Unequal nondisjunction frequencies of trivalent chromosomes in male mice heterozygous for two Robertsonian translocations

Robertsonian (Rb) translocation heterozygosity may cause pairing problems during prophase and segregation irregularities at anaphase of meiosis I. These stages of meiosis I were studied in male mice doubly heterozygous for the two Rb chromosomes Rb(9.19)163H and Rb(16.17)8Lub. At pachytene both Rb chromosomes similarly showed pairing irregularities like unpaired segments. However, highly different nondisjunction frequencies of chromosomes forming the respective trivalents were found. The nondisjunction frequency of the Rb8Lub trivalent chromosomes was about 40%, whereas a very low frequency of nondisjunction was found in combination with the Rb163H trivalent. Since both trivalents were together in the same cell, differences in kinetochore function are assumed to be responsible for the diverse frequency of nondisjunction.     

Non-random patterns of non-disjunctional orientation in trivalents of multiple Robertsonian heterozygotes of Dichroplus pratensis (Acrididae)

Metaphase I orientation of centric fusion trivalents was studied in 24 single, 19 double and 3 triple heterozygotes of Dichroplus pratensis. Different populations of this South American melanopline grasshopper are polymorphic for seven Robertsonian fusions, and the polymorphisms seem to be stable. Several cytogenetic factors involved in the orientation and segregation of the meiotic configurations such as chromosomal length, symmetry and number and position of chiasmata, have been analysed in previous works. In this paper we study another factor that is relevant in the above respect in individuals with more than one heterozygous fusion: interaction among configurations regarding orientation.Our results indicate that, when there are two or three trivalents present in the MI cell, there is an interaction in such a way that the number of metaphases in which the two or three trivalents are non-disjunctionally oriented is always significantly higher than expected under a hypothesis of independence. However, the number of cells in which all trivalents are disjunctionally oriented does not decrease significantly, so an increase of unbalanced gametes due to this factor is not expected. The stability of the polymorphisms would thus not be affected.Both authors are affiliated with the CONICET (Argentina)     

Meiotic behavior of Robertsonian heterozygotes in populations of Dichroplus pratensis (Acrididae) with different fusion frequencies

The meiotic behavior of Robertsonian heterozygotes of Dichroplus pratensis was analyzed in order to establish the nature of the fusion polymorphisms (stable or transient) that occur in the species. The range of fusion frequencies varies widely for each fusion studied and populations with extreme frequencies exist, which could indicate a tendency for the loss or fixation of a given rearrangement. Our results revealed that no significant correlation exists between orientation (convergent and non-convergent) in PMI and MI and fusion frequency, nor between aneuploidy and fusion frequency. Thus, orientation and segregation seem to be independent of the frequencies of the fusions, which also do not appear to affect severely the fertility of heterozygotes. Our data suggest that the polymorphisms are, at least, cytogenetically stable.Both authors are affiliated to the CONICET (Argentina)     

Metaphase I orientation of Robertsonian trivalents in the water-hyacinth grasshopper, Cornops aquaticum (Acrididae, Orthoptera)

Trivalents resulting from polymorphic Robertsonian rearrangements must have a regular orientation in metaphase I if the polymorphisms are to be maintained. It has been argued that redistribution of proximal and interstitial chiasmata to more distal positions is necessary for a convergent orientation, the only one that produces viable gametes. Cornops aquaticum is a South-American grasshopper that lives and feeds on water-hyacinths, and has three polymorphic Robertsonian rearrangements in its southernmost distribution area in Central Argentina and Uruguay. The orientation of trivalents in metaphase I, the formation of abnormal spermatids and the frequency and position of chiasmata in the trivalents, was analysed in a polymorphic population of C. aquaticus. In this study we observed a correlation between the number of trivalents with the frequency of abnormal spermatids; additionally, the number of chiasmata, especially proximal and interstitial ones, was strongly correlated with the frequency of the linear orientation. Therefore we confirmed our previous assumption, based on other evidence, that the chiasmata redistribution in fusion carriers is essential to the maintenance of the polymorphisms.     

