Somatic hybrids and cybrids of Senecio fuchsii Gmel. (x) jacobaea L.

Summary In situ hybridisation and restriction fragment length polymorphism (RFLP) analysis were used to determine the relative location of the translocation breakpoint and the size of the integrated chromatin segment in hexaploid wheat-Lophopyrum translocation stocks. Three 7el₂-7D recombinant stocks were Robertsonian translocations, 7DS.7el. The remaining recombinant stock (KS10-2) was 7elS.7el-7DL and contained only the distal one-half of the long arm of 7D. The recombinant stock with 7el₁ (K11695) could be designated 7DS.7DL-7el where approximately the distal one-half of 7DL was replaced. RFLP analysis indicated that on the 7DL RFLP map the breakpoints for K11695 and KS10-2 are in different locations and that the two recombinants contain an overlapping region (a common region) of the Lophopyrum chromosome 7 in which Lr19, a leaf-rust resistant gene, is located. RFLP analysis also indicated that RFLP markers which mapped to within 1.5 cm of the centromere of chromosome 7D are located in the distal half of the long arm.     

A ''hot-spot'' for Ty transposition on the left arm of yeast chromosome III.

The small ring derivative of Saccharomyces cerevisiae chromosome III, which was formed by a cross-over between HML on the left arm and HMR on the right arm, contains three Ty elements. The class II element Ty 1-17 lies immediately centromere-distal to LEU2 on the left arm while two class I elements are tandemly arranged distal to PGK on the right arm. We have sequenced the regions of chromosome III surrounding Ty 1-17 and have defined a region where a number of transposition events have occurred. This region is flanked by the 5' ends of two tRNA genes, tRNA3Glu on the centromere distal side and tRNA3Leu immediately in front of LEU2. Close to the tRNA3Glu gene there is a region containing degenerate delta sequences organised in opposite orientations. Immediately distal to Ty 1-17 there are two complete solo delta elements, one inserted into the other. The sequence indicates that these two delta sequences were inserted into chromosome II by separate transposition events. A model is presented to explain how this structure arose and the role of solo delta elements in transposon propagation and maintenance is discussed.     

A near terminal pericentric inversion leads to nitrogen metabolite derepression in Aspergillus nidulans

Summary The mutation xprD-1, previously shown to be an allele of the areA gene and to lead to nitrogen metabolite derepression in Aspergillus nidulans, is shown to be associated with a near terminal pericentric inversion in linkage group III. The left arm break-point is between the adI and sC genes, and the right arm break-point is between the ornC and areA genes but just centromere proximal to areA. In crosses of xprD-1 strains to inversion-free strains one class of duplication-deficiency progeny is recovered. These progeny have two copies of the distal portion of the left arm beginning just before sC but lack a copy of areA and the region distal to it on the right arm. The viability of these duplication-deficiency progeny indicates that no indispensable gene can lie distal to areA, suggesting proximity of areA to the telomere. The inversion might increase expression of areA which, given the positive acting nature of this regulatory gene, would result in nitrogen metabolite derepression. If increased expression be the result of fusion to (or creation of) a more efficient promoter and/or ribosome binding sequence, areA must be transcribed towards the right arm telomere.     

Mapping the nucleolus organizer region,seed protein loci and isozyme loci on chromosome 1R in rye

Summary The nucleolus organizer region located on the short arm of chromosome 1R of rye consists of a large cluster of genes that code for ribosomal RNA (designated the Nor-R1 locus). The genes in the cluster are separated by spacer regions which can vary in length in different rye lines. Differences in the spacer regions were scored in two families of F₂ progeny. Segregation also occurred, in one or both of the families, at two seed protein loci and at two isozyme loci also located on chromosome 1R. The seed protein loci were identified as the Sec 1 locus controlling -secalins located on the short arm of chromosome 1R and the Sec 3 locus controlling high-molecular-weight secalins located on the long arm of 1R. The two isozyme loci were the Gpi-R1 locus controlling glucose-phosphate isomerase isozymes and the Pgd 2 locus controlling phosphogluconate dehydrogenase isozymes. The data indicated linkage between all five loci and map distances were calculated. The results indicate a gene order: Pgd 2 ... Sec 3 ... [centromere] ... Nor-R1 ... Gpi-R1 ... Sec 1. Evidence was obtained that rye possesses a minor 5S RNA locus (chromosome location unknown) in addition to the major 5S RNA locus previously shown to be located on the short arm of chromosome 1R.     

