Radiation-induced meiotic autosomal non-disjunction in male mice The effects of low doses of fission neutrons and X-rays in meiosis I and II of a Robertsonian translocation heterozygote

Male mice, heterozygous for the Rb(11.13)4Bnr translocation, were irradiated for 14.5 min with either a dose of 15-rad fission neutrons or 60-rad X-rays. Animals of this karyotype are known to show high levels of spontaneous autosomal non-disjunction (20–30%) after anaphase I. The effects of the irradiation on this process were determined after 2 and 3 in air-dried preparations.The length of the period from the end of meiosis I till the end of meiosis II was assessed autoradiographically, with the aid of cells showing a labelled Y chromosome only and appeared to last less than 3 h. Inter-mouse variation with regard to the duration of the period “last premeiotic S-phase till diakinesis/ methaphase I” prevented a more accurate estimate.On the basis of this 3-h datum, the induced effects were studied at intervals of 2 and 3 h after the start of the irradiation. The influence of irradiation was assessed by scoring: (1) univalents in primary spermatocytes, (2) deletions, aneuploid chromosome counts and precocious centromere separation in secondary spermatocytes, and (3) chromatid gaps and breaks in both cell types. Both radiation types induced comparable levels of chromosomal damage. A neutron-X-rays RBE value for these parameters was calculated to be 5.4 for the MI stage and 3.3 for the MII stage. The significantly higher incidence of cells showing damage at MII than at diakinesis/MI is not believed to indicate a difference in radiation sensitivity, but is believed to be merely the consequence of the different chromosomal processes taking place during the irradiation-fixation time interval.     

Defective imprint resetting in carriers of Robertsonian translocation Rb (8.12)

Meiotic silencing of unsynapsed chromatin (MSUC) occurs in the germ cells of translocation carriers and may cause meiotic arrest and infertility. We hypothesized that if bypassing meiotic checkpoints MSUC may cause epigenetic defects in sperm. We investigated the meiotic behavior of the Robertsonian translocation Rb (8.12) in mice. The unsynapsed 8 and 12 trivalent was associated with the XY body during early and mid-pachynema in heterozygous Rb (8.12) carriers, suggesting possible silencing of pericentromeric genes, such as the Dnmt3a gene. In wild-type mice, DNMT3A protein showed a dramatic accumulation in the nucleus during the mid-pachytene stage and distinct association with the XY body. In translocation carriers, DNMT3A was less abundant in a proportion of pachytene spermatocytes that also had unsynapsed pericentromeric regions of chromosomes 8 and 12. The same mice had incomplete methylation of the imprinted H19 differentially methylated region (DMR) in sperm. We propose that impaired H19 imprint establishment results from lack of synapsis in chromosomes 8 and 12 probably through transient silencing of a chromosome 8 or 12 gene during pachynema. Furthermore, our findings support the notion that imprint establishment at the H19 locus extends into pachynema.     

Robertsonian heterozygosity in wild mice: fertility and transmission rates in Rb(16.17) translocation heterozygotes

Breeding data show that there is no distortion of the transmission ratio of chromosomal types in the progeny of wild mice heterozygous for Rb(16.17) nor was litter size significantly affected by chromosomal heterozygosity. The relevance of these results to those obtained with artificial hybrids and to processes of chromosomal differentiation are discussed.     

Wild-derived Robertsonian translocation in mice. Chromosome 17, Rb (16:17)7, shows novel interactions with t-alleles

We have studied the effects of wild-derived (Rb7) and laboratory-derived (Rb1) Robertsonian translocations involving chromosome 17 on t-complex determined transmission ratio distortion and crossing-over suppression in mice. The Rb7 chromosome is significantly unlike all other wild-type chromosome 17s tested, while Rb1 is not. t0/Rb7 males are uniformly extremely high distorters (greater than 96 percent) while th2/Rb7 males are uniformly extremely low distorters. t0/Rb7 animals allow genetic recombination in the centromere to t-lethal region interval. These observations could be explained if the Rb7 chromosome contains one or more t-like regions.     

