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At least 19 recordsLinked to original sources
PubMed2024

BRCA1 safeguards genome integrity by activating chromosome asynapsis checkpoint to eliminate recombination-defective oocytes.

In the meiotic prophase, programmed DNA double-strand breaks are repaired by meiotic recombination. Recombination-defective meiocytes are eliminated to preserve genome integrity in gametes. BRCA1 is a critical protein in somatic homologous recombination, but studies have suggested that BRCA1 is dispensable for meiotic recombination. Here we show that BRCA1 is essential for meiotic recombination. Interestingly, BRCA1 also has a function in eliminating recombination-defective oocytes. Brca1 knockout (KO) rescues the survival of Dmc1 KO oocytes far more efficiently than removing CHK2, a vital component of the DNA damage checkpoint in oocytes. Mechanistically, BRCA1 activates chromosome asynapsis checkpoint by promoting ATR activity at unsynapsed chromosome axes in Dmc1 KO oocytes. Moreover, Brca1 KO also rescues the survival of asynaptic Spo11 KO oocytes. Collectively, our study not only unveils an unappreciated role of chromosome asynapsis in eliminating recombination-defective oocytes but also reveals the dual functions of BRCA1 in safeguarding oocyte genome integrity.

Oocytes↗
PubMed2000

Evidence that sex chromosome asynapsis, rather than excess Y gene dosage, is responsible for the meiotic impairment of XYY mice.

There is extensive evidence for the existence of a meiotic checkpoint that acts to eliminate spermatocytes that fail to achieve full sex chromosome synapsis at the pachytene stage of the first meiotic prophase. XYY mice are nearly always sterile, with clear signs of meiotic impairment, and sex chromosome asynapsis has been proposed to underlie this impairment. However, a study of XYY*(X) mice (mice having three sex chromosomes but only a single dose of Y genes) revealed that these mice are fertile, and thus implicated Y gene dosage as a major factor in the sterility of XYY mice. To address this question further, sex chromosome synapsis and spermatogenic proficiency were compared between XYY*(X) and XYY mice generated in the same litters. This established that differences in spermatogenic proficiency within and between the two genotypes correlated with the frequency of radial trivalent formation (full sex chromosome synapsis); XYY*(X) males, as a group, had double the radial trivalent frequency of XYY males. This observation provides strong support for the view that sex chromosome asynapsis (or some consequence thereof), rather than Y gene dosage, is the major factor leading to the meiotic impairment of XYY mice.

Animals↗
PubMed1993

Meiotic chromosome asynapsis in a boar with a reciprocal translocation and acquired testicular degeneration.

During a period of performance testing, a boar used for artificial insemination purposes was found responsible for reduced litter size and slightly increased incidence of repeat breeding. The qualitative and quantitative semen characteristics, however, were within normal limits. Later, evaluation of the semen during a period of 3 months revealed a decreased sperm concentration, but sperm cell morphology and mobility were normal. At castration, the boar, which had suffered from scrotal inflammation, was found to have hypoplastic gonads, the right testis being smaller than the left one. Histologically there was spermatogenic arrest at the primary spermatocyte level in almost all tubules of the right testis. In the left testis, histology was more heterogeneous, with some tubules containing all developmental stages, while others had an almost complete spermatogenic arrest. Cytogenetically the boar was carrying a translocation, rcp(2;14)(p14;q23). Synaptonemal complex analysis revealed complete quadrivalent pairing in 21 out of 43 cells analysed. The remaining cells demonstrated quadrivalents with axes moderately or extensively unpaired. There was a distinct difference between cells from the right vs. the left testis. The latter cells showed more completely paired translocation configurations. Moreover, in 14 and 7 cells analysed from the right and left testis, respectively, the translocation configuration could not be identified. This was due to chromosome asynapsis and frequent occurrence of gaps in the chromosomes, including others than those of the translocation, plus an abundant cellular deposit which formed a dense cell background. Association or pairing of the quadrivalent with the sex bivalent was seen in one cell only. At diakinesis-MI, most cells had a ring-shaped quadrivalent. It is believed that the asynapsis of chromosomes during the prophase of meiosis was to a large extent due to the degeneration caused by the inflammation processes.

Animals↗
PubMed1975

[Comparative study of salivary gland polytene chromosome asynapsis in larvae of Drosophila melanogaster, Drosophila simulans and their hybrids].

