Chromosome evolution by robertsonian translocation in Gibasis (Commelinaceae)

The plant species Gibasis schiedeana (Kunth) D. R. Hunt sens. lat. contains two cytotypes viz. a self-sterile diploid with 2n=10 (x=5) and a selffertile cytological autotetraploid with 2n=16 (x=4). Single chromosome sets of these plants consist of 2 metacentrics +3 acrocentrics, and 3 metacentrics +1 acrocentric chromosomes respectively suggesting a Robertsonian relationship between them. Their artificial F₁ hybrids show the pairing of acrocentrics with metacentric arms confirming the supposed nature of the chromosome affinities. Both breeding systems and ploidy levels show that the direction of the change has been from x=5 to x=4 by a translocation of the Robertsonian type.     

Allozyme differentiation of populations of the dogwhelk Nucella lapillus, (L.): the relative effects of geographic distance and variation in chromosome number

Geographic patterns of genie differentiation were compared with differentiation between karyotypes in the intertidal snail Nucella lapillus. Samples from 24 sites covering the species range in Europe and North America were analysed for allozyme variation at 16 soluble enzyme loci. Two homokaryotypes have been identified with diploid numbers 2n = 26 and 2n= 36 (variation is Robertsonian and hybrids have intermediate chromosome numbers) and samples were classified (on the basis of published data) according to karyotype. Group 1 consisted of samples from three English Channel populations of higher chromosome number (on average 2n > 32) and Group 2 consisted of the remaining 21 samples (presumed to be 2n= 26). Karyotype variation accounts for roughly the same amount of the absolute allozyme variance as geographic variation (46.3 °, and 53.7°, respectively). Yet the patterns of differentiation seen between karyotypes and with geographic separation are very different. In samples classified as 2n= 26 (Group 2), while there is a significant amount of heterogeneity (F_ST per locus averaged 0.128 for 10 polymorphic loci), allozyme variation occurs independently at different loci so mean genetic identity (Nei) is high: 0.972. There is only a slight decline in genetic identity with distance (genetic identity averaged 0.965 for amphi-atlantic comparisons) indicating that passive transport of juveniles or adults may contribute significantly to gene flow. Conversely, allozyme variation between karyotypes was concordant. High chromosome number populations possessed a suite of alleles at four allozyme loci (Esl-3, Lap-2, Mdh-1 and Pep-2) which were absent or rare in Group 2 samples resulting in high F_ST values for these loci (from 0.294 to 0.472) when karyotypic classes were combined. Consequently the mean genetic identity between these Robertsonian races is low, 0.856, and falls within the range more usually associated with congeneric comparisons than with con-specific comparisons. The mechanisms maintaining this genie difference are unclear. However the distribution of the karyotypes and physiological and morphological differences (in shell shape) between them strongly suggest that karyotypic variation in Nucella is adaptive.     

Chromosome cytology and karyotype change inGalaxia (Iridaceae)

Basic chromosome number inGalaxia is believed to be x = 9, and this number, or multiples, occurs in all species of subgenusGalaxia. In subgenusEurystigma, G. barnardii has n = 8,G. versicolor n = 8 and 7,G. citrina n = 8, 7 and 17 whileG. variabilis has n = 7 exclusively. Karyotypes in forms ofG. versicolor with n = 7 and inG. variabilis are quite different and clearly originated independently. Karyotypic features provide evidence for the hypothesis that changes in chromosome number were accomplished through chromosome fusion either by classical Robertsonian translocation, or unequal reciprocal translocation.     

Patterns of evolution in Graomys griseoflavus (Rodentia,Muridae). IV. A case of rapid speciation