Inheritance and meiotic behaviour of a de novo chromosome fusion in the aphid Myzus persicae (Sulzer)

A de novo tandem fusion between autosomes 2 and 3 (A2+3), arising in the course of laboratory crosses of sexual morphs of two clones of the aphid Myzus persicae, was stable through more than 180 generations of parthenogenetic (clonal) reproduction. Studies of its inheritance through the sexual phase, and segregation from an amplified esterase marker gene, showed that crossing over occurred during oogenesis, but not in spermatogenesis, confirming previous cytological observations. Only a small number of progeny resulted from attempts at selfing fusion heterozygotes, and none of these was homozygous for the fusion. A2+3 paired in parallel alignment with the separate A2 and A3 to form a trivalent at prophase I of spermatogenesis. Fusion heterozygotes had a segregation problem at anaphase I of meiosis, A2+3 forming a chromatin bridge between the daughter spermatocytes in about 42% of dividing cells, which could be attributed to alternate orientation in the trivalent (A2 and A3 paired with opposite sides of A2+3) in the preceding metaphase I. Males heterozygous for an A2 dissociation were also studied and found to have much less of a segregation problem, despite showing similar orientation patterns at metaphase I. Possible reasons for this difference and the significance of the findings in relation to karyotype evolution in aphids are discussed.     

Meiotic chromosome segregation in human t(11;22)(q23;q11) carriers: a theoretical consideration

The t(11;22)(q23;q11) translocation is the most frequently encountered familial reciprocal translocation in humans. In the majority of reported cases ascertainment has been through the birth of a child with the chromosomal constitution 47,XX,+der(22) or 47,XY,+der(22), i.e., tertiary trisomy. Previous segregation analysis of familial cases showed a number of interesting features. Thus, euploid unbalanced genotypes resulting from adjacent segregation are absent in the progeny, and only tertiary trisomic offspring are recovered. To explain this unusual progeny output we present here a model for the meiotic behavior of this translocation in the carriers based on an analysis of cytogenetic data of progeny of carriers. This model predicts the formation of a chain trivalent with chromosome order 11-der(11)-22 during prophase I and its predominant alternate orientation at metaphase I.     

The effect of multiple simple Robertsonian heterozygosity on chromosome pairing and fertility of wild-stock house mice (Mus musculus domesticus)

The influence of Robertsonian (Rb) heterozygosity on fertility has been the subject of much study in the house mouse. However, these studies have been largely directed at single simple heterozygotes (heterozygous for a single Rb metacentric) or complex heterozygotes (heterozygous for several to many metacentrics which share common chromosome arms). In this paper we describe studies on male multiple simple heterozygotes, specifically the F(1) products of crosses between wild-stock mice homozygous for four or seven metacentrics and wild-stock mice with a standard all-acrocentric karyotype; these F(1) products were characterized by four and seven trivalents at meiosis I, respectively. Mice with the same karyotype, but two different genetic backgrounds were examined. Although a range of meiotic and fertility studies were conducted, particular emphasis was paid to analysis of chromosome pairing, previously not well-described in multiple simple heterozygous mice. The progression of spermatocytes through prophase I was followed by electron microscopy of surface spread material. As previously shown for single simple Rb heterozygotes, the trivalents that characterize multiple simple heterozygotes initially showed delayed pairing of the centromeric region and later showed side arm formation, resulting from non-homologous pairing by the centromeric ends of the acrocentric chromosomes. In the four trivalent groups of mice, 15 and 32% of trivalents showed unpairing in the centromeric region at mid pachytene; equivalent values were 29 and 39% for the seven trivalent groups. Pairing abnormalities (largely attachments and interlocks between trivalents and between a trivalent and the XY configuration) were observed in 18 and 23% of mid pachytene cells in the four trivalent groups and 36 and 49% of cells in the seven trivalent groups. The greater level of pachytene irregularity (unpairing and pairing abnormalities) in seven versus four trivalent heterozygotes was mirrored in terms of higher anaphase I nondisjunction frequency and lower germ cell counts. However, while pachytene irregularities appear to contribute to germ cell death, examples of male sterility in our material undoubtedly also involve genic incompatibilities.     

Male meiosis and behaviour of sex chromosomes in different populations of Rumex acetosa L. from the Western Himalayas, India

Detailed meiotic analysis in 28 North West Himalayan populations of dioecious plant Rumex acetosa L. was carried out. The species is generally discussed as an important plant having sex chromosomes. Male meiosis in all the studied populations clearly showed the formation of six bivalents and one trivalent during diakinesis and metaphase-I. The sex chromosomes in male plants exhibit a chain of trivalent (Y₁–X–Y₂). In addition, among the presently investigated populations ring-shaped trivalents were also observed for the first time in the species. Varied frequency of abnormal segregation of sex trivalent was also observed leading to XY:Y segregation instead of normal X:Y₁Y₂ segregation. A majority of the populations exhibit normal meiosis. Plants of six populations show meiotic abnormalities like cytomixis, laggards, bridges, chromatin stickiness, etc., leading to reduced pollen fertility. Translocation between an autosome and sex chromosomes was also observed in some of the populations. 0–1B chromosomes were noticed in one population. This is the first ever meiotic analysis of the species from India.     

