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.     

Two new B-10 translocations involved in the control of nondisjunction of the B chromosome in maize

A B-A translocation, TB-10(18), has been established involving breakpoints in the proximal region of the long arm of chromosome 10 and the minute short arm of the maize B chromosome. TB-10(18) differs in its nondisjunctional behavior at the second microspore division from TB-10(19), which has a breakpoint in the same region of 10 but in the heterochromatic region of the long arm of B, in the following ways: (1) Nondisjunction of the B¹⁰ chromosome of the TB-10(18) translocation occurs in the absence of the reciprocal element (10^B), albeit at low frequency. (2) Presence of 10^B increases the frequency of B¹⁰ nondisjunction but not to the level found for TB-10(19) and certain other translocations. (3) The frequency of B¹⁰ nondisjunction varies among closely related sublines both when 10^B is present and when it is absent. It is inferred that the B¹⁰ of TB-10(18) carries all the components of B necessary for nondisjunction but that expression is weak in the absence of 10^B, suggesting the existence in the B chromosome short arm of a factor influencing efficient nondisjunction.     

A spontaneous derivative of abnormal chromosome 10 in maize

Summary A new type of abnormal chromosome 10 has been found among maize plants grown from seeds sent by Dr. Y. C. Ting of Harvard University. This chromosome deviates in its morphology from the orthodox abnormal chromosome 10 described by Rhoades (1952) and from the one described by Ting (1958b). It produces a low degree of neo-centric activity.Cytological observations of plants heterozygous for the new abnormal chromosome 10 and either an orthodox abnormal chromosome 10 or a normal one, have suggested that the new type was derived from an orthodox abnormal 10 through spontaneous breakage and loss of an important piece of its long arm. The delection involved the distal part of the long arm of orthodox abnormal chromosome 10, proximally limited by the third most distal dissimilar and prominent chromomere. This corresponds approximately to the extra segment at the end of orthodox abnormal chromosome 10 which remains unpaired in heterozygotes with the normal 10. It bears a large heterochromatic knob. The missing piece is a part of the larger fraction of the long arm of orthodox abnormal chromosome 10 that remains unaffected by crossingover in a heteromorphic bivalent having a normal chromosome 10 (telo-segment). The telo-segment has its proximal limit at the left of the most proximal of the 3 dissimilar chromomeres, probably between the R and Sr ₂ loci. It has been proposed that a factor or factors responsible for neo-centric activity are located in the portion of the telosegment between its proximal limit and the third most distal dissimilar chromomere (3 dissimilar chromomere region).Since the telo-segment of the orthodox abnormal 10 also bears a large knob in its distal half, it has been suggested that this segment has a dual role in neo-centric activity. The factor or factors located in the proximal piece of the telo-segment would stimulate over-abundance of fiber-forming substance, whereas local production of chromosomal fibers would depend ultimately on the knob's activity.If the large knob is absent, its role in neo-centric activity would be transferred to the next smaller and distally located hetero-chromatic mass, such as the knob-like body near the end of the new abnormal 10 which results from the fusion of the two most proximal prominent chromomeres of the telo-segment.This work has been partly done in the United States, under an I.C.A. — National Academy of Sciences fellowship.     

Sporophytic nondisjunction of the maize B chromosome at high copy numbers

It has been known for decades that the maize B chromosome undergoes nondisjunction at the second pollen mitosis.Fluorescence in-situ hybridization (FISH) was used to undertake a quantitative study of maize plants with differing numbers of B chromosomes to observe if instability increases by increasing B dosage in root tip tissue.B chromosome nondisjunction was basically absent at low copy number,but increased at higher B numbers.Thus,B nondisjunction rates are dependent on the dosage of B's in the sporophyt...     

Sporophytic nondisjunction of the maize B chromosome at high copy numbers

It has been known for decades that the maize B chromosome undergoes nondisjunction at the second pollen mitosis.Fluorescence in-situ hybridization(FISH)was used to undertake a quantitative study of maize plants with differing numbers of B chromosomes to observe if instability increases by increasing B dosage in root tip tissue.B chromosome nondisjunction was basically absent at low copy number,but increased at higher B numbers.Thus,B nondisjunction rates are dependent on the dosage of B's in the sporophyte.Differences in nondisjunction were also documented between odd and even doses of the B.In plants that have inherited odd humbered doses of the B chromosome,B loss is nearly twice as likely as B gain in a somatic division.When comparing plants with even doses of B's to plants with odd doses of B's,plants with even numbers had a significantly higher chance to increase in number.Therefore,the B's nondisjunctive capacity,previously thought to be primarily restricted to the gametophyte,is present in sporophytic cells.     

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.     