Genetic linkage between C-bands and storage protein genes in chromosome 1B of tetraploid wheat

Summary Genetic mapping of polymorphic C-bands allows direct comparisons between genetic and physical maps. Eleven C-bands and two seed storage protein genes on chromosome 1B, polymorphic between Langdon durum and four accessions of T. dicoccoides, were used to study the distribution of recombination along the entire length of the chromosome. Recombination in the short arm was almost completely restricted to the satellite, two-thirds of the arm's length from the centromere; the Gli-B1 gene was found to be tightly linked to the telomeric C-band. In the long arm, the distal 51.4% of the arm accounted for 88% of recombination; the proximal half of the arm accounted for the remaining 12%. While the amount of crossing-over differed significantly between the four T. dicoccoides 1B chromosomes, there were no significant differences in the relative distributions of crossing-over along the chromosome. Consequently, the genetic maps obtained from the four individual T. dicoccoides chromosomes were combined to yield a consensus map of 14 markers (including the centromere) for the chromosome.     

Mitotic crossing over in chromosome I disomics of Aspergillus nidulans

Summary The frequency and pattern of homologous recombination in chromsome I disomics of Aspergillus nidulans is presented. Approximately 6% of randomly selected haploid breakdown sectors are recombinant. Most of these arise from double exchange events, one of which is located in the centromere region, the other distal on the left arm. Other marked regions are rarely involved in a recombination event. Reciprocal genotypes arise in approximately equal frequencies indicating that exchange results in reciprocally recombined non-sister chromatids at the four strand stage of mitosis. Possible theories for the extreme localisation of exchange events are discussed.     

The Entire Compound Autosomes of DROSOPHILA MELANOGASTER

Three new unusual compound chromosomes have been synthesized in Drosophila melanogaster. They consist of two homologous autosomes joined together in the new order: right arm, left arm, centromere, left arm, right arm, for each of the two major autosomes, and one in which chromosomes 2 and 3 have been combined in the order: right arm of 2, left arm of 2, centromere, left arm of 3, right arm of 3. The attachments of the autosomal arms were accomplished by obtaining chromosome breaks at or very close to the ends of the left arms of the autosomes such that no essential chromosome material has been removed; the compounds derived from them are therefore referred to as entire compounds. These large chromosomes are recovered in progeny with frequencies lower than expectation partly because of zygote mortality associated with these chromosomes, and partly because of a failure of spermiogenesis.     

Precocious Meiotic Centromere Separation of a Novel Yeast Chromosome

In Saccharomyces cerevisiae, a reciprocal translocation between chromosome II and a linear plasmid carrying a centromere (CEN6) has split chromosome II into two fragments: one, approximately 530 kilobase pairs (kbp) in size, has the left arm and part of the right arm of chromosome II; the other, a telocentric fragment approximately 350 kbp in size, has CEN6 and the rest of the right arm of chromosome II. A cross of this yeast strain with a strain containing a complete chromosome II exhibits a high frequency of precocious centromere separation (separation of sister chromatids during meiosis I) of the telocentric fragment. Precocious centromere separation is not due to the position of the centromere per se, since diploids that are homozygous for both fragments of chromosome II segregate the telocentric fragment with normal meiotic behavior. The precocious centromere separation described here differs from previously described examples in that pairing and synapsis of this telocentric chromosome seem to be normal. One model of how centromeres function in meiosis is that replication of the centromere is delayed until the second meiotic division. Data presented in this paper indicate that replication of the centromere is complete before the first meiotic division. The precocious separation of the centromere described here may be due to improper synapsis of sequences flanking the centromere.     

