Double insertion of homogeneously staining regions in chromosome 1 of wild Mus musculus musculus: effects on chromosome pairing and recombination

An examination of the meiotic pattern of chromosome 1 isolated from a feral mouse population and containing a double insertion (Is) of homogeneously staining regions (HSRs) was carried out. The region delineated by the proximal breakpoint of Is(HSR;1C5) 1Icg and the distal breakpoint of Is(HSR;1E3)2Icg is desynapsed during the early pachytene stage and heterosynapsed at the midpachytene, as shown by electron microscopic analysis of synaptonemal complexes. The HSRs have no effect on the segregation of chromosome 1 in heterozygous mice. The lack of homosynapsis in the region under study causes chiasmata redistribution in heteromorphic bivalents. In normal males, single chiasmata are located in the medial part of the chromosome. In heterozygotes, this segment is heterosynapsed and unavailable for recombination. This leads to a significant decrease in the frequency of bivalents bearing single chiasmata. The total number of chiasmata per bivalent is much higher in heterozygous males than in normal ones. The recombination frequency between proximal markers fz and In also is higher in heterozygous animals. The increase in the total chiasma number in the heteromorphic bivalent is due to the addition of double chiasmata located mostly at precentromeric and pretelomeric regions of the chromosome.     

Chromosomal control of chiasma distribution in house mice. Analysis of chiasma distribution in homo- and heterozygotes by inversion in chromosome 1

Examination of chiasma distribution in the chromosome 1 in male mice homo- and heterozygous for distal inversion In(1)12Rk and in normal mice was carried out. No differences in chiasma distribution was found between homozygotes for the inversion and homozygotes for normal chromosome 1. A drastic change in this trait was revealed in heterozygous animals. In heterozygotes, the telomeric segments of SC were asynapsed and unavailable for recombination. This leads to significant decrease in the frequency of bivalents bearing chiasmata in pretelomeric region. In turn, it produced chiasma redistribution in proximal noninverted portion of the bivalent 1. These results could be interpreted as evidence for chromosomal control of chiasma distribution pattern: the distance of certain part of the chromosome from telomere and interference (which also operates at the chromosomal level) are more important for determination of the chiasmata frequency in the given region, than its genetic content.     

Near-normal chiasma formation and maintenance in a short distal translocated segment in maize

Study of successful crossover pairing and chiasma formation is informatively extended to a very short translocated segment. Contrary to previous suggestion it now seems likely that the extreme distal region of the long arm of maize chromosome 1 is not deficient in intrinsic capacity for the initiation of crossover pairing. In addition, chiasmata formed in this short region appear to be efficiently maintained.     

Chiasma maintenance and terminalization across a homoeologous bivalent segment

In maize microsporocytes Heterozygous for a chromosome 2 interchange chromosome (which carries a homoeologous Tripsacum chromosome segment substituted for the distal half of its short arm), normal synapsis usually occurs at pachytene throughout the bivalent, but crossing over is almost entirely restricted to the homologous proximal region of the arm, (where it apprently occurs frequently). At diakinesis chiasmata were very often found to be located immediately proximal to an easily observable terminal knob of the Tripscacum chromosome segment. It was concluded that chiasmata, initiated in the homologous proximal region, had been maintained while terminalizing across the homoeologous region. It was also noted that heterozygosity for a telomere (and distal region) foreign to maize does not seem to inhibit pairing effective for crossing over in the homologous proximal portion of the chromosome arm.     

