Chromosomal fission accounts for small‐scale radiations in Zamia (Zamiaceae; Cycadales)

Zamia is unique among Cycadales in its diversity of morphology, ecology and chromosome numbers. The chromosome numbers in Zamia range from 16 to 28, excluding 20, manifest as both interspecific and intraspecific series. It has long been recognized that Robertsonian transformations (chromosomal fission or fusion) probably dominate karyotype evolution in Zamiaceae, although it has been debated whether chromosome numbers are increasing or decreasing. We re‐analyse published karyotypes of Zamia spp., relating both chromosome forms and sizes to recent phylogenetic data. We show that karyotype evolution is most probably moving towards increased asymmetry, with higher numbers of smaller chromosomes, thus supporting chromosomal fission. We also address additional hypotheses for increasing chromosome numbers, namely pericentric inversions and unequal translocations. Finally, we discuss the role of these chromosomal changes in evolutionary radiations. © 2011 The Linnean Society of London, Botanical Journal of the Linnean Society, 2011, 165 , 168–185.     

Studies in individual spawnings of Salmo salar: a model to explain chromosome polymorphism patterns

Summary Atlantic salmon fry from nine offspring belonging to individual spawnings were karyotyped. Different patterns of Robertsonian chromosome polymorphism were obtained. A theoretical model is developed to explain the different chromosome polymorphism patterns in Salmo salar offspring in terms of the chromosome numbers of the parents.     

Cytological variation and evolution withinPelargonium sectionHoarea (Geraniaceae)

Chromosome numbers of 65 species of sect.Hoarea have been determined. These show three basic chromosome numbers, x = 11, 10 and 9. Only a few species are tetraploid. In five species both diploid and tetraploid cytotypes are reported. Several cases of deviations in chromosome numbers and cytological abnormalities were found, most of these being related to the presence of B chromosomes that occur in eight species. Evidence is presented to suggest that the basic chromosome numbers of x = 10 and x = 9 are derived from x = 11 by centric fusion. Although variation in basic chromosome number withinPelargonium has been the subject of detailed study, this is the first time that evidence has been found for a mechanism of change in basic number, that of centric fusion by Robertsonian translocation. For the species of sect.Hoarea with x = 9, where the evidence for Robertsonian translocation is greatest, this process has probably taken place quite recently. In contrast to results from other sections of the genusPelargonium, the three different basic numbers of sect.Hoarea do not contradict its delimitation as a natural taxon.     

Karyotype evolution in the genus Ellobius (Microtinae, Rodentia)]

Comparative analysis was undertaken of the pattern of G-dyed chromosome sets of three Ellobius species: E. tancrei, E. fuscocapillus, E. lutescens with respective diploid chromosome numbers 54, 36, 17. From the data obtained one can envisage probable evolutionary pathway of the Ellobius karyotype. Variability in chromosome numbers of this genus species was shown to be a result of both centromeric and centromeric-telomeric as well as telomeric translocations of originally acrocentric chromosomes. No combinations of acrocentric chromosomes of the E. tancrei Robertsonian fan were found in the karyotypes of E. fuscocapillus and E. lutescens, which points to independence of the evolutionary processes in the Ellobius genus taking different routes. The data are obtained to the effect that evolution of the genus Ellobius was accompanied by increase in the amount of C-heterochromatin.     

Characterization of a centromeric marker on mouse Chromosome 11 and its introgression in a <Emphasis Type="Italic">domesticus/musculus</Emphasis> hybrid zone

It has been proposed that the distribution of Robertsonian chromosome fusions and the Chromosome 11 Nucleolar Organizer Region (NOR) in the Danish hybrid zone between M. m. musculus and M. m. domesticus stems from centromeric incompatibilities between the two subspecies. To test this hypothesis, we identified and characterized a diagnostic subspecific marker closely linked to the centromere on mouse Chromosome 11. Using an allele-specific PCR assay, we investigated the introgression pattern of this centromere in a large sample of mice from a North–South transect of the hybrid zone in Jutland. Domesticus alleles were found to introgress far away from the center of the zone on the musculus side. These results suggest there is no incompatibility between the domesticus centromere of Chromosome 11 in the musculus genomic background.Two first authors contributed equally to this work.     

