Incipient Genome Differentiation in Gossypium. I. Chromosomes 14, 15, 16, 19 and 20 Assessed in G. HIRSUTUM, G. RAIMONDII and G. LOBATUM by Means of Seven a-D Translocations

The genus Gossypium is favorable for study of genome divergence at several levels. Early stages of divergence have been studied among four D genomes by comparing chiasma frequencies (reciprocal exchanges) between pairs of genomes and between individual counterpart chromosomes marked by heterozygous translocations. D₅ (G. raimondii) shows barely detectable differentiation from from D_h (G. hirsutum), whereas D₇ (G. lobatum) is considerably less closely related to D_h than is D₅. Fragmentary data suggest that D_2–2 (G. harknessii) falls between D₅ and D₇ in its relationship to D_h. Since chiasma frequencies in individual chromosomes and marked regions exhibit the same order of relationships as their corresponding whole genomes, it is concluded that the genome differentiation is generalized (i.e., nucleus-wide) rather than localized in specific chromosomes or chromosome regions. Estimates of relationships based on reciprocal exchange frequencies agree with those based upon preferential synapsis in allohexaploids reported previously. Since preferential synapsis and reciprocal exchange frequencies reveal the same order of relationships, it is concluded that to some extent they reflect common underlying changes in chromosome properties, despite recent evidence that synapsis and crossing over are under independent genetic control.     

Incipient Genome Differentiation in Gossypium. III. Comparison of Chromosomes of G. HIRSUTUM and Asiatic Diploids Using Heterozygous Translocations

Hybrids between upland cotton (G. hirsutum, genome constitution 2A_hD_h) and either A-genome or D-genome diploid species exhibit 26 paired and 13 unpaired chromosomes at metaphase I. The A_h and D_h genomes are therefore considered homoeologous with those of the respective diploids. Previous studies, nevertheless, revealed a low level of ("incipient") differentiation between D_h and various diploid D genomes. The diploid A genomes have been regarded as more closely homologous to A_h on the basis of low preferential pairing and autotetraploid segregation ratios in allohexaploids.—The present study addressed the following questions: Are the diploid A genomes differentiated from A_h in meiotic homology? If so, is the differentiation manifested equally by all 13 chromosomes or is it localized in certain chromosomes?—Three diploid A-genome lines representing G. herbaceum and G. arboreum were hybridized by in ovulo culture of embryos (1) with a standard line of G. hirsutum, which differs from G. herbaceum by two and from G. arboreum by three naturally occurring reciprocal translocations involving chromosomes 1–5, and (2) with six lines homozygous for experimental translocations involving chromosomes 6, 7, 10, 11, 12 and 13. Chiasma frequencies in hybrids were compared with those in appropriate G. hirsutum controls. In every comparison overall chiasma frequencies were slightly lower in the hybrids. Therefore A_h appears to be differentiated from the diploid A genomes. No localized differentiation was detected in chromosomes marked by experimental translocations. The differentiation may be localized mainly in chromosomes 4 and 5.     

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.     

Genetic Control of Chromosome Elimination during Haploid Formation in Barley

Genetic control over chromosome stability in the interspecific hybrid embryos of Hordeum vulgare and H. bulbosum has been hypothesized to reside on specific chromosomes. In this study, crosses between the primary trisomic lines for the seven different H. vulgare chromosomes and tetraploid H. bulbosum revealed that both chromosomes 2 and 3 of H. vulgare were involved in the control of chromosome elimination. Subsequent crosses using the available monotelotrisomics for chromosomes 2 and 3 led to the conclusion that both arms of chromosome 2 and the short arm of chromosome 3 most likely contain major genetic factors.—From the results of this study and the genome balance observed in the interspecific crosses between H. vulgare and H. bulbosum at the diploid and tetraploid cytotypes, it appears that the factors causing the elimination of the bulbosum chromosomes are located on the H. vulgare chromosome. These factors are offset or balanced by factors on the H. bulbosum chromosomes which, when present in sufficient dosage, either neutralize the effects of the vulgare factors or are able to "protect" the bulbosum chromosomes.     