Centromere function and nondisjunction are independent components of the maize B chromosome accumulation mechanism

Supernumerary or B chromosomes are selfish entities that maintain themselves in populations by accumulation mechanisms. The accumulation mechanism of the B chromosome of maize (Zea mays) involves nondisjunction at the second pollen mitosis, placing two copies of the B chromosome into one of the two sperm. The B chromosome long arm must be present in the same nucleus for the centromere to undergo nondisjunction. A centromere, containing all of the normal DNA elements, translocated from the B chromosome to the short arm of chromosome 9 was recently found to be epigenetically silenced for centromeric function. When intact B chromosomes were added to this genotype, thus supplying the long arm, the inactive centromere regained the property of nondisjunction causing the translocation chromosome 9 to be differentially distributed to the two sperm or resulted in chromosome breaks in 9S, occasionally producing new translocations. Translocation of the inactive B centromere to chromosome 7 transferred the nondisjunction property to this chromosome. The results provide insight into the molecular and evolutionary basis of this B chromosome accumulation mechanism by demonstrating that nondisjunction is caused by a process that does not depend on normal centromere function but that the region of the chromosome required for nondisjunction resides in the centromeric region.     

The relationship of abnormal chromosome 10 to B-chromosomes in maize

Abnormal chromosome 10 (K10) is known to increase recombination in maize and to induce preferential segregation in knobbed heterozygotes during megasporogenesis. In spite of the considerable interest generated by these findings, the origin of the K10 chromosome is unknown. It has been postulated that the extra segment of K10 arose by simple translocation between normal 10 and a B-chromosome. This hypothesis was tested by comparing meiosis in haploids with either K10 or the normal 10 and carrying a single B-chromosome. The frequency of bivalent configurations was found to be similar in the two types of haploids suggesting that the K10 and B-chromosomes do not share homologies that lead to chiasma formation. These results lend no support to the hypothesis that the K10 chromosome came from a B type. The implications of these results to the action of K10 at meiosis are also discussed.     

Four loci on abnormal chromosome 10 contribute to meiotic drive in maize

We provide a genetic analysis of the meiotic drive system on maize abnormal chromosome 10 (Ab10) that causes preferential segregation of specific chromosomal regions to the reproductive megaspore. The data indicate that at least four chromosomal regions contribute to meiotic drive, each providing distinct functions that can be differentiated from each other genetically and/or phenotypically. Previous reports established that meiotic drive requires neocentromere activity at specific tandem repeat arrays (knobs) and that two regions on Ab10 are involved in trans-activating neocentromeres. Here we confirm and extend data suggesting that only one of the neocentromere-activating regions is sufficient to move many knobs. We also confirm the localization of a locus/loci on Ab10, thought to be a prerequisite for meiotic drive, which promotes recombination in structural heterozygotes. In addition, we identified two new and independent functions required for meiotic drive. One was identified through the characterization of a deletion derivative of Ab10 [Df(L)] and another as a newly identified meiotic drive mutation (suppressor of meiotic drive 3). In the absence of either function, meiotic drive is abolished but neocentromere activity and the recombination effect typical of Ab10 are unaffected. These results demonstrate that neocentromere activity and increased recombination are not the only events required for meiotic drive.     

The effect of abnormal chromosome 10 on transposition of modulator from the R locus in maize

Transposition of the non-specific repressor element, Modulator, from the R locus on chromosome 10 in maize, is enhanced by coupling with the K10 segment at a distance of at least 35 map units from R. There is no detectable interaction in the repulsion phase. The K10 effect appears to be relatively greater in the earlier somatic cell generations during ear development. The transposition rate also is affected by the direction of crosses, being somewhat higher on the ears of F₁ plants which received the compound mutable R allele from the pollen parent. The significance of the behavior of Modulator and other instability phenomena of higher plants is discussed in relation to chromosome organization.     

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.     

Chromosome nondisjunction and instabilities in tapetal cells are affected by B chromosomes in maize

Abnormal mitosis occurs in maize tapetum, producing binucleate cells that later disintegrate, following a pattern of programmed cell death. FISH allowed us to observe chromosome nondisjunction and micronucleus formation in binucleate cells, using DNA probes specific to B chromosomes (B's), knobbed chromosomes, and the chromosome 6 (NOR) of maize. All chromosome types seem to be involved in micronucleus formation, but the B's form more micronuclei than do knobbed chromosomes and knobbed chromosomes form more than do chromosomes without knobs. Micronuclei were more frequent in 1B plants and in a genotype selected for low B transmission rate. Nondisjunction was observed in all types of FISH-labeled chromosomes. In addition, unlabeled bridges and delayed chromatids were observed in the last telophase before binucleate cell formation, suggesting that nondisjunction might occur in all chromosomes of the maize complement. B nondisjunction is known to occur in the second pollen mitosis and in the endosperm, but it was not previously reported in other tissues. This is also a new report of nondisjunction of chromosomes of the normal set (A's) in tapetal cells. Our results support the conclusion that nondisjunction and micronucleus formation are regular events in the process of the tapetal cell death program, but B's strongly increase A chromosome instability.     

