Somatic association of telocentric chromosomes carrying homologous centromeres in common wheat

Summary Measurements of distances between telocentric chromosomes, either homologous or representing the opposite arms of a metacentric chromosome (complementary telocentrics), were made at metaphase in root tip cells of common wheat carrying two homologous pairs of complementary telocentrics of chromosome 1 B or 6 B (double ditelosomic 1 B or 6 B). The aim was to elucidate the relative locations of the telocentric chromosomes within the cell. The data obtained strongly suggest that all four telocentrics of chromosome 1 B or 6 B are spacially and simultaneously co-associated. In plants carrying two complementary (6 B ^S and 6 B ^L) and a non-related (5 B ^L) telocentric, only the complementary chromosomes were found to be somatically associated. It is thought, therefore, that the somatic association of chromosomes may involve more than two chromosomes in the same association and, since complementary telocentrics are as much associated as homologous, that the homology between centromeres (probably the only homologous region that exists between complementary telocentrics) is a very important condition for somatic association of chromosomes. The spacial arrangement of chromosomes was studied at anaphase and prophase and the polar orientation of chromosomes at prophase was found to resemble anaphase orientation. This was taken as good evidence for the maintenance of the chromosome arrangement — the Rabl orientation — and of the peripheral location of the centromere and its association with the nuclear membrane. Within this general arrangement homologous telocentric chromosomes were frequently seen to have their centromeres associated or directed towards each other. The role of the centromere in somatic association as a spindle fibre attachment and chromosome binder is discussed. It is suggested that for non-homologous chromosomes to become associated in root tips, the only requirement needed should be the homology of centromeres such as exists between complementary telocentrics, or, as a possible alternative, common repeated sequences of DNA molecules around the centromere region.Dedicated to Professor Dr. Marcus M. Rhoades on his 70th birthday.     

Localization of the active type I DNA topoisomerase gene on human chromosome 20q11.2-13.1, and two pseudogenes on chromosomes 1q23-24 and 22q11.2-13.1

Summary Different subfragments of a cDNA coding for DNA topoisomerase I were used as probes to determine the chromosomal localization of topoisomerase I sequences in human cells. Southern blotting of restricted DNA from a panel of rodent-human somatic cell hybrids revealed the localization of the complete gene on chromosome 20 and the presence of two truncated topoisomerase I pseudogene sequences on chromosomes 1 and 22. In situ chromosome hybridzation experiments confirmed these results showing the location of the complete gene on band q11.2–13.1 of chromosome 20, and the location of the pseudogene sequences on band q23–24 of chromosome 1 and q11.2–13.1 of chromosome 22.     

Non-random association of trypsin-banded human acrocentric chromosomes

Summary This paper deals with a computer-aided study of the associations between acrocentric chromosomes as well as between those other chromosomes which in our investigations were revealed to be significantly closer to each other than random. The chromosome pairs were identified by a trypsinbanding technique. The method used has been elaborated previously with the specific aim of determining associations in a manner that avoids all subjective criteria.The tendency for association between homologous chromosomes is in decreasing order: 21 and 13>1>14, 18 and 19>17. Among the nonhomologous acrocentric chromosomes the significant tendencies for associations are between D-D: 13–14>13–15>14–15; between D-G: 13–21>14–21>13–22>15–22.The implication of the different tendencies to associate are dicussed in terms of aetiology of numerical and structural chromosome abnormalities.     

Chromosome territory arrangement and homologous pairing in nuclei of <Emphasis Type="Italic">Arabidopsis thaliana</Emphasis> are predominantly random except for NOR-bearing chromosomes

Differential painting of all five chromosome pairs of Arabidopsis thaliana revealed for the first time the interphase chromosome arrangement in a euploid plant. Side-by-side arrangement of heterologous chromosome territories and homologous association of chromosomes 1, 3 and 5 (on average in 35–50% of nuclei) are in accordance with the random frequency predicted by computer simulations. Only the nucleolus organizing region (NOR)-bearing chromosome 2 and 4 homologs associate more often than randomly, since NORs mostly attach to a single nucleolus. Somatic pairing of homologous 100 kb segments occurs less frequently than homolog association, not significantly more often than expected at random and not simultaneously along the homologs. Thus, chromosome arrangement in Arabidopsis differs from that in Drosophila (characterized by somatic pairing of homologs), in spite of similar genome size, sequence organization and chromosome number. Nevertheless, in up to 31.5% of investigated Arabidopsis nuclei allelic sequences may share positions close enough for homologous recombination.Electronic Supplementary Material Supplementary material is available for this article at .     

Comparative study of the somatic conjugation of chromosomes in the neural ganglia of male and female Drosophila melanogaster D-32

The frequency of somatic conjugation of the 2nd and 3d chromosome pairs has been analysed in male and female neural ganglia from Drosophila melanogaster larvae D-32 line. The frequency of conjugation found for males was 1.5 times lower than for females. On this basis a supposition concerning less duration of homologous conjugation in males has been put forward. It is suggested that it is due to the differences in space arrangement of homologous chromosomes holding their conjugation.     