Preferential Breakpoints in the Recovery of Broken Dicentric Chromosomes in Drosophila melanogaster

We designed a system to determine whether dicentric chromosomes in Drosophila melanogaster break at random or at preferred sites. Sister chromatid exchange in a Ring-X chromosome produced dicentric chromosomes with two bridging arms connecting segregating centromeres as cells divide. This double bridge can break in mitosis. A genetic screen recovered chromosomes that were linearized by breakage in the male germline. Because the screen required viability of males with this X chromosome, the breakpoints in each arm of the double bridge must be closely matched to produce a nearly euploid chromosome. We expected that most linear chromosomes would be broken in heterochromatin because there are no vital genes in heterochromatin, and breakpoint distribution would be relatively unconstrained. Surprisingly, approximately half the breakpoints are found in euchromatin, and the breakpoints are clustered in just a few regions of the chromosome that closely match regions identified as intercalary heterochromatin. The results support the Laird hypothesis that intercalary heterochromatin can explain fragile sites in mitotic chromosomes, including fragile X. Opened rings also were recovered after male larvae were exposed to X-rays. This method was much less efficient and produced chromosomes with a strikingly different array of breakpoints, with almost all located in heterochromatin. A series of circularly permuted linear X chromosomes was generated that may be useful for investigating aspects of chromosome behavior, such as crossover distribution and interference in meiosis, or questions of nuclear organization and function.     

Multiple Ty-mediated chromosomal translocations lead to karyotype changes in a wine strain of Saccharomyces cerevisiae

Enological strains of Saccharomyces cerevisiae display a high level of chromosome length polymorphism, but the molecular basis of this phenomenon has not yet been clearly defined. In order to gain further insight into the molecular mechanisms responsible for the karyotypic variability, we examined the chromosomal constitution of a strain known to possess aberrant chromosomes. Our data revealed that the strain carries four rearranged chromosomes resulting from two reciprocal translocations between chromosomes III and I, and chromosomes III and VII. The sizes of the chromosomal fragments exchanged through translocation range from 40 to 150?kb. Characterization of the breakpoints indicated that the translocations involved the RAHS of chromosome III, a transposition hot-spot on the right arm of chromosome I and a region on the left arm of chromosome VII. An analysis of the junctions showed that in all cases Ty elements were present and suggested that the translocations result from recombination between transposable Ty elements. The evidence for multiple translocations mediated by Ty elements in a single strain suggests that spontaneous Ty-driven rearrangement could be quite common and may play a major role in the alteration of karyotypes in natural and industrial yeasts.     

Genetic Lengths and Break Points in Twelve Chromosomes of GOSSYPIUM HIRSUTUM Involved in Ten Reciprocal Translocations

Chromosome configurations were recorded in about 5500 pollen mother cells (PMC's) in 2n and 2n-1 (missing the intact A-genome chromosome) heterozygotes of ten reciprocal translocations involving six A-genome chromosomes (H1, H2, H3, H4, H6 and H7) and six D-genome chromosomes (H14, H15, H16, H19, H20 and H21) of Gossypium hirsutum. From these records, chiasma frequencies at each of six positions were determined for nine translocations and at two positions for one. These frequencies were used to calculate recombination frequencies in different chromosome regions, and from these distances the breakpoints in 15 chromosome arms were mapped relative to each other and to their respective centromeres, insofar as the data permitted. The karyotype so derived for twelve chromosomes is in reasonably good agreement with data from genetic mapping, telosome and monosome mapping, and the mitotic idiogram.     

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.     