Molecular organization of Chlorella vulgaris chromosome I: presence of telomeric repeats that are conserved in higher plants

The unicellular green alga Chlorella vulgaris (strain C-169) has a small genome (38.8 Mb) consisting of 16 chromosomes, which can be easily separated by CHEF gel electrophoresis. We have isolated and characterized the smallest chromosome (chromosome 1, 980 kb) to elucidate the fundamental molecular organization of a plant-type chromosome. Restriction mapping and sequence analyses revealed that the telomeres of this chromosome consist of 5-TTTAGGG repeats running from the centromere towards the termini; this sequence is identical to those reported for several higher plants. This sequence is reiterated approximately 70 times at both termini, although individual clones exhibited microheterogeneity in both sequence and copy number of the repeats. Subtelomeric sequences proximal to the termini were totally different from each other: on the left arm, unique sequence elements (14–20 bp) which were specific to chromosome I, form a repeat array of 1.7 kb, whereas a 1.0 kb sequence on the right arm contained a poly(A)-associated element immediately next to the telomeric repeats. This element is repeated several times on chromosome I and many times on all the other chromosomes of this organism.     

Evidence for a genetic duplication involving alcohol dehydrogenase genes inCeratitis capitata

AnAdh duplication is described in the medflyCeratitis capitata. Evidence is presented for two separateAdh ₁ andAdh ₂ structural loci mapping at a distance of 0.49 recombination unit from each other. By deletion mapping theAdh region has been cytologically located near the free end of the left arm of the second chromosome within an area between 2C;3A segments of the polytene chromosome. The genetic analysis of the region aroundAdh has identified seven neighboring genes (Acon ₁,Mpi, Est ₆,Aox, Xdh, Mdh ₂,Lsp _I) which identify the linkage group D. The orientation of loci with regard to the centromere sets the origin of the map of the left arm of the second chromosome close to the twoAdh loci.Research supported by National Research Council of Italy, Special Project RAISA, Subproject N.2, Paper N.200. Grants from I.A.E.A. (International Atomic Energy Agency, Vienna, Austria) and from European Communities Commission, Second R&D Programme Science and Technology for Development also contributed to this work.     

Physical mapping of the 5S ribosomal RNA genes on rice chromosome 11

One 5S ribosomal RNA gene (5S rDNA) locus was localized on chromosome 11 of japonica rice by in situ hybridization. The biotinylated DNA probe used was prepared by direct cloning and direct labeling methods, and the locus was localized to the proximal region of the short arm of chromosome 11 (llpl.l) by imaging methods. The distance between the signal site and the centromere is 4.0 arbitrary units, where the total length of the short arm is 43.3 units. The 5S rDNA locus physically identified and mapped in rice was designated as 5SRrn. The position of the 5S rDNA locus reported here differs from that in indica rice; possible reasons for this difference are discussed. DNA sequences of 5S rDNA are also reported.     

Intrachromosomal mapping of crossability genes in wheat (Triticum aestivum)

Summary Intrachromosomal mapping studies were used to locate the positions of the genes Kr1 and Kr2, which control the crossability of wheat with Hordeum bulbosum, on chromosomes 5B and 5A, respectively. The location of Kr1 was established using the telocentric mapping technique and found to be on the long arm of chromosome 5B, distal to the centromere with a mean recombination frequency of 44.8±3.28%. Kr2 was located on the long arm of chromosome 5A by linkage with the major gene markers Vrn1, controlling vernalization requirement, and q, controlling ear morphology. Kr2 is closely linked to Vrn1, with a mean recombination frequency of 4.8±4.66%, and is distal to q with a mean recombination frequency of 38.1±10.60%. The similar locations of Kr1 and Kr2 on homoeologous chromosomes suggest that these two loci are homoeoallelic. Significant correlations between Hordeum bulbosum and rye crossability confirmed that Kr1 and Kr2 control the crossability of wheat with both species.     

Investigation of Homologous Crossing over and Sister Chromatid Exchange in the Wheat Nor-B2 Locus Coding for Rrna and Gli-B2 Locus Coding for Gliadins

