Neocentric Activity Research Articles

Activation of Rye 5RL Neocentromere by an Organophosphate Pesticide

An interstitial constriction located on the long arm of rye chromosome 5R (5RL) shows neocentromeric activity at meiosis. In some meiocytes this region is strongly stretched orienting with the true centromere to opposite poles at metaphase I, and keeping sister chromatid cohesion at anaphase I. We found previously that the frequency of neocentric activity varied dramatically in different generations suggesting the effect of environmental factors. Here we studied the behavior of the 5RL neocentromere in mono- and ditelosomic 5RL, and mono-, and disomic 5R wheat-rye addition lines, untreated and treated with an organophosphate pesticide. The treated plants form neocentromeres with an about 4.5-fold increased frequency compared to untreated ones, demonstrating that the pesticide promotes neocentric activity. The neocentromere was activated irrespectively of the pairing configuration or the presence of a complete or truncated 5R centromere. Fluorescence in situ hybridization (FISH) with 2 repetitive sequences (UCM600 and pSc119.2) present at the constriction showed kinetic activity at several locations within this region. Immunostaining with anti-α-tubulin showed that treated plants have abnormal spindles in 46% of the metaphase I cells, indicating that disturbances in spindle formation might promote neocentromere activation.

No Neocentric Activity on Aegilops markgrafii Chromosome E

Classical neocentromeres, proven mainly in Poaceae species, cause meiotic drive in higher plants. Here I present data indicating that the morphological stretching of a chromatin domain at the long arm of Aegilops markgrafii chromosome E during meiosis is not due to a supposed neocentric activity. The stretching is visible at the alien univalent in metaphase I and anaphase I pollen mother cells of Triticum aestivum – Ae. markgrafii monosomic addition line E and at one univalent of the 49-chromosome-containing plants containing the haploid Ae. markgrafii genome added to wheat. The absence of both centromeric histone variant CENH3 and attachment of spindle fibers, proven by immunostaining with antibodies against CENH3 and tubulin, respectively, revealed that the univalent elongation of chromosome E is not caused by a classical plant neocentromere.

Neocentric activity in open-pollinated cultivars of rye

Kavander, T. and Viinikka, Y. 1987. Neocentric activity in open-pollinated cultivars of rye. —Hereditas 107 229–233. Lund, Sweden. ISSN 0018–0661. Received February 23,1987 Neocentric activity has been observed in meiosis of nine plants representing three cultivars of winter rye. In addition, the activity was examined in spring rye cultivated under different conditions. The highest frequencies, up to 55 % of cells at first anaphase, were observed in plants cultivated in growth beds from August to September. The active site was located at the telomere of the short arm of a chromosome from the group 4–5–6R. The results indicate that neocentric activity can be occasionally found in open-pollinated plants in addition to the regular occurrence in certain inbred lines of rye.

Neocentric activity in chromosome 5R of rye revealed by haploidy

Schlegel, R. 1987. Neocentric activity in chromosome 5R of rye revealed by haploidy. - Hereditas 107: 1–6. Lund, Sweden. ISSN 0018–0661. Received October 2, 1986 In metaphase I and anaphase I of haploid ‘Petka’ rye a particular univalent chromosome with a proximal constriction can be observed in most of the pollen mother cells. A similarly shaped chromosome was also found in polyhaploid wheat-rye hybrids, triticale × wheat hybrids. and in a monosomic wheat-rye addition line derived from several rye genotypes. This chromosome behaves as a dicentric since co-orientation of the ordinary centromere and a second centromere-like structure is evident. However, the neocentric activity does not originate in the telomeres, but is confined to the proximal region of the long arm of this submeta-centric chromosome, which has been identified as chromosome 5R by the C-banding pattern (5Rq21).

C-band polymorphism in the inbred lines showing neocentric activity in rye

C-banding patterns of five inbred lines originating from cv.‘Stâlrâg’ with different intensity of neocentric activity were examined. Differences between the lines were observed in the length of telomeric C-bands as well as in the band located adjacent to the nuceleolus-organizing region. However, the band on the short arm of 4R, which has earlier been shown to be the primary site of activity in the line with the highest intensity, differed in size in only one line. The deviating line is exceptional showing the activity in up to six chromosomes instead of the usual one. No other possible correlations between the size of bands and the intensity of neocentric activity could be established.

