Structural basis for Robertsonian translocations in man: association of ribosomal genes in the nucleolar fibrillar center in meiotic spermatocytes and oocytes

Effects of chromosomal translocation characteristics on fertilization and blastocyst development - a retrospective cohort study

OBJECTIVE

To determine the effect of different translocation characteristics on fertilization rate and blastocyst development in chromosomal translocation patients.

METHODS

This retrospective cohort study was conducted at the Third Affiliated Hospital of Zhengzhou University From January 2017 to December 2022.All couples were diagnosed as reciprocal translocation or Robertsonian translocation by karyotype of peripheral blood lymphocytes test. After adjusting for confounding factors, the effect of chromosomal rearrangement characteristics, such as carrier sex, translocation type, chromosome length and break sites, on fertilization rate and embryo development were analysed separately using multiple linear regression.

RESULTS

In cases of Robertsonian translocation (RobT), the carrier sex plays an independent role in fertilization rate, and the male carriers was lower than that of female carriers (76.16% vs.86.26%, P = 0.009). In reciprocal translocation (RecT), the carrier sex, chromosome types and break sites had no influence on fertilization rate, blastocyst formation rate (P > 0.05). However, patients with human longer chromosomal (chromosomes 1-5) translocation have a lower available blastocyst formation rate (Group AB vs. Group CD: 41.49%vs.46.01%, P = 0.027). For male carriers, the translocation types was an independent factor affecting the fertilization rate, and the RobT was the negative one (B = - 0.075, P = 0 0.009). In female carriers, we did not observe this difference (P = 0.227).

CONCLUSIONS

In patients with chromosomal translocation, the fertilization rate may be influenced by carrier sex and translocation type, chromosomes 1-5 translocation may adversely affect the formation of available blastocysts. Break sites have no role in fertilization and blastocyst development.

The p-Arms of Human Acrocentric Chromosomes Play by a Different Set of Rules

The p-arms of the five human acrocentric chromosomes bear nucleolar organizer regions (NORs) comprising ribosomal gene (rDNA) repeats that are organized in a homogeneous tandem array and transcribed in a telomere-to-centromere direction. Precursor ribosomal RNA transcripts are processed and assembled into ribosomal subunits, the nucleolus being the physical manifestation of this process. I review current understanding of nucleolar chromosome biology and describe current exploration into a role for the NOR chromosomal context. Full DNA sequences for acrocentric p-arms are now emerging, aided by the current revolution in long-read sequencing and genome assembly. Acrocentric p-arms vary from 10.1 to 16.7 Mb, accounting for ∼2.2% of the genome. Bordering rDNA arrays, distal junctions, and proximal junctions are shared among the p-arms, with distal junctions showing evidence of functionality. The remaining p-arm sequences comprise multiple satellite DNA classes and segmental duplications that facilitate recombination between heterologous chromosomes, which is likely also involved in Robertsonian translocations.

Interaction of acrocentric chromosome involved in translocation and sex of the carrier influences the proportion of alternate segregation in autosomal reciprocal translocations

STUDY QUESTION

Are meiotic segregation patterns of reciprocal translocations affected by the combined effect of chromosome type and carrier's sex?

SUMMARY ANSWER

Interaction of an acrocentric chromosome (Acr-ch) involved in the translocation and sex of the carrier influences the proportion of alternate segregation for normal or balanced chromosome contents during meiotic segregation in autosomal reciprocal translocations.

WHAT IS KNOWN ALREADY

Carriers of reciprocal translocations are at a significantly increased risk of fertility problems due to the generation of unbalanced gametes in meiotic segregation of a quadrivalent. Previous studies have reported that meiotic segregation patterns of a quadrivalent can be affected by factors such as a carrier's sex and age and the chromosome type. However, the reported proportion of alternate segregation does not differ significantly, except in one study, and whether combined effects between these factors exist is unclear.

STUDY DESIGN, SIZE, DURATION

A retrospective study of array comparative genomic hybridization (aCGH) outcome data from patients with autosomal reciprocal translocations was conducted to analyse meiotic segregation patterns and blastocyst euploidy rates. We enroled 473 couples whose embryos were tested between January 2013 and September 2016.

PARTICIPANTS/MATERIALS, SETTING, METHODS

Meiotic segregation patterns of 2101 blastocysts from 243 female carriers, including 76 cases with translocations involving Acr-ch, and 230 male carriers, including 88 cases with translocations involving Acr-ch, were analysed according to chromosome type, carrier's sex and age.