Recombination was investigated within the Nor-B2 locus of wheat chromosome 6B that contains several thousand of the 18S-5.8S-26S rRNA (rDNA) repeated units. Additionally, recombination was assessed for several chromosome regions, in arm 6Bq between the centromere and the B2 locus (awn suppressor) and in arm 6Bp between the centromere and Nor-B2, between Nor-B2 and a distal C-band and between Nor-B2 and Gli-B2 coding for gliadins. The experimental design permitted the distinction between crossing over between homologous chromosomes and exchange between sister chromatids. No homologous crossing over within the Nor-B2 locus was found in a sample of 446 chromosomes, but one exchange with the attributes of unequal sister chromatid exchange was identified. The molecular characteristics of this presumed sister chromatid exchange indicate that the spacer variants present in the Nor-B2 locus are clustered. No homologous recombination was detected within the distal Gli-B2 locus containing repeated genes coding for gliadin seed-storage proteins. Both arms of chromosome 6B showed low crossing-over frequency in the proximal regions. The distance from the centromere to Nor-B2 was only from 0.3 to 2.2 cM although it accounts for about two-thirds of the metaphase chromosome arm, which shows a great distortion of the metaphase map of the arm. The level of homologous recombination within the Nor-B2 locus is lower than in the chromosome region immediately distal to it. Whether it is comparable to that in the chromosome region proximal to it could not be determined. Recombination frequencies of different pairs of chromosome 6B in all but one interval paralleled the frequencies of their metaphase I pairing: Lower pairing at metaphase I was paralleled by lower crossing-over frequency. This relationship indicated that reduced metaphase I pairing between 6B chromosomes from different populations is due to impaired crossing-over and not due to precocious chiasma terminalization.     

Linkage and cytogenetic maps of genes controlling endosperm storage proteins and isozymes in rye (Secale cereale L.)

Summary An F₁ plant fromSecale cereale ssp.ancestrale xtelocentric substitution lines3R of the cultivated rye Petkus spring was used as female in a cross with the inbred line Riodeva (I28), which has the standard chromosome arrangement. Single plants from this backcross progeny were analyzed for chromosome constitution, storage protein, and isozymic patterns. The seed protein loci were identified asSec-1a andSec-1b loci controlling 40-K-secalins and-secalins, respectively. These loci are located on the short arm of chromosome1R. TheSec-3 locus controlling high-molecular-weight secalins is located on the long arm of chromosome1R. A further seed protein locus,Pr-3 (55-K protein), was located on the short arm of chromosome1R. A linkage was found between the6Pgd-2 isozyme locus controlling 6-phosphogluconate dehydrogenase isozymes located on the long arm of chromosome1R and the four seed protein loci. The results favor the gene order:6Pgd-2 ...Sec-3 ... [centromere] ...Pr-3 ...Sec-1b ...Sec-1a. Other linkages detected werePer-3a andPer-3b (0.33±0.33 cM),Est-8 andEst-12 (0.33±0.33 cM), andGot-3 and centromere (20.57±2.42 cM). The proxidase (Per), glutamate oxaloacetate transaminase (Got), and esterase (Est) loci were located on chromosome arms2RS,3RL, and6RL, respectively. The distances and the maps obtained are compared with data available in the literature.     

Distribution of genes and recombination on wheat homoeologous group <Emphasis Type="Italic">6</Emphasis> chromosomes: a synthesis of available information

Presence of genes in gene-rich regions on wheat chromosomes has been widely reported. However, there is a lack of information on the precise characterization of these regions with respect to the distribution of genes and recombination. We attempted to critically analyze the available data to characterize gene-rich regions and to study the distribution of genes and recombination on wheat homoeologous group 6 chromosomes which are a reservoir of several useful genes controlling traits of economic importance. Consensus physical and genetic linkage maps were constructed for homoeologous group 6 using physical and genetic mapping data. Five major gene-rich regions were identified on homoeologous group 6 chromosomes, with two on the short and three on long arm. More than 90% of marker or gene loci were present in these five gene-rich regions, which comprise about 30% of the total physical chromosomal length. The gene-rich regions were mainly present in the distal 60% regions of the chromosomes. About 61% of the total loci map in the most distal regions which span only about 4% of the physical length of the chromosome. A range of sub-microscopic regions within each gene-rich region were also identified. Comparisons of the consensus physical and genetic linkage maps revealed that recombination occurred mainly in the gene-rich regions. Seventy percent of the total recombination occurred in the two most distally located regions that span only 4% of the physical length of the chromosomes. The relationship of recombination to the gene-rich region is not linear with distance from the centromere, especially on the long arm. The kb/cM estimates for group 6 chromosomes ranged from 146 kb in the gene-rich to about 10 Mb in the gene-poor region. The information obtained here is vital in understanding wheat genome structure and organization, which may lead in developing better strategies for positional cloning in wheat and related cereals.This revised version was pubished online in April 2005 with corrections to the page numbering.     

