Locating a site on the maize B chromosome that controls preferential fertilization.

In maize, the B chromosome can undergo nondisjunction at the second pollen mitosis, producing sperm with two B chromosomes and sperm with zero B chromosomes. Preferential fertilization is the ability of the sperm carrying two B chromosomes to transmit more frequently to the embryo of a kernel than the sperm lacking the B chromosome. A translocation involving the B chromosome and chromosome 9, TB-9Sb, has been used to study preferential fertilization. The B-9 chromosome has the same properties of nondisjunction and preferential fertilization as the standard B chromosome. Deletion derivatives of B-9, which lack the centric heterochromatin and possibly some adjacent euchromatin, were tested for their ability to induce preferential fertilization. They were found to lack the capacity for preferential fertilization.     

Unstable telocentric chromosome produced after centric misdivision of a 21q/21q translocated element

Summary An unstable telocentric chromosome was found in an individual with Down's syndrome and an unusual chromosomal mosaic, 46,XX, t(21q21q)+,21-/46,XX,21p-/45,XX,21-. As the 21q/21q chromosome was of paternal origin, based on the characteristics of its centromeric heterochromatin and on the characteristics of both 21 chromosomes of the father, it was concluded that its formation involved centric breakage and loss of centromeric material. The cell line with the 21p- chromosome may have originated from the translocation by an asymmetric misdivision of the reduced centromeric material. Of the two telocentrics produced by this fracture, one, possessing the smaller amount of centromeric apparatus, would be immediately lost; the other would be retained, but complete activity of its centromere would not be restored. It would therefore be unstable and might be lost.     

Centric fission consequences in man

The authors summarise the consequences of centric fission in man as follows: classical (monocentric) isochromosomes; usually either for p or q, exceptionally for both arms; stable telocentrics for either one or both arms; isochromosome for one arm, stable telocentric for the other; isochromosome for one arm concurring with translocation of the telocentric for the other; telocentric/isochromosome mosaicism for the same arm; stable telocentric for a part of one arm, the remaining of the chromosome forming a smaller element (obviously this rearrangement requires an additional break outside the centromere), and whole-arm translocations. These events are discussed in the light of current notions about centromere structure and function.     

Structural chromosome abnormalities in Down syndrome: a study of two families.

Two families were ascertained through a proband with Down syndrome and a structural rearrangement involving two chromosomes 21. It is suggested that in one patient the chromosome is an isochromosome formed by misdivision of the centromere of a maternal telocentric chromosome 21 and that in the other a Robertsonian translocation involving chromosome 21 was inherited from the mother, who is a 46,XX/46,XX, -21,+t(21q21q) mosaic. The origin of the mosaicism is discussed and considered to be likely to be the result of breakage and reunion at the chromatid, rather than the chromosome, level.     

Efficient recovery of centric heterochromatin P-element insertions in Drosophila melanogaster

Approximately one-third of the human and Drosophila melanogaster genomes are heterochromatic, yet we know very little about the structure and function of this enigmatic component of eukaryotic genomes. To facilitate molecular and cytological analysis of heterochromatin we introduced a yellow(+) (y(+))-marked P element into centric heterochromatin by screening for variegated phenotypes, that is, mosaic gene inactivation. We recovered >110 P insertions with variegated yellow expression from approximately 3500 total mobilization events. FISH analysis of 71 of these insertions showed that 69 (97%) were in the centric heterochromatin, rather than telomeres or euchromatin. High-resolution banding analysis showed a wide but nonuniform distribution of insertions within centric heterochromatin; variegated insertions were predominantly recovered near regions of satellite DNA. We successfully used inverse PCR to clone and sequence the flanking DNA for approximately 63% of the insertions. BLAST analysis of the flanks demonstrated that either most of the variegated insertions could not be placed on the genomic scaffold, and thus may be inserted within novel DNA sequence, or that the flanking DNA hit multiple sites on the scaffold, due to insertions within different transposons. Taken together these data suggest that screening for yellow variegation is a very efficient method for recovering centric insertions and that a large-scale screen for variegated yellow P insertions will provide important tools for detailed analysis of centric heterochromatin structure and function.     