Quantitative comparative study of larvae salivary gland chromosome asynapsis has been performed on 3L arm for sibling species of Drosophila melanogaster, D. simulans and their hybrids (D. melanogaster female X D. simulans hermaphrodite). A frequency of asynapsis occurence, an asynapsis localization and distributions of asynapsis terminal points and asynapsis lengths were determined. It is revealed that parent species are very similar as regards to quantitative characteristics studied. For hybrids there are some essential differences with either of parent species. These are: 1) reliably larger frequency of asynapsis, 2) different character of asynapsis localization, and 3) two new "peaks" of distal terminal points of asynapsis and three new "peaks" of proximal terminal points of asynapsis are revealed, main parents "peaks" being kept.

Animals↗
PubMed2003

Megalospermatocytes in the human testis exhibit asynapsis of chromosomes.

Testis biopsies of three infertile patients were identified, which showed a predomination of megalospermatocytes in the seminiferous tubules. Megalospermatocytes are very large primary spermatocytes indicating a spermatogenic arrest. Because of the high percentage of these germ cells it was possible to apply a whole-mount spreading technique to investigate the chromosomal pairing behaviour in prophase I of meiosis. It could be shown that most of the megalospermatocytes exhibited extensive chromosomal asynapsis, suggesting that a characteristic meiotic disorder may give rise to reduced fertility, or even infertility.

Adult↗
PubMed2001

Telomere dysfunction triggers developmentally regulated germ cell apoptosis.

Telomere dysfunction results in fertility defects in a number of organisms. Although data from fission yeast and Caenorhabditis elegans suggests that telomere dysfunction manifests itself primarily as defects in proper meiotic chromosome segregation, it is unclear how mammalian telomere dysfunction results in germ cell death. To investigate the specific effects of telomere dysfunction on mammalian germ cell development, we examined the meiotic progression and germ cell apoptosis in late generation telomerase null mice. Our results indicate that chromosome asynapsis and missegregation are not the cause of infertility in mice with shortened telomeres. Rather, telomere dysfunction is recognized at the onset of meiosis, and cells with telomeric defects are removed from the germ cell precursor pool. This germ cell telomere surveillance may be an important mechanism to protect against the transmission of dysfunctional telomeres and chromosomal abnormalities.

Animals↗
PubMed2001

[Chromosomal polymorphism of populations of Anopheles arabiensis (Diptera: Culicidae) from Reunion island and cross-fertility among continental African populations].

Cytological examination of a sample of Anopheles gambiae complex mosquitoes from Reunion island revealed the presence of An. arabiensis only. Chromosomal polymorphisms were observed only for inversion 3Ra, the standard homozygote form being predominant. Cross-mating experiments with laboratory specimens originating from continental Africa produced viable and fertile offspring with no chromosomal asynapsis observed in the F1 female progeny. There was no evidence for speciation of the Reunion island populations. The results are discussed with regard to the behaviour of the vector and its influence on the vectorial capacity of this species, and the history of malaria and malaria control in the South-West islands of the Indian Ocean and on Reunion island in particular.

Africa↗
PubMed2003

Meiosis in male PL/J mice: a genetic model for gametic aneuploidy.

Sperm from mice of the PL/J strain have a high frequency of sperm-head morphology abnormalities. Fluorescence in situ hybridization (FISH) methods revealed that PL/J sperm are also characterized by a high frequency of aneuploidy. The traits of abnormal sperm head morphology and aneuploidy are associated with numerous meiotic abnormalities. Spermatocytes of PL/J mice exhibit chromosome asynapsis during meiotic prophase as well as reduced crossing over, revealed by analysis of both MLH1 foci in pachytene spermatocytes and chiasmata seen at the first meiotic metaphase. During the first meiotic division, roughly one-third of the PL/J spermatocytes exhibit aberrant spindle morphology, with abnormalities including monopolar spindles, split spindle poles, and incomplete spindle formation and centrosomal abnormalities. F1 progeny of a cross between PL/J and C57BL/6J did not exhibit a high frequency of either sperm aneuploidy or sperm head morphology aberrations, as would be expected if the PL/J traits were dominant. Among progeny of a backcross of F1 mice to PL/J, none of 16 males assessed exhibited elevated frequencies of sperm head morphology abnormalities. Four of the individuals exhibited elevated sperm aneuploidy, but not at the levels of the PL/J parents. Thus, it is likely that the aberrant PL/J traits are due to several genes and/or modifiers affecting the generation of both sperm aneuploidy and abnormal sperm head morphology.

