Synapsis and crossing over within a paracentric inversion in the grasshopper,Camnula pellucida

A male grasshopper, Camnula pellucida (Scudder), was found to be heterozygous for a paracentric inversion occupying approximately 10% of the length of one of the two longest chromosomes. Analysis of 297 cells in pachytene revealed inversion loops, suspected inversion loops, asynapsis, and straight pairing in 1.0, 2.7, 8.4, and 87.9% of the analyzable cells, respectively. The frequency of straight pairing (87.9%) indicated a high degree of non-homologous pairing. Analysis of 603 cells in anaphase I and II, and in telophase I and II for the presence of acentric fragments and dicentric chromatid bridges indicated that crossing over within the inversion region occured in about 8% of the cells. The difference between the frequency of the observed plus suspected inversion loops in pachytene and that of the dicentric chromatid bridges and acentric fragments in anaphase I or subsequent stages was not statistically significant. The correspondence between the presence of inversion loops and crossovers within the region of the inversion is thus similar to that observed by Maguire (1966) for a short paracentric inversion in maize. The reasons for this correspondence are considered.Supported by grants GB 1585 and GB 6745 from the National Science Foundation, Washington, D.C.     

Inversion heterozygosity and sub-chromatid exchange in Agave stricta

A plant of Agave stricta Salm. (2n=60) has a bimodal complement of 10 L, 4 M and 46 S chromosomes. It is heterozygous for a paracentric inversion which involves the middle third of the long arm of one of the L chromosomes. It produces at anaphase I bridges and fragments and also loops and fragments, both single and double. Breakage and reunion at the sub-chromatid and at the chromatid level produce side-arm bridges and bridges and fragments respectively at anaphase I. A method is given, based on chiasma frequency, which will in certain cases of inversion heterozygosity provide a reasonable estimate of the position and the length in genetic map units of an inverted segment with respect to the whole chromatid arm.     

Increased variability in nuclear DNA content of testis cells and spermatozoa from mice with irregular meiotic segregation.

Variability in DNA content to testis cells and sperm from F₁ hybrids between the laboratory mouse (M. muscullus) and the tobacco mouse (M. poschiavinus), has been determined by flow cytometry (FMC). The F₁ hybrid mouse is known to be heterozygous for seven metacentric chromosomes produced by Robertsonian fusion. Enriched populations of nuclei from late pachytene spermatocytes and round spermatids were obtained by velocity sedimentation. These nuclei, as well as epididymal sperm nuclei and spleen cells, were stained by the acriflavin-Feulgen technique for DNA and measured by FCM. Peaks in the fluorescence intensity frequency distributions resulting from these measurements were analyzed to determine their mean fluorescence intensities and their widths (coefficients of variation). Because mean intensities of corresponding cell types from M. musculus and the F₁ hybrids were identical, the average DNA contents were taken to be the same. The average coefficients of variation of the peaks to fluorescence from the pachytene, spermatid, and sperm nuclei and spleen cells from M. muscullus animals were about 5%. While the peaks of fluorescence from spleen cells and pachytene nuclei from f₁ hybrids also had average coefficients of variation of 5%, post-meiotic nuclei from spermatids and spermatozoa had coefficients of variationof 8%. From these results we conclude that, in these F₁ hybrids, abnormal meiotic segregation causes an increased variability of 6% in the amount of DNA in the spermatozoa.     

A pachytene analysis of two male-fertile paracentric inversions in chromosome 1 of the mouse and in the male-sterile double heterozygote

Meiotic studies have been made at pachytene on two paracentric inversions in chromosome 1 of the mouse. Surface-spread preparations of primary spermatocytes have been analysed at the light microscope level in males heterozygous for the inversions In(1)1Rk and In(1)12Rk and in the double heterozygote In(1)1RK/In(1)12Rk. In singly heterozygous form, neither inversion produces any serious effect on male fertility. In the double heterozygote, spermatogenesis is arrested in the majority of cells at the spermatocyte stage and males are rendered totally sterile by azoospermia. In the double heterozygote, a complex loop, indicating the inversion bivalent, is found in 90% of pachytene cells analysed. In the In(1)1Rk/+ heterozygote, a looped bivalent was seen in 47 per cent of pachytene cells but in In(1)12Rk/+ no cells containing loops could be found. -80% of pachytene spermatocytes from the In(1)1Rk/In (1)12Rk double heterozygote showed apposition of the inversion bivalent to the sex bivalent. Such an association was rarely seen in pachytene cells of either of the fertile single heterozygotes. Spermatogenic failure in the double heterozygote may be related to interference, by the inversion bivalent, with X chromosome inactivation at meiotic prophase.     

