Dominant X-Chromosome Nondisjunction Mutants of CAENORHABDITIS ELEGANS

Eight dominant X-chromosome nondisjunction mutants have been identified and characterized. Hermaphrodites (XX) heterozygous for any one of the mutations produce 20–35% male (XO) self-progeny compared with the wild-type frequency of 0.2%. Seven of the eight mutants carry X-autosome translocations. Three of these, represented by mnT2, involve linkage group (LG) II and show severe crossover suppression for X-linked markers. The two half-translocations comprising mnT2 are separable and of very unequal size. The smaller one includes the left tip of X and the right end of LGII and can exist as a free duplication, being present in addition to the normal chromosome complement, in either hermaphrodites or males; it has no effect on X nondisjunction. The reciprocal half-translocation of mnT2 includes the bulk of both LGII and X chromosomes; it disjoins regularly from a normal LGII and confers the property of X-chromosome nondisjunction. A fourth translocation, mnT10(V;X), is also reciprocal and consists of half-translocations that recombine with V and X, respectively. Either half-translocation of mnT10 can exist in heterozygous form in the absence of the other to give heterozygous duplication-deficiency animals; the property of X-chromosome nondisjunction is conferred, in homozygotes as well as heterozygotes, solely by one of the half-translocations, which is deficient for the left tip of the X. The final three translocations have X breakpoints near the right end of X and autosomal breakpoints near the right end of LGIV, the left end of LGV and the right end of LGI, respectively. All three are homozygous inviable. Males hemizygous for the X portion of any of the seven translocations are viable and fertile. The final mutant, mn164, maps as a point at or near the left tip of the X and causes X-chromosome nondisjunction in both heterozygotes and homozygotes. In heterozygotes, mn164 promotes equational nondisjunction of itself but not its wild-type allele. The mutants are discussed in light of the holocentric nature of the C. elegans chromosomes. It is proposed that the left end of the X chromosome plays a critical structural role in the segregation of X chromosomes during meiosis in XX animals.     

A Drosophila melanogaster cell line tested for the presence of active NORs by silver staining

Silver staining was used to detect active NORs in a Drosophila melanogaster cell line (C1 82) characterized by dimorphic X chromosomes (XX_L), one of the two Xs showing a marked increase in heterochromatin where the nucleolar organizer (NO) is located. The Q-banding technique was used to determine the karyotype characteristics of the line. Ag-positive NORs appeared only on structurally changed X chromosomes (X_L), both in diploid and tetraploid cells, indicating that rRNA genes of X_L are more active or numerous than those on normal homologues. A possible relationship between NOR stainability, the presence of an increased heterochromatic portion and the selective advantage of XX_L cells, recurrent in numerous Drosophila female lines, is discussed.     

Nondisjunction Mutants of the Nematode CAENORHABDITIS ELEGANS

The frequency of males (5AA; XO) among the self progeny of wild-type Caenorhabditis elegans hermaphrodites (5AA; XX) is about one in 500. Fifteen him (for "high incidence of males") mutations have been identified that increase this frequency by a factor of ten to 150, as a result of increased X-chromosome nondisjunction. The mutations define ten complementation groups, which have been mapped: nine are autosomal, and one sex linked. Most of the mutants are superficially wild type in anatomy and behavior; however, him-4 mutants display gonadal abnormalities, and unc-86 mutants, which have a Him phenotype, exhibit a variety of anatomical and behavioral abnormalities. All the mutants segregate fertile 3X hermaphrodite progeny as well as XO male progeny. Some produce large numbers of inviable zygotes. Mutants in all ten genes produce diplo-X and nullo-X exceptional ova, and in the four strains tested, diplo-X and nullo-X exceptional sperm are produced by 2X "transformed" males. It appears likely that most of the mutants have defects in both gamete lines of the hermaphrodite. XO males of him strains other than him-4 and unc-86 are similar to wild-type males in anatomy and behavior, and all produce equal or almost equal numbers of haplo-X and nullo-X sperm, and no diplo-X sperm. Male fertility is reduced to varying extents in all him mutants. In four of the strains, nondisjunction during oogenesis has been shown to occur at a reductional division, and in three of these strains, abnormalities in recombination have been demonstrated. One mutant also exhibits autosomal nondisjunction, but many of the others probably do not. Therefore, the X chromosome of C. elegans may differ from the autosomes in the mechanisms controlling its meiotic behavior.——3X hermaphrodites are shorter and less fertile than 2X hermaphrodites, and they produce many inviable zygotes among their self progeny: these are probably 4X zygotes. Haplo-X and diplo-X ova are produced in 2:1 ratio by 3X hermaphrodites. him mutations are expressed in these animals, increasing the frequency of self-progeny males and 2X hermaphrodites.     

Late replication patterns in adult and embryonic mice carrying Searle's X-autosome translocation

Bromodeoxyuridine-dye technique analysis of X chromosome DNA synthesis in female adult and fetal mice carrying the balanced form of the T(X; 16) 16H translocation demonstrated that the structurally normal X chromosome was late replicating (and hence presumably inactive) in 93% of the adult cells and 99% of the 9-day embryo cells, with the X¹⁶ chromosome late replicating in the remaining cells. We conclude from these results that in T16H/+ females either there is preferential inactivation of the normal X chromosome or that, if inactivation is random, cell selection takes place before 9 days of development. Two 9-day female embryos with an unbalanced karyotype were also studied; both had two late-replicating chromosomes in most of their cells, one being the chromosome 16^X, the other a normal X chromosome. These results, together with the presence of a late-replicating X¹⁶ chromosome in T16H/+ adult and fetal mice, support the concept that more than one inactivation center is present on the X chromosome of the mouse because the X¹⁶ and the 16^x chromosomes can be late replicating.     