The positioning of rye homologous chromosomes added to wheat through the cell cycle in somatic cells untreated and treated with colchicine

The arrangement of chromosome pairs 5RL and 7R added to the wild type and the ph1b mutant line of hexaploid wheat are analyzed in 2N somatic root tip cells during the cell cycle relative to the arrangement that chromosomes 5RL show in 4N tapetal cells produced after colchicine treatment. Both homologous chromosome pairs are identified at interphase and mitosis by fluorescence in situ hybridization. In nuclei at interphase, chromosomes appear as discrete domains that show the Rabl orientation. Homologous chromosomes are predominantly non-associated and their positioning seems not to be influenced by the Ph1 gene that suppresses homoeologous meiotic pairing. This pattern of arrangement contrasts with the high level of somatic pairing that sister chromosomes show in the interphase that follows chromosome duplication induced by colchicine. Disruption of pairing observed in some 4N nuclei is produced at c-anaphase which suggests no topological redistribution of homologues during conformation of the new nucleus. Homologous chromosomes show no predominant arrangement in ellipsoidal metaphase plates, which contrasts with the preferential opposite location of homologues in human prometaphase rosettes. Differences between chromosomes in the variation of the length through the cell cycle and in the chromatin morphology when the Ph1 is absent suggest different patterns of chromatin condensation in both chromosomes.     

Mapping quantitative trait loci (QTL) in sheep. II. Meta-assembly and identification of novel QTL for milk production traits in sheep

An (Awassi × Merino) × Merino backcross family of 172 ewes was used to map quantitative trait loci (QTL) for different milk production traits on a framework map of 200 loci across all autosomes. From five previously proposed mathematical models describing lactation curves, the Wood model was considered the most appropriate due to its simplicity and its ability to determine ovine lactation curve characteristics. Derived milk traits for milk, fat, protein and lactose yield, as well as percentage composition and somatic cell score were used for single and two-QTL approaches using maximum likelihood estimation and regression analysis. A total of 15 significant (P < 0.01) and additional 25 suggestive (P < 0.05) QTL were detected across both single QTL methods and all traits. In preparation of a meta-analysis, all QTL results were compared with a meta-assembly of QTL for milk production traits in dairy ewes from various public domain sources and can be found on the ReproGen ovine gbrowser http://crcidp.vetsci.usyd.edu.au/cgi-bin/gbrowse/oaries_genome/. Many of the QTL for milk production traits have been reported on chromosomes 1, 3, 6, 16 and 20. Those on chromosomes 3 and 20 are in strong agreement with the results reported here. In addition, novel QTL were found on chromosomes 7, 8, 9, 14, 22 and 24. In a cross-species comparison, we extended the meta-assembly by comparing QTL regions of sheep and cattle, which provided strong evidence for synteny conservation of QTL regions for milk, fat, protein and somatic cell score data between cattle and sheep.     

Preferential occurrence of wheat-rye meiotic pairing between chromosomes of homoeologous group 1

Summary Wheat-rye homoeologous pairing in both ABRR and (0–7)A(0–7)BRR plants takes place preferentially between homoeologous chromosomes of group 1. This suggests either a much greater affinity between wheat and rye chromosomes for this group or more efficient pairing initiation because of common nucleolar organizer activities. 1A–1R associations were more frequent than 1B–1R associations though in both cases pairing was restricted mostly to the long arms. From the variation in these particular chromosome arms the three following factors might hinder the wheat-rye pairing: regular homologous pairing of rye chromosomes, presence of prominent telomeric C-bands in rye chromosomes or occurrence of wheat-wheat homoeologous pairing.     

Food composition of Clarias gariepinus (= C. lazera) (Cypriniformes,Clariidae) in Lake Kinneret (Israel)

The feeding behaviour of the freshwater piscivore, Clarias gariepinus (C. lazera) (C & V 1840) was studied over two periods: 1973–1975 and 1981–1982, in Lake Kinneret (Israel). The total number of fish analysed was 264 and their sizes (SL) and weights varied between 238 and 830 mm (146 to 5728 g). More than fifty species of plants and animals from the plankton, benthos and nekton of Lake Kinneret were identified in the intestines of C. gariepinus. Preyed fish were the most abundant food component (81%) and constituted the highest biomass, with Mirogrex terraesanctae representing the majority (although other species were also found). The potential impact of piscivory in the Kinneret ecosystem is considered.     