Über das Verhalten eines Ringchromosoms in der Mitose und Meiose von Antirrhinum majus L.

Summary The mitotic and meiotic behaviour of a ring-chromosome in Antirrhinum majus was analysed. 26.5% mitotic anaphases showed bridges demonstrating the occurrence of a crossing-over-like process in meristematic cells. From pachytene studies the ring-chromosome could be identified as chromosome 6.An attempt was made to derive the details of the crossing-over process from the various anaphase configurations in pollen mother cells with a heterozygous ring-rod-bivalent. The observed frequencies could only be brought in approximate correspondance with theoretical values by postulating (i) the occurrence of sister-strand and non-sister-strand crossing-over in certain quantitative combinations, and (ii) an unexplained loss or irricognizability of most double bridges in anaphase I.The frequency of plants heterozygous for the ring-chromosome in progenies after seifing was 16.8%. The rate of chromosome mutations in these progenies was not increased. Chromosomal aberrations resulting from meiotic disturbances in the ring plants are probably lost by gonal elimination of unbalanced chromosome sets.     

The X-autosome translocation in the common shrew (Sorex araneus L.): late replication in female somatic cells and pairing in male meiosis

Common shrews have an XX/XY₁Y₂ sex chromosome system, with the X chromosome being a translocation (tandem fusion) between the original X and an autosome; in males this autosome is represented by the Y₂ chromosome. From G-banded chromosomes, the Y₂ is homologous to the long arm and centromeric part of the short arm of the X. The region of the X that is homologous to the Y₂ and also the telomeric region of the short arm of the X were found to be early replicating in somatic cells from a female shrew after 5-bromo-2-deoxyuridine (BrdU) treatment in vitro. The remainder of the short arm of the X was shown to be late replicating. Electron microscopic examination of synaptonemal complexes in males at pachytene revealed pairing of the Y₂ axis with the long arm of the X, and Y₁ with the short arm. At early stages of pachytene, there is apparently extensive nonhomologous pairing between the X and Y₁. In essence, the short arm of the shrew X chromosome behaves like a typical eutherian X chromosome (it is inactivated in female somatic cells and is paried with the Y₁ during male meiosis) while the long arm behaves like an autosome (escapes the inactivation and pairs with the Y₂).     

Male gametophyte development and two different DNA classes of pollen grains in <Emphasis Type="Italic">Rumex acetosa</Emphasis> L., a plant with an XX/XY<Subscript>1</Subscript>Y<Subscript>2</Subscript> sex chromosome system and a female-biased sex ratio

Female-biased sex ratio is an interesting phenomenon observed in Rumex acetosa, a dioecious plant with an XX/XY₁Y₂ sex chromosome system. Previous authors have suggested that the biased sex ratio in this species is conditioned not only postzygotically (sex-differential sporophytic mortality) but also prezygotically, because the sex ratio of seeds is also female-biased, although to a lesser extent than the sex ratio of flowering plants. The mechanisms underlying female bias in Rumex seeds are only poorly understood. To gain more knowledge of them, we analysed male gametophyte development and used flow cytometry to determine the frequency of female-determining (n = 7, A + X) and male-determining (n = 8, A + Y₁Y₂) pollen grains in anthers. Embryological studies showed a regular course of male gametophyte development in R. acetosa. There were no signs of degeneration of microspores or disturbances in pollen divisions (irregular nuclei, micronuclei, delayed chromosomes and anaphase bridges). The Alexander test revealed only 1.6% nonviable pollen grains within anthers. All mature pollen grains were uniformly equipped with starch granules. The two sexes were shown to substantially differ in nuclear 2C DNA amount in somatic tissues (7.00 pg in 2A + XX females and 7.50 pg in 2A + XY₁Y₂ males), and two clearly different DNA classes of mature pollen grains, with lower and with higher DNA amounts (16.8% difference) were found. Most probably the grains with the lower DNA amount possess seven chromosomes, and grains with the higher DNA amount eight of them. The quantitative ratio of these grains in anthers at anthesis was 1:1.2, very close to the sex ratio of seeds observed by the majority of previous authors. All these observations support the opinion that the sex-ratio bias in Rumex is determined prezygotically to some extent.     