The South American group of rodents known as Graomys griseoflavus comprises two sibling species differing only in diploid chromosomal complement: G. griseoflavus (2n = 36, 37 and 38) and G. centralis (2n=42). Reproductive barriers comprising postzygotic as well as precopulatory mechanisms prevent gene exchange between these species. We have studied genetic polymorphism in two populations of G. centralis and four of G. griseoflavus by means of gel electrophoresis of enzymes and other proteins giving information on a total of 30 loci. Values of interspecific genetic identity were similar to those obtained for conspecific populations, suggesting that fixation of Robertsonian fusions would have occurred without significant bottlenecks. It would also indicate that the cladogenetic process must be relatively recent. F_IS values showed no evidence of inbreeding. Fixation indices (F_ST) for the ancestral species showed a tendency to form demes with very low gene flow among them, while in the derived species such tendency was not shown. However, because of the characteristics of the region they occupy, populations are of moderate size, and genic flow is low. Lack of correlation between gene flow levels and geographical distance between population pairs would indicate a recent and fast colonization of its distribution areas by the derived species. It is possible that fixation of Robertsonian fusions occurred in a marginal deme of the ancestral species, e.g. in a parapatric geographical context.     

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.     

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.     

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.     

Genomic relationships among diploid wild perennial species of the genus Glycine Willd. subgenus Glycine revealed by crossability,meiotic chromosome pairing and seed protein electrophoresis

Summary The nomenclature of species beased on classical taxonomy can be verified from cytogenetic, biochemical and molecular studies. The objective of the study presented here was to provide further information on genomic affinities among species of the genus Glycine Willd. based on crossability, meiotic chromosome pairing of F₁ hybrids and seed-protein profiles. Meiotic chromosome pairing data revealed no genomic similarity between G. microphylla (BB) and G. falcata (FF), nor between G. tomentella (2n = 38; EE) and G. microphylla (BB). Despite morphological similarity between G. cyrtoloba (CC) and G. curvata no F₁ hybrid was obtained, although 748 flowers were pollinated. The seed-protein banding patterns showed G. latrobeana to be closer to the A-genome species than to others. Based on these results we assign genome symbol A₃A₃ to G. latrobeana. Likewise, G. curvata was allotted the designation C₁C₁ because the seed-protein banding patterns of G. curvata and G. cyrtoloba are similar. The genome designations of Glycine species based on cytogenetic investigations may be further extended by results obtained from biochemical and molecular approaches.Research supported in part by the Illinois Agricultural Experiment Station and US Department of Agriculture Competitive Research Grant 88-37231-4100     

Fertile progeny of a hybridization between soybean [Glycine max (L.) Merr.] and G. tomentella Hayata

Summary A colchicine-doubled F₁ hybrid (2n=118) of a cross between PI 360841 (Glycine max) (2n=40) x PI 378708 (G. tomentella) (2n=78), propagated by shoot cuttings since January 1984, produced approximately 100 F₂ seed during October 1988. One-fourth of the F₂ plants or their F₃ progeny have been analyzed for chromosome number, pollen viability, pubescence tip morphology, seed coat color, and isoenzyme variation. Without exception, all plants evaluated possessed the chromosome number of the G. max parent (2n=40). Most F₂ plants demonstrated a high level of fertility, although 2 of 24 plants had low pollen viability and had large numbers of fleshy pods. One F₂ plant possessed sharp pubescence tip morphology, whereas all others were blunt-tipped. All evaluated F₂ and F₃ plants expressed the malate dehydrogenase and diaphorase isoenzyme patterns of the G. max parent and the endopeptidase isoenzyme pattern of the G. tomentella parent. Mobility variants were observed among progeny for the isoenzymes phosphoglucomutase, aconitase, and phosphoglucoisomerase. This study suggests that the G. Tomentella chromosome complement has been eliminated after genetic exchange and/or modification has taken place between the genomes.Journal Paper No. J-13776 of the Iowa Agriculture and Home Economics Experiment Station, Ames, IA, USA, Project 2763     

Multiple sex chromosome system of X1X1X2X2/X1X2)Y type in lutjanid fish, Lutjanus quinquelineatus (Perciformes)