The use of Robertsonian translocations in the mouse for studies on non-disjunction

In the mouse, gametes with gross chromosome duplications and deficiencies can complement each other to give viable zygotes (with some notable exceptions involving particular chromosomes). These complementation-type offspring can be recognised in intercrosses between translocation heterozygotes in which one parent is homozygous for a recessive genetic marker not carried by the other. This system has beeN used by Lyon and colleagues (1976) to study non-disjunction in heterozygotes for tobacco mouse and laboratory-derived Robertsonian translocations. Although non-disjunction is frequent in the former group, still higher frequencies are needed for a workable test system in which wild type mice are treated and mated to a tester stock generating many aneuploid gametes. Possible approaches include (1) use of semidominant markers, (2) marking both arms, (3) combining two or three independent Robertsonians in the tester stock, (4) use of compounds of Robertsonians wih monobrachial homology, since these give very high frequencies of non-disjunction, (5) generation of a compound of three Robertsonians with tribrachial homology, which should produce aneuploid gametes only. This last seems the most promising approach, if the compound proves fertile, and would be analogous to the isochromosome system of Drosophila.     

Spam1 (PH-20) mutations and sperm dysfunction in mice with the Rb(6.16) or Rb(6.15) translocation

In mice bearing the Rb(6.16) or Rb(6.15) Robertsonian translocation (Rb), sperm dysfunction associated with the Rbs has been shown to lead to transmission ratio distortions (TRDs) in heterozygotes. The severity of the TRDs is directly related to the severity in the alteration of expression of the gene for the Sperm Adhesion Molecule 1 (Spam1), which maps to proximal mouse Chromosome 6 (Chr 6) near the translocation junction and encodes a sperm antigen with hyaluronidase activity. Here we demonstrate that there is a significantly reduced fertility in the Rb homozygotes (P < 0.001), based on litter size; and that with the Sperm Select Penetration assay Rb-bearing sperm have significantly decreased (P < 0.02–0.001) rates of penetration of hyaluronic acid. Catalytic kinetics studies indicate that reduced Spam1 (PH-20) hyaluronidase activity in the Rb(6.15) mice results from a qualitative defect, while for Rb(6.16) with the greater TRD both a qualitative and a quantitative deficiency (confirmed by Western analysis) of Spam1 exist. Six point mutations were shown to be clustered in the Spam1 hyaluronic acid-binding domain in Rb(6.15). For Rb(6.16) which has a gross genomic alteration at the Spam1 locus, 11 point mutations are scattered in the 5′ and 3′ UTRs and the coding region, where one leads to the replacement of a conserved residue. Entrapment of spontaneous Spam1 mutations, owing to recombination suppression near the Rb junctions, is proposed as the major underlying defect of the sperm dysfunction. Received: 19 April 2001 / Accepted: 5 July 2001     

A first exploration of a Robertsonian translocation heterozygote in the mouse for its usefulness in cytological evaluation of radiation-induced meiotic autosomal non-disjunction.

In this report some data concerning the male meiotic system of mice heterozygous for Rb(11.13)4Bnr are presented and compared with those of a chromosomally normal Swiss random-bred stock. Change of the genetic background from a C3H/Swiss hybrid situation to the fourth backcross generation (to the Swiss random-bred stock), did not alter the average frequency of aneuploid secondary spermatocytes. This was confirmed by studies on post-implantation loss. Spermatogenic characteristics of Rb4/+ mice, such as testis weight, sperm production and the number of diplotene-metaphase-I figures found in stage XII of the seminiferous epithelium, suggest delay and cell death during this period. These data support our working hypothesis that such an aberrant chromosome system may be more prone to radiation effects and therefore is promising in our cytological studies into the causes of spontaneous and in our cytological studies into the causes of spontaneous and induced autosomal non-disjunction during meiosis in the mouse.     

A new type of nonhomologous synapsis in T(X;4)1R1 translocation male mice

The synaptonemal complexes of T(X;4)1R1 (abbreviated R1) translocation heterozygotes have been examined by electron microscopy and compared with those of two X-7 translocations: R5 and R6. The X chromosome breakpoint of R1 is estimated to lie between 78 and 82% from the proximal end of the X, in the same general region as the R5 and R6 breakpoints. The position of the autosomal breakpoint of R1, like that of R6, is about 30% from the proximal end of the respective autosome. R1 is also similar to R6 in that there is extensive nonhomologous synapsis both in quadrivalents and heteromorphic bivalents. We have recently found that the location of breakpoints with respect to the position of the G-bands appears to be related to the synaptic behavior seen in translocation heterozygotes. If both breaks of a reciprocal translocation lie in G-light bands, as was the case with R5, synapsis is confined to homology. However, if one break lies in or immediately adjacent to a G-dark band, there is nonhomologous synapsis, as occurs with R1 and R6. Comparison of the synaptic behavior of R1 with R5 and R6 leads to the conclusion that this G-band-related nonhomologous synapsis is of a different type than the "synaptic adjustment" phenomenon that has been described by Moses (1977a). This G-band-related nonhomologous synapsis is not substage-specific, but competes with homologous synapsis during zygotene-early pachytene.     