The relative arrangement of chromosomes in mitotic interphase and metaphase in Haplopappus gracilis

The relative position of mitotic metaphase chromosomes in Haplopappus gracilis is studied by direct observation of undisturbed metaphase cells in root tips: the homologous chromosomes lay always adjacent to each other, whereas the relative position of the pairs is not constant. — The relative position of interphase chromosomes is inferred from the frequency of radiation-induced mutual rearrangements between any possible pair of chromosomes. — It is concluded that the relative position of interphase chromosomes is reflected by the relative position of metaphase chromosomes in Haplopappus gracilis.     

Homologous somatic association in radial metaphases of Crepis species

The chromosome arrangement in radial metaphases has been analyzed in root tips of Crepis capillaris (2n=6), C. taraxicifolia (2n=8) and C. rubra (2n=10) by using two statistical approaches: 1) measuring the distances between both members of each pair of homologues as the number of intervening chromosomes on the equatorial ring, and 2) applying a new statistical approach developed by Lacadena et al. (1977) which permits to analyze homologous somatic association considering as a whole the n chromosome pairs of the complement. The occurrence of somatic association of homologous chromosomes is clearly demonstrated with both approaches. Previous results obtained by other authors in different materials and with different statistical methods are discussed. The karyotypes of C. taraxifolia and C. rubra are described with numerical data.     

An ordered arrangement of chromosomes in the somatic nucleus of common wheat Triticum aestivum L.

Spatial relationships between chromosomes of the same genome, both homologous and non-homologous, were studied in root-tip cells of common wheat, Triticum aestivum (2n = 6x = 42). Mean distance between members of all the 21 homologous pairs (seven in each of the three genomes) and of 45 out of the 63 possible non-homologous combinations of two (21 in each genome) were determined. To minimize disruption of nuclear chromosomal arrangement, the cells were pretreated with cold temperature either in tap water or in a physiological medium (White solution) and distances between cytologically marked chromosomes were measured at metaphase. Comparison of distances for homologues with those for non-homologues indicated clearly that, within each genome, the homologous chromosomes were significantly closer to one another than were the non-homologues. Distances between homologues were similar in all three genomes, as were distances between non-homologues. The data are consistent with the hypothesis that the chromosomes of each genome of common wheat are arranged in the somatic nucleus in a highly specific ordered pattern. In this hypothetical arrangement, homologous chromosomes are closely associated, while the nonhomologues occupy definite positions with respect to one another. The universality of the phenomenon and its cellular mechanism and biological significance are discussed.     

Heterochromatin and highly repeated DNA sequences in rye (Secale cereale)

Secale cereale DNA, of mean fragment length 500 bp, was fractionated by hydroxylapatite chromatography to allow recovery of a very rapidly renaturing fraction (C₀t 0–0.02). This DNA fraction was shown to contain several families of highly repeated sequence DNA. Two highly repeated families were purified; (1) a fraction which renatured to a density of 1.701 g/ cc and comprised 2–4% of the total genome, and (2) polypyrimidine tract DNA which comprised 0.1% of the total genome. The 1.701 g/cc DNA consisted of short sequence repeat units (5–50 bp long) tandemly repeated in blocks 30 kb long, while a portion of the polypyrimidine tract DNA behaved as part of a much larger block of tandemly repeated sequences. The chromosomal location of these sequences was determined by the in situ hybridisation of radioactive, complementary RNA to root tip mitotic chromosomes and showed the 1.701 g/cc sequences to be largely limited to the telomeric blocks of heterochromatin, accounting for 25–50% of the DNA present in these parts of the chromosomes. The polypyrimidine tracts were distributed at interstitial locations with 20–30% of the sequences at three well defined sites. The combined distributions of the 1.701 g/cc DNA sequences and polypyrimidine tracts effectively individualised each rye chromosome thus providing a sensitive means of identifying these chromosomes. The B chromosomes present in Secale cereale cv. Unevita, did not show defined locations for the sequences analysed. — The data are discussed in terms of the structure of the rye genome and the generality of the observed genomic arrangement of highly repeated sequence DNA.     

Analysis of chromosome positions in the interphase nucleus of Chinese hamster cells by laser-UV-microirradiation experiments

Summary Unsynchronized cells of an essentially diploid strain of female Chinese hamster cells derived from lung tissue (CHL) were laser-UV-microirradiated (=257 nm) in the nucleus either at its central part or at its periphery. After 7–9 h postincubation with 0.5 mM caffeine, chromosome preparations were made in situ. Twenty-one and 29 metaphase spreads, respectively, with partial chromosome shattering (PCS) obtained after micro-irradiation at these two nuclear sites, were Q-banded and analyzed in detail. A positive correlation was observed between the frequency of damage of chromosomes and both their DNA content and length at metaphase. No significant difference was observed between the frequencies of damage obtained for individual chromosomes at either site of microirradiation. The frequency of joint damage of homologous chromosomes was low as compared to nonhomologous ones. Considerable variation was noted in different cells in the combinations of jointly shattered chromosomes. Evidence which justifies an interpretation of these data in terms of an interphase arrangement of chromosome territories is discussed. Our data strongly argue against somatic pairing as a regular event, and suggest a considerable variability of chromosome positions in different nuclei. However, present data do not exclude the possibility of certain non-random chromosomal arrangements in CHL-nuclei. The interphase chromosome distribution revealed by these experiments is compared with centromere-centromere, centromere-center and angle analyses of metaphase spreads and the relationship between interphase and metaphase arrangements of chromosomes is discussed.     