The karyotype and other chromosomal markers as revealed by C-banding and Ag-staining were studied in Lutjanus quinquelineatus and L. kasmira (Lutjanidae, Perciformes). While in latter species, the karyotype was invariably composed of 48 acrocentric chromosomes in both sexes, in L. quinquelineatus the female karyotype had exclusively 48 acrocentric chromosomes (2n = 48) but that of the male consisted of one large metacentric and 46 acrocentric chromosomes (2n = 47). The chromosomes in the first meiotic division in males showed 22 bivalents and one trivalent, which was formed by an end-to-end association and a chiasmatic association. Multiple sex chromosome system of X₁X₁X₂X₂/X₁X₂Y type resulting from single Robertsonian fusion between the original Y chromosome and an autosome was hypothesized to produce neo-Y sex chromosome. The multiple sex chromosome system of L. quinquelineatus appears to be at the early stage of the differentiation. The positive C-banded heterochromatin was situated exclusively in centromeric regions of all chromosomes in both species. Similarly, nucleolus organizer region sites were identified in the pericentromeric region of one middle-sized pair of chromosomes in both species. The cellular DNA contents were the same (3.3 pg) between the sexes and among this species and related species.     

Relationship between parental chromosomic contribution and nuclear DNA content in the coffee interspecific hybrid C. pseudozanguebariae×C. liberica var ‘dewevrei’

 F₁ hybrids were obtained between two coffee species with the same chromosome number (2n=22) but with different nuclear DNA contents [C. pseudozanguebariae (PSE) 2C=1.13 pg and C. liberica var ‘dewevrei’ (DEW) 2C=1.42 pg]. G2 hybrids were obtained by open-pollination of the F₁ hybrids. Genomic in situ hybridisation (GISH) and flow cytometry were used on six F₁ hybrids and seven G2 hybrids to determine their parental chromosomic contribution and their nuclear DNA content (qDNA), respectively. GISH efficiently identified chromosomes from both species. F₁ hybrids had a qDNA intermediate between that of the parental species and contained the expected 11 chromosomes from each species. There was a linear relationship between the number of PSE chromosomes and the nuclear DNA content, which indicates that flow cytometry can be used to give a rough estimate of the parental chromosomic contribution in G2 hybrids. Received: 1 August 1997/Accepted: 25 August 1997     

Robertsonian differentiation and preferential pairing revealed in species and F1 hybrids of theGibasis linearis group (Commelinaceae)

Three closely related species of theG. linearis group are differentiated by their basic numbers and ploidy levels. The change in basic number involves Robertsonian fusion, but the species also differ by interchange and cryptic structural changes revealed by meiotic pairing in F₁ hybrids.Chromosome Evolution in theGibasis linearis Group (Commelinaceae), II.—Part I:Kenton (1981a).     

Crossing relationships and chromosome numbers of Nordic populations of Draba (Brassicaceae), with emphasis on the D. alpina complex

101 populations of Nordic Draba were investigated for chromosome numbers and crossing relationships. The populations were referred to 16 currently recognized species of Sect. Draba, Sect. Chrysodraba (including the D. alpina complex with D. alpina, D. oxycarpa, and D. corymbosa), and Sect. Drabella. The chromosome data suggest that all populations and species are homoploid. Three species are diploid, and 13 species are polyploids ranging from tetraploid to 16-ploid. Draba alpina is decaploid, D. oxycarpa is octoploid, and D. corymbosa is 16-ploid. The chromosome numbers of D. micropetala (n = 16,4x) and D. adamsii (n = 24,2n = 48, 6x) of Sect. Chrysodraba and D. cinerea (n = 24, 6x) of Sect. Draba were determined for the first time based on Nordic material. The relationships inferred from the crossing data largely agree with those inferred from previously published molecular data, but correspond poorly to relationships inferred from morphology. Inter-populational F₁ hybrids in D. fladnizensis were entirely sterile, suggesting that this predominantly inbreeding diploid species comprises at least two sibling species, possibly isolated by genic barriers. Sterile to semifertile F₁ hybrids with intermediate chromosome numbers were obtained in 19 interspecific combinations. F₂ hybrids were obtained in seven of these combinations. Both Sect. Chrysodraba and the D. alpina complex are probably polyphyletic. The crossing data suggest that the morphologically very similar polyploids D. alpina (10x) and D. oxycarpa (8x) do not have any diploid progenitors in common, and that Sect. Draba forms an intricate allopolyploid complex that also includes D. alpina and D. corymbosa of Sect. Chrysodraba. Draba corymbosa (16x) is probably an intersectional, polyphyletic alloploid derived from D. alpina (10x) and various hexaploids of Sect. Draba. Crossing data also suggest that D. norvegica (6x) is one of the progenitors of the Nordic endemic D. cacuminum (8x). The phenotypic expression of genes encoding taxonomically important indument characters did not follow consistent patterns in interspecific hybrids. This result may explain the discrepancy between genetic and taxonomic relationships in Nordic Draba, and supports the use of wide taxonomic concepts in this highly reticulate genus.     