Karyotypes of three species of the family Cottidae (Scorpaeniformes)

Karyotypes and cellular DNA contents of three species of the family Cottidae viz.Icelus cataphractus, Gymnocanthus intermedius andAlcichthys alcicornis were analyzed. Structural modifications within the family were supposedly by Robertsonian translocations. The diploid chromosome numbers were determined to be 48 inAlcichthys alcicornis, 44 inGymnocanthus intermedius and 40 inIcelus cataphractus. The DNA contents ranged from 1.46 to 1.50pg/cell in the three species. The karyotype ofIcelus cataphractus is unique in having the smallest chromosome number (2n = 40) and 14 large-sized chromosomes. From the chromosome number and the existence of some large chromosome pairs, Robertsonian translocations seem to have occurred frequently inIcelus cataphractus andGymnocanthus intermedius.     

Robertsonian polymorphism and constitutive heterochromatin distribution in chromosomes of the rainbow trout (Salmo gairdneri).

White-blood-cell culture was used to examine the chromosomes of 53 rainbow trout (Salmo gairdneri) from three locations in the Pacific Northwest of the United States. A Robertsonian chromosome polymorphism is present, resulting in diploid numbers of 60, 59, or 58 in different individuals with 104 chromosome arms. The low level of intraindividual Robertsonian variation, differences in the number of subtelocentric chromosomes between individuals with different chromosome numbers, and frequencies of fish with different chromosome numbers in one population suggest that the interindividual differences are inherited and not somatic. C-banding shows that constitutive heterochromatin is localized near the centromeres and near the secondary constriction one chromosome pair.     

Robertsonian polymorphism in plaice, Pleuronectes platessa L., and cod, Gadus morhua L., (Pisces Pleuronectiformes and Gadiformes)

Karyotypic analysis of plaice, Pleuronectes platessa L., and cod, Gadus morhua L., revealed that the chromosome numbers in both species vary but that chromosome arm numbers (NF) stay constant. The C-, Q- and R-banding patterns also confirmed that the population of plaice studied shows Robertsonian polymorphism. The tendency of reduction in chromosome number in fishes suggests that Robertsonian fusions play a role in karyotype evolution in fishes.     

Analysis of meiotic segregation in a man heterozygous for a 13;15 Robertsonian translocation and a review of the literature

Summary Meiotic segregation was studied in a male heterozygous for a 13;15 Robertsonian translocation using in vitro sperm penetration of hamster eggs. Sixty-seven sperm chromosome complements were obtained and R-banded. Alternate segregation produced equal numbers of normal (31) and balanced (29) gametes, as was theoretically expected. Incidence of unbalanced complements was 10.4%, and the frequency of abnormalities unrelated to the translocation was 7.4%. This study confirms the predominance of alternate meiotic segregation in Robertsonian translocation carriers. Four sperm studies of Robertsonian translocation have been previously reported. A review of the combined results points out the low incidence of imbalance in the sperm of Robertsonian translocation carrier and the lack of evidence for an interchromosomal effect.     

Comparative cytogenetics of six Indo‐Pacific moray eels (Anguilliformes: Muraenidae) by chromosomal banding and fluorescence in situ hybridization

A comparative cytogenetic analysis, using both conventional staining techniques and fluorescence in situ hybridization, of six Indo‐Pacific moray eels from three different genera (Gymnothorax fimbriatus, Gymnothorax flavimarginatus, Gymnothorax javanicus, Gymnothorax undulatus, Echidna nebulosa and Gymnomuraena zebra), was carried out to investigate the chromosomal differentiation in the family Muraenidae. Four species displayed a diploid chromosome number 2n = 42, which is common among the Muraenidae. Two other species, G. javanicus and G. flavimarginatus, were characterized by different chromosome numbers (2n = 40 and 2n = 36). For most species, a large amount of constitutive heterochromatin was detected in the chromosomes, with species‐specific C‐banding patterns that enabled pairing of the homologous chromosomes. In all species, the major ribosomal genes were localized in the guanine‐cytosine‐rich region of one chromosome pair, but in different chromosomal locations. The (TTAGGG)_n telomeric sequences were mapped onto chromosomal ends in all muraenid species studied. The comparison of the results derived from this study with those available in the literature confirms a substantial conservation of the diploid chromosome number in the Muraenidae and supports the hypothesis that rearrangements have occurred that have diversified their karyotypes. Furthermore, the finding of two species with different diploid chromosome numbers suggests that additional chromosomal rearrangements, such as Robertsonian fusions, have occurred in the karyotype evolution of the Muraenidae.     