Techniques for Manipulating Chromosomal Rearrangements and Their Application to DROSOPHILA MELANOGASTER. II. Translocations

Translocations have long been valued for their segregational properties. This paper extends the utility of translocations by considering recombinational derivatives of pairs of simple reciprocal translocations. Three major derivative structures are noted. One of these derivatives is suitable for use in half-tetrad experiments. A second should find use in recombining markers with translocation breakpoints. The third is an insertional-tandem duplication: it has a section of one chromosome inserted into a heterologue with a section of the latter chromosome tandemly repeated about the breaks of the insert. All of these structures are contained in "constellations" of chromosomes that regularly segregate aneuploid-1 products (informationally equivalent to nonrecombinant adjacent-1 segregants) for one of the parental translocations but do not segregate euploid products. This is in contrast to the parental T₁/T₂ constellations which segregate euploid products but not aneuploid-1 products. Methods are described for selecting translocation recombinants on the basis of this dichotomy. Several examples of translocation recombinants have been recovered with these techniques, and the recombination frequencies seem to be consistent with those observed for crossovers between inversion breakpoints. Recombinant chromosomes tend to disjoin, but it is observed that the tendency may vary according to the region involved in the recombination, and it is suggested that this difference reflects a difference in chiasmata terminalization times. Special consideration is given to insertional-tandem duplications. Large insertional-tandem duplications are useful in cytogenetic screens. Small insertional-tandem duplications are useful in gene dosage studies and other experiments that require an insert from one chromosome to another. Large duplications can be deleted to form small duplications. To generate a small insert for a specified region, it is only necessary to have one translocation with a breakpoint flanking the region of interest. The second translocation can have a breakpoint quite far from the region: an insertional-tandem duplication containing the region that has one closely flanking breakpoint can be deleted to create a smaller duplication that has two closely flanking breakpoints.     

The classification of the chromosomes of rye (Secale cereale L.): A translocation tester set

The difficulties encountered in classifying the seven chromosomes of rye (Secale cereale L.) are discussed. Unequivocal classification is possible only with a standard testing system such as a translocation tester set. In the present paper a set is described which contains seven reciprocal translocations. Each chromosome participates at least once. The translocated chromosomes can be visually recognized. The size of the chromosome arms was measured and is expressed as % of the total complement length. Which chromosomes were involved in the translocations was studied by using a special graphic method based on the arm ratios of the mitotic chromosomes, and also by intercrossing followed by an analysis of the meiotic cofigurations in the F₁'s.     

Production of wheat-rye substitution lines and identification of chromosome composition of karyotypes using C-banding, GISH, and SSR markers

Based on the cross (Triticum aestivum L. × Secale cereale L.) × T. aestivum L., wheat-rye substitution lines (2n = 42) were produced with karyotypes containing, instead of a pair of homologous wheat chromosomes, a homeologous pair of rye chromosomes. The chromosome composition of these lines was described by GISH and C-banding methods, and SSR analysis. The results of genomic in situ hybridization demonstrated that karyotype of these lines included one pair of rye chromosomes each and lacked wheat-rye translocations. C-banding and SSR markers were used to identify rye chromosomes and determine the wheat chromosomes at which the substitution occurred. The lines were designated 1R(1D), 2R(2D)₂, 2R(2D)₃, 3R(3B), 6R(6A)₂. The chromosome composition of lines 1R(1A), 2R(W)₁, 5R(W), 5R(5A), and 6R(W)₁, which were earlier obtained according to the same scheme for crossing, was characterized using methods of telocentric analysis, GISH, C-banding, and SSR analysis. These lines were identified as 1R(1A), 2R(2D)₁, 5R(5D), 5R(5A), and 6R(6A)₁, C-banding of chromosomes belonging to line 1R(1A) revealed the presence of two translocated chromosomes (3DS.3DL-del. and 4AL.W) during simultaneous amplification of SSR markers located on 3DL and 4AS arms. The “combined” long arm of the newly derived chromosome 4A is assumed to be formed from the long arm of chromosome 4AS itself and a deleted segment 3DL. All examined lines are cytologically stable, except for 3R(3B), which does not affect the stability of rye 3R chromosome transfer. Chromosome identification and classification of the lines will permit them to be models for genetic studies that can be used thereafter as promising “secondary gene pools” for the purpose of plant breeding.     