Genomic divergence of allopatric sibling species studied by molecular cytogenetics of their F1 hybrids

Despite their similar karyotype morphology and close taxonomic affinity, the genomes of allopatric sibling species, Gibasis karwinskyana and Gibasis consobrina, are clearly distinguished in metaphases of their F₁ hybrids by genomic in-situ hybridization (GISH). The reduced ability of chromosomes from one species to bind labelled total DNA from the other involves almost the whole chromosome complement, and is equally pronounced in euchromatin and heterochromatin. The only region strongly conserved in the two species is an AT-rich band proximal to each nucleolus organizer. Molecular differentiation is accompanied by chromosome pairing failure in the F₁ interspecific hybrids, although the reason remains open to question. The two species also differ in their numbers of detectable sites for rRNA genes. The greater number of such sites in G. consobrina may be linked with a propensity for interchange heterozygosity. The ability to discriminate rapidly and reliably between the chromosomes of close relatives with almost identical karyotypes makes GISH invaluable in preliminary studies of phytogeny. Detection of even small conserved chromosome bands using GISH confirms the sensitivity of the technique and demonstrates its potential use in evolutionary cytogenetics. This will allow rapid re-evaluation of many important genetic systems exposed by classical cytogenetics in previous decades.     

Chromosomal location of a gene suppressing powdery mildew resistance genes Pm8 and Pm17 in common wheat (Triticum aestivum L. em. Thell.)

The chromosomal location of a suppressor for the powdery mildew resistance genes Pm8 and Pm17 was determined by a monosomic set of the wheat cultivar Caribo. This cultivar carries a suppressor gene inhibiting the expression of Pm8 in cv Disponent and of Pm17 in line Helami-105. In disease resistance assessments, monosomic F₁ hybrids (2n=41) of Caribo x Disponent and Caribo x Helami-105 lacking chromosome 7D were resistant, whereas monosomic F₁ hybrids involving the other 20 chromosomes, as well as disomic F₁ hybrids (2n=42) of all cross combinations, were susceptible revealing that the suppressor gene for Pm8 and Pm17 is localized on chromosome 7D. It is suggested that genotypes without the suppressor gene be used for the exploitation of genes Pm8 and Pm17 in enhancing powdery mildew resistance in common wheat.     

Interspecific hybridization of perennial Medicago species using ovule-embryo culture

Summary New interspecific hybrids between alfalfa (Medicago sativa L.) and several perennial Medicago species were obtained by embryo rescue techniques. The methodology, designated ovule-embryo culture, involved preculturing the fertilized ovule (10 to 20 days post-pollination) for a period of six to 12 days followed by excision and direct culture of the embryo. Placement of the hybrid embryo directly onto culture medium without the interim ovule culture was unsuccessful. Ovule culture to germination without removing the embryo also was unsuccessful. Ovule-embryo culture was essential for recovering interspecific hybrids between diploid alfalfa (2n=2x=16) and the following diploid (2n=2x=16) species: M. hybrida Traut., M. marina L., M. papillosa Boiss., M. rhodopea Velen. and M. rupestris M.B. In addition, trispecies hybrids between M. sativa x M. dzhawakhetica Bordz. F₁ hybrids (2n=3x=24) and either M. cancellata M.B. (2n=6x=48) or M. saxatilis M.B. (2n=6x=48) were obtained from ovuleembryo culture. Media manipulations using M. sativa x M. rupestris F₁ and first backcross generation embryos demonstrated the optimum concentration of 12.5 mM NH₄ ⁺ for successful embryo rescue; ammonium salt formulation (whether chloride, nitrate or sulfate) was not critical. From a few thousand crosses, hybrids between M. sativa and either M. rhodopea or M. rupestris were recovered relatively efficiently with 157 and 66 hybrids, respectively. However, only 13 hybrids between M. sativa and M. papillosa were obtained from more than 2,000 crosses, and just two hybrids each have been recovered from the combinations M. sativa x M. hybrida and M. sativa x M. marina from 2,000 to 3,000 crosses. The predominant chromosome number between diploid alfalfa and the other diploid perennial species was 2n=2x=16. Morphology of the hybrids was generally intermediate. Electrophoretic analysis of the F₁ hybrids and parental clones on uniform or gradient polyacrylamide gels demonstrated that peroxidase phenotypes could be used to confirm hybridity. For all interspecific combinations there was at least one peroxidase isozyme unique to the wild species that was present in the F₁ interspecific hybrid.     