Meiotic chromosomal aberrations in wild populations of Podophyllum peltatum

Meiotic chromosomal aberrations observed in wild populations of the plant Podophyllum peltatum include incomplete homologous pairing, non-homologous pairing, and inversion heterozygosity in pachytene; univalents, asymmetrical bivalents, and translocation heterozygosity in metaphase-I; bridge and fragments in anaphase-I; and non-disjunction as detected in anaphase-II. Incomplete homologous pachytene pairing is believed to result in non-homologous pairing and in the formation of metaphase-I univalents. The unequal distribution and precocious division of univalents in anaphase-I leads to non-disjunction. Non-disjunction chromosomes (varying in frequency from 0.0 to 24.6%) appear to be distributed among the genome on the basis of chromosome length. Asymmetrical bivalents and anaphase-I side-arm bridges are believed to be caused by chromatid breakage and fusion rather than inversion heterozygosity. Of the 135 clones examined, 20 were found to be heterozygous for translocations. The possibility of widespread distribution of some translocations is suggested.     

Spermiogenesis of inversion heterozygotes in backcross hybrids between Drosophila buzzatii and D. serido

Interspecific F1 hybrid females of D. serido and D. buzzatii are fertile, but hybrid males are sterile. By successive backcrossing of hybrid females to D. buzzatii males it is possible to diminish the genomic contribution of D. serido to the hybrid karyotype. Finally, only selected chromosome sections of D. serido known as inversions restricted to this species were individually left in the otherwise D. buzzatii karyotype, namely: 2 C2b-F4a (j⁹m⁹n⁹), 2 B2c-F4a (j⁹k⁹), 3 C5a-G1b (k²), 4 E2a-G2f (m) and 5 C5d-F2h (w). The present paper deals with the influence of these chromosome sections on sperm differentiation. Any of them produces hybrid male sterility in heterozygous condition. We analyzed spermiogenesis using the DNA specific fluorescence dye BAO in hybrid males which were heterozygous either for only one inversion, as in chromosomes 3, 4 and 5, or for a series of inversions on the same chromosome, as in chromosome 2. The abnormalities recorded included abnormal formation of the cysts, lower than normal number of cysts, abnormal number of nuclei per cyst, incomplete elongation of the cyst, incomplete elongation of the nuclei, displacement of the nuclei from the head region of the cyst and lack of individualization. In no case was there any contents in the seminal vesicle. The section from chromosome 2 of D. serido had the most drastic effect; the disruption produced by the chromosome section corresponding to inversion 3 k² was only a little more severe than that due to 5 w, and both may be distinguished only quantitatively; inversion 4 m produced the slightest deviation from normal spermiogenesis. The larger the serido section introduced in the hybrid, the more severe were the abnormalities it produced. An interpretation in terms of a balance genic theory on the functioning of the genetic system is given.This is paper No. VII in the series The evolutionary history of Drosophila buzzatii.     

Synapsis of a chromosomal pair heterozygous for a pericentric inversion and the presence of a heterochromatic short arm

Analysis of meiotic pairing configurations in a deer mouse heterozygous for both a pericentric inversion and the presence of a heterochromatic short arm at chromosome 15 revealed straight-paired synaptonemal complexes with equal axial lengths in a majority of the pachytene nuclei. Nonhomologous pairing in this bivalent occurs by direct heterosynapsis of the inverted segments followed by synaptic adjustment of the heterochromatin heteromorphism.     

Heterosynapsis and suppression of chiasmata within heterozygous pericentric inversions of the Sitka deer mouse

The patterns of chromosomal pairing and chiasma distribution were analyzed in male Sitka deer mice (Peromyscus sitkensis) polymorphic for terminally positioned pericentric inversions of chromosomes 6 and 7. Gand C-banding of somatic metaphases indicated that the inversions involved 30% and 40% of chromosomes 6 and 7, respectively. Analysis of silver-stained synaptonemal complexes in surface-spread zygotene and pachytene nuclei from heterozygous individuals revealed that inversion loops were not formed. The inverted segments proceeded directly to heterosynapsis without an intervening homosynaptic phase, and the heteromorphic bivalents remained straight-paired throughout pachynema. C-banded pachytene nuclei corroborated the occurrence of heterosynapsis, as the heteromorphic bivalents exhibited nonaligned centromeres. Analysis of diplonema and diakinesis indicated that crossing over had not occurred within the heterosynapsed inverted segments. The observation of chiasma suppression within the inversions indicates that pericentric inversion heterozygosity does not lead to the production of unbalanced gametes. Heterosynapsis of the inverted segments during zygonema and pachynema and the resulting chiasma suppression therefore represent a meiotic mechanism for the maintenance of pericentric inversion polymorphisms in this population of P. sitkensis.     

