two chromosomes (7a and 7b/16p) and are conserved in carnivores. The subchromosomal organization of the ancestral primate segment 7a shared by a lemur and higher Old World monkeys is the result of a paracentric inversion. The ancestral higher primate chromosome form was then derived by a fission of 7b/16p, followed by a centric fusion of 7a/7b as observed in the orangutan. In hominoids two further inversions with four distinct breakpoints were described in detail: the pericentric inversion in the human/African ape ancestor and the paracentric inversion in the common ancestor of human and chimpanzee. FISH analysis employing BAC probes confined the 7p22.1 breakpoint of the pericentric inversion to 6.8 Mb on the human reference sequence map and the 7q22.1 breakpoint to 97.1 Mb. For the paracentric inversion the breakpoints were found in 7q11.23 between 76.1 and 76.3 Mb and in 7q22.1 at 101.9 Mb. All four breakpoints were flanked by large segmental duplications. Hybridization patterns of breakpoint-flanking BACs and the distribution of duplicons suggest their presence before the origin of both inversions. We propose a scenario by which segmental duplications may have been the cause rather than the result of these chromosome rearrangements.

Male and Female Meiosis in a Natural Population of Dichroplus Pratensis (Acrididae) Polymorphic for Robertsonian Translocations: A Study of Chiasma Frequency and Distribution

Chiasma frequency and distribution were comparatively analysed in males and females from an Argentinian population of the Acridid grasshopper Dichroplus pratensis. This population is polymorphic for four centric fusions among the six L-autosomes of the standard 2n = 193♂/209♀ karyotype. Males and females carrying three fusions, L1/L2, L3/L4, and L5/L6 in homozygous or heterozygous condition, showed similar chiasma frequencies and distributions, which contrasted sharply from those of standard (all-telocentric) individuals from two different Patagonian populations. The fusions induce the same type of chiasma repatterning in both sexes, producing a significant decrease of chiasma frequency in all L-chromosome arms irrespective of relative size. Mean chiasma frequency per bivalent varies between 1.64 and 1.05 for the largest and smallest L-autosomes in standard individuals while, when fused, all have a frequency of about 1.0. Furthermore, males and females from Cerro Ceferino display the same distal chiasma pattern, while standard bivalents show a typical proximal-distal chiasma distribution. The X bivalent of females was analysed individually: it has a chiasma pattern not different from that of standard telocentric autosomes. Our results suggest that, in this species, Robertsonian translocations exert parallel effects on recombination in both sexes, and that, if there exist separate controls of recombination for males and females, these are overruled by the effects of the rearrangements, which, in turn, would control balanced orientation and segregation of trivalents and, thus, the maintenance of the polymorphisms in nature.

Chromosomal localization of conglutinin (CGN1) gene to river buffalo by sequential RBH-banding and FISH.

Chromosomal localization of conglutinin (CGN1) gene to river buffalo by sequential RBH-banding and FISH.

Trends of Karyotype Evolution in Neotropical Oedionychina (Coleoptera: Chrysomelidae: Alticinae)

Although the cytology of the Oedionychina flea beetles is unorthodox in many ways, their karyotypes are conservative in chromosome number and sex chromosome system: typically 2n = 22(X, y); 10 + X + y. Differences between karyotypes are expressed mainly in arm length ratios. This study describes instances where this conservatism is broken and new karyotypes have emerged or seem to be emerging. The cases include distance sex multivalents up to 7x + Y (Asphaera spp. and Aspicela scutata), a full series of autosomal centric fusions: 5 + X + y (Alagoasa equestris), up to 17 B chromosomes (“Al. transparent” and Al. oblecta), and erosion of achiasmate arms (Al. januaria).

A new Robertsonian translocation in cattle, rob(16;20).

A new centric fusion translocation, rob(16;20), was discovered in a phenotypically normal bull. C-banding revealed the dicentric nature of this centric fusion. This bull is a descendant of a German red pied bull and a Czechoslovakian red pied cow. Its mother and 26 half-brothers had normal karyotypes, indicating that this translocation arose "de novo".