The lampbrush chromosomes of the chicken. Cytological maps of the macrobivalents

The lampbrush chromosomes (LBC) were prepared from growing oocytes 0.75-1.50 mm in diameter. A map of 6 autosomes and the ZW sex bivalents is presented. Several types of landmarks were noticed: lumpy loops (LL), telomeric bow-like loops (TBL), some large loops in interstitial regions (marker loops--ML). Supposedly, the centromeres of LBC in the chicken are at one of the axial bars bearing no loops. The landmarks PBL and DBL mark the proximal and distal boundaries of bars. LBC-A (probably, chromosome 1 of the chicken karyotype) is about 185 microns. There are 7.3 +/- 0.2 chiasmata. Chiasmata are distributed at quasi-random. In LBC-A one chiasma is localized in a telomere, as a rule. Coordinates of 13 of the 14 different landmarks in LBC-A have been estimated. LBC-B (probably, chromosome 2) is about 151 microns, there are 5.50 +/- 0.23 chiasmata. The LBC-B may be identified by LL-21 and LL-22. LBC-C (probably, chromosome 3) is 128 microns; there are 4.70 +/- 0.18 chiasmata. The chromosome can be identified by characteristic loops LL-31, an unlooped chromomere bar near the telomere (T-32), a characteristic distribution of normal loops along LBC-C: about one half of this LBC bears large loops, and the other one--small loops. LBC-D (chromosome 4?) is 107 microns; there are 3.80 +/- 0.31 chiasmata. Double-loop bridges appear frequently near ML-41. LBC-E (chromosome 5?) is about 72 microns with 2.50 +/- 0.28 chiasmata. There are characteristic TBL loops with abundant RNP material thus being like LL-loops. LBC-F (chromosome 8?) is about 36.5 microns; there are 2 chiasmata. This LBC can be identified by giant telomeric loops GML-F1 and by unlooped bar in the middle of LBC.     

Simultaneous negative and positive chiasma interference across the break point in interchange heterozygotes

The impossibility to obtain real roots from equations published earlier for estimating chiasma frequencies in the two translocated segments from configuration frequencies in interchange heterozygotes, was shown to be a result of lack of independence of chiasma formation. This is interpreted as negative interference. Similarly, negative interference could be shown to operate between the two interstitial segments. In all cases where a sufficient number of bivalents was formed by the interchange complex, chiasma frequency in the interstitial segments was strikingly higher in bivalents (having no chiasmata in the translocated segments) than in multivalents (with chiasmata in one or both translocated segments). This indicates strong positive interference between the interstitial and translocated segments.Negative interference between opposite-and positive interference between adjacent segments across the break point of the interchange occurred simultaneously in the cell populations. The phenomenon was attributed to complications in effective chromosome pairing at the point of partner exchange which in interchanges is determined by the breakpoint.The material was Secale cereale where five interchanges were analysed in a total of 12000 PMC's from 14 plants.     

Chiasmata distribution in the normal karyotype of mice

The number of chiasmata per cell and variance of chiasmata numbers were studied, as well as the recombinational interaction between different bivalents in CBA/Lac mice male line. No competition of bivalents for chiasmata was discovered in mice; at the same time, the chiasmata within one arm of the chromosome interfere with each other. The number of chiasmata per bivalent is estimated for each chromosome independently. The number of chiasmata per chromosome is limited both from below (minimum one chiasma independently of its size) and from above (positive interference of chiasmata).     

Meiotic recombination and spermatogenic impairment in Mus musculus domesticus carrying multiple simple Robertsonian translocations

We quantitatively analyzed the spermatogenic process, including evaluation of seminiferous tubules with defective cycles, rates of germ cell death and sperm morphology, in adult male mice with standard telocentric chromosomes (2n = 40, CD1 strain), homozygous (2n = 24, Mil II population) and heterozygous (2n = 24 x 40) for Robertsonian (Rb) rearrangements. The animals were analyzed at three different ages: three, five and seven months after birth. The number and position of crossover events were also determined by chiasmata counting and immunostaining with an antibody against mouse MLH1 protein. Our analysis of spermatogenesis confirms the impairment of the spermatogenic process in multiple simple heterozygotes due to both germ cell and abnormal sperm morphology. The detrimental effects exerted by Rb heterozygosities were found to be at least partially buffered with time: the frequency of defective tubules was lower and germ cell survival and sperm morphology better in 7-month-old animals than in the 3- and 5-month-old mice. While there are previously published data on germ cell death in multiple simple heterozygotes, this is the first report of a partial rescue of spermatogenesis with time. The mean frequency of MLH1 foci was lower in Rb homozygous and heterozygous mice than in mice carrying all telocentric chromosomes. The lower number of foci in Rb mice can be ascribed to a decrease in the number of multiple chiasmata and the maintenance of single chiasmata preferentially located in the terminal region of both the telocentric and metacentric chromosomes.     