Aneuploidy↗
PubMed1976

[Cytological map of the terminal points of the asynapsis in the polytene chromosomes of the salivary glands of Drosophila melanogaster].

A cytological map of the terminal points of the asynapsis (TPA) has been composed for X, 2 (L, R) and 3 (L, R) salivary gland polytene chromosomes of the third instar larvae of the wild type outbred line K-1 Drosophila melanogaster. TPA are distributed along the chromosome length unevenly forming "peaks". These peaks, as a rule, are situated in the intercalary heterochromotin. The coincidence of the main peaks with the regions of an increased crossing-over frequency is noticed. The preferential unidirectionless of termination is characteristic of TPA peaks; in the number of cases, the "discrimination" effect of asynapsises of definite lengths takes place. For the third chromosome, the frequency of simultaneous occurrence of asynapsises adjacent to the chromocenter is shown to exceed the frequency of an accidental combination. The centromeric region is not always, presumably, an obligatory board of the asynapsis, and the chromosome in the synapsis prosess can function as continuous.

Animals↗
PubMed2006

Pachytene asynapsis drives meiotic sex chromosome inactivation and leads to substantial postmeiotic repression in spermatids.

Transcriptional silencing of the sex chromosomes during male meiosis (MSCI) is conserved among organisms with limited sex chromosome synapsis, including mammals. Since the 1990s the prevailing view has been that MSCI in mammals is transient, with sex chromosome reactivation occurring as cells exit meiosis. Recently, we found that any chromosome region unsynapsed during pachytene of male and female mouse meiosis is subject to transcriptional silencing (MSUC), and we hypothesized that MSCI is an inevitable consequence of this more general meiotic silencing mechanism. Here, we provide direct evidence that asynapsis does indeed drive MSCI. We also show that a substantial degree of transcriptional repression of the sex chromosomes is retained postmeiotically, and we provide evidence that this postmeiotic repression is a downstream consequence of MSCI/MSUC. While this postmeiotic repression occurs after the loss of MSUC-related proteins at the end of prophase, other histone modifications associated with transcriptional repression have by then become established.

Animals↗
PubMed1988

Cytogenetic evidence for a fifth species within the taxon Anopheles dirus in Thailand.

Crossbreeding and chromosomal evidence are presented for the existence of a fifth sibling species within the taxon of Anopheles dirus in Thailand. The new species is morphologically identifiable as Anopheles balabacensis "Fraser's Hill form." Structural differences in mitotic chromosomes and extensive asynapsis in hybrid polytene chromosomes indicate that significant genetic divergence exists between this species and its closest relatives, An. dirus species A, B, C and D and An. balabacensis.

Animals↗
PubMed1994

Cytogenetic variation in natural populations of the Old World screwworm fly Chrysomya bezziana (Diptera: Calliphoridae).

The existence of sibling species in the Old World screwworm fly Chrysomya bezziana would raise serious problems in eradicating this pest if it entered Australia. Cytogenetic variation in C. bezziana was investigated by analyzing pupal trichogen polytene chromosomes. Natural populations of C. bezziana spanning its range from southern Africa to Papua New Guinea were examined as well as hybrids between a New Guinea laboratory strain and natural populations. No evidence of sibling species was found. All populations exhibited the same basic banding pattern as the standard sequence established from a Papua New Guinea strain. Extensive asynapsis of chromosome homologues was found in some hybrid crosses and was therefore measured in all populations and hybrids to detect systematic variation. Asynapsis levels in most hybrids could not be statistically distinguished from those present in the parent populations except for crosses between populations at the ends of the range. This result does not permit asynapsis levels to be used in establishing the origin of introduced flies by estimating their distance from known populations. One inversion polymorphism and six band polymorphisms spread over three chromosomes were analyzed. Populations in each sampled region had characteristic combinations of band polymorphisms. This may offer a diagnostic method for determining the origin of flies accidentally introduced to Australia.

Animals↗
PubMed1992

[Hybridization experiments using Anopheles minimus from Guangxi and Yunnan].