The B chromosome polymorphism of the grasshopper Eyprepocnemis plorans in North Africa: III. mutation rate of B chromosomes

B chromosome variation in nine Moroccan populations of the grasshopper Eyprepocnemis plorans was analysed for 3 consecutive years. In addition to B1, which was the predominant B chromosome in all nine populations, we found 15 other B variants, albeit at very low frequency. Eight variants were found in adults caught in the wild, four appeared in adults reared in the laboratory and seven were found in embryo progeny of controlled crosses between a 0B male and a B-carrying female. Some variants were found in more than one kind of material. At least the seven B variants that appeared in embryo progeny of females carrying a different B type arose de novo through mutation of the maternal B chromosome. The mutation rate of B chromosomes was 0.73%, on average, which explains the high variety of morphs and banding patterns found. The most frequent de novo mutations observed in these chromosomes were centromere misdivision with or without chromatid nondisjunction, which generates iso-B-chromosomes or telocentric Bs, respectively, as well as translocations with A and B chromosomes and deletions. But the whole variation observed, including that found in adult individuals, suggests that other mutations such as duplications, inversions and centric fusions do usually affect B chromosomes. Finally, B chromosome mutation rate was remarkably similar in both Moroccan and Spanish populations, which suggests that it might be dependent on B chromosome intrinsic factors.     

毛冠鹿种内异染色质变化与染色体多态

采用原代和传代培养方法对8头毛冠鹿(Elaphodus cephalophus)的皮肤细胞进行了染色体研究,发现了一种核型与以前所报道的几种核型不一致,确定为一新核型。在该核型中,染色体众数2n=47,2条X染色体异型,一条为端着丝粒,另一条为近端着丝粒。C-带显示该核型中异染色质除了分布在2条X染色体长臂中之外,在第一对大的端着丝粒染色体中的一条近着丝粒区出现一异染色质“柄”。结合C-带及薄层扫描结果对毛冠鹿种内常染色体、性染色体中异染色质的含量和分布与染色体多态的关系进行了探讨。     

The B-chromosome system of Allium schoenoprasum

Nine morphologically distinct euchromatic B-chromosomes have been identified in Allium schoenoprasum from the River Wye, South Wales. The most common type (89%) is telocentric (B^t–1) and it is likely that the non-standard Bs are derivatives of B^t–1 by deletion, centric shift and/or centric misdivision. New B-types have also been produced from standard Bs in controlled crosses. In general, the Bs are mitotically extremely stable, although occasional plants, particularly those carrying non-standard Bs, are conspicuously variable in their B-constitution between root-tip cells. In addition, B-chromosome number is enhanced in some anthers of about one third of plants. Behaviour of B-chromosomes during meiosis is described. Although there is little bivalent formation, less than 5% of the Bs are lost during meiosis in anthers. There is, however, no evidence of B-chromosome accumulation in the offspring of controlled crosses, usually a slight loss, and Bs have deleterious effects on aspects of vigour and fertility. Thus, no satisfactory explanation for populations with up to 65% B-containing individuals has yet been found.     

Cytological and molecular analysis of centromere misdivision in maize.

B chromosome derivatives suffering from breaks within their centromere were examined cytologically and molecularly. We showed by high resolution FISH that misdivision of the centromere of a univalent chromosome can occur during meiosis. The breaks divide the centromere repeat sequence cluster. A telocentric chromosome formed by misdivision was found to have the addition of telomeric repeats to the broken centromere. A ring chromosome formed after misdivision occurred by fusion of the broken centromere to the telomere. Pulsed-field electrophoresis analyses were performed on the telocentric and ring chromosomes to identify fragments that hybridize to both the telomeric repeat and the B-specific centromeric repeat. We conclude that healing of broken maize centromeres can be achieved through the mechanisms of addition or fusion of telomeric repeat sequences to the broken centromere.     