MAIN RESULTS AND THE ROLE OF CHANCE

In cases with translocations involving the Acr-ch subgroup, the proportion of alternate segregation (53.9 vs 33.4%, P < 0.0001) was significantly higher in male carriers than in female carriers, with the proportion of 3:1 segregation (6.8 vs 16.3%, P < 0.0001) being significantly lower. The proportions of alternate segregation were similar between sexes in cases with translocations not involving the Acr-ch subgroup. Meanwhile, in the female carrier subgroup, the proportion of alternate segregation (33.4 vs 45.2%, P < 0.001) was significantly lower and the proportion of 3:1 segregation (16.3 vs 8.2%, P < 0.001) was significantly higher in cases with translocations involving Acr-ch than in those not. In the male carrier subgroup, the proportion of alternate segregation (53.9 vs 46.9%, P = 0.031) was higher and the proportion of adjacent-1 segregation (27.1 vs 37.3%, P < 0.001) was significantly lower in cases with translocations involving Acr-ch than in those not. Carrier's age did not affect the meiotic segregation patterns. However the euploidy rates were significantly lower in couples with advanced compared to young maternal age respectively.

LIMITATIONS, REASONS FOR CAUTION

Mosaic embryos were not identified using aCGH in this study. Patients with complex chromosome rearrangements and translocations involving sex chromosomes were excluded. Interchromosomal effect was not analysed.

WIDER IMPLICATIONS OF THE FINDINGS

The findings of this study provide detailed information for genetic counselling of couples with autosomal reciprocal translocations on their chances of producing euploid gametes.

STUDY FUNDING/COMPETING INTEREST(S)

This research was supported by the National Key Research and Development Program of China (2016YFC1000202); the National Natural Science Foundation of China (81671522); the Natural Science Foundation of Shandong Province in China (ZR2016HP09); and the Innovative Foundation of Reproductive Hospital Affiliated to Shandong University (20171114, 20171111). No competing interests are declared.

TRIAL REGISTRATION NUMBER

N/A.

Effects of a carrier's sex and age on the segregation patterns of the trivalent of Robertsonian translocations

PURPOSE

To investigate the effects of a carrier's sex and age on the segregation patterns of the trivalent of Robertsonian translocations.

METHODS

This retrospective study was designed to analyze the segregation patterns of the trivalent and euploidy rates of blastocysts. Data were collected from 154 couples with Robertsonian translocation (77 with a female carrier and 77 with a male carrier). Embryos were diagnosed via array comparative genomic hybridization between January 2013 and July 2017. The segregation patterns of the trivalent of 604 blastocysts were analyzed according to the carrier's sex and age.

RESULTS

The proportion of alternate segregation was significantly higher (82.9% vs. 55.2%) in the male carriers than in the female carriers of Robertsonian translocation, and the proportion of adjacent segregation was significantly lower (16.8% vs. 42.6%), with no difference in 3:0 segregation. The segregation patterns were similar in same-sex carriers when analyzed according to the type of translocation. The carrier's age had no influence on the segregation patterns of the trivalent.

CONCLUSIONS

The proportions of the trivalent's meiotic segregation pattern differ significantly according to the carrier's sex in Robertsonian translocations and are independent of the carrier's age. A significantly higher proportion of alternate segregation for normal or balanced chromosome contents was observed in the blastocysts of the male carriers than in those of the female carriers.

Chromosomal fusion and life history-associated genomic variation contribute to within-river local adaptation of Atlantic salmon

Chromosomal inversions have been implicated in facilitating adaptation in the face of high levels of gene flow, but whether chromosomal fusions also have similar potential remains poorly understood. Atlantic salmon are usually characterized by population structure at multiple spatial scales; however, this is not the case for tributaries of the Miramichi River in North America. To resolve genetic relationships between populations in this system and the potential for known chromosomal fusions to contribute to adaptation, we genotyped 728 juvenile salmon using a 50 K SNP array. Consistent with previous work, we report extremely weak overall population structuring (Global F_ST  = 0.004) and failed to support hierarchical structure between the river's two main branches. We provide the first genomic characterization of a previously described polymorphic fusion between chromosomes 8 and 29. Fusion genomic characteristics included high LD, reduced heterozygosity in the fused homokaryotes, and strong divergence between the fused and the unfused rearrangement. Population structure based on fusion karyotype was five times stronger than neutral variation (F_ST  = 0.019), and the frequency of the fusion was associated with summer precipitation supporting a hypothesis that this rearrangement may contribute local adaptation despite weak neutral differentiation. Additionally, both outlier variation among populations and a polygenic framework for characterizing adaptive variation in relation to climate identified a 250-Kb region of chromosome 9, including the gene six6 that has previously been linked to age-at-maturity and run-timing for this species. Overall, our results indicate that adaptive processes, independent of major river branching, are more important than neutral processes for structuring these populations.

The biogenesis of chromosome translocations

Chromosome translocations are catastrophic genomic events and often play key roles in tumorigenesis. Yet the biogenesis of chromosome translocations is remarkably poorly understood. Recent work has delineated several distinct mechanistic steps in the formation of translocations, and it has become apparent that non-random spatial genome organization, DNA repair pathways and chromatin features, including histone marks and the dynamic motion of broken chromatin, are critical for determining translocation frequency and partner selection.