Assignment of RFLP linkage groups to chromosomes using monosomic F1 analysis in hexaploid oat

The availability of molecular genetic maps in oat (Avena spp.) and improved identification of chromosomes by C-banding are two recent developments that have made locating linkage groups to chromosomes possible in cultivated hexaploid oat, 2n=6x=42. Monosomic series derived from Avena byzantina C. Koch cv Kanota and from Avena sativa L. cv Sun II were used as maternal plants in crosses with the parents, Kanota-1 and Ogle-C, of the oat RFLP mapping population. Monosomic F₁ plants were identified by root-tip cell chromosome counts. For marker analysis, DNAs of eight F₂ plants from a monosomic F₁ were combined to provide a larger source of DNA that mimicked that of the monosomic F₁ plant. Absence of maternal alleles in monosomic F₁s served to associate linkage groups with individual chromosomes. Twenty two linkage groups were associated with 16 chromosomes. In seven instances, linkage groups that were independent of each other in recombination analyses were associated with the same chromosome. Five linkage groups were shown to be associated with translocation differences among oat lines. Additionally, the results better-characterized the oat monosomic series through the detection of duplicates and translocation differences among the various monosomic lines. The F₁ monosomic series represents a powerful cytogenetic tool with the potential to greatly improve understanding of the oat genome. Received: 24 April 2000 / Accepted: 10 May 2000     

Gamete composition and chromosome variation in pollen-derived plants from octoploid triticale x common wheat hybrids

Summary Anther culture of secondary octoploid triticale (AABBDDRR) and F₁ hybrids (AABBDDR) of octoploid triticale x common wheat crosses was carried out, and 96 pollen-derived plants were developed and studied cytologically. In addition to the 8 types of pollen-derived plants with the theoretically predicted chromosome number, plants with the chromosome constitutions of 2n = 38, 43, 45, 47, 74, and mixoploids were obtained. The haploids and the diploids had different distributions. The frequencies of plants with one and two (pairs of) rye chromosomes were extremely high, and anther culture may be an expeditious route for creating alien addition lines of distant hybrid F₁s. Chromosome aberrations, including deletions, inversions, translocations, as well as isochromosomes and ring chromosomes, were observed in some plants. Abnormal meioses, such as chromosome non-disjunction, were also found. The reasons for the chromosome aberrations are discussed.     

The founder effect theory: quantitative variation and mdg-1 mobile element polymorphism in experimental populations of Drosophila melanogaster

The chromosomes of 14 specimens of the genus Reithrodon from three different localities of Argentina and two localities of Uruguay were studied using G-and C-banding techniques. Specimens of Uruguay showed a karyotype of 2n=28 chromosomes having a large metacentric X, and a telocentric Y chromosome. This karyotype is very similar to that recently described in a sample from southern Brazil, differing only in the nature of the Y chromosome, which is metacentric in the Brazilian form. All specimens from Argentina showed a 2n=34 karyotype, differing from the Brazilian karyotype by two centric fusions, an acquisition of chromosome material, and at least one pericentric inversion, and by the telocentric nature of both the X and the Y chromosomes. G-and C-banding suggest that the metacentric gonosomes in the Brazilian form resulted from a double autosomal-X-Y Robertsonian translocation. The Uruguayan cytotype is interpreted as derived from a hypothetical neo-X/Y₁Y₂ ancestral form by the secondary loss of the Y₁ chromosome. The karyotypic differences between the Brazilian-Uruguayan and the Argentinian forms afford evidence of species differentiation. It is proposed to assign the former to Reithrodon typicus, and the later to R. auritus.