Sequences associated with A chromosome centromeres are present throughout the maize B chromosome

Maize chromosome spreads containing the supernumerary B chromosome were hybridized with probes from various repetitive elements including CentC, CRM, and CentA, which have been localized to centromeric regions on the A chromosomes. Repetitive elements that are enriched or found exclusively near the centromeres of A chromosomes hybridized to many sites distinct from the centromere on the B chromosome. To examine whether these elements recruit kinetochore proteins at locations other than the canonical B centromere, cells were labeled with antibodies against CENH3, a key kinetochore protein. No labeling was detected outside the normal centromere and no evidence of B chromosome holocentromeric activity was observed. This finding suggests that, as in other higher eukaryotes, DNA sequence alone is insufficient to dictate kinetochore location in plants. Additionally, examination of the B centromere region in pachytene chromosomes revealed that the B-specific element ZmBs hybridizes to a much larger region than the site of hybridization of CentC, CRM, and CentA and the labeling by anti-CENH3 antibodies.This revised version was published online in December 2004 with corrections to Table 1.     

Nondisjunction and isochromosome formation in the B chromosome of maize

Bianchi et al. (1961) found that sectored losses of B-translocation chromosomes occur at a significant rate during early development of the endosperm and sporophyte. The losses were attributed to nondisjunction of the chromomosome, since B type chromosomes are known to undergo nondisjunction at the second pollen mitosis. Sector formation was further analyzed in the present paper, using the translocation, TB-9b. It was found that losses of the B⁹ chromosome during early endosperm mitoses occur only if the 9^B chromosome is present. In addition, sectors are produced in the sporophyte only if the 9^B and B⁹ chromosomes are inherited from the male parent. Both of these findings suggest that nondisjunction is indeed responsible for the B⁹ losses (see text). However, cytological observation of sectored plants demonstrates that isochromosome formation, rather than nondisjunction, produces most B⁹ losses in the sporophyte. The conflicting results can be reconciled by assuming that the same basic event, perhaps stickiness of the B⁹ chromosome, produces nondisjunction at the second pollen mitosis and isochromosome formation in the developing sporophyte. Observation of the isochromosome in pachytene reveals that a heterochromatic region corresponding to the short arm of the normal B⁹ is missing. The normal B⁹ chromosome is, therefore, an acrocentric chromosome.     

Minichromosomes derived from the B chromosome of maize

Fourteen minichromosomes derived from the B chromosome of maize are described. The centromeric region of the B chromosome contains a specific repetitive DNA element called the B repeat. This sequence was used to determine the transmission frequency of the different types of minichromosomes over several generations via Southern blot analysis at each generation. In general, the minichromosomes have transmission rates below the theoretical 50% frequency of a univalent chromosome. The gross structure of each minichromosome was determined using fluorescence in situ hybridization (FISH) on root tip chromosome spreads. The presence of the B centromeric repeat and of the adjacent heterochromatic knob sequences was determined for each minichromosome. In two cases, the amount of the centromeric knob repeat is increased relative to the progenitor chromosome. Other isolates have reduced or undetectable levels of the knob sequence. Potential uses of the minichromosomes are discussed.     

A comparative cytogenetic study of chromosome homology between chicken and Japanese quail

In order to construct a chicken (Gallus gallus) cytogenetic map, we isolated 134 genomic DNA clones as new cytogenetic markers from a chicken cosmid DNA library, and mapped these clones to chicken chromosomes by fluorescence in situ hybridization. Forty-five and 89 out of 134 clones were localized to macrochromosomes and microchromosomes, respectively. The 45 clones, which localized to chicken macrochromosomes (Chromosomes 1-8 and the Z chromosome) were used for comparative mapping of Japanese quail (Coturnix japonica). The chromosome locations of the DNA clones and their gene orders in Japanese quail were quite similar to those of chicken, while Japanese quail differed from chicken in chromosomes 1, 2, 4 and 8. We specified the breakpoints of pericentric inversions in chromosomes 1 and 2 by adding mapping data of 13 functional genes using chicken cDNA clones. The presence of a pericentric inversion was also confirmed in chromosome 8. We speculate that more than two rearrangements are contained in the centromeric region of chromosome 4. All 30 clones that mapped to chicken microchromosomes also localized to Japanese quail microchromosomes, suggesting that chromosome homology is highly conserved between chicken and Japanese quail and that few chromosome rearrangements occurred in the evolution of the two species.