Synapsis in Robertsonian heterozygotes and homozygotes of Dichroplus pratensis (Melanoplinae, Acrididae) and its relationship with chiasma patterns

Dichroplus pratensis has a complex system of Robertsonian rearrangements with central-marginal distribution; marginal populations are standard telocentric. Standard bivalents show a proximal-distal chiasma pattern in both sexes. In Robertsonian individuals a redistribution of chiasmata occurs: proximal chiasmata are suppressed in fusion trivalents and bivalents which usually display a single distal chiasma per chromosome arm. In this paper we studied the synaptic patterns of homologous chromosomes at prophase I of different Robertsonian status in order to find a mechanistic explanation for the observed phenomenon of redistribution of chiasmata. Synaptonemal complexes of males with different karyotypes were analysed by transmission electron microscopy in surface-spread preparations. The study of zygotene and early pachytene nuclei revealed that in the former, pericentromeric regions are the last to synapse in Robertsonian trivalents and bivalents and normally remain asynaptic at pachytene in the case of trivalents, but complete pairing in bivalents. Telocentric (standard) bivalents usually show complete synapsis at pachytene, but different degrees of interstitial asynapsis during zygotene, suggesting that synapsis starts in opposite (centromeric and distal) ends. The sequential nature of synapsis in the three types of configuration is directly related to their patterns of chiasma localisation at diplotene-metaphase I, and strongly supports our previous idea that Rb fusions instantly produce a redistribution of chiasmata towards chromosome ends by reducing the early pairing regions (which pair first, remain paired longer and thus would have a higher probability of forming chiasmata) from four to two (independently of the heterozygous or homozygous status of the fusion). Pericentromeric regions would pair the last, thus chiasma formation is strongly reduced in these areas contrary to what occurs in telocentric bivalents.     

A novel transchromosomic system: stable maintenance of an engineered Mb-sized human genomic fragment translocated to a mouse chromosome terminal region

Transchromosomic (Tc) technology using human chromosome fragments (hCFs), or human artificial chromosomes (HACs), has been used for generating mice containing Mb-sized segments of the human genome. The most significant problem with freely segregating chromosomes with human centromeres has been mosaicism, possibly due to the instability of hCFs or HACs in mice. We report a system for the stable maintenance of Mb-sized human chromosomal fragments following translocation to mouse chromosome 10 (mChr.10). The approach utilizes microcell-mediated chromosome transfer and a combination of site-specific loxP insertion, telomere-directed chromosome truncation, and precise reciprocal translocation for the generation of Tc mice. Human chromosome 21 (hChr.21) was modified with a loxP site and truncated in homologous recombination-proficient chicken DT40 cells. Following transfer to mouse embryonic stem cells harboring a loxP site at the distal region of mChr.10, a ~4 Mb segment of hChr.21 was translocated to the distal region of mChr.10 by transient expression of Cre recombinase. The residual hChr.21/mChr.10ter fragment was reduced by antibiotic negative selection. Tc mice harboring the translocated ~4 Mb fragment were generated by chimera formation and germ line transmission. The hChr.21-derived Mb fragment was maintained stably in tissues in vivo and expression profiles of genes on hChr.21 were consistent with those seen in humans. Thus, Tc technology that enables translocation of human chromosomal regions onto host mouse chromosomes will be useful for studying in vivo functions of the human genome, and generating humanized model mice.     

Chromosome Rearrangements That Involve the Nucleolus Organizer Region in Neurospora

In ~3% of Neurospora crassa rearrangements, part of a chromosome arm becomes attached to the nucleolus organizer region (NOR) at one end of chromosome 2 (linkage group V). Investigations with one inversion and nine translocations of this type are reported here. They appear genetically to be nonreciprocal and terminal. When a rearrangement is heterozygous, about one-third of viable progeny are segmental aneuploids with the translocated segment present in two copies, one in normal position and one associated with the NOR. Duplications from many of the rearrangements are highly unstable, breaking down by loss of the NOR-attached segment to restore normal chromosome sequence. When most of the rearrangements are homozygous, attenuated strands can be seen extending through the unstained nucleolus at pachytene, joining the translocated distal segment to the remainder of chromosome 2. Although the rearrangements appear genetically to be nonreciprocal, molecular evidence shows that at least several of them are physically reciprocal, with a block of rDNA repeats translocated away from the NOR. Evidence that NOR-associated breakpoints are nonterminal is also provided by intercrosses between pairs of translocations that transfer different-length segments of the same donor-chromosome arm to the NOR.     

