Pachytene analysis in a 17;21 reciprocal translocation carrier: role of the acrocentric chromosomes in male sterility

Summary Pachytene analysis was undertaken in an infertile male, heterozygous for a 17;21 reciprocal translocation. The quadrivalent was identified by its configuration and chromomere pattern. A non-random association was found between the quadrivalent and the sex vesicle in 77% of the pachytene nuclei analysed. In 13.1% of the cells the contact with the sex vesicle was established by the terminal chromomere of the two chromosomes 21; in 63.9% of the cells, the entire region of the breakpoints was completely hidden by the sex vesicle. In some nuclei asynapsis was found in the region of the breakpoints. The nature of the contact between the quadrivalent and the sex vesicle is discussed in this paper. It is proposed that the acrocentric chromosome favours the contact between the quadrivalent and the sex vesicle, and increases the risk of sterility in male carriers of Robertsonian translocations and of reciprocal translocations involving one acrocentric chromosome.     

Asynapsis in the salivary gland chromosomes ofDrosophila hydei

An investigation of the asynapsis phenomenon in salivary gland chromosomes ofDrosophila hydei was undertaken. Asynapsis was found to occur in fixed and stained as well as in surviving chromosomes.Frequency of asynapsis was found to be much higher in the X-chromosome than in any of the autosomes. The total asynapsis frequency of autosomes is slightly higher in females than in males. Among the four large autosomes, no consistent preference or sequence of frequency was encountered. In none of the chromosomes a preference of asynapsis for a certain region was found. Crowding of the cultures had no influence on frequency or distribution of non-pairing.In hybrids between wild stocks of different geographic origin a slightly higher asynapsis frequency, compared to the wild stocks was found.     

Genetic and cytogenetic analysis of the fruit fly Rhagoletis cerasi (Diptera: Tephritidae).

The European cherry fruit fly, Rhagoletis cerasi, is a major agricultural pest for which biological, genetic, and cytogenetic information is limited. We report here a cytogenetic analysis of 4 natural Greek populations of R. cerasi, all of them infected with the endosymbiotic bacterium Wolbachia pipientis. The mitotic karyotype and detailed photographic maps of the salivary gland polytene chromosomes of this pest species are presented here. The mitotic metaphase complement consists of 6 pairs of chromosomes, including one pair of heteromorphic sex chromosomes, with the male being the heterogametic sex. The analysis of the salivary gland polytene complement has shown a total of 5 long chromosomes (10 polytene arms) that correspond to the 5 autosomes of the mitotic nuclei and a heterochromatic mass corresponding to the sex chromosomes. The most prominent landmarks of each polytene chromosome, the "weak points", and the unusual asynapsis of homologous pairs of polytene chromosomes at certain regions of the polytene elements are also presented and discussed.     

Histone H2AFX Links Meiotic Chromosome Asynapsis to Prophase I Oocyte Loss in Mammals

Chromosome abnormalities are common in the human population, causing germ cell loss at meiotic prophase I and infertility. The mechanisms driving this loss are unknown, but persistent meiotic DNA damage and asynapsis may be triggers. Here we investigate the contribution of these lesions to oocyte elimination in mice with chromosome abnormalities, e.g. Turner syndrome (XO) and translocations. We show that asynapsed chromosomes trigger oocyte elimination at diplonema, which is linked to the presence of phosphorylated H2AFX (γH2AFX). We find that DNA double-strand break (DSB) foci disappear on asynapsed chromosomes during pachynema, excluding persistent DNA damage as a likely cause, and demonstrating the existence in mammalian oocytes of a repair pathway for asynapsis-associated DNA DSBs. Importantly, deletion or point mutation of H2afx restores oocyte numbers in XO females to wild type (XX) levels. Unexpectedly, we find that asynapsed supernumerary chromosomes do not elicit prophase I loss, despite being enriched for γH2AFX and other checkpoint proteins. These results suggest that oocyte loss cannot be explained simply by asynapsis checkpoint models, but is related to the gene content of asynapsed chromosomes. A similar mechanistic basis for oocyte loss may operate in humans with chromosome abnormalities.     