Species delimitation in the Acomys cahirinus–dimidiatus complex (Rodentia, Muridae) inferred from chromosomal and morphological analyses

Our earlier chromosome banding studies of Acomys cahirinus and Acomys dimidiatus (the latter long considered to be a subspecies of the former) revealed that, despite very close diploid numbers (36 vs. 38), these taxa possess sharply different karyotypes and undoubtedly belong to different species. In this context, the taxonomic status and the relationship between the two chromosomal forms in Sinai (2 n  = 36) and Israel (2 n  = 38), chromosomally homozygous across a vast range except for a very narrow hybrid zone, remain poorly documented. Neither of these forms have previously been studied by chromosome banding; thus, the exact nature of chromosomal differences as well as the species to which these forms should be assigned remain unknown. Here, we present the data on comparative G-banding analysis and morphometrics of Acomys from Israel, Sinai, and Saudi Arabia, and a hybrid obtained in laboratory crosses between latter two. The analysis revealed that karyotype of Acomys from Israel is identical to that of Acomys from Saudi Arabia and both are different from that of Acomys from Sinai by one Robertsonian fusion. Therefore, karyotypically, all three are very different from A. cahirinus . It follows from the study that Sinai and probably Arabian peninsula and Minor Asia must be excluded from geographical distribution of A. cahirinus , which is limited from West Sahara to Egypt along Nile river (except Sinai). Furthermore, the synthesis of chromosomal and recent molecular data suggests a phylogeographical scenario explaining the modern distribution of Acomys in the Sinai and Arabian peninsulas and permits the update of the taxonomic status of these populations.  © 2007 The Linnean Society of London, Biological Journal of the Linnean Society , 2007, 91 , 203–214.     

Chromosomal variation in the southern short-tailed shrew (Blarina carolinensis)

Chromosomal polymorphism was assessed in the southern short-tailed shrew (Blarina carolinensis) using standard metaphase chromosome and G-banding techniques. Twenty-one animals (11 males, 10 females) from the Meeman Biological Station in Shelby Co., Tennessee, were examined for diploid number. Results showed diploid numbers of 35, 36, 37, 38, 39, 40 and 41 and fundamental numbers of 41, 42, 43, 44 and 45. No diploid numbers or fundamental numbers were unique to a specific collecting locality. The first G-banded karyotypes are reported for the species. These results indicate that Robertsonian polymorphisms, inversions, and possibly other events are responsible for chromosomal variation in B. carolinensis.     

Chromosomes in a hybrid zone of Israeli mole rars (Spalax, Radentia)

Chromosomal novelties and the level of meiotic and mitotic abnormalities were studied in a hybrid zone between two chromosomally differentiated Spalax cytotypes of 2n = 58 and 2n = 52. These cytotypes differ by five Rb fusions, four centromeric shifts accompanied by heterochromatin deletion, one paracentric inversion, and the Y-chromosome reorganization. Among 149 specimens studied, 82 were hybrids with 64 different karyotypes ranging in diploid numbers from 2n = 50 to 2n = 60. Nine hybrid specimens were mosaics for the chromosome numbers due to occurrence of cell lines with different Robertsonian chromosome arrangements, and six specimens possessed variable number of B-chromosomes. Mosaicism of B-chromosomes was found also in meiotic cells however chromatid breaks and abnormal chromosome pairing during meiosis occurred very rarely. All these results imply some local genomic instability resulting in the spontaneous process of reversible Rb fusions.     