Movement ability of rye terminal neocentromeres

Rye terminal neocentromeres were analyzed in various aspects. Plants with and without neocentromeres were crossed to determine the possible genetic control on their formation. The segregation obtained in our work is consistent with the hypothesis of two trans-acting genes determining neocentric activity in such a way that individuals with no neocentromeres at all would carry all non-activating alleles, whereas one activating allele might permit the activation of a few neocentromeres. Individuals with four activating alleles would show the maximum frequency of neocentromeres per cell. Anti-tubulin immunolabelling was used to visualize the interaction between the neocentromeres and the microtubules. In most cases an end-on interaction between neocentromeres and microtubules was observed, but a few neocentromeres were observed free of them. Spikes were irradiated at early meiosis to determine whether acentric fragments carrying subtelomeric heterochromatin were able to behave as neocentromeres. In no case were acentric fragments observed to form an extension polewards as they did in whole chromosomes. Broken chromosomes joined by a thin thread of chromatin to the centromeric region showed neocentric activity, strongly suggesting that subtelomeric sequences need a cis-acting centromere to be active as neocentromeres.

Human centromere repositioning "in progress".

Centromere repositioning provides a potentially powerful evolutionary force for reproductive isolation and speciation, but the underlying mechanisms remain ill-defined. An attractive model is through the simultaneous inactivation of a normal centromere and the formation of a new centromere at a hitherto noncentromeric chromosomal location with minimal detrimental effect. We report a two-generation family in which the centromeric activity of one chromosome 4 has been relocated to a euchromatic site at 4q21.3 through the epigenetic formation of a neocentromere in otherwise cytogenetically normal and mitotically stable karyotypes. Strong epigenetic inactivation of the original centromere is suggested by retention of 1.3 megabases of centromeric alpha-satellite DNA, absence of detectable molecular alteration in chromosome 4-centromereproximal p- and q-arm sequences, and failure of the inactive centromere to be reactivated through extensive culturing or treatment with histone deacetylase inhibitor trichostatin A. The neocentromere binds functionally essential centromere proteins (CENP-A, CENP-C, CENP-E, CENP-I, BUB1, and HP1), although a moderate reduction in CENP-A binding and sister-chromatid cohesion compared with the typical centromeres suggests possible underlying structural/functional differences. The stable mitotic and meiotic transmissibility of this pseudodicentric-neocentric chromosome in healthy individuals and the ability of the neocentric activity to form in a euchromatic site in preference to a preexisting alphoid domain provide direct evidence for an inherent mechanism of human centromere repositioning and karyotype evolution "in progress." We discuss the wider implication of such a mechanism for meiotic drive and the evolution of primate and other species.

Rye terminal neocentromeres: characterisation of the underlying DNA and chromatin structure.

We have studied rye plants with neocentromeres on the terminal regions of the chromosomes. These neocentromeres only appear in meiosis, they are active together with the normal centromere and move the chromosomal arms polewards from prometaphase to anaphase at both the first and second meiotic divisions. All chromosomes of the normal set may show neocentric activity, but chromosomal arms with terminal heterochromatic blocks, as assessed by C-banding, are significantly more susceptible than those that do not have them. At least three repetitive sequences underlie the neocentromeres: pSc34, pSc74 and pSc200. These sequences are not detectable in B chromosomes, which never showed neocentric activity. Fluorescence in situ hybridisation with these sequences used as probes revealed elongated chromatin extensions on the neocentromeres that have not been observed using other staining techniques. These extensions were never observed in control plants. They suggest a modified chromatin structure, which might be responsible for the interaction with proteins involved in chromosomal movement on the spindle.

Characterization of the constriction with neocentric activity of 5RL chromosome in wheat.

In wheat-5RL monotelosomic and ditelosomic addition lines, a proximal constriction located on the long arm of rye chromosome 5R shows neocentric activity at metaphase I of meiosis. In some pollen mother cells this region is unusually stretched, acquires kinetic activity and co-orients with the true centromeres. In the work described here we characterized the putative neocentric constriction of 5RL using various approaches. Fluorescence in situ hybridization (FISH) revealed that the rye subtelomeric repetitive DNA sequence pSc119.2 is a constituent of the 5RL constriction. This FISH site corresponds with a heterochromatic C-band in normal rye. Other subtelomeric (pSc34, pSc74, pSc200), centromeric (CCS1, Bilby) and Arabidopsis-type telomeric sequences produce no detectable hybridization signal on the constriction. Immunolocalization with anti-alpha-tubulin antibodies showed that microtubules are bound to the constriction in a similar way to their binding to true centromeres. Silver staining demonstrated that proteins are accumulated at the constriction, the signal being more prominent than that observed at the centromere and telomeres of 5RL. The frequency of neocentric activity in different plants varied dramatically in different generations and in siblings grown in different years, suggesting that activation of the neocentric site is dependent on internal features and environmental conditions.