Characteristics of the first meiotic division in hamster hybrids obtained by backcrossing Phodopus sungorus and Phodopus campbelli]

Spermatocytes of fertile, subfertile, and sterile hamster hybrids obtained by backcrossing Phodopus sungorus and Ph. campbelli were analyzed under light and electron microscopes. Light microscopy showed that early meiosis was blocked in pachitene in the spermatocytes of sterile hybrids. The X and Y chromosomes were dissociated in metaphase I in several fertile and subfertile animals. Electron microscopic analysis of the synaptonemal complex (SC) revealed a disturbed synapsis of sex chromosomes and autosomes in all hybrids. Dissociation of the sex chromosomes, terminal and interstitial asynapsis, and interlocking of autosomes were observed. Disturbed synapsis in hybrids was assumed to result from the difference between Ph. sungorus and Ph. campbelli in not only their chromosomes, but their genes as well.     

Extensive meiotic asynapsis in mice antagonises meiotic silencing of unsynapsed chromatin and consequently disrupts meiotic sex chromosome inactivation

Chromosome synapsis during zygotene is a prerequisite for the timely homologous recombinational repair of meiotic DNA double-strand breaks (DSBs). Unrepaired DSBs are thought to trigger apoptosis during midpachytene of male meiosis if synapsis fails. An early pachytene response to asynapsis is meiotic silencing of unsynapsed chromatin (MSUC), which, in normal males, silences the X and Y chromosomes (meiotic sex chromosome inactivation [MSCI]). In this study, we show that MSUC occurs in Spo11-null mouse spermatocytes with extensive asynapsis but lacking meiotic DSBs. In contrast, three mutants (Dnmt3l, Msh5, and Dmc1) with high levels of asynapsis and numerous persistent unrepaired DSBs have a severely impaired MSUC response. We suggest that MSUC-related proteins, including the MSUC initiator BRCA1, are sequestered at unrepaired DSBs. All four mutants fail to silence the X and Y chromosomes (MSCI failure), which is sufficient to explain the midpachytene apoptosis. Apoptosis does not occur in mice with a single additional asynapsed chromosome with unrepaired meiotic DSBs and no disturbance of MSCI.     

Meiotic Cohesin SMC1β Provides Prophase I Centromeric Cohesion and Is Required for Multiple Synapsis-Associated Functions

Cohesin subunit SMC1β is specific and essential for meiosis. Previous studies showed functions of SMC1β in determining the axis-loop structure of synaptonemal complexes (SCs), in providing sister chromatid cohesion (SCC) in metaphase I and thereafter, in protecting telomere structure, and in synapsis. However, several central questions remained unanswered and concern roles of SMC1β in SCC and synapsis and processes related to these two processes. Here we show that SMC1β substantially supports prophase I SCC at centromeres but not along chromosome arms. Arm cohesion and some of centromeric cohesion in prophase I are provided by non-phosphorylated SMC1α. Besides supporting synapsis of autosomes, SMC1β is also required for synapsis and silencing of sex chromosomes. In absence of SMC1β, the silencing factor γH2AX remains associated with asynapsed autosomes and fails to localize to sex chromosomes. Microarray expression studies revealed up-regulated sex chromosome genes and many down-regulated autosomal genes. SMC1β is further required for non-homologous chromosome associations observed in absence of SPO11 and thus of programmed double-strand breaks. These breaks are properly generated in Smc1β^−/− spermatocytes, but their repair is delayed on asynapsed chromosomes. SMC1α alone cannot support non-homologous associations. Together with previous knowledge, three main functions of SMC1β have emerged, which have multiple consequences for spermatocyte biology: generation of the loop-axis architecture of SCs, homologous and non-homologous synapsis, and SCC starting in early prophase I.     