The Cytogenetics and Cytotaxonomy of Clrrysolina Mots. and Oreina Chevr. (Coleoptera, Chrysomelidae, Chrysomelinae)

Thirteen species of Chrysolina and four of Oreina have been chromosomally analyzed. C. (Euchrysolina) graminis and three species of Oreina, O. (s.sfr.) basilea, O. (Chrysochlou) speciossissima and O. (Prororina) ludocicae have shown a diploid number of 24 chromosomes and a 11 + Xyp male meioformula. Oreina (s.str.) viridis has an asymmetrical karyotype of 9 + Xyp meioformula, probably derived from the former 11 + Xyp by two centric fusions. The Chrysolina of subgenus Stidioptera are chromosomally heterogeneous but all of them share asymmetrical karyotypes a fact which probably supports their common ancestry, C. latecineta and C. ripoceanensis have 11 + Xyp C. gypsophilae 15 + Xyp and C. sunguinolenta 16 + XypC. (Hypercoa)hyperici and C. (Hypericia) quadrigemina display 38 chromosomes in agreement with another species of that subgenus. The recent proposal of joining C. fuliginosa and C. (Heliostola) within the subgenus Allochrysolina is reinforced by their common diploid number of 42 and 20 + Xyp meioformula. C. (Heliostola) Montana shares the same 19 + Xyp meioformula with the species of the somewhat related subgenus Chalcoidea which also lives on Asteraceae. C. (Cruspeda) limbata shows a 20 + Xyp meiformula, quite similar to the 19 + Xyp previously found in species of the subgenera Colaphodes and Ovosoma which also feed on Plantaginaceae C. (Threnosoma) helopioides presents the highest chromosome number, 2n = 47 and 23 + X male meioformula, in agreement with that found in another species of Threnosoma. The proposed chromosomal evolution of Chrysolina, mainly due to centric fissions and coupled to their host-plant shifts, is not contradicted by the molecular phylogeny even though the latter provides a better resolution. The possible joining of Chrysolina and Oreina within a same genus might be supported by the cytogenetic data but should await enlarged analyses of molecular phylogenies.

A new centric fusion translocation in cattle: rob (13;19).

A new Robertsonian translocation has been found in cattle. A bull from Marchigiana breed (central Italy) was found to be a heterozygous carrier of a centric fusion translocation involving cattle chromosomes 13 and 19 according to RBA-banding and cattle standard nomenclatures. CBC-banding revealed the dicentric nature of this new translocation, underlining the recent origin of this fusion. In fact, both the bull's parents and relatives had normal karyotypes. In vitro fertilization tests were also performed in the bull carrying the new translocation, in two bulls with normal karyotypes (control) and in four other bulls carrying four different translocations.

Extraordinary chromosomal polymorphism with 28 different karyotypes in the neotropical species Akodon cursor (Muridae, Sigmodontinae), one of the smallest diploid number in rodents (2n = 16, 15 and 14).

All available published cytogenetic data show the presence of 28 different karyotypes in 311 specimens of A. cursor as an exceptional example of high karyotype variability in a single species. Our present sample of 116 animals collected in the rain forest of the Atlantic coast of the states of São Paulo and Bahia, Brazil, show 25 karyotype constitutions. The diploid number (2n) ranged from 16 to 14, and the number of autosomal arms (NF) from 26 to 18, because of centric fusion and pericentric inversions involving two autosome pairs, pericentric inversions in three other chromosome pairs, trisomy in the pair 7 and the presence of two XO females. Synaptonemal complex analysis, associated with data from experimental cross-breeding, suggested that heterozygous individuals for pericentric inversions have normal fertility. In this paper, we have reviewed the chromosomal data of this species, and have thus standardized the karyotype description and chromosome numbering. We discuss about karyotype evolution of Akodon cursor based on the frequency and constitution of karyotypes of all different geographical samples described so far in the literature.

A new species of Calomys (Rodentia, Sigmodontinae) from Central Brazil identified by its karyotype.