A chromosomal analysis of some water beetle species recently transferred from Agabus Leach to Ilybius Erichson, with particular reference to the variation in chromosome number shown by I. montanus Stephens (Coleoptera: Dytiscidae)

The karyotypes of seven Ilybius species are described and illustrated. All except I. wasastjernae have a basic karyotype of 34 autosomes plus sex chromosomes which are X0 ( male symbol ), XX ( female symbol ), with the X chromosome among the largest in the nucleus. This karyotype appears to be the norm for Ilybius and supports the transfer of the species concerned from Agabus to Ilybius. I. wasastjernae has 36 autosomes and the X chromosome is the smallest in the nucleus and its karyotype is unlike any other known karyotype in either Ilybius or Agabus. In most of the species studied no intraspecific variation has been detected. Exceptions are I. chalconatus, where there is one inversion polymorphism in one of the autosomes, and I. montanus whose autosome number has been found to vary from 29 to 34. Such variation is highly unusual among Coleoptera. The variation results from fusion-fission polymorphisms involving three different pairs of autosomes. In each case the fusions may be homozygous, heterozygous or absent. All populations investigated were polymorphic for some of the fusions, but only one (La Salceda, Spain) included individuals lacking all fusions. The frequencies of fused and unfused chromosomes were analysed in three English populations. In only one case was there a departure from the values expected from the Hardy-Weinberg equilibrium, and this population also showed a significant difference from the other two. Meiosis in males heterozygous for fusions involves the production of trivalents in first division, but results in the production of abundant sperm, with no evidence of chromosomal abnormalities in second metaphase, or of degenerating cells as a result of failed meiosis. The three fusions sites are consistent in all the populations studied, and it is concluded that these fusions represent unique historical events rather than current chromosomal instability.     

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.     

An analysis of meiotic impairment and of sex chromosome associations throughout meiosis in XYY mice

The existing XYY meiotic data for mice present a very heterogeneous picture with respect to the relative frequencies of different sex chromosome associations, both at pachytene and diakinesis/metaphase I. Furthermore, where both pachytene and diakinesis/MI data are available for the same males, the frequencies of the different configurations at the two stages are very different. In the present paper we utilise "XYY" and "XY/XYY" mosaic mice with cytologically distinguishable Y chromosomes to investigate the factors responsible for this heterogeneity between different males and between the two meiotic stages. It is concluded (1) that the initial pattern of synapsis is driven by the relatedness of the three pseudoautosomal regions (PARs); (2) that the order and extent of PAR synapsis within radial trivalents are also affected by PAR relatedness and that this leads to chiasmata being preferentially formed between closely related PARs; (3) that trivalents with a single chiasma resolve into a bivalent + univalent by the diakinesis stage; (4) that although many spermatocytes with asynapsed sex chromosomes are eliminated between pachytene and diakinesis, those that survive this phase of elimination progress to the first meiotic metaphase (MI) and accumulate in large numbers, leading to an over-representation of those with univalents as compared to radial trivalents; and (5) that the arrested MI cells are eventually eliminated, so that very few "XYY" cells contribute products to MII.     

TRIVALENT FORMATION IN MULTIPLE TRISOMICS OF CLARKIA UNGUICULATA

Vasek, F. C. (U. California, Riverside.) Trivalent formation in multiple trisomics of Clarkia unguiculata. Amer. Jour. Bot. 50(3): 244–247. 1963.—A series of multiple trisomics, ranging from 2n + 1 to 2m + 7, was scored for univalents and trivalents at first metaphase. The mean number of trivalents per cell per extra chromosome was 0.43, 0.46, 0.46, 0.56, 0.54, 0.55 and 0.59 for plants with 1, 2, 3, 4, 5, 6 and 7 extra chromosomes respectively. These trivalent frequencies are interpreted to fall into 2 classes, indicating that the cytological differences between triploids and single trisomies may be based on a threshold effect. The results are compared with available data from other genera, and it is suggested that the observed increase in trivalent formation per extra chromosome may be associated with an increase in chiasma frequency.     

Synaptonemal complex pairing and metaphase I association in a telo-substituted telotrisomic of rye (Secale cereale L.)