Analysis of male meiotic "sex body" proteins during XY female meiosis provides new insights into their functions

During male meiosis in mammals the X and Y chromosomes become condensed to form the sex body (XY body), which is the morphological manifestation of the process of meiotic sex chromosome inactivation (MSCI). An increasing number of sex body located proteins are being identified, but their functions in relation to MSCI are unclear. Here we demonstrate that assaying male sex body located proteins during XY female mouse meiosis, where MSCI does not take place, is one way in which to begin to discriminate between potential functions. We show that a newly identified protein, "Asynaptin" (ASY), detected in male meiosis exclusively in association with the X and Y chromatin of the sex body, is also expressed in pachytene oocytes of XY females where it coats the chromatin of the asynapsed X in the absence of MSCI. Furthermore, in pachytene oocytes of females carrying a reciprocal autosomal translocation, ASY associates with asynapsed autosomal chromatin. Thus the location of ASY to the sex body during male meiosis is likely to be a response to the asynapsis of the non-homologous regions [outside the pseudoautosomal region (PAR)] of the heteromorphic X-Y bivalent, rather than being related to MSCI. In contrast to ASY, the previously described sex body protein XY77 proved to be male sex body specific. Potential functions for MSCI and the sex body are discussed together with the possible roles of these two proteins.     

The meiotic behaviour of autosomal heterochromatic segments in hedgehogs

Male meiosis in the two species of hedgehogs Erinaceus europaeus and Aethechinus algirus, possessing respectively three and two pairs of autosomes with large blocks of heterochromatin, has been studied. The heterochromatic segments pair homologously till the end of pachytene, but separate during diplotene, owing to lack of chiasmata in these regions. They also organize the nucleolus in both species. The sex chromosomes (sex vesicle) are not associated with the nucleolus. The lack of chiasmata in the heterochromatic segments is interpreted as possible mechanism for the conservation of vital genes, such as ribosomal cistrons.     

Chromosomes and DNA of Mus

Using C-banded preparations of Mus dunni it is possible to study the behavior of constitutive heterochromatin in early stages of meiotic prophase. The X and the Y chromosomes, both of which contain a large amount of heterochromatin, lie apart in leptotene but move toward each other during zygotene. They then form the sex vesicle at late zygotene. In autosomes zygotene pairing appears to start from the telomeric ends. The centromere of the Y chromosome associates end-to-end with the terminal end of the long arm of the X chromosome. The autosomal heterochromatic short arms show forked morphology in certain bivalents at pachytene, suggesting probable incomplete synapsis.     

C-Banding/DAPI and in situ hybridization reflect karyotype structure and sex chromosome differentiation in Humulus japonicus Siebold & Zucc

Japanese hop (Humulus japonicus Siebold & Zucc.) was karyotyped by chromosome measurements, fluorescence in situ hybridization with rDNA and telomeric probes, and C-banding/DAPI. The karyotype of this species consists of sex chromosomes (XX in female and XY1Y2 in male plants) and 14 autosomes difficult to distinguish by morphology. The chromosome complement also shows a rather monotonous terminal distribution of telomeric repeats, with the exception of a pair of autosomes possessing an additional cluster of telomeric sequences located within the shorter arm. Using C-banding/DAPI staining and 5S and 45S rDNA probes we constructed a fluorescent karyotype that can be used to distinguish all autosome pairs of this species except for the 2 largest autosome pairs, lacking rDNA signals and having similar size and DAPI-banding patterns. Sex chromosomes of H. japonicus display a unique banding pattern and different DAPI fluorescence intensity. The X chromosome possesses only one brightly stained AT-rich terminal segment, the Y1 has 2 such segments, and the Y2 is completely devoid of DAPI signal. After C-banding/DAPI, both Y chromosomes can be easily distinguished from the rest of the chromosome complement by the increased fluorescence of their arms. We discuss the utility of these methods for studying karyotype and sex chromosome evolution in hops.     

X-Y-autosome translocation, chromosome compaction, NOR expression and heterochromatin insulation in the Scarabaeid beetle Dynastes hercules hercules

The karyotype of the giant beetle Dynastes hercules hercules is composed of only 16 autosomes and large sex chromosomes. Meiotic studies in the males showed that a large part of the sex chromosomes undergo synapsis at pachynema similarly to autosomes, demonstrating that both derived from an autosome-gonosome translocation. Therefore, karyotype formula is 18,neoXY. The heterochromatisation of the neoX short arm at pachynema indicates that it corresponds to the ancestral X. It carries the nucleolar organizer region (NOR) in its proximal part, which is undercondensed, especially in male mitotic and meiotic cells. In female mitotic cells, both NOR staining and undercondensation were more difficult to observe in the neoX short arms. In somatic interphase nuclei, NOR expression strongly varies with the sex. Two separated compact groups of silver dots were observed in female nuclei, while a single dispersed and large group of silver deposit exists in the males. Both the lower condensation and the higher NOR expression of the single neoX of the males, compared to each of the two neoXs of the females, is interpreted to be a consequence of dosage compensation, a mechanism not yet described in Coleoptera. In mammals as well as in Coleoptera, the carriers of gonosome-autosome translocations not exhibiting deleterious phenotypes show constitutive heterochromatin at the autosome-gonosome junction. Thus, heterochromatin may play an important universal role by clearly separating chromosome segments with different regulations of gene expression, such as inactivation or dosage compensation of the X chromosome on the one side and a conventional autosomal structure on the other side.     