Molecular distinction between true centric fission and pericentric duplication-fission

Centromere (centric) fission, also known as transverse or lateral centric misdivision, has been defined as the splitting of one functional centromere of a metacentric or submetacentric chromosome to produce two derivative centric chromosomes. It has been observed in a range of organisms and has been ascribed an important role in karyotype evolution; however, the underlying mechanisms remain unknown. We have investigated four cases of apparent centric fission in humans. Two cases show a missing chromosome 22 or 18 that is replaced by two centric ring products, a third case shows two chromosome-10-derived telocentric chromosomes, whereas a fourth case involves the formation of two chromosome-18-derived isochromosomes. In all four cases, results of gross cytogenetic and fluorescence in situ hybridisation analyses were consistent with a simple centric fission event. However, detailed molecular analyses provided evidence in support of centromere duplication as a predisposing mechanism for the observed chromosomal breakage in two of the cases. Results for the third case are consistent with direct centric fission not involving centromere pre-duplication as the likely mechanism. Insufficient material has precluded the further study of the fourth case. The data provide the first molecular evidence for centromere pre-duplication as a possible mechanism to explain the classically assumed simple centric fission events in clinical cytogenetics, karyotype evolution and speciation.     

Instability of the maize B chromosome

Summary The B ⁹ chromosome of maize exhibits a very ordered type of instability at the second pollen mitosis, when nondisjunction may reach a level of 95%. Much less commonly the chromosome is unstable during early development of the kernel. Instability in the kernel produces recessive sectors in either the endosperm or the sporophyte, reflecting the absence of dominant markers carried by the B ⁹. The causes of B ⁹ loss in the endosperm and the sporophyte were investigated for the two observable classes of sectoring: fractional loss (single event) and multiple loss (mosaic pattern). The fractional class represents isochromosome formation by the B ⁹ (Carlson, 1970, 1971). Data presented here suggest that the isochromosome is a by-product of telocentric formation at the second pollen mitosis, and does not arise directly from the B ⁹ chromosome. The chromosomal basis for the mosaic pattern of B ⁹ loss is not completely known. However, one class of mosaic kernels displays a heritable instability of the B ⁹ chromosome which apparently results from ring chromosome formation by the B ⁹. The time of origin of the ring B ⁹ chromosome is prior to the second pollen mitosis, since the unstable chromosome generated in the male parent is transmitted to both the endosperm and the sporophyte. Finally, a genetic factor controlling B ⁹ stability in the developing endosperm has been found. A single plant (1818-1), crossed as a female parent to a B ⁹-containing stock, induced a mosaic pattern of B ⁹ loss in the endosperm at a very high rate. The characteristics of this plant are being investigated.Dedicated with much appreciation and respect to Dr. M. M. Rhoades on the occasion of his 70th birthday.     

Stable telocentric chromosomes produced by centric fission in chinese hamster cells in vitro

Metaphase examination of pseudodiploid Chinese hamster cells revealed that spontaneous breaks or fission occurred rather frequently (2.9%) at the centromeric regions of subtelo- or metacentric chromosomes, resulting in the production of telocentric chromosomes. The centromeric fission appeared to occur in every member of the chromosome complement. An attempt was made to isolate cells possessing thus derived telocentrics from the cell population and gave two clonal lines which were retaining one and two telocentric chromosomes, respectively. Both banding and labeling patterns of these chromosomes indicated unequivocally their X chromosome origin. They were transmitted successively to the daughter cells during a 3-month culture period, showing no tendency to fuse to produce a metacentric chromosome.Contribution No. 897 from the National Institute of Genetics, Japan.     

Induced translocation of an A and a B Chromosome in Lolium perenne

A reciprocal translocation between an A and a B chromosome was identified in the progeny of X-irradiated heads of Lolium perenne. The resulting centric B^A fragment was found to have lost the capacity to undergo non-disjunction at pollen grain mitosis and consequently was transmitted in the normal mendelian manner. The synthesis of a stable population of 16 chromosome L. perenne plants is therefore a possibility. —Analysis of the effect of both the translocated B segment and the centric B fragment on meiosis in diploid L. temulentum x L. perenne hybrids showed that both parts are capable, to a certain extent at least, of suppressing homoeologous chromosome association at first metaphase.     