Neocentric activity of rye 5RL chromosome in wheat.

The neocentric activity of a constriction located on the long arm of rye 5R chromosome (5RL) was analysed. It is not observed in normal rye but it is unusually stretched in bivalents involving 5RL telosomes in wheat-ditelosomic 5RL addition lines. In 20% of metaphase I cells, the 5RL bivalent presents the centromeres oriented to one pole and the constrictions oriented towards the opposite pole with a strong tension. In 5% of the cells, the constriction was able to orient the bivalent to the poles without tension in the centromeres. Sister chromatid cohesion, which is one of the distinct features of centromeric function, is persistent at the constriction in delayed 5RL chromosomes at anaphase I. Neither the elongation of the constriction nor the neocentric activity was observed at second meiotic division or mitosis. FISH studies showed that the 5RL constriction lacked detectable quantities of two repetitive DNA sequences, CCS1 and the 180-bp knob repeat, present at cereal centromeres and neocentromeres, respectively. We propose that, under special conditions, such as the wheat background, the normally non-centromeric DNA present at this region of 5RL acquires a specific chromatin structure, differentiated as an elongated constriction, which is able to function as a centromere.

Steric hindrance and its effect on chromosome movement inVicia faba somatic cells

The arrangement of chromosome arms in metaphases and anaphases has been studied inVicia faba root meristem cells. During metaphase, the long chromosome arms are aligned parallel to the spindle axis. As a consequence, at the onset of anaphase, one chromatid can move straight ahead to the spindle pole whereas the other has to invert its orientation. Specially in narrow cells it has been observed frequently that some chromatids move in a reverse orientation to the pole, i.e., they move telomere-first instead of centromere-first. This behaviour results in a chromatid which protrudes beyond the main group of late anaphase or telophase chromatids. It is dicussed that the most likely explanation for the phenomenon is that in narrow cells chromatid behaviour is influenced by steric hindrance by the tightly packed surrounding chromatids and microtubules. When there is insufficient room, some chromatids are unable to make the required U-turn. Under such conditions the kinetochore of a non-inverted chromatid pulls the chromatid in a reverse orientation to the pole. An alternative explanation, i.e., protruding chromatids being the result of a neocentric activity at the telomere end of a reverse-directed chromatid or the lateral associations of spindle microtubules, failed to find support by electron microscopical studies.

Identification of the chromosomes showing neocentric activity in rye

Chromosomes showing neocentric activity were identified with the aid of the C-banding technique in an inbred line of rye, 1940-129 of cv. 'Stålråg'. The frequency of cells with neocentrics varied from 34 to 35.7% at first anaphase, and from 32 to 68% at second metaphase. In most cells only one chromosome showed the activity. It always belonged to the group 4-5-6R, and in cells where individual identification of chromosomes was possible the activity was invariably located at the telomere of the arm 4RS. When in rare cases two or three chromosomes were active, one chromosome again belonged to the group 4-5-6R, the other was identified in four cells as a member of the group 2-3-7R, but it is possible that all the telomeres with a prominent C-band can occasionally show the activity. It is assumed that certain telomeric regions remain in the active state at both meiotic divisions and offer secondary accumulation sites for kinetochoric proteins.

A new triploid race ofPennisetum orientale Rich exhibiting chromosomal numerical mosaicism and neo-centric activity

An allotriploid ofP. orientale Rich with 2n=27 chromosomes was located. Cytomorphological studies revealed that it is a new chromosomal race exhibiting chromosomal numerical mosaicism and neo-centric activity. Heterochromatic knobs present on the chromosomes are considered as sites of neo-centromeres. The possible causes of chromosomal mosaicism and neo-centric activity are discussed. Polyploidy coupled with hybridization seems to play a major role in the evolution of this taxon and the aberrant meiotic behaviour is not a barrier for the survival of the species since it is an apomict.