Precise mapping of a locus affecting grain protein content in durum wheat

Grain protein content (GPC) is an important factor in pasta and breadmaking quality, and in human nutrition. It is also an important trait for wheat growers because premium prices are frequently paid for wheat with high GPC. A promising source for alleles to increase GPC was detected on chromosome 6B of Triticum turgidum var. dicoccoides accession FA-15-3 (DIC). Two previous quantitative trait locus (QTL) studies found that the positive effect of DIC-6B was associated to a single locus located between the centromere and the Nor-B2 locus on the short arm of chromosome 6B. Microsatellite markers Xgwm508 and Xgwm193 flanking the QTL region were used in this study to develop 20 new homozygous recombinant substitution lines (RSLs) with crossovers between these markers. These 20 RSLs, plus nine RSLs developed in previous studies were characterized with four new RFLP markers located within this chromosome segment. Grain protein content was determined in three field experiments organized as randomized complete block designs with ten replications each. The QTL peaks for protein content were located in the central region of a 2.7-cM interval between RFLP markers Xcdo365 and Xucw67 in the three experiments. Statistical analyses showed that almost all lines could be classified unequivocally within low- and high- protein groups, facilitating the mapping of this trait as a single Mendelian locus designated Gpc-6B1. The Gpc-6B1 locus was mapped 1.5-cM proximal to Xcdo365 and 1.2-cM distal to Xucw67. These new markers can be used to reduce the size of the DIC chromosome segment selected in marker-assisted selection programs. Markers Nor-B2 and Xucw66 flanking the previous two markers can be used to select against the DIC segment and reduce the linkage drag during the transfer of Gpc-6B1 into commercial bread and pasta wheat varieties. The precise mapping of the high GPC gene, the high frequency of recombinants recovered in the targeted region, and the recent development of a tetraploid BAC library including the Gpc-6B1 DIC allele are the first steps towards the map-based cloning of this gene.Communicated by J. Dvorak     

Molecular hybridization of iodinated 4S, 5S, and 18S + 28S RNA to salamander chromosomes

4S, 5S, AND 18S + 28S RNA from the newt Taricha granulosa granulosa were iodinated in vitro with carrier-free 125I and hybridized to the denatured chromosomes of Taricha granulosa and Batrachoseps weighti. Iodinated 18S + 28S RNA hybridizes to the telomeric region on the shorter arm of chromosome 2 and close to the centromere on the shorter arm of chromosome 9 from T. granulosa. On this same salamander the label produced by the 5S RNA is located close to or on the centromere of chromosome 7 and the iodinated 4S RNA labels the distal end of the longer arm of chromosome 5. On the chromosomes of B. wrighti, 18S + 28S RNA hybridizes close to the centromeric region on the longer arm of the largest chromosome. Two centromeric sites are hybridized by the iodinated 5S RNA. After hybridization with iodinated 4S RNA, label is found near the end of the shorter arm of chromosome 3. It is concluded that both ribosomal and transfer RNA genes are clustered in the genome of these two salamanders.     

Comparison between responses to gametophytic and sporophytic recurrent selection in maize (Zea mays L.)

Summary Experiments were conducted to determine the chromosomal location of the gene conditioning overproduction of a methionine-rich, 10-K zein in maize kernels of line BSSS53. In addition, the chromosomal location of the structural gene encoding the overproduced protein was determined. Whereas the structural gene, designated Zps10/(22), was found to be located on the long arm of chromosome 9 near the centromere, the locus regulating overproduction of the zein protein was mapped to the short arm of chromosome 4. This regulatory gene has been designated Zpr10/(22). Regulation of 10-K zein production by Zpr10/(22) is, therefore, via a trans-acting mechanism.     

Meiotic mapping of the nuclear determinant of cell lysis of the osmotic dependent Saccharomyces cerevisiae mutant VY1160

Summary A recessive nuclear mutation, sorb ^-, which determines the ability for lysis of the osmotic dependent Saccharomyces cerevisiae mutant VY1160 has been mapped on the right arm of chromosome I. sorb ^- is not centromere linked and is approximately 31 recombination units from ade1.