Construction of midget chromosomes in wheat.

To test the usefulness of breakage-fusion-bridge (BFB) cycles in generating new chromosome aberrations in bread wheat (Triticum aestivum L.) and to extend the range of aberrations available, a series of midget chromosomes was produced from the long arm of chromosome 1B. Using a reverse tandem duplication initiated chromatid type BFB cycle, the 1BL arm was broken and fused with centromeres of either chromosome 5BL or 1RS to form dicentric chromosomes. The 1R and 5B centromeres were broken by centric misdivision. Among the progenies of plants with dicentric chromosomes, two classes of monocentric chromosomes were selected: deficient chromosomes 1B and chromosomes that had 1RS or 5BL for one arm and various fragments of 1BL for the other arm. Following centric misdivision of these monocentrics, midget chromosomes 1BL were isolated: deficient and deletion telocentrics and telocentrics derived from interstitial regions of 1BL. By chance, one deficient chromosome 1BS and one deletion chromosome 1BS were identified in unrelated lines of the same wheat. Following centric misdivision of these chromosomes, two midget chromosomes covering the whole of 1BS were added to the set.     

A procedure for localizing genetic factors controlling mitotic nondisjunction in the B chromosome of maize

The B chromosome of maize undergoes nondisjunction at the second pollen mitosis at rates as high as 98% (Roman, 1948; Carlson, 1969a). Nondis-junction is controlled by at least two separable regions on the B chromosome (Roman, 1949; Longley, 1956; Carlson, 1969b; Ward, 1972). A procedure for identifying and localizing the chromosomal sites required for nondisjunction is reported here. A translocation between the B and chromosome 9 (TB-9b) was utilized. Plants carrying TB-9b were screened for mutants of nondisjunction, i.e. translocations in which nondisjunction does not occur. Two such translocations were identified in a small screening. While the mutant translocations have not been analyzed in pachytene, they are most likely deletions or rearrangements of regions on the B chromosome vital to nondisjunction. Diminutive and rearranged B chromosomes are known to arise spontaneously in small populations (Randolph, 1941; Longley, 1956). — Also reported here are the nondisjunctional properties of the B⁹ isochromosome (Carlson, 1970) and several telocentric (or subtelocentric) derivatives of this chromosome. Some derivatives of the isochromosome are virtually incapable of nondisjunction, and should provide information on the role of the centromere in nondisjunction.     

Construction and uses of new compound B-A-A maize chromosome translocations

Maize B-A translocations result from reciprocal interchanges between a supernumerary B chromosome and an arm of an essential A chromosome. Because of the frequent nondisjunction of the B centromere at the second pollen mitosis, B-A translocations have been used to locate genes to chromosome arms and to study the dosage effects of specific A segments. Compound B-A translocations (B-A-A translocations) are created by bringing together a simple B-A translocation with an A-A translocation in which breakpoints in the A-A and B-A translocations are in the same arm. Recombination in the region of shared homology of these A chromosome segments creates a B-A-A translocation. Success in creating and testing for a new B-A-A translocation requires that the B-A translocation be proximal to the A-A translocation and that the A-A translocation be proximal to the tester locus. The breakpoints of most of the A-A translocations have been cytologically defined by earlier investigators. Previous investigators have produced 16 B-A-A translocations and one B-A-A-A translocation, which collectively define 35 A chromosome breakpoints. We have enlarged this group by creating 64 new B-A-A translocations. We present a summary of the total of 81 B-A-A translocations showing their distribution among the chromosome arms and the 163 cytologically defined chromosome segments delimited by them. We also illustrate the method of construction of these B-A-A stocks and their uses.     