Evidence against somatic association in hexaploid wheat

In mitotic interphase nuclei, nucleolus-organizing regions of homologous chromosomes lay no closer to each other than nucleolus-organizing regions of non-homologous chromosomes. — Mitotic metaphase studies of distances between satellites and also between telocentrics showed no greater nearness of homologues than of non-homologues. — These studies fail to support the concept of somatic association in hexaploid wheat.     

Population structure and responses to disturbance of the basidiomycete Resinicium bicolor

Summary Resinicium bicolor (Alb. & Schw. ex Fr.) Parm. [=Odontia bicolor (Alb. & Schw. ex Fr.) Bres.] is an outcrossing resupinate basidiomycete associated with root and butt rots of trees, but is itself only very weakly pathogenic. The distribution of genets among every spruce stump in a 70-year-old 1250 m² spruce stand was analysed using somatic incompatibility testing. R. bicolor was present on 40% of 8-to 10-year-old stumps. Nineteen genets were found occupying 32 stumps; yielding probabilities of colonisation following establishment by basidiospores of 0.20–0.24 and by mycelial extension or dispersal of 0.16–0.20. The probability of colonisation decreased with increasing distance from a point of establishment. R. bicolor responded to both enrichment and destructive disturbances by the formation of an extensive cord system which enabled it to colonise discontinuously distributed resources and to overgrow fungi adjacent to it in a single resource unit, including Heterobasidion annosum.     

Genes for the ‘H’ subunit of human ferritin are present on a number of human chromosomes

Summary DNa has been extracted from hamster-human and mouse-human hybrid cell lines, restricted with EcoRI, and hybridised to a probe for the H subunit of human ferritin, pDBR2. Sequences highly homologous to this probe have been found on at least eight human chromosomes: 1, 2, 3, 6p216cen, 11, 14, 20, and Xq23–25Xqter. Only the gene on chromosome 11 appears to be expressed in these hybrids Southern blotting of DNA from somatic cell hybrids containing different subsets of human chromosomes. The study shows that H subunit sequences are found on at least nine different chromosomes.     

Chromosome organization revealed upon the decondensation of telophase chromosomes inAllium

Chromosomes of root tip cells ofAllium cepa andAllium sativum were studied in early, middle and late telophase to examine the organization of mitotic chromosomes, taking advantage of the naturally occurring chromosome dispersion during the process of decondensation in telophase. Longitudinal and transverse sections of telophase chromosomes viewed under the transmission electron microscope showed that mitotic chromosomes inAllium were composed of helically coiled 400–550 nm chromatin fibres. In some regions of the longitudinal sections, these chromatin fibres were seen to be orientated parallel to one another but formed roughly a right angle to the long axis of the chromosome. In transverse sections, the telophase chromosome appeared to have a hollow centre encircled by the 400–550 nm chromatin fibre which in turn was a hollow tube structure formed by the coiling of a thinner fibre of 170–200 nm. In addition, cross views of chromatin fibres of 170–200 nm and 50–70 nm were also identified in telophase chromosome preparations. These two organizational levels of chromatin fibres also showed a hollow centre. The process of decondensation of telophase chromosomes is described, and some morphological characteristics associated with the activities of chromosome decondensation are analysed. Based on the observations made onAllium chromosomes in this study, various models of chromosome organization are discussed.     

The ordered arrangement of chromosomes in the Chinese hamster spermatocyte nucleus

Summary The question of chromosome distribution in the mammalian nucleus is addressed, and data are provided in support of the ordered arrangement of chromosomes in the Chinese hamster spermatocyte. Testicular cells were dispersed and air-dried without prior fixation, then stained and karyotyped. The position of chromosome telomeres in 217 pachytene spermatocytes was determined in relation to four concentric rings which equally divided the nuclear area. The distribution of telomeres showed a progressive decline from the central to the peripheral rings. This was particularly pronounced for chromosomes 1–7, but was reversed for the XY chromosomes. The distribution of the total as well as of the individual chromosomes was significantly different from that expected on the basis of random distribution. The only exceptions to this were chromosomes 8–10, which exhibited random distribution. Thus, while chromosomes 1–7 had a central position, the XY pair had a peripheral localization. The mean ring position appeared to be related to chromosome length, except for the XY chromosomes, suggesting that chromosome length may determine chromosome position.