The consequences of an unusual Robertsonian translocation in celery (Apium graveolens var dulce)

A Robertsonian translocation found in a cultivar of celery is described which is unusual because half the small product of the exchange survives as a telocentric chromosome. The change has no drastic effect on the overall fitness of the translocation homozygote. The consequences of such a change for karyotype evolution are shown to be, karyotype asymmetry, the possible genesis of B-chromosomes and the possibility of a chromosome change becoming established even though it has no adaptive value.     

The influence of females on male territorial dominance and female preference in dwelling place in laboratory mice

The influence of female mice (Mus musculus) on intermale aggression and the female choice between dominant and subordinate males were tested. Normal male mice were paired with a castrated male or a female. Two pairs of the same kind were housed together after acclimation for 3 days in half (home cage) of the apparatus, which was composed of eight cages connected by tunnels. One of the two normal males became dominant. The dominant males became more aggressive in the presence of females than in the presence of castrated males. The two normal males dwelt in their own home cages. The two females dwelt together and preferred to dwell in the home cages of the dominant male. The two castrated males dwelt together and seemed to decide their dwelling cage more freely than the subordinate normal male. Received: August 20, 1999 / Accepted: December 6, 1999     

Nondisjunction and transmission ratio distortion ofChromosome 2 in a (2.8) Robertsonian translocation mouse strain

Aneuploidy results from nondisjunction of chromosomes in meiosis and is the leading cause of developmental disabilities and mental retardation in humans. Therefore, understanding aspects of chromosome segregation in a genetic model is of value. Mice heterozygous for a (2.8) Robertsonian translocation were intercrossed with chromosomally normal mice and Chromosome 2 was genotyped for number and parental origin in 836 individuals at 8.5 dpc. The frequency of nondisjunction of this Robertsonian chromosome is 1.58%. Trisomy of Chromosome 2 with two maternally derived chromosomes is the most developmentally successful aneuploid karyotype at 8.5 dpc. Trisomy of Chromosome 2 with two paternally derived chromosomes is developmentally delayed and less frequent than the converse. Individuals with maternal or paternal uniparental disomy of Chromosome 2 were not detected at 8.5 dpc. Nondisjunction events were distributed randomly across litters, i.e., no evidence for clustering was found. Transmission ratio distortion is frequently observed in Robertsonian chromosomes and a bias against the transmission of the (2.8) Chromosome was detected. Interestingly, this was observed for female and male transmitting parents.     

The origin of a Robertsonian chromosomal translocation in house mice inferred from linked microsatellite markers

The western European house mouse, Mus domesticus, includes many distinct Robertsonian (Rb) chromosomal races. Two competing hypotheses may explain the distribution of Rb translocations found in different populations: they may have arisen independently multiple times, or they may have arisen once and been spread through long-distance dispersal. We investigated the origin of the Rb 5.15 translocation using six microsatellite loci linked to the centromeres of chromosomes 5 and 15 in 84 individuals from three Rb populations and four neighboring standard-karyotype populations. Microsatellite variation on the 5.15 metacentric chromosomes was significantly reduced relative to the amount of variation found on acrocentric chromosomes 5 and 15, suggesting that linked microsatellite loci can track specific mutational events. Phylogenetic analyses resulted in trees which are consistent with multiple origins of the 5.15 metacentric chromosomes found in the three Rb populations. These results suggest that cytologically indistinguishable mutations have arisen independently in natural populations of house mice.     

Molecular detection of a translocation (Y;15) in a 45,X male

Summary A 45,X male individual was shown to have a translocation of Y-chromosome material to the short arm or proximal long arm of chromosome 15. This translocation was detected by genomic DNA blotting and in situ hybridization with Y-chromosome-specific DNA probes.