Molecular cytogenetic mapping of Cucumis sativus and C. melo using highly repetitive DNA sequences

Chromosomes often serve as one of the most important molecular aspects of studying the evolution of species. Indeed, most of the crucial mutations that led to differentiation of species during the evolution have occurred at the chromosomal level. Furthermore, the analysis of pachytene chromosomes appears to be an invaluable tool for the study of evolution due to its effectiveness in chromosome identification and precise physical gene mapping. By applying fluorescence in situ hybridization of 45S rDNA and CsCent1 probes to cucumber pachytene chromosomes, here, we demonstrate that cucumber chromosomes 1 and 2 may have evolved from fusions of ancestral karyotype with chromosome number n = 12. This conclusion is further supported by the centromeric sequence similarity between cucumber and melon, which suggests that these sequences evolved from a common ancestor. It may be after or during speciation that these sequences were specifically amplified, after which they diverged and specific sequence variants were homogenized. Additionally, a structural change on the centromeric region of cucumber chromosome 4 was revealed by fiber-FISH using the mitochondrial-related repetitive sequences, BAC-E38 and CsCent1. These showed the former sequences being integrated into the latter in multiple regions. The data presented here are useful resources for comparative genomics and cytogenetics of Cucumis and, in particular, the ongoing genome sequencing project of cucumber.

The behavior during pachynema of a normal and an inverted Y chromosome in Microtus agrestis

The pachytene behavior of the chromosomes of Microtus agrestis (L.) (Rodentia, Arvicolidae) males carrying either the standard, or the pericentrically inverted Lund Y chromosome have been examined by electron microscopy of microspread spermatocytes. There is no synapsis between the X and either the standard or the Lund Y chromosomes during any substage of pachynema. Since synapsis is generally considered a prerequisite for crossing over, there appears to be no opportunity for crossover or chiasma formation between the X and Y in this species. The G-, C- and NOR-banded mitotic karyotypes of animals carrying the standard and Lund Y are also presented.

Additional nucleoli and NOR activity during meiotic prophase I in larch (Larix decidua Mill.)

Transcriptional activity was investigated in successive stages of prophase I (male meiosis) of larch meiocytes. Br-UTP incorporated into RNA was detected by light and electron microscopy. Two peaks of RNA synthesis were identified in the nucleolus. The first occurred during the zygotene-pachytene stage and the second (not previously described in plant meiocytes) in the diplotene. These processes correlated with a considerable increase in nucleolus volume during these periods. At the end of the zygotene, several perinucleolar structures lying close to each other and containing rRNA, argyrophilic proteins, U3 small nucleolar RNA, and fibrillarin were observed. The occurrence of newly formed RNA was also observed in these structures. This suggests that the observed perinucleolar structures correspond to the additional nucleoli known from animals.

Whole-arm translocations between chromosome 1 and acrocentric G chromosomes are associated with a poor prognosis for spermatogenesis: two new cases and review of the literature

OBJECTIVE

To analyze unusual translocations involving a chromosome 1 whole arm and an acrocentric G chromosome p arm found in two men with azoospermia.

DESIGN

Case report with review of the scientific literature.

SETTING

Cytogenetics department.

PATIENT(S)

Two men with azoospermia and normal hormonal levels.

INTERVENTIONS(S)

Peripheral blood lymphocytes were obtained for karyotype, and metaphases were studied by standard GBG, RBG, and CBG banding procedures.

MAIN OUTCOME MEASURE(S)

Karyotype GBG, RBG, and CBG banding.

RESULT(S)

Karyotype revealed balanced translocation involving a chromosome 1 whole arm and an acrocentric G chromosome p arm: 46,XY,t(1;21)(q11;p13) (patient 1) and 46,XY,t(1;22)(q11;p11) (patient 2).

CONCLUSION(S)

With regard to published cases of whole-arm translocation of human chromosome 1 with an acrocentric p arm and a maternal origin of these abnormalities, we argue for an impairment of meiosis resulting in a high probability of quadrivalent-XY-body interaction. Male factor infertility might be due to two poor prognostic factors, first the involvement of human chromosome 1 (and its heterochromatic region) and second the involvement of an acrocentric chromosome p-arm breakpoint. This probable interaction between the pachytene quadrivalent and XY body might explain azoospermia.