Hybridization experiments were conducted by forced mating between Anopheles minimus from Guangxi (G) and Yunnan (Y). F1 hybrid females were all fertile with normally developed ovaries. (Y female x G male) F1 males were all sterile, with abnormally enlarged testes. (G female x Y male) F1 males, though fertile, showed somewhat atrophic testes, and when they were back crossed with parental females, the latter produced eggs with very low hatching rate. Ovarian nurse cell polytene chromosomes from F1 hybrid females showed partial asynapsis in X chromosome as well as in autosonles, asynapsis at 38 zone in 3L being the most constant. It was obvious that there was definite, though partial, reproductive isolation between An minimus from Guangxi and that from Yunnan. Whether they are sibling species remains to be further investigated (Figs. 1-8).

Animals↗
PubMed1983

Experiments in crossing two strains of Anopheles barbirostris van der Wulp 1884 (Diptera: Culicidae) in Thailand.

Crossing experiments between two strains of Anopheles (Anopheles) barbirostris from Chumphon and Chon Buri provinces in Thailand were done by induced copulation in order to determine the genetic relationship. On comparison of the F1 hybrids and those of their parent species as the control, there was a difference in the number of eggs laid, hatchability and viability. The low viability of the F1 hybrids with high larval and pupal mortalities, producing only a few F hybrids, and the fact that F1 hybrids' salivary chromosome showed asynapsis suggest there exists reproductive isolations between both strains. The data presented suggest that these two strains exhibit possible presence of a species complex in An. barbirostris.

Animals↗
PubMed1996

[Homologue pairing: initiation sites and effects on crossing over and chromosome disjunction in Drosophila melanogaster].

The role of homologue pairing and chromocentral association of chromosomes in recombination and segregation during cell division is discussed. Peculiarities of mitotic and meiotic chromosome pairing in Drosophila males and females are considered. On the basis of our own and published data, the presence and localization of sites of homologue pairing initiation in euchromatin are substantiated. The effects of transfer of initiation sites along a chromosome (exemplified by inversions) on chromosome pairing (asynapsis), crossing over (intrachromosomal, interchromosomal, and centromeric effects), and segregation are discussed. To record the effects of pairing sites on crossing over, a method of comparing crossing-over frequencies in an inverted region with those in a region of the same size and position with regard to the centromere on cytological maps was proposed. Chromosomes orient toward opposite division poles during paracentromeric heterochromatin pairing. This occurs after successful euchromatin pairing, during which the chromocentral circular structure is reorganized. If heterochromatin pairing is disrupted because of structural or locus mutations, nonexchange bivalents segregate randomly. In this case, chromosome coordination may occur due to proximal chiasmata or chromocentral associations between homologues.

Animals↗
PubMed1998

An analysis of meiotic pairing in trisomy 21 oocytes using fluorescent in situ hybridization.

We report the use of chromosome 21-specific painting probes to analyze early stages of oogenesis in nine trisomy 21 fetuses. The proportion of cells in zygotene and pachytene in the trisomic ovaries ranged from 8 to 70% with a mean of 42% +/- 19 while the comparable values of euploid specimens ranged from 34 to 90% with a mean of 65% +/- 19. The low proportion of pairing cells may be the basis for the ovarian dysgenesis observed in some trisomy infants. Five percent of trisomic pachytene cells exhibited complete asynapsis which is an order of magnitude higher than that observed in euploid cells. A large fraction of the asynaptic cells were atretic which is consistent with the hypothesis of meiotic pairing as a signal for atresia. In addition, the asynaptic cells exhibited asynapsis of chromosomes other than 21, which we interpret as an interchromosomal effect of trisomy 21.

Chromosomes, Human, Pair 21↗
PubMed1997

The X bivalent in fetal bovine oocytes.

Wholemount preparations of oocytes from fetal bovine ovaries were examined in an attempt to study the incidence and type of chromosomes involved in pairing irregularities during different stages of early ovarian differentiation. Synaptonemal complexes exhibiting pairing irregularities were noted in meiocytes of all age groups. However, asynapsis and partial synapsis were more frequently noted in X chromosomes at the onset of meiosis in fetal bovine ovaries while the frequencies of similar errors in autosomes were relatively low and remained unchanged in the age groups included in this study. The significance and mechanisms of X chromosome asynapsis in excess of that expected on the basis of numerical ratio of the X to the bovine meiotic complement, are not known at present. We hypothesize that changes in the transcriptional status involving activation, inactivation, and reactivation of the X chromosomes during embryonic and ovarian differentiation on the conceptus, in addition to the inactivation undergone by the paternal X chromosome prior to fertilization, could be a factor rendering them susceptible to structural changes which, in turn may increase the incidence of sex chromosome asynapsis at the onset of meiosis in female fetuses.

Animals↗