Chromosomes in a hybrid zone of Israeli mole rars (Spalax, Rodentia)

Chromosomal novelties and the level of meiotic and mitotic abnormalities were studied in a hybrid zone between two chromosomally differentiated Spalax cytotypes of 2n = 58 and 2n = 52. These cytotypes differ by five Rb fusions, four centromeric shifts accompanied by heterochromatin deletion, one paracentric inversion, and the Y-chromosome reorganization. Among 149 specimens studied, 82 were hybrids with 64 different karyotypes ranging in diploid numbers from 2n = 50 to 2n = 60. Nine hybrid specimens were mosaics for the chromosome numbers due to occurrence of cell lines with different Robertsonian chromosome arrangements, and six specimens possessed variable number of B-chromosomes. Mosaicism of B-chromosomes was found also in meiotic cells however chromatid breaks and abnormal chromosome pairing during meiosis occurred very rarely. All these results imply some local genomic instability resulting in the spontaneous process of reversible Rb fusions.     

Cytogenetic control of a hybrid zone between two <Emphasis Type="Italic">Sorex araneus</Emphasis> chromosome races before breeding season

Two chromosome races of common shrew, Moscow and Seliger, differ in the arm combination in 11 diagnostic chromosomes (Robertsonian metacentrics/acrocentrics). Homozygotes of both pure races, simple Robertsonian heterozygotes of Seliger race, and complex heterozygotes (F₁ hybrids) were detected in the found earlier hybrid zone of these races, in the spring before the breeding season. The g/o heterozygote was first discovered in race Seliger, whose chromosome formula typically contains acrocentrics g and o. The m/q heterozygote was recorded for the second time. Meiosis was studied in 16 males representing five detected karyotypic categories. No abnormal in pairing of homologs in either sex trivalent common for the species (XY1Y2) or autosome trivalents (g/o and m/q) was detected at diakinesis-metaphase I. Two hybrids displayed a theoretically expected and unimpaired meiotic configuration in a form of a very long chain comprising 11 monobrachial homologs (g/gm/mq/qp/pr/rk/ki/ih/hn/no/o). The results are discussed in terms of hypotheses on fertility of complex heterozygotes and limited gene flow in hybrid zone.     

Chromosome rearrangements and survival of androgenetic rainbow trout (Oncorhynchus mykiss)

The purpose of this work was to quantify the impact of spontaneous and X-radiation-induced chromosome rearrangements on survival rate of androgenetic rainbow trout (Oncorhynchus mykiss). Various doses of X irradiation (50, 150, 250, 350 Gy) were used for inactivation of nuclear DNA in oocytes. After the irradiation, eggs were inseminated with normal sperm from 4 males derived from a strain characterized by Robertsonian rearrangements and length polymorphism of the Y chromosome. The haploid zygotes were exposed to a high hydrostatic pressure (7000 psi) to duplicate the paternal DNA. Neither Robertsonian chromosome polymorphism nor the Y chromosome morphology impaired the viability of the androgenetic embryos and alevins. Moreover, survival of eyed embryos of the androgenetic rainbow trout increased significantly with increasing doses of oocyte X irradiation. After 6 months of rearing, only specimens from the 250 and 350 Gy variants survived. The number of fingerlings with remnants of the maternal genome in the forms of chromosome fragments was higher in the 250 Gy group. Intraindividual variation of chromosome fragment number was observed, and some individuals exhibited haploid/diploid mosaicism and body malformations. Individuals irradiated with less than 250 Gy died, presumably because of the conflict between intact paternally derived chromosomes and the residues of maternal genome in the form of chromosome fragments.     

The role of chromosome rearrangements in the evolution of mole voles of the genus Ellobius (Rodentia,Mammalia)

Modern mole voles of the genus Ellobius are characterized by species-specific features of autosomes and sex chromosomes. Owing to the use of the Zoo-FISH method, the nomenclature of chromosomes was refined and nonhomologous Robertsonian translocations indistinguishable by G-staining were identified for Ellobius tancrei, which is a species with a wide chromosome variation of the Robertsonian type. The electron-microscopic analysis of synaptonemal complexes in F₁ hybrids of forms with 2n = 50 and 2n = 48 revealed the formation of a closed SC-pentavalent composed of three metacentrics with monobrachial homology and two acrocentrics. Segregation of chromosomes of such complex systems is impeded by disturbances in the nucleus architecture leading to the formation of unbalanced gametes and to a dramatic reduction in fertility of hybrids. Our data support the hypothesis that the formation of monobrachial homologous metacentric chromosomes can be considered as a way of chromosomal speciation.     