Meiotic recombination and spatial proximity in the etiology of the recurrent t(11;22)

Although balanced translocations are among the most common human chromosomal aberrations, the constitutional t(11;22)(q23;q11) is the only known recurrent non-Robertsonian translocation. Evidence indicates that de novo formation of the t(11;22) occurs during meiosis. To test the hypothesis that spatial proximity of chromosomes 11 and 22 in meiotic prophase oocytes and spermatocytes plays a role in the rearrangement, the positions of the 11q23 and 22q11 translocation breakpoints were examined. Fluorescence in situ hybridization with use of DNA probes for these sites demonstrates that 11q23 is closer to 22q11 in meiosis than to a control at 6q26. Although chromosome 21p11, another control, often lies as close to 11q23 as does 22q11 during meiosis, chromosome 21 rarely rearranges with 11q23, and the DNA sequence of chromosome 21 appears to be less susceptible than 22q11 to double-strand breaks (DSBs). It has been suggested that the rearrangement recurs as a result of the palindromic AT-rich repeats at both 11q23 and 22q11, which extrude hairpin structures that are susceptible to DSBs. To determine whether the DSBs at these sites coincide with normal hotspots of meiotic recombination, immunocytochemical mapping of MLH1, a protein involved in crossing over, was employed. The results indicate that the translocation breakpoints do not coincide with recombination hotspots and therefore are unlikely to be the result of meiotic programmed DSBs, although MRE11 is likely to be involved. Previous analysis indicated that the DSBs appear to be repaired by a mechanism similar to nonhomologous end joining (NHEJ), although NHEJ is normally suppressed during meiosis. Taken together, these studies support the hypothesis that physical proximity between 11q23 and 22q11--but not typical meiotic recombinational activity in meiotic prophase--plays an important role in the generation of the constitutional t(11;22) rearrangement.

Pericentric inversion of chromosome 9: prevalence in 300 Down syndrome families and molecular studies of nondisjunction

The incidence of Down syndrome (DS) families where one of the parents is an heterozygous carrier of pericentric inversion of the heterochromatic region of chromosome 9-inv(9) (qh) - was determined in 3 independent groups of 100 families each. The total number of 17 such families found in the sample is significantly greater than the expected number of 5.73 for a sample of non-DS families of equal size. Consequently, the statistical association of the presence of inv (9) (qh) in one parent with the birth of a DS offspring, and the correlative 3-fold increased risk of a DS child for such families, seem to be demonstrated. A study of the origin of nondisjunction, using restriction fragment length polymorphism (RFLP) segregation analysis with a sufficient number of chromosome 21 specific probes, has provided complete information in 7 of 8 available families. Although the statistical interpretation of the results is not straightforward, due to the small size of the sample, the observed data do not contradict the assumption that the presence of inv (9) (qh) in a parent increases, by a factor of about 3, the chance that the offspring will inherit an extra chromosome 21 from that parent. Nevertheless, gathering further data appears desirable because stronger evidence would have relevance both for clinical implications and for the understanding of the function of heterochromatin, particularly with respect to meiotic and mitotic processes.

FISHing for acrocentric associations between chromosomes 14 and 21 in human oogenesis

OBJECTIVE

The purpose of this study was to search for cytologic evidence of robertsonian translocation formation that involves chromosomes 14q and 21q in human oogenesis with the use of dual color fluorescent in situ hybridization with whole chromosome paints.

STUDY DESIGN

The oocytes from a chromosomally normal fetus at 23.5 weeks of gestation underwent cohybridization with chromosome specific DNA libraries from chromosomes 14 and 21. The nuclei were scored for the proportion of meiosis I prophase substages and for hybridization efficiency and were evaluated for the presence of hybridization signals that were suggestive of heterologous associations between chromosomes 14q and 21q in zygotene, pachytene, and diplotene.

RESULTS

A total of 1769 meiotic nuclei were analyzed. Of 272 informative nuclei at zygotene, pachytene, and diplotene, 1 nucleus at pachytene demonstrated hybridization signals for chromosomes 14 and 21 that could be consistent with a robertsonian translocation.

CONCLUSION

A heterologous association between chromosomes 14q and 21q that possibly represent robertsonian translocation formation was observed cytologically with the use of fluorescent in situ hybridization.

Somatic pairing, endomitosis and chromosome aberrations in snakes (Viperidae and Colubridae)

The positioning of macrochromosomes of Bothrops jararaca and Bothrops insularis (Viperidae) was studied in undistorted radial metaphases of uncultured cells (spermatogonia and oogonia) not subjected to spindle inhibitors. Colchicinized metaphases from uncultured (spleen and intestine) and cultured tissues (blood) were also analyzed. We report two antagonic non-random chromosome arrangements in untreated premeiotic cells: the parallel configuration with homologue chromosomes associated side by side in the metaphase plate and the antiparallel configuration having homologue chromosomes with antipolar distribution in the metaphase ring. The antiparallel aspect also appeared in colchicinized cells. The spatial chromosome arrangement in both configurations is groupal size-dependent and maintained through meiosis. We also describe, in untreated gonia cells, endomitosis followed by reductional mitosis which restores the diploid number. In B. jararaca males we observed that some gonad regions present changes in the meiotic mechanism. In this case, endoreduplicated cells segregate the diplochromosomes to opposite poles forming directly endoreduplicated second metaphases of meiosis with the suppression of first meiosis. By a successive division, these cells form nuclei with one set of chromosomes. Chromosome doubling in oogonia is known in hybrid species and in parthenogenetic salamanders and lizards. This species also presented chromosome rearrangements leading to aneuploidies in mitosis and meiosis. It is suggested that somatic pairing, endomitosis, meiotic alterations, and chromosomal aberrations can be correlated processes. Similar aspects of nuclei configurations, endomitosis and reductional mitosis were found in other Viperidae and Colubridae species.