The Relationship of Homologous Synapsis and Crossing over in a Maize Inversion

Frequency of homologous synapsis at pachytene for a relatively short heterozygous inversion was compared to the frequency of crossover occurrence within the inversion and to the frequency of the presence of a recombination nodule within the homologously synapsed inverted region. Crossover frequencies were estimated from bridge-fragment frequencies at anaphase I and anaphase II. Recombination nodules (RNs) were observed in electron micrographs. Results show very similar frequencies of homologous synapsis and the occurrence of reciprocal recombination within the inverted region, consistent with the interpretation that establishment of homologous synapsis in this case is related to at least commitment to the form of resolution of crossover intermediates which gives rise to reciprocal recombination, not conversion only, events. An RN was generally found at pachytene in homologously synapsed inverted regions.     

Karyotype,heterochromatin content and meiotic features ofPoecilocera picta (Orthoptera,Acrididae)

Cytological study of three distinctly separated populations ofPoecilocera picta revealed a chromosome number of 2N = 18 + XO/ XX. Except for the hemizygosity of a procentric heterochromatic block in the M₆ pair of the Bangalore population, the basic karyotype of the three populations is markedly similar. The autosomal karyotype formula is 2Lt + 4Mt + 1 Mst + 2S st and the telocentric X chromosome is the longest of the complement. All bivalents at pachytene carried procentric heterochromatic blocks. The M₄ is the nucleolus organiser with the NOR region situated interstitially but proximal to the centromere. About 11 μm (4%) of the total (290 μm) autosomal pachytene complement is heterochromatic; a major portion of it is contributed by the S₉ pair which is mostly heterochromatic. Chiasmata are localized proximally and distally and in the S₉ pair their formation is confined to the short procentric euchromatic segment of the long arm. Female meiosis did not reveal any chromomere pattern at pachytene and, unlike in the male, the sex bivalent in the female is indistinguishable from the autosomal bivalents. G- and C-banding patterns in males showed procentric bands in all the chromosomes. In addition there are eight telomeric and two interstitial bands which are C negative. The S₉ pair showed only two bands. The G-banding pattern of the sex chromosome in meiosis showed only a centric band while the heterochromatic body of the facultatively heterochromatic X remained G negative.     

Paracentric inversion polymorphism in the grasshopper Boonacris alticola

Paracentric inversion heterozygosity can be detected at pachytene by observation of frequent regions of asynapsis and reinforced by observation of the elimination of a chiasma in the region of the inversion at diplotene and by a low level of bridges and fragments at anaphase. We present evidence that paracentric inversion polymorphism can persist in a grasshopper population despite a low level of crossing over within the inverted region in heterozygotes. Lethality resulting from aneuploidy due to limited crossing over within the region of the inversion appears to be more than compensated for by heterosis.     

The Influence of Chromosome Content on the Size and Shape of Sperm Heads in DROSOPHILA MELANOGASTER and the Demonstration of Chromosome Loss during Spermiogenesis

The volumes of sperm heads were estimated from three-dimensional reconstructions of serially sectioned bundles of nearly mature spermatid nuclei. Cysts from males in which all sperm are expected to have comparable amounts of chromatin (X/Y and In(3LR)/+) show unimodal frequency distributions of nuclear volumes, whereas cysts from males in which meiotic segregation is expected to deliver unequal amounts of chromatin material to spermatid nuclei show two (XY/O and XY/Y) or more (T(2;3)/⁺ and C(2L);C(2R)) modes. The mean volumes of the subpopulations in these cases are related in the same proportions as the metaphase lengths of their chromosomal complements. Thus the volumes of sperm nuclei are proportional to their DNA content. Sperm head shape, on the other hand, does not appear to be very sensitive to chromosomal constitution, as heads of different size do not vary greatly in shape.—The numbers of sperm heads in the various size classes in a cyst depart from mendelian expectations; these departures are caused by the elimination, during individualization, of chromosomes contained within micronuclei that are formed in spermatids at the end of the second meiotic division. The effect of this chromosome loss is to increase the proportion of nullosomic gametes in the sperm pool.—The relative frequencies of XY-bearing and nullo-X, nullo-Y sperm in XY/O males were estimated from the volume measurements. Taking this estimate as a measure of the fertilizing population, it is possible to infer from the change in sex ratio over time following insemination, that XY-bearing sperm have an advantage of 1.5 over nullo-X, nullo-Y sperm in leaving the seminal receptacle of the female for fertilization of ova.     