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。     

Genetic and cytogenetic analysis of the olive fruit fly Bactrocera oleae (Diptera: Tephritidae)

The genetic and cytogenetic characteristics of one of the major agricultural pests, the olive fruit fly Bactrocera oleae, are presented here. The mitotic metaphase complement of this insect consists of six pairs of chromosomes including one pair of heteromorphic sex chromosomes, with the male being the heterogametic sex. The analysis of the polytene complements of three larval tissues, the fat body, the salivary glands and the Malpighian tubules of this pest has shown (a) a total number of five long chromosomes (10 polytene arms) that correspond to the five autosomes of the mitotic nuclei and a heterochromatic mass corresponding to the sex chromosomes, (b) the constancy of the banding pattern of the three somatic tissues, (c) the absence of a typical chromocenter as an accumulation of heterochromatin, (d) the existence of reverse tandem duplications, and (e) the presence of toroid tips of the chromosome arms. The in situhybridization of genes or DNA sequences to the salivary gland polytene chromosomes of B. oleaeprovided molecular markers for all five autosomes and permitted the establishment of chromosomal homologies among B. olea, B. tryoniand Ceratitis capitata. The heat shock response of B. oleae, as revealed by heat-inducible puffing and protein pattern, shows a higher thermotolerance than Drosophila melanogaster.     

Polytene chromosome maps of the melon fly Bactrocera cucurbitae (Diptera: Tephritidae).

Standard photographic maps of the polytene chromosomes are presented for the melon fly Bactrocera cucurbitae, a serious pest of fleshy fruits and vegetables. Five larval salivary gland polytene chromosomes (10 polytene arms) were isolated, and their characteristic features and landmarks have been recognized. Banding patterns of each of the polytene arms are presented, where variation in band intensity and puffs appear to reflect fundamental differences in chromosomes. The whole polytene genome has been typically mapped by dividing it into 100 sections and the subsections were lettered. The mitotic chromosomes of larval brain ganglia are also examined, five pairs of autosomes and an XX/XY sex chromosome pair. In addition, a heterochromatic mass corresponding to the sex chromosomes are observed in the polytene nuclei of salivary gland tissue. This investigation showed that B. cucurbitae has excellent cytological material for polytene chromosome analysis and proved to be very useful for obtaining more detailed genetic information on the pest's natural populations.     

Comparative studies on sex chromosomes in different species

InLocusta migratoria (XO),Mus musculus, Rattus norvegicus, Mesocricetus auratus, Cricetulus griseus andHomo sapiens typical sex vesicle structures are visible in early meiotic prophase stages up to pachynema. The structures include whole sex chromosomes or parts thereof. The heterologous parts and the solitary X chromosome ofLocusta pass diplonema, diakinesis and first metaphase nearly in mitotic shape. Entirely heterologous sex chromosomes are kept together by a unilateral and achiasmatic end connection. The sex vesicle is interpreted as a special structure of allocyclic sex chromosomes or parts of them, corresponding in early meiotic stages to the chromocenters of mitotic interphase nuclei. The formation of the sex vesicle is independent of the orthoploidy of nuclei and of the DNA ratio between autosomes and sex chromosomes. Heteropycnotic behaviour of sex chromosomes in spermatids is interpreted as a condition capable of blocking genetic activity, like in the Barr bodies of female somatic nuclei, giving equal chances of fertilization to both types of gametes.Based on a paper read at the XI International Congress of Genetics, of which an abstract has appeared in the congress proceedings, Genetics Today, Vol. 1, p. 299 (1963).