Synaptic process in the rat (Rattus norvegicus): Influence of methodology on results

Synaptonemal complex (SC) analysis is a widely used method for assessing the effects of genotoxic agents in germ cells. Although the evolution of the SCs and their related annexed structures, such as nucleoli, has been well established, sometimes it is difficult to assess whether the abnormal features observed correspond to genotoxic effects or to an artefact related to the method used to obtain the SC preparations. In this article, we describe a new method of obtaining SC preparations for electron microscopy, as well as the results of a study of the first meiotic prophase in oocytes and spermatocytes of the rat (Rattus norvegicus Sprague Dawley) in which we analysed how the methodology used can influence the results. Besides important sex-specific differences, mainly during desynapsis (diplotene), a relationship between several bivalents and nucleolar structures, that in some cases could disturb the synaptic process, was observed in oocytes. At the same time, the characteristic SC fragmentation in oocytes was verified, but this fragmentation, in addition to a sex-specific component, was influenced by the method itself. By reducing to a minimum the artefacts produced by the method, it is possible to optimise the analysis of SCs as a method of testing genotoxic effects in the germ line.

Chromosome fission associated with growth of ribosomal DNA in Neodiprion abietis (Hymenoptera: Diprionidae)

The haploid complement consists of seven metacentric chromosomes in most diprionid species but has evolved to n = 8 by fission in Neodiprion abietis. This fission generated a small telocentric chromosome and a large pseudoacrocentric chromosome with a short arm carrying a satellite. In situ hybridization indicated that the location of the rRNA gene cluster corresponds to the whole short arm. This suggests that (i) the breaking point was located close to an rRNA gene cluster, and (ii) fission was associated with growth of rDNA. These results suggest rDNA as a preferential breaking point but with a role in the healing of naked chromosome ends.

Bivalent 15 regularly associates with the sex vesicle in normal male meiosis

Using fluorescent in-situ hybridization, we investigated the positioning of different human bivalents at the pachytene stage of normal male meiosis. We showed that, in about 35% of nuclei, the pericentromeric region of bivalent 15 is closely associated with the sex vesicle (SV). This behaviour may be linked to the presence of three domains in the pericentromeric region of chromosome 15: a large imprinted domain, a nucleolar organizing region (NOR), and a heterochromatic block. In order to define the domains of chromosome 15 involved in this association, we analysed the meiotic behaviour of other bivalents with similar domains: human bivalent 11 and mouse bivalent 7, bearing imprinted domains, other human acrocentric bivalents bearing a NOR, and the human bivalents 1, 9 and 16 containing a heterochromatic region. None of these bivalents were as frequently associated with the SV as the human bivalent 15. Nevertheless, we suggest that the bivalent 15 heterochromatin may be responsible for the association because of two properties: its telomeric location on chromosome 15 and its strong sequence homology with the Yq heterochromatin. This phenomenon could explain the high frequency of translocations between the chromosome 15 and the X or Y chromosomes.

Stable chromosome fission associated with rDNA mobility

A spontaneous chromosome fission in the plant Hypochoeris radicata has been characterized by Feulgen staining, in situ hybridization of the rDNA probe pTA71 and silver staining for active nucleolus organizing regions. The parental acrocentric chromosome has no detectable ribosomal genes at the centromere, but both fission derivatives possess active NORs at their centric ends. In fission heterozygotes, pachytene configurations studied by synaptonemal complex spreading show that the ribosomal cistrons form short arms on each telocentric which pair together to form a triradial. The paired short arms are associated with the single nucleolus at pachytene. It is proposed that the origin and stabilization of the fission rearrangement involved transposition of rDNA from the nucleolus organizing region of chromosome 3 into the centromeric region of chromosome 1.

Biparental inheritance of chromosome 21 polymorphic markers indicates that some Robertsonian translocations t(21;21) occur postzygotically

Robertsonian translocations between acrocentric chromosomes are the most common structural chromosomal rearrangements in humans and many other organisms, and several mechanisms for their formation have been proposed. We have analyzed highly informative DNA polymorphisms in a family with a non-mosaic de novo Robertsonian translocation 21q;21q, to determine the parental origin of the two 21q arms of the rearranged chromosome. The genotypes indicated a biparental origin, i.e. one 21q was paternal and the other maternal. These results imply that in some cases the formation of the rob(21q;21q) occurs in the zygote or in the first few postzygotic mitotic divisions.