B chromosomes: the troubles of integration

Starting with a spontaneous B-A centric fusion found in a natural population of the grasshopper Eyprepocnemis plorans, we have obtained different strains carrying the rearrangement in various conditions and doses. Using this material, we have analyzed the meiotic behavior of the translocated chromosome in living cultured spermatocytes, simulating the successive steps of a hypothetical process of integration of a B chromosome into the standard genome via B-A centric fusion. Remarkably, the behavior of fusion heterozygotes, the initial step of the integration process, is much more regular than that of any other configuration, including homozygotes. The reasons for the failure of B chromosome integration into the normal complement by translocation are discussed.     

The behavior of isochromosomes and telocentrics in wheat

Summary Genetic tests with chromosome IX of common wheat showed 13.3% formation of isochromosomes from telocentrics as against 7.0% for the same long arm of normal IX, and 7.0% formation of telocentrics from each arm of iso-IX instead of 10.8% from the same arm of normal IX. The differences are not significant statistically.No differences in frequency of formation of isochromosomes were found among 16 independently produced telocentrics for the long arm of chromosome IX.Somatic loss of both telocentrics and isochromosomes for chromosome IX and other wheat chromosomes has been observed, with loss of the telo being somewhat more frequent. A telocentric gave rise, to an isochromosome somatically. and isochromosomes added a telocentric. Two complex aberrations involving loss and duplication of parts of arms were observed, one of a telo and one of an iso; but these aberrations probably occurred at meiosis.Senior Geneticist, Division of Cereal Crops and Diseases, Bureau of Plant Industry, Soils, and Agricultural Engineering, Agricultural Research Administration, U. S. Department of Agriculture; and Research Associate, Field Crops Department, University of Missouri. This work was supported in part by funds obtained under Bankhead-Jones Project SRF 2–95, Combining in Wheat the Disease Resistance and Other Desirable Characters of Related Grass Species. Journal Paper No. 1269 of the Missouri Agricultural Experiment Station.     

SIMILAR UNSTABLE MUTATIONS IN THREE SPECIES OF GRACILARIA (RHODOPHYTA)1

Unstable mutants with similar variegated pigmentation were genetically characterized in the red algae. Gracilaria tikvahiae (McLachlan), G. foliifera (Forsk.) Børg. and. G. sjoestedtii (Kylin). All three mutants were green plants with flecks of red tissue where cells had reverted to wild type. The mutant green phenotypes were all recessive, and their genetic behavior in crosses indicated that each was the result of a single, unstable, nuclear gene. Wild-type revertant tissue was stable one it arose. Revertant plants obtained from spores and revertant fronds taken from variegated plants could not be distinguished from the normal wild type, either phenotypically or genetically. Reversion to wild type occurred during all phases of the life cycle. In crosses between the mutants and wild type, most of the F₁ tetrasporophytes were heterozygous wild-type plants, an observation consistent with the recessive nature of the mutations; however, a low frequency of homozygous unstable-green F₁ tetrasporophytes was also obttained from these crosses. The molecular basis of neither the mutant instability, i.e. the reversion to wild type, nor of the process producing the unstable green F₁ tetrasporophytes can yet be deduced, but the phenotype of the plants and genetic results suggest the involvement of transposable genetic elements.     

An asymptotic determination of minimum centromere size for the maize B chromosome

The maize B chromosome is a dispensable chromosome and therefore serves as a model system to study centromere function. The B centromere region is estimated to be approximately 9,000 kb in size and contains a 1.4 kb repeat that is specific to this centromere. When maintained as a univalent, the B chromosome occasionally undergoes centric misdivision. Consecutive misdivision analysis of the maize B chromosome centromere has generated a collection of functional centromeres that are greatly reduced in complexity. These small centromeres are often correlated with strongly reduced meiotic transmission. Molecular analyses of the misdivision collection have revealed that the smallest functional maize B centromere is a minimum of 110 kb in size. Considering the collection as a whole, meiotic transmission becomes severely compromised when the estimated centromere size is reduced to a few hundred kilobases.