Ten species of small rodents of genus Calomys are found in South America. Three of these ten species are known to occur in Brazil: C. tener, C. laucha and C. expulsus (= C. callosus expulsus). Almost all Calomys karyotypes are made up of acrocentric pairs. In this paper we describe a new karyotype with 2n = 46 (FN = 66), including 11 meta/submetacentric and 11 acrocentric autosomal pairs. This is not related to any described Calomys karyotype. The X chromosome is a medium submetacentric and the Y is a small acrocentric. This new karyotype is briefly compared to karyotype of the sympatric species C. tener (2n = 66, FN = 66). The reduced diploid number and small amount of pericentromeric heterochromatin observed in the biarmed chromosomes that contrasts to large blocks seen in acrocentrics seem to indicate that centric fusion and loss of constitutive heterochromatin have led to the new karyotype. Cytogenetic evidence suggests strongly that a new species with 2n = 46 from Central Brazil should be described in the genus Calomys.

Chromosomal polymorphism, morphometric traits and mating success in Leptysma argentina Bruner (Orthoptera).

The South-American species Leptysma argentina Bruner is polymorphic for a centric fusion between chromosomal pairs 3 and 6 (fusion 3/6). Cytogenetic and morphological studies revealed that fusion 3/6 significantly increases some morphometric variables in males. A selection components study showed that the fusion is positively selected for longevity, the direct effect being exerted on thorax height. Moreover, a unisexual approach revealed that sexual male selection acts by increasing third femur length. In the present study the effect of fusion 3/6 on morphometric variables and the effect of body size and karyotype on mating success in both males and females were analysed through a bisexual approach. Total body length (TL) and third femur length (FL) were significantly larger amongst the fusion carrier females. Besides, the largest females and the carriers of the fusion have increased mating success. The selection differential suggests that directional sexual selection favours the females with increased TL and fusion dosage; however, the selection gradient revealed that none of the variables here analysed is a direct target of sexual selection. The male sexual selection study agrees with previous results, showing the importance of the third pair of legs in male mating success among grasshoppers.

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.

Establishment of two hormone-responsive mouse mammary carcinoma cell lines derived from a metastatic mammary tumor.

We report the establishment of two mouse mammary cancer cell lines, MC7-2A and MC7-2B obtained from a mouse mammary carcinoma induced by medroxyprogesterone acetate (MPA) and maintained by syngeneic transplantation in BALB/c mice. They are epithelial (express cytokeratins) and express both estrogen receptors alpha (ERalpha) and progesterone receptors (PRs) isoforms A and B (western blots). In vitro, MPA inhibited 3H-thymidine uptake, starting from concentrations as low as 10(-13) M in MC7-2A and 10(10) M in MC7-2B; the antiprogestin RU 486 exerted a stimulatory effect at 10(-14) M in both cell lines; 17-beta-estradiol (E2) also exerted a stimulatory effect starting at 10(-10) M in MC7-2A and at 10(-13) M in MC7-2B. When transplanted in syngeneic mice, both cell lines originated adenocarcinomas that gave rise to lung metastases within 3 months. In in vivo studies, in MC7-2A, the antiprogestin inhibited completely tumor growth, E2 induced a slight although significant ( p < 0.05) stimulatory effect and MPA stimulated tumor growth while MC7-2B cells were unresponsive to all treatments. ER and PR were also expressed in tumors as assessed by immunohistochemistry. Two marker chromosomes were identified by FISH as translocations between chromosomes 4 and 7, and between chromosomes X and 2; the third marker chromosome remains unidentified. All these markers were also present in the parental tumor. A new marker, a centric fusion of chromosomes 2, was acquired in both cell lines. Considering that there are very few murine breast carcinoma responsive cell lines, these cells represent new tools in which the regulatory effect of hormones can be studied.

Autosomal rearrangements in Graomys griseoflavus (Rodentia): a model of non-random Robertsonian divergence.

The south American rodent Graomys griseoflavus exhibits a remarkable chromosome polymorphism as a consequence of four Robertsonian fusions. Focusing on the genetic analysis of the taxon, genome organization of all karyomorphs was studied at chromosome and molecular organization level. Cytogenetic (G, NOR and Re banding) and molecular (satellite and mitochondrial DNAs) events accompanying chromosome divergence allowed tracing a phylogenetic relationship among all karyomorphs. Available data led to propose that chromosome evolution of G. griseoflavus occurred in a non-random sequence of centric fusions, supporting the hypothesis of single origin for Robertsonian karyomorphs.