Parental origin determination in thirty de novo Robertsonian translocations

Cytogenetic heteromorphisms and restriction fragment length polymorphisms were used to assign the parental origins of 30 de novo non-homologous Robertsonian translocations. The balanced and unbalanced translocations studied included 20 rob(14q21q) four rob(13q14q)four rob(15q21q) one rob(13q15q), and one rob(13q21q). Significantly more maternally (26/30) than paternally (4/30) derived de novo translocations were noted and all rob(14q21q) ascertained through unbalanced probands (20/20) were maternal in origin. Interestingly, 12/13 probands who were trisomic and informative for proximal chromosome 21q loci were homozygous for the markers tested. Segregation (2:1) of the Robertsonian translocation into one daughter cell in meiosis I and subsequent failure of the chromosome 21 chromatids to separate in meiosis II may account for our observation of homozygosity for proximal chromosome 21 loci in the majority of de novo rearrangements tested.

Nucleoli, nucleolar chromosomes and ribosomal genes in the human spermatocyte

The formation and development of nucleoli and their connections with the nucleolar chromosomes were studied in human spermatocytes using electron microscopy, silver staining of nucleolus organizer regions (NORs), high resolution autoradiography and in situ hybridization in order to localize rRNA genes and their transcription in the different stages of meiotic prophase I. At leptotene, new nucleoli were formed, consisting of a fibrillar centre surrounded by a cap of dense fibrillar component. Following [3H]uridine uptake, label was found only over the dense fibrillar component. In situ hybridization revealed rDNA mainly in the dense fibrillar component and in the chromatin. During zygotene, nucleoli increased in size. The fibrillar centre was connected with the secondary constriction region of the nucleolar bivalent and was partially surrounded by dense fibrillar component. This shell of dense fibrillar component merged into a fibrillo-granular mesh that extended away from the fibrillar centre. Autoradiography following [3H]uridine uptake again showed the label overlaying the dense fibrillar component and the proximal part of the fibrillo-granular strands. With in situ hybridization in both the light and electron microscope, signal was mainly found in the dense fibrillar component. A small quantity of label was observed in the peripheral region of the fibrillar centre and in the adjacent chromatin. From early to late pachytene segregation of nucleolar components occurred, with a reduction in the dense fibrillar component that formed a narrow rim around the fibrillar centre with small extensions along the granular component. [3H]uridine incorporation progressively decreased. In situ hybridization showed signal located mainly in the dense fibrillar component and in the chromatin corresponding to the condensed short arm of the nucleolar bivalent. Our results indicate that the majority of rDNA is located and transcribed in the dense fibrillar component; only a small amount is present in the peripheral part of the fibrillar centre and may be transcribed there. Moreover, from leptotene to zygotene, rDNA unravels from the nucleolar chromosome into the nucleolar dense fibrillar component. From zygotene to late pachytene a progressive return to the condensed acrocentric short arm is observed.

Down syndrome due to de novo Robertsonian translocation t(14q;21q): DNA polymorphism analysis suggests that the origin of the extra 21q is maternal

Down syndrome is rarely due to a de novo Robertsonian translocation t(14q;21q). DNA polymorphisms in eight families with Down syndrome due to de novo t(14q;21q) demonstrated maternal origin of the extra chromosome 21q in all cases. In seven nonmosaic cases the DNA markers showed crossing-over between two maternal chromosomes 21, and in one mosaic case no crossing-over was observed (this case was probably due to an early postzygotic nondisjunction). In the majority of cases (five of six informative families) the proximal marker D21S120 was reduced to homozygosity in the offspring with trisomy 21. The data can be best explained by chromatid translocation in meiosis I and by normal crossover and segregation in meiosis I and meiosis II.

Nuclear architecture of human pachytene spermatocytes: quantitative analysis of associations between nucleolar and XY bivalents