Molecular characterization of Fusarium resistance from <Emphasis Type="Italic">Elymus repens</Emphasis> introgressed into bread wheat

A cross was made of Elymus repens onto the wheat cultivar Crocus and BC₁ progeny advanced to BC₁F₇ by single seed descent. Sixteen lines were selected based on agronomic performance and evaluated in an FHB epiphytotic nursery. Eight lines with FHB resistance were selected. Based on GISH analysis, line PI 142-3-1-5 had 42 chromosomes with one pair of chromosomes showing telomeric translocations on both arms. This chromosome was identified as 3D by using SSR markers. An evaluation of lines with single translocations revealed that FHB resistance was contributed by the translocation on the long arm of chromosome 3D. That line has minimal linkage drag and should be amenable to applications inbreeding for disease resistance.     

Gene Duplication in SACCHAROMYCES CEREVISIAE

Five indepdendent duplications of the acid-phosphatase (aphtase) structural gene (acp1) were recovered from chemostat populations of S. cerevisiae that were subject to selection for in vivo hyper-aphtase activity. Two of the duplications arose spontaneously. Three of them were induced by UV. All five of the duplication events involved the transpositioning of the aphtase structural gene, acp1, and all known genes distal to acp1 on the right arm of chromosome II, to the terminus of an arm of other unknown chromosomes. One of the five duplicated regions of the right arm of chromosome II was found to be transmitted mitotically and meiotically with very high fidelity. The other four duplicated regions of the right arm of chromosome II were found to be unstable, being lost at a rate of about 2% per mitosis. However, selection for increased fidelity of mitotic transmission was effective in one of these strains. No tandem duplications of the aphtase structural gene were found.     

Spontaneous chromosomal rearrangements in cultivated and wild barleys

Summary Four of 1,240 cultivated barley lines collected from different regions of the world and 3 of 120 lines of wild barley, Hordeum spontaneum C. Koch, carry spontaneous reciprocal translocations. Break-point positions and rearrangements in the interchanged chromosomes have been examined by both test crosses and Giemsa banding techniques. The four translocation lines in cultivated barley were all of Ethiopian origin and have the same translocation involving chromosomes 2 and 4. The breakpoints are at the centromeres of both chromosomes, resulting in interchanged chromosomes 2S+4S and 2L+4L (S=short arm, L=long arm). A wild barley line, Spont.II, also has translocated chromosomes 2 and 4 which are broken at the centromeres. The resultant chromosomes are, however, 2S+4L and 2L+4S. Another wild barley line, Spont.S-4, has interchanged chromosomes with breakpoints in the short arm of chromosome 3 and the long arm of chromosome 7. In addition, this line has a paracentric inversion in the short arm of chromosome 7 that includes a part of nucleolar constriction, resulting in two tandemly arranged nucleolar constrictions. The third wild barley line, Spont.S-7, has interchanged chromosomes with breakpoints in the long arms of both chromosomes 3 and 6. The translocated chromosome 3 is metacentric and the translocated chromosome 6 has a long arm similar in length to the long arm of chromosome 7.     

Structural rearrangement in chromosome 2M of Aegilops comosa has prevented the utilization of the Compair and related wheat-Ae. comosa translocations in wheat improvement

The genetic constitutions of chromosome 2M of Aegilops comosa and the derived wheat-Ae. comosa translocations were analyzed by molecular cytogenetic techniques. Hybridization of 15 RFLP markers covering the entire length of the group-2 chromosomes revealed that chromosome 2M was structurally rearranged compared to the homoeologous chromosomes of wheat by either a pericentric inversion or a terminal intrachromosomal translocation. The breakpoint of the rearrangement was located in a region between the loci Xpsr131 and Xcdo405, resulting in the translocation of 47% of 2MS to 2ML. This aberrant structure of 2M allowed homoeologous recombination between 2M and its wheat counterpart only in the translocated segment on 2ML. C-banding and genomic in situ hybridization analyses confirmed that all translocation chromosomes consisted of the complete 2MS arm, a large part of 2ML, and very small distal segments derived from 2AS or 2DS, as expected from the aberrant structure of chromosome 2M. Thus, the translocation in the line 2A-2M?4/2 can be described as T2AS-2M?1L???2M?1S and the translocations in the lines Compair and 2D-2M?3/8 as T2DS-2M?1L???2M?1S. RFLP analysis determined the breakpoints in these translocation chromosomes to be within the telomeric 16% of the wheat chromosome arms. The breakpoint of the 2A/2M translocation was between Xbcd348 and Xcdo783, and that of the 2D/2M translocation was between Xcdo783 and Xpsr666. Because the translocation chromosomes retain the structural aberration found in chromosome 2M, further exploitation of the wheat-Ae. comosa translocations for cultivar improvement is questionable.     