A paracentric inversion suppresses genetic recombination at the <Emphasis Type="Italic">FON3</Emphasis> locus with breakpoints corresponding to sequence gaps on rice chromosome 11L

Paracentric inversion is known to inhibit genetic recombination between normal and inverted chromosomal segments in heterozygous arrangements. Insect inversion polymorphisms have been studied to reveal adaptive processes for maintaining genetic variation. We report the first paracentric inversion in rice (Oryza sativa), which was discovered in our effort to clone the floral organ number gene FON3. Recombination at the FON3 locus on the long arm of chromosome 11 was severely suppressed over a distance of more than 36 cM. An extensive screening among 8,242 F₂ progeny failed to detect any recombinants. Cytological analysis revealed a loop-like structure on pachytene chromosomes, whereas FISH analysis showed the migration of a BAC clone from a distal location to a position closer to the centromere. Interestingly, the locations where the genetic recombination suppression began were coincided with the positions of two physical gaps on the chromosome 11, suggesting a correlation between the physical gaps, the inversion breakpoints. Transposons and retrotransposons, and tandemly arranged members of gene families were among the sequences immediately flanking the gaps. Taken together, we propose that the genetic suppression at the FON3 locus was caused by a paracentric inversion. The possible genetic mechanism causing such a spontaneous inversion was proposed.     

The mitotic, polytene, and meiotic chromosomes of Drosophila ananassae.

The mitotic chromosome complement of D. ananassae consists of four structurally distinguishable submetacentric pairs and all four have been identified with their linkage groups. For the polytene chromosome complement of six arms representing the X, second and third chromosomes, an improved reference map has been constructed and used to describe selected cytogenetically useful rearrangements. In meiotic prophase of spermatocytes, chromosomes 2 and 3 form pachytene-diplotene bivalents whose arms may be associated by chiasmata in postdiplotene stages, but the X, Y and fourth chromosomes participate in a complex multivalent. No correlation was detected between meiotic chromosome behavior and specific genes that regulate crossing over in males. In male inversion heterozygotes having high levels of genetically monitored crossing over, no unequivocal evidence was found for formation of either pachytene inversion loops or anaphase bridges and fragments.     

Evidence of crossing-over inhibition in rye anthers cultured with colchicine

In order to investigate wheter colchicine affects crossing-over, rye anthers of an inbred line of rye forming bridges and fragments at anaphase I produced by erroneous chiasmata, and anthers of plants heterozygous for a conspicuous heterochromatin band, were cultured in a medium with colchicine. Anthers planted at zygotene did not show bridges at AI in the inbred line. In the heterozygotes no difference between associated chromatids in respect to the heterochromatin band, resulting from crossingover, were observed. In anthers planted at pachytene both bridges and chromosomes showing difference between associated chromatids were observed at a stage equivalent to AI with the same frequency as in anaphase I cells of untreated anthers. This demonstrates that crossing-over or a prerequisite to crossing-over is established at zygotene, and also that absence of chiasmate association at later stages is not due to precocious slipping off of chiasmata.     

Molecular cytogenetic characterization of <Emphasis Type="Italic">Rumex papillaris</Emphasis>, a dioecious plant with an XX/XY<Subscript>1</Subscript>Y<Subscript>2</Subscript> sex chromosome system

Rumex papillaris Boiss, & Reut., an Iberian endemic, belongs to the section Acetosa of the genus Rumex whose main representative is R. acetosa L., a species intensively studied in relation to sex-chromosome evolution. Here, we characterize cytogenetically the chromosomal complement of R. papillaris in an effort to enhance future comparative genomic approaches and to better our understanding of sex chromosome structure in plants. Rumex papillaris, as is common in this group, is a dioecious species characterized by the presence of a multiple sex chromosome system (with females 2n = 12 + XX and males 2n = 12 + XY₁Y₂). Except for the X chromosome both Y chromosomes are the longest in the karyotype and appear heterochromatic due to the accumulation of at least two satellite DNA families, RAE180 and RAYSI. Each chromosome of pair VI has an additional major heterochromatin block at the distal region of the short arm. These supernumerary heterochromatic blocks are occupied by RAE730 satellite DNA family. The Y-related RAE180 family is also present in an additional minor autosomal locus. Our comparative study of the chromosomal organization of the different satellite-DNA sequences in XX/XY and XX/XY₁Y₂ Rumex species demonstrates that of active mechanisms of heterochromatin amplification occurred and were accompanied by chromosomal rearrangements giving rise to the multiple XX/XY₁Y₂ chromosome systems observed in Rumex. Additionally, Y₁ and Y₂ chromosomes have undergone further rearrangements leading to differential patterns of Y-heterochromatin distribution between Rumex species with multiple sex chromosome systems.