Nucleolar association and heterochromatin coalescence have both been invoked as mechanisms involved in the origin of chromosomal associations between nucleolar bivalents themselves, as well as between these bivalents and the XY pair, during meiotic prophase in human spermatocytes. However, these mechanisms do not satisfactorily explain how associating bivalents meet each other within the nuclear space. To elucidate this problem, we have characterized different types of nucleolar-nucleolar and nucleolar-XY bivalent associations, and their frequencies, in light and electron microscope serial sections of spermatocyte nuclei. In the pachytene nucleus, nucleolar bivalent associations were found to involve only one nucleolar sphere of RNP granules connected through a fibrillar center to a chromatin mass composed of two, or more, nucleolar-bivalent short arms. Structural relationships between these elements were examined using 3D computer models of various nucleolar associations. XY and nucleolar bivalents were usually located towards the nuclear periphery associated with the inner face of the nuclear envelope. Some nucleolar bivalents, whether single or associated appeared beside or over XY chromatin. When nucleolar-bivalent short arms (BK) were found over nucleolar or over XY chromatin, their telomeres were unattached to the nuclear envelope and the corresponding synaptonemal complexes were not observed. Ninety nucleoli were found in sixty pachytene nuclei. Thirty six percent of these nucleoli were bound to associated BKs and the remaining 64% to single BKs. Over 40% of individual spermatocytes showed at least one cluster of associated BKs and about 20% presented single or multiple BKs associated with the XY pair. The frequencies of random BK associations, over the total or restricted areas of the nuclear envelope, were calculated according to a probabilistic nuclear model. A correspondence was found in comparing the observed frequencies of associated BKs with those calculated on the basis of bouquet formation. Such an analysis strongly suggests that the occurrence of associations between nucleolar bivalents may arise at random within the bouquet. Thus, the architecture of the meiocyte nucleus, particularly the organization of the bouquet, may be the primary mechanism by which nucleolar bivalents meet each other and, consequently, become associated either through common nucleolus formation or by heterochromatin coalescence.

Nucleolar structure and synthetic activity during meiotic prophase and spermiogenesis in the rat

The ultrastructure of nucleoli was examined in developing rat spermatocytes and spermatids, with the help of serial sections. In addition, the radioautographic reaction of nucleoli as examined in rats sacrificed 1 hr after intratesticular injection of 3H(5')-uridine and taken as an index of the rate of synthesis of ribosomal RNA (rRNA). Primary spermatocytes from preleptotene to zygotene have small nucleoli typically composed of fibrillar centers, a fibrillar component, and a granular component, within which are narrow interstitial spaces. During early and mid-pachytene, nucleoli enlarge to about nine times their initial size, with the fibrillar and granular components forming an extensive network of cords--a nucleolonema--within which are wide interstitial spaces. Meanwhile, there appear structures identical to the granular component but distinct from nucleoli; they are referred to as extranucleolar granular elements. Finally, from late pachytene to the first maturation division, nucleoli undergo condensation, as shown by contraction of fibrillar centers into small clumps, while fibrillar and granular components condense and segregate from each other, with a gradual decrease in interstitial spaces. In secondary spermatocytes, nucleoli are compact and rather small, while in young spermatids they are also compact and even smaller. Nucleoli disappear in elongating spermatids. In 3H-uridine radioautographs, nucleolar label is weak in young primary spermatocytes, increases progressively during early pachytene, is strong by the end of mid pachytene, but gradually decreases during late pachytene up to the first maturation division. In secondary spermatocytes and spermatids, there is no significant nucleolar label. In conclusion, rRNA synthesis by nucleoli is low in young spermatocytes. During pachytene, while nucleoli enlarge and form a lacy nucleolonema, rRNA synthesis increases gradually to a high level by the end of mid pachytene. However, during the condensation and segregation of nucleolar components occurring from late pachytene onward, the synthesis gradually decreases and disappears. The small, compact spermatids arising from the second maturation division do not synthesize rRNA.

Analysis of meiotic segregation in a man heterozygous for a 13;15 Robertsonian translocation and a review of the literature

Meiotic segregation was studied in a male heterozygous for a 13;15 Robertsonian translocation using in vitro sperm penetration of hamster eggs. Sixty-seven sperm chromosome complements were obtained and R-banded. Alternate segregation produced equal numbers of normal (31) and balanced (29) gametes, as was theoretically expected. Incidence of unbalanced complements was 10.4%, and the frequency of abnormalities unrelated to the translocation was 7.4%. This study confirms the predominance of alternate meiotic segregation in Robertsonian translocation carriers. Four sperm studies of Robertsonian translocation have been previously reported. A review of the combined results points out the low incidence of imbalance in the sperm of Robertsonian translocation carrier and the lack of evidence for an interchromosomal effect.

Nucleolar structures in chromosome and SC preparations from human oocytes at first meiotic prophase

We describe a comparative study of the behavior of nucleolar structures and their relationship with nucleolar chromosomes and synaptonemal complexes at first meiotic prophase of human oocytes in an attempt to elucidate the nature of this cellular organization and to learn more about maternal nondisjunction. The number of main nucleoli varies along the different stages of prophase I and is usually low. It shows an increase from leptotene to pachytene and a decrease from pachytene to diplotene related to a decrease and an increase of main nucleoli volume, respectively. The methodology employed has enabled us to analyze in detail dark bodies, round bodies, dense bodies, and main nucleoli in chromosome or synaptonemal complex spreads. The relationship between nucleolar chromosomes or synaptonemal complexes and the nucleoli implies the existence, in a very reduced space, of chromosomal regions that contain homologous sequences and that are often unpaired. This situation may facilitate the production of heterologous pairing and chromosomal exchanges between nonhomologous chromosomes and finally result in aneuploidy. Thus, the situation explained above together with the differences between the oocyte and spermatocyte NOR cycles could be one of the reasons for the higher incidence of aneuploidies of maternal origin at meiosis I.