Regional control of nondisjunction of the B chromosome in maize

Control of nondisjunction in the maize B chromosome was studied using a set of B-10 translocations. The study focused on the possible effect of the proximal region of the B long arm. The experimental procedure utilized a combination of a 10^B chromosome from one translocation with a B¹⁰ from another translocation. The breakpoints of the two translocations were so located that combination of the two elements created a deletion in the proximal region of the B chromosome, but no deletion in chromosome 10. Two different types of deletions were established; one involved a portion of the euchromatic region and the other the entire heterochromatic portion comprising the distal half of the B long arm, except for the small euchromatic tip. Deletion of the heterochromatic portion did not exert any effect on nondisjunction. Deletions of different portions of the euchromatic region produce different responses. Some deletions resulted in typical B nondisjunctional activity; others resulted in the disappearance of this activity. It is concluded that a region within the euchromatic portion of the chromosome is critical for the nondisjunction of B chromosomes. Among 22 translocations with breakpoints in the euchromatic regions, three were proximal to the critical region, 16 were distal and the position of three others was not determined.     

The cytogenetic localization of the alcohol dehydrogenase-1 locus in maize

The alcohol dehydrogenase-1 (Adh) locus in maize has been positioned relative to thirteen reciprocal translocations that have breakpoints in the long arm of chromosome 1(1L). The methods of Gopinath and Burnham (1956) to produce interstitial segmental trisomy with overlapping translocations and of Rakha and Robertson (1970) to produce compound B-A translocations were coupled with the co-dominant nature of the ADH isozymes to allow the cytological placement. The results of several crosses are consistent with Adh being in the region of 0.80-0.90 of 1L.--The duplication that results from the overlap of translocations 1-3(5267) and 1-3(5242) and that includes Adh was studied with respect to meiotic segregation and pollen transmission. When heterozygous with normal chromosomes, a low level of recombination within the duplicated regions is detectable and the duplication and normals are recovered with equal frequencies through the female. In the pollen, the hyperploid grains cannot compete equally with the euploids in achieving fertilization.--The use of co-dominant heteromultimeric isozymes as genetic markers for the development of a series of interstitial segmental trisomics in maize is discussed.     

Multiple Ty-mediated chromosomal translocations lead to karyotype changes in a wine strain of Saccharomyces cerevisiae.

Enological strains of Saccharomyces cerevisiae display a high level of chromosome length polymorphism, but the molecular basis of this phenomenon has not yet been clearly defined. In order to gain further insight into the molecular mechanisms responsible for the karyotypic variability, we examined the chromosomal constitution of a strain known to possess aberrant chromosomes. Our data revealed that the strain carries four rearranged chromosomes resulting from two reciprocal translocations between chromosomes III and I, and chromosomes III and VII. The sizes of the chromosomal fragments exchanged through translocation range from 40 to 150 kb. Characterization of the breakpoints indicated that the translocations involved the RAHS of chromosome III, a transposition hot-spot on the right arm of chromosome I and a region on the left arm of chromosome VII. An analysis of the junctions showed that in all cases Ty elements were present and suggested that the translocations result from recombination between transposable Ty elements. The evidence for multiple translocations mediated by Ty elements in a single strain suggests that spontaneous Ty-driven rearrangement could be quite common and may play a major role in the alteration of karyotypes in natural and industrial yeasts. Received: 18 December 1998 / Accepted: 26 March 1999