Synapsis and chiasma distribution in mice heterozygous for translocations in chromosomes 16 and 17

Electron microscopic analysis of synaptonemal complexes and analysis of chiasmata distribution in male mice heterozygous for Robertsonian translocation T(16; 17)7Bnr - (Rb7), for synaptonemal reciprocal translocation T(16;17)43H - (T43), in double heterozygotes for these translocations and in males with partial trisomy of the proximal region of chromosome 17 was carried out. Synaptic disturbances around the breakpoints of the translocations, such as asynapsis of homologous regions of partners and non-homologous synapsis of centromeric regions of acrocentric chromosomes, were revealed. Synaptic regularity in the proximal part of the chromosome 17 appeared to be affected by no t12 haplotype. Good coincidence between sizes of mitotic chromosomes and corresponding lateral elements of synaptonemal complexes was found for all chromosomes, with the exception of Rb7 in trisomics. In the latter karyotype, the proximal part of chromosome 17 involved in Robertsonian fusion seems to be shortened in the course of zygotene and never synapted with homologous segment of neither the acrocentric chromosome 17 nor large product of reciprocal translocation. Drastic increase in chiasmata frequency in the proximal part of chromosome 17 was revealed in heterozygotes for T43H and in trisomics, as compared with the double heterozygotes Rb7/T43. The latter finding was explained by the existence of two independent pairing segments in the former karyotypes.     

The behaviour of chromosomal axes in Searle's X-autosome translocation

The sex vesicle-autosomal complex of mice heterozygous for Searle's X-autosome translocation has been reconstructed by serial sectioning and model building. The chromosomal axes of the five reconstructed models showed a characteristic pattern. The four axes present were identified as corresponding to: an unchanged autosome (A₁), the Y chromosome (Y) and the two translocation products, Xt, that has the X centromere, and A₂t that has an autosomal centromere. The axes of these translocated chromosomes have a mixed path, inside the sex vesicle and autosomal chromatin. The axes pair among themselves according to a pattern which agrees with that predicted by Ford and Evans (1964). It has been shown that the pairing region of the X chromosome of mice is the distal region and that the nucleolus is attached near its centromeric region. In some cells a slightly different pattern of the axes (type B) was observed. These cells have an anomalous synaptonemal complex between A₂t and Xt, that is, between portions of the X axis. It has been shown that autosomal chromatin becomes heteropycnotic in the proximity of the X–Y chromatin, and that this effect is stronger in the proximal part of A₂t. This effect explains the enlarged volume of the sex vesicle.     

Identification of DNA sequences flanking the breakpoint of human t(14q21q) Robertsonian translocations.

We have employed molecular probes and in situ hybridization to investigate the DNA sequences flanking the breakpoint of a group of t(14q21q) Robertsonian translocations. In all the families studied, the probands were patients with Down syndrome who carried a de novo t(14q21q) translocation. The DNA probes used were two alphoid sequences, alphaRI and alphaXT, which are specific for the centromeres of chromosomes 13 and 21 and of chromosomes 14 and 22, respectively; a satellite III sequence, pTRS-47, which is specific for the proximal p11 region of chromosomes 14 and 22; and a newly defined satellite III DNA, pTRS-63, which is specific for the distal p11 region of chromosome 14. The two alphoid probes detected approximately the same amount of autoradiographic signal on the translocated chromosomes as was expected for chromosomes 14 and 21 of the originating parent, suggesting that there has been no loss of these centromeric sequences during the translocation events. Results with the two satellite III probes indicated that the domain corresponding to pTRS-47 was retained in the translocated chromosomes, whereas the domain for pTRS-63 was lost. These results have allowed us to place the translocation breakpoint between the pTRS-47 and pTRS-63 domains within the p11 region of chromosome 14.     

Mapping of thec-sis oncogene on human chromosome 22 with respect to the breakpoint associated with chronic myeloid leukaemia

Chronic myeloid leukaemia (CML) cells are often characterized by the presence of a small chromosome 22, in which most of the q arm has been translocated to chromosome 9. Using cell hybrids containing different parts of chromosome 22 I have mapped the c-sis oncogene, which is known to be situated on chromosome 22, to a region distal to the CML breakpoint (22q112) and proximal to 22q13. This demonstrates that c-sis is translocated to chromosome 9 in CML cells.     