Differential associative behaviour of mitotic and meiotic acrocentric chromosomes

A comparative study of the association of mitotic acrocentric chromosomes and acrocentric bivalents at the pachytene stage shows that at least two factors can act in the associative behaviour of these chromosomes: (1) Nor activity and (2) the presence of satellite DNA in the short arms of these chromosomes. These factors do not act with the same intensity in the two cell lines studied. In lymphocytes, Nor activity prevails, whereas satellite DNA plays the main role in the association of acrocentric chromosomes in germ cells at the pachytene stage.

Robertsonian translocation and an extra microchromosome: aetiology and effect on meiotic segregation

A familial Robertsonian translocation accompanied by a marker chromosome t(14q;21q), + iso dic (?14?21)(pter----q11.1) is described. The evolution of the marker is considered in relation to a model proposed by ultrastructural studies of acrocentric bivalents for the formation of Robertsonian-type translocations. Previous reports have suggested that a marker chromosome may be of pathogenetic importance for non-disjunction of chromosome 21. The effect of the marker chromosome on meiotic segregation in the family described and its implications upon genetic counselling are considered.

Nucleolus, nucleolar chromosomes, and nucleolus-associated chromatin from early diplotene to dictyotene in the human oocyte

The shape, relationships, relative DNA content, and nucleolar activity of the short arm of acrocentric bivalents were studied in human oocytes from early diplotene to dictyotene. At the beginning of diplotene, the short arms of the previously paired chromosomes were again separated and displayed the same morphological features as in mitotic prophase chromosomes. They were connected only with the nucleolus. In situ hybridization and silver staining showed that the nucleolar organizer regions (NORs) were located in the peripheral region of the nucleolus. Tritiated-uridine incorporation was active. At birth, the relationships of the acrocentric short arms showed increasing complexity. The chromosomes ended in nucleolus-associated chromatin blocks of irregular shape, containing large quantities of DNA as demonstrated by intense binding of 3H-actinomycin D. The number of chromosomes converging on these chromatin blocks exceeded the number of acrocentrics, suggesting that heterochromatic regions of other chromosomes were associated with the short arm of acrocentrics. In the electron microscope, the NORs were represented by fibrillar centers located on the periphery of the nucleolus and consistently connected with the blocks of dense chromatin. These relationships remained unchanged in the primordial oocyte in the adult ovary. Persistence of 3H-uridine uptake showed that the oocyte was not at a "resting" stage. The possible cytogenetic consequences of these observations are discussed.

A de novo translocation, 14q21q, with a microchromosome-14p21p

A familial translocation, t(14;21)(14p21p;14q21q), in a mother and her child is described. The translocation was ascertained through the birth of a Down syndrome baby with the chromosome constitution 47,XX,-14, +der 14, +der 21,t(14;21)(q11;p12) mat. A 1:3 segregation in the maternal meiosis is suggested for the evolution of the unbalanced chromosome state. The main translocated chromosome 14q21q mimics the product of a Robertsonian translocation, while the 14p21p chromosome has the morphology of a satellited microchromosome. The cytogenetic nature of this translocation is discussed.

The association of the nucleolus and the short arm of acrocentric chromosomes with the XY pair in human spermatocytes: its possible role in facilitating sex-chromosome acrocentric translocations

Sex vesicle-nucleolus association was observed in 12% of zygotene and pachytene human spermatocytes using Giemsa and NOR-silver stained preparations. The silver-positive area of the nucleolus, corresponding to the nucleolus organizer (NOR), was usually close to the XY pair. C-banding frequently showed the terminal chromomere, formed by the condensed short arm of an acrocentric bivalent, attached to the sex vesicle. When a nucleolus produced by transcription of rDNA was connected to the short arm, it seemed to be secondarily associated with the sex vesicle. Non-transcribed ribosomal genes, which did not form a nucleolus, were revealed by in situ hybridization. Autoradiographs showed the rDNA-containing short arm of acrocentric bivalents associated with the sex vesicle in 18% of spermatocytes. The difference with the frequency of nucleolus-XY pair association was partially explained by the presence of inactive ribosomal genes. Moreover, electron microscopy showed that the dimensions of the newly formed nucleoli at early zygotene did not exceed 0.5 micron; they can be missed in light microscope investigations. From early zygotene to late pachytene, close relationships were observed between the sex vesicle chromatin and that of the associated acrocentric bivalent, especially in the short arm region. These relationships might explain the frequent involvement of acrocentrics in Y-autosome and X-autosome translocations occurring during male meiosis.