Chromosome evolution in the Brachytheciaceae

Twenty-eight moss species have been investigated cytologically. Mitotic karyograms have been presented for most of these species and these show a remarkable similarity in the n = 11 karyotype. Nine of the eleven chromosomes have terminal or sub-terminal centromeres. Lightly staining heterochromatic bands frequently occur along the axis of the chromosomes and very often these are the sites of chromosome bending; it has been suggested that these may be areas of neocentric activity. The karyotypes investigated show a reduction series of chromosome number which is paralleled by an increase in the number of long metacentrics; since the number of major chromosome arms is maintained in most of the species it has been suggested that Robertsonian fusion has been an important mechanism in the evolution of the Bracytheciaceae, and probably in all Diplolepidous mosses. Polyploidy has also played an important role in speciation. Finally it has been proposed that n = 11 is the primary basic number in the Diplolepideae.

Highly repeated DNA sequence limited to knob heterochromatin in maize

A highly repeated DNA sequence has been isolated from the maize genome as a satellite in actinomycin D/CsCl gradients. By using maize stocks differing in their heterochromatin content we have established that the sequence is a major constituent of one class of heterochromatin, knob heterochromatin, which can occur at 23 locations in the chromosome complement. The repeating unit, of 185 base pairs, has been cloned in plasmid pBR322 and its nucleotide sequence has been determined. The presence of this DNA sequence in knob heterochromatin and its absence from centromeric, nucleolar, and B chromosome heterochromatin parallels the cytogenetic differentiation previously described for these classes of heterochromatin in maize. Because knob heterochromatin has a distinctive cytological appearance and is unique in showing neocentric activity at meiosis, its association with a particular repeated DNA sequence may reflect a functional role for the sequence in the cell cycle.

Asynapsis and spontaneous centromeric breakage in an inbred line ofPennisetum americanum (L.) Leeke

In one of the plants of the inbred line IP 1475 ofPennisetum americanum (L.) Leeke, spontaneous chromosome breakage was observed. Distribution of the breaks was non-random and was confined to the centromeres and to the chromosome ends. Breakage in the centric region was twice as frequent as that in the end segments. Further, nucleolar chromosomes were involved less frequently than others in breakage and stickiness. Other meiotic abnormalities like asynapsis, exaggeration and stretching of the centromere, chromosome stickiness and neocentric activity were also observed in the same plant. All these abnormalities were interpreted to be the features of breakdown of the genetic system resulting from the enforced inbreeding of the normally cross pollinated species.

Further correlations between chiasmata and U-type exchanges in rye meiosis

Earlier studies have demonstrated convincing correlations between the distribution patterns of chiasmata and of U-type exchanges within bivalents. On the basis of this evidence and other considerations it has been proposed that these contrasting meiotic exchange events are related in origin, and that U-type exchanges, giving rise to bridge and fragment configurations, arise as errors in crossing-over and chiasma formation. This hypothesis is given further consideration in the present report and further correlated distributions of chiasmata and U-type exchanges are presented. These correlations involve the distribution patterns of exchanges between bivalents and between the two arns of one particular bivalent which is consistently marked by the presence of localised neocentric activity. The relationships of these exchange distributions to chromosome length are also investigated and as a result it becomes clear that a mutual dependence of the two types of exchange on chromosome length cannot account for the observed correlations. The total evidence relating to the hypothesis of a causal connection between chiasmata and U-type exchanges is reviewed and critically assessed.

Cytological studies on a haploid cultivar of pelargonium, and its colchicine-induced diploids

Kleine Liebling is a haploid of unusual interest; not only is it a monoploid, but at the same time a successful horticultural variety. Analysis made of pollen mother cell meiosis revealed a very low frequency of bivalents, and evidence of neocentric activity among the univalents. Colchicine-induced diploid plants were synthesised, but contrary to expectation, microsporogenesis was very erratic, and pairing never complete. As in the haploid from which they were derived, a high proportion of dyads formed the final product of meiosis. It was concluded that both these irregularities were the result of mutations effecting the genetic control of meiosis.

Genetic control of neocentric activity in rye

Genetic control of neocentric activity in rye

Cytogenic Behavior of a Knobbed Chromosome 10 in Maize

Unlike maize plants heterozygous the abnormal chromosome 10 (K10) and the normal chromosome 10 (k10), those heterozygous for a knobbed chromosome 10 (K(T)10/k10) do not undergo preferential segregation or "meiotic drive." Ting's chromosome 10, however, does show neocentric activity, although the extent of this is not as pronounced as is the case in K10 homozygotes. The K(T)10 chromosome is considerably shorter than and is cytologically distinguishable from the K10 and the k10 chromosomes.