The synaptonemal complexes in pollen mother cells (PMCs) of rye in which one chromosome 1R was replaced by the two corresponding telocentrics, and where one additional telocentric 1RS was present, showed approximately the expected 21 ratio of 1R-1RL-1RS trivalent with 1RS univalent versus heteromorphic 1R-1RL bivalent with 1RS bivalent. In addition, however, many cells with a partner exchange were found, several even including bivalents other than 1R. At metaphase I1R-1RL-1RS trivalents predominated, cells with two univalent telocentrics were relatively frequent but partner exchange configurations were extremely rare. It is concluded that the almost consistent failure to form chiasmata in the interstitial region of 1RS after partner exchange, combined with much more frequent chiasma formation in the terminal segment, is the main reason for the unexpected metaphase I configuration frequencies. Possible causes are discussed. The shift observed does not yet explain the erratic variation in relative frequencies of metaphase I configurations reported earlier in similar material. Frequent pairing partner exchange may play a role there also.     

Meiosis in autopolyploid Crepis capillaris

Meiotic chromosome pairing of triploid and trisomic Crepis capillaris was analysed by electron microscopy in surface-spread prophase I nuclei and compared with light microscopic observations of metaphase I. This system allows identification and separate analysis of each chromosome of the C. capillaris genome. Prophase I trivalent frequencies are very high in all three trisomes and only slightly dependent on chromosome size. At metaphase I, on the other hand, trivalent frequencies are much lower and strongly dependent on chromosome size. There is no evidence for trivalent elimination during prophase I in this system, and the reduction in trivalent frequency at metaphase I can be explained by an insufficiency of appropriately placed chiasmata. The high prophase I trivalent frequencies far exceed the two-thirds expected on a simple model with two terminal independent pairing initiation sites per trisome, suggesting that multiple pairing initiation occurs. Direct observations reveal high frequencies of pairing partner switches (PPSs) in prophase I trisomes, which confirms this supposition. The numbers of PPSs per trisome shows a better fit to the Poisson than to the binomial distribution and their positional distribution along trisomes is random and non-localized. All these observations favour a model of pairing initiation in trisomes based on a large number of evenly distributed autonomous pairing sites each with a uniform and low probability of generating a PPS.by C. Heyting     

Sex-dependent synaptic behaviour in triploid turbot,Scophthalmus maximus (Pisces,Scophthalmidae)

A surface-spreading synaptonemal complex (SC) technique was used to analyse spermatocytes and oocytes of triploid turbot (Scophthalmus maximus) in order to visualise the process of chromosome synapsis. The most conspicuous characteristic of triploid oocytes is that, in the trivalents, the lateral elements of the SC were frequently associated in threes, either completely along the length of the trivalent, or partially, forming a variety of forked structures. In these nuclei, synapsis usually occurred among homologous chromosomes and the number of bivalents observed was significantly higher than that expected under the assumption of random chromosome association among all partners. However, the frequency of trivalents was very low in triploid spermatocytes, triple synapsis being also scarce. In these nuclei chromosomes that were excluded from homologous synapsis become engaged in random SC formation, and, therefore a considerable number of non-homologous associations are produced. The causes of the synaptic differences observed in triploid males and females of turbot and their possible relation to the sterility displayed by these animals are discussed.     

Immunocytogenetics. III. Analysis of trivalent and multivalent configurations in mouse pachytene spermatocytes by human autoantibodies to synaptonemal complexes and kinetochores

Mice heterozygous for one or more Robertsonian (Rb) translocation chromosomes have been used to analyze synaptonemal complex (SC) configurations and kinetochore arrangements in trivalents and multivalents. Rb heterozygosity without arm homologies leads to the formation of heteromorphic trivalents in meiosis I; alternating homology of the chromosome arms produces ringlike or chainlike multivalents. Immunofluorescence double-labeling with human antibodies to SCs and kinetochores was performed on surface-spread pachytene spermatocytes. Both Rb bivalents and Rb trivalents clearly showed that metacentrics possess only one centromere. In heteromorphic trivalent SCs, the nonhomologous kinetochores of the two acrocentrics were closely paired in a cis-configuration and juxtaposed opposite the kinetochore of the metacentric; the latter appeared to be an integral part of the longitudinal SC axis. Meiotic multivalents of interpopulation hybrids included up to 36 chromosome arms. In multivalent SCs, the kinetochores always lay together, with the SC arms arranged away from the central centromere cluster. The paracentromeric regions of the Rb chromosomes appeared to remain unsynapsed on both sides of the centromeres. The SC arms were often linked by end-to-end associations. Following desynapsis of the multivalent SC, the kinetochores of the Rb metacentrics showed a highly nonrandom topologic distribution within the nucleus, reminiscent of their arrangement during synapsis.     