Genetic mapping: X chromosome

Starting with the male chiasma distribution for chromosome 2, a significantly better fit is obtained to lod scores for the X chromosome if terminalization of distal chiasmata is assumed. The linkage data are not consistent with a uniform distribution of chiasmata, absence of terminalization, or restriction of terminalization to the distal band. As information about the genetic map of the X chromosome increases, the map will be freed from assumptions about chiasma distribution. At present, however, even fragmentary data on the male are useful to construct a genetic map that, by converting physical assignments to equivalent genetic recombinations, has no inconsistencies between genetic and physical map orders.     

Pseudo-multiple formation as a consequence of prolonged non-homologous chromosome association in Metrioptera brachyptera

Multiple configurations involving the L ₃ and L ₄ homologues have been observed in all individuals from a population sample of Metrioptera brachyptera. These associations which persist up to and including metaphase of the first meiotic division are non-homologous and achiasmate in character. They are conditioned by the persistent association of large distally located heterochromatic blocks on the L ₃ and L ₄ chromosomes and are not, as White has proposed, the result of crossing over in translocated terminal duplications. When the L ₃ and L ₄ chromosomes form bivalents the distal heterochromatin restricts crossing over in this region and chiasmata are localised proximally. In other members of the complement chiasmata are localised at either or both the centric and telomeric ends. A relationship is demonstrated between the pattern of chiasma localisation and that of chromosome pairing.     

Zebrafish: chiasmata and interference.

With immunofluorescence microscopy, the positions of centromeres and MLH1 (MutL homolog) foci representing the sites of presumptive chiasmata are shown for zebrafish (Danio rerio Hamilton 1822) synaptonemal complexes (SCs) in spermatocyte nuclei at meiotic prophase. Most SCs have a single focus and a few (7 of 140) have 2 chiasmata. MLH1 foci tend to be in the distal regions of SCs, with progressively fewer occurring towards the middle of the SCs. This non-random distribution suggests chiasma interference. Synaptic initiation, as well as replication protein A (RPA) foci at the chromosome ends, correlates with the distal localization of MLH1 foci. These observations may provide the physical basis for the reported limited genetic recombination in the centromeric region of androgenetic offspring of a male.     

Chiasmata and chromosome breakages are related to crossing over in Drosophila ananassae males.

A cytogenetic analysis of male crossing over in Drosophila ananassae revealed that cytological exchanges resulted in genetic crossing over, and that chiasma frequency and the genetic recombination correlated positively in chromosomes 2 and 3. Furthermore, the frequency of chromosome breakages correlated positively with chiasma frequency. Paracentric inversion heterozygosity had no detectable influence on the chromosome pairing or exchange events within the inversion loop at meiosis. Scoring of the chiasma demonstrated that males homozygous for the previously mapped enhancers of male crossing over had low frequencies of chiasmata, whereas higher frequencies of chiasmata were observed in males heterozygous for enhancers. The results presented here indicate that the genetic factors controlling male crossing over are involved in the origin of chromosome breakages and in exchange events.     

Chiasma redistribution in the presence of different sized supernumerary segments in a grasshopper: dependence on nonhomologous synapsis.

Eight different sized supernumerary segments located at distal ends of the long arms of chromosomes M4, M5, M6, and S8 of the grasshopper Stenobothrus festivus were studied in males with regard to the synaptic process and chiasma distribution in the bivalents that carry them. The M4, M5, and M6 bivalents heterozygous for extra segments were always monochiasmate, in contrast to their bichiasmate condition observed in basic homozygotes. Furthermore, the presence of any of these extra segments led to chiasma redistribution in the carrier bivalents, so that such chiasmata were formed preferentially further away from the extra segment. The intensity of this effect is dependent on the size of the segment. Not all heteromorphic bivalents exhibited synaptonemal complexes with equalized axes at pachytene, but there was always a variable proportion of heterosynapsis around the distal ends of the long arms that was dependent on both the size of the segment and the size of the carrier chromosome. It is proposed that the absence of chiasmata in nonhomologous synapsed regions is responsible for the results obtained. Length measurements of the different extra segments and their carrier chromosomes between pachytene and diplotene indicated that synaptonemal complex is underrepresented in supernumerary heterochromatin.