Fixation of metacentric chromosomes in eastern Europe populations of the common shrew <Emphasis Type="Italic">Sorex araneus</Emphasis> L.

In this review, we discuss the processes of fixation of Robertsonian chromosome fusions in populations of the common shrew Sorex araneus L. Various Robertsonian fusions, accumulating in populations, create an illusion of large chromosomal rearrangements, reciprocal translocations of complete chromosome arms. The use of these rearrangements for phylogenetic reconstructions results in false conclusions. Robertsonian fusions accumulate in populations at such stages of the species evolution, when large open or subdivided populations prevail (populations of warm periods of Pleistocene and many present-day populations) and are fixed in small isolated populations and glacial refugia. The formation of monomorphic chromosome races requires a long time, several glaciation epochs during the whole Pleistocene.     

Ultrastructure,meiotic behavior,and evolution of sex chromosomes of the genus Ellobius

The whole-mount SC preparations from males of three species of the genus Ellobius (Ellobius fuscocapillus, Ellobius lutescens), and Ellobius tancrei were studied by electron microscopy. In the males of Ellobius fuscocapillus, behavioral peculiarities of the sex bivalent (viz. the normal male heterozygosity) are characterized by early complete desynapsis of sex chromosomes (X, Y), occurring at late pachytene-early diplotene. The karyotype of species Ellobius lutescens is unique for mammals. In both sexes it is characterized by an odd number of chromosomes (2n=17). At prophase I the unpaired chromosome 9 is not involved in synapsis with other chromosomes and forms a sex body at the end of pachytene.The complete Robertsonian fan has been described for superspecies Ellobius tancrei. As shown on the basis of G-band patterns the male and female sex chromosomes are cytologically indistinguishable.Analysis of whole-mount SC preparations revealed the formation of a closed sex SC bivalent and showed some morphological differences in the axes of sex chromosomes at meiotic prophase I. A number of assumptions are made about the relationship between the behavior of sex chromosomes, their evolution and the sex determination system in the studied species of genus Ellobius.

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Natural hybridization between extremely divergent chromosomal races of the common shrew (Sorex araneus, Soricidae, Soricomorpha): hybrid zone in European Russia

The Moscow and Seliger chromosomal races of the common shrew differ by Robertsonian fusions and possibly whole‐arm reciprocal translocations (WARTs) such that their F₁ hybrids produce a chain‐of‐eleven configuration at meiosis I and are expected to suffer substantial infertility. Of numerous hybrid zones that have been described in the common shrew, those between the Moscow and Seliger races involve the greatest chromosomal difference. We collected 211 individuals from this zone to generate a total dataset of 298 individuals from 187 unique global positioning system (GPS) locations within the vicinity of interracial contact. We used a geographic information system (GIS) to map the location of the hybrid zone, which follows a direct route between two lakes, as would be anticipated from tension zone theory. Even within the central area of the hybrid zone, there is a much higher frequency of pure race individuals than hybrid, making this a clear example of a bimodal zone in the sense of Jiggins & Mallet (2000) . The zone runs through good habitat for common shrews, but nevertheless it is very narrow (standard cline widths: 3–4 km), as would be anticipated from low hybrid fitness. There is clear potential for an interruption to gene flow and build‐up of reproductive isolation. As found in some other hybrid zones, there is a high frequency of novel genetic variants, in this case, new chromosomal rearrangements. Here, we report a de novo Robertsonian fission and a de novo reciprocal translocation, both for the first time in the common shrew. There is an extraordinarily high frequency of de novo mutations recorded in F₁ hybrids in the zone and we discuss how chromosomal instability may be associated with such hybrids. The occurrence of a de novo Robertsonian fission is of considerable significance because it provides missing evidence that fissions are the basis of the novel acrocentric forms found and apparently selected for in certain common shrew hybrid zones.