Chromosome-wide control of meiotic crossing over in C. elegans

A central event in sexual reproduction is the reduction in chromosome number that occurs at the meiosis I division. Most eukaryotes rely on crossing over between homologs, and the resulting chiasmata, to direct meiosis I chromosome segregation, yet make very few crossovers per chromosome pair. This indicates that meiotic recombination must be tightly regulated to ensure that each chromosome pair enjoys the crossover necessary to ensure correct segregation. Here, we investigate control of meiotic crossing over in Caenorhabditis elegans, which averages only one crossover per chromosome pair per meiosis, by constructing genetic maps of end-to-end fusions of whole chromosomes. Fusion of chromosomes removes the requirement for a crossover in each component chromosome segment and thereby reveals a propensity to restrict the number of crossovers such that pairs of fusion chromosomes composed of two or even three whole chromosomes enjoy but a single crossover in the majority of meioses. This regulation can operate over physical distances encompassing half the genome. The meiotic behavior of heterozygous fusion chromosomes further suggests that continuous meiotic chromosome axes, or structures that depend on properly assembled axes, may be important for crossover regulation.     

棕色田鼠罗伯逊易位的研究(简报)

棕色田鼠(Microtus mandarinus Milne-Edwards,1871)又称北方田鼠,主要分布于我国。前苏联、蒙古的少数地区亦有分布,前苏联学者称该鼠为中国田鼠。关于该鼠的染色体研究国内外已有报道。仅推测罗伯逊易位是引起该鼠第一对常染色体多态及其染色体数目多态的主要原因。本文详细研究了该鼠第一对常染色体多态类型与个体染色体数目之间的一一对应关系、性个体(2n=51)的G带带型及其联会复合作中三价体的存在,完全证实了     

Impact of Robertsonian translocation on meiosis and reproduction: an impala (Aepyceros melampus) model

The captive bred animal populations showing centric fusion polymorphism can serve as a model for analysis of the impact of the rearrangement on meiosis and reproduction. The synapsis of homologous chromosomes and the frequency and distribution of meiotic recombination events were studied in pachytene spermatocytes of captive bred male impalas (Aepyceros melampus) polymorphic for der(14;20) by immunofluorescent analysis and fluorescence in situ hybridization. The chromosomes 14 and 20 involved in the centric fusion were significantly shorter due to the loss of sat I repeats indicating ancient origin of the rearrangement. The fused chromosome and the normal acrocentric chromosomes 14 and 20 formed trivalent in pachynema which showed either protruding proximal ends of the acrocentric chromosomes or single axis with synaptic adjustment in the pericentromeric region. There was no significant difference in the number of recombination events per cell between the group of translocation heterozygotes and the animals with normal karyotype. A significant reduction in the number of recombination events was observed in the trivalent chromosomes compared to the normal chromosomes 14 and 20. The level of the recombination reduction was related to the trivalent configuration. The centric fusion der(14;20) was not apparently demonstrated by any spermatogenic defects or reproductive impairment in heterozygous impalas. However, the high incidence of the chromosomal polymorphism within the captive bred population shows the importance of cytogenetic examinations in captive breeding and wildlife conservation programs, especially in the case of reintroduction of the endangered species.     

Cytogenetic control of a hybrid zone between two <Emphasis Type="Italic">Sorex araneus</Emphasis> chromosome races before breeding season

Two chromosome races of common shrew, Moscow and Seliger, differ in the arm combination in 11 diagnostic chromosomes (Robertsonian metacentrics/acrocentrics). Homozygotes of both pure races, simple Robertsonian heterozygotes of Seliger race, and complex heterozygotes (F₁ hybrids) were detected in the found earlier hybrid zone of these races, in the spring before the breeding season. The g/o heterozygote was first discovered in race Seliger, whose chromosome formula typically contains acrocentrics g and o. The m/q heterozygote was recorded for the second time. Meiosis was studied in 16 males representing five detected karyotypic categories. No abnormal in pairing of homologs in either sex trivalent common for the species (XY1Y2) or autosome trivalents (g/o and m/q) was detected at diakinesis-metaphase I. Two hybrids displayed a theoretically expected and unimpaired meiotic configuration in a form of a very long chain comprising 11 monobrachial homologs (g/gm/mq/qp/pr/rk/ki/ih/hn/no/o). The results are discussed in terms of hypotheses on fertility of complex heterozygotes and limited gene flow in hybrid zone.