In vitro induction of a trisomic of Agave tequilana Weber var. Azul (Agavaceae) by para-fluorophenylalanine treatment

The effect of para-fluorophenylalanine (PFP) on the production of trisomic plants of Agave tequilana Weber var. Azul produced through somatic embryogenesis was investigated. Normal diploid plants with 2n = 2x = 60 were obtained in the control treatment and with 4 mg L⁻¹ PFP exposure, while use of 8 and 12 mg L⁻¹ PFP led to production of trisomics with 2n = 2x = 61. Normal diploid plants showed a bimodal karyotype with five pairs of large chromosomes and 25 pairs of small chromosomes. Trisomic plants also had a bimodal karyotype with a group of three chromosomes in position five of the chromosome set. More than 13 homologous chromosome pairs exhibited structural changes. Differences in chromosome arm ratio (long arm/short arm) were also found in eight chromosome pairs; all these aberrations in the chromosome complement of trisomic plants were probably caused by inversions, deletions, and/or duplications produced by high concentrations of PFP. The gross chromosome structural changes and the presence of a single extra chromosome could have been induced by the effect of PFP on the mitotic spindle by inducing nondisjunction of sister chromatids, resulting in hyperploids (2n + x) and hypoploids (2n − x). Flow cytometric analysis of nuclear DNA content was performed using nuclei isolated from young leaves of normal and trisomic plants. The 2C DNA content of 8.635 pg (1Cx = 4,223 Mbp of trisomic plants was different (p < 0.001) than that of normal plants (2C DNA = 8.389 pg (1Cx = 4,102 Mbp). The difference in genome size was correlated with the large structural changes in the trisomic plant genomes.     

The cytogenetics of a triploid Hordeum bulbosum and of some of its hybrid and trisomic derivatives

Summary The progeny from a cross between diploid H. vulgare and triploid H. bulbosum were mostly triploid (VBB) hybrids, the other progeny were haploid (V) barley (H. vulgare). From a cross between diploid and triploid H. bulbosum, four of the seven possible trisomic lines were isolated. The Giemsa banded karyotype of H. bulbosum was produced, and two of the lines were identified as trisomic for chromosomes 6 and 7. The cytology and transmission rates of the trisomics were examined.     

Trisomics and aneuploids of ryegrass

Summary Primary trisomics (2 n + 1 = 15), double trisomics (2 n + 1 + 1 = 16) and aneuploids with 24 to 30 chromosomes, as well as a diploid and tetraploids, were found in the progeny of a hypertriploid (2 n = 22) plant of perennial ryegrass, Lolium perenne L. Trisomics and double trisomics differed in their mean chromosome association, chiasma number and spike morphology. A few aneuploids and tetraploids had reciprocal translocations. The diploid, primary trisomics and tetraploids were more fertile than the double trisomics and aneuploids. Most trisomics and aneuploids were probably produced through female transmission. One double trisomic had a high univalent number, a low chiasma number and loose chromosome coiling. Both the extra chromosomes carried secondary constrictions. The gene for desynapsis might be located on one of these chromosomes.     

Multiple sex chromosome system of X1X1X2X2/X1X2)Y type in lutjanid fish, Lutjanus quinquelineatus (Perciformes)

The karyotype and other chromosomal markers as revealed by C-banding and Ag-staining were studied in Lutjanus quinquelineatus and L. kasmira (Lutjanidae, Perciformes). While in latter species, the karyotype was invariably composed of 48 acrocentric chromosomes in both sexes, in L. quinquelineatus the female karyotype had exclusively 48 acrocentric chromosomes (2n = 48) but that of the male consisted of one large metacentric and 46 acrocentric chromosomes (2n = 47). The chromosomes in the first meiotic division in males showed 22 bivalents and one trivalent, which was formed by an end-to-end association and a chiasmatic association. Multiple sex chromosome system of X₁X₁X₂X₂/X₁X₂Y type resulting from single Robertsonian fusion between the original Y chromosome and an autosome was hypothesized to produce neo-Y sex chromosome. The multiple sex chromosome system of L. quinquelineatus appears to be at the early stage of the differentiation. The positive C-banded heterochromatin was situated exclusively in centromeric regions of all chromosomes in both species. Similarly, nucleolus organizer region sites were identified in the pericentromeric region of one middle-sized pair of chromosomes in both species. The cellular DNA contents were the same (3.3 pg) between the sexes and among this species and related species.     

Fragmentation par les rayons X de l'organisateur d'une différenciation de chromosome en écouvillon (lampbrush), chez Pleurodeles waltlii

X-rays have been used to induce heritable changes in the specific morphology of the lampbrush chromosomes in the newt Pleurodeles waltlii. The karyotype organization of female progeny of irradiated males was studied. Nine out of ten females were found to have chromosomal aberrations. In one of the nine, one of the breaks occurred at the sphere organizer, the sphere being part of the striking morphological features of chromosome IV. On irradiation the normal sphere organizer had been broken into two fragments each of which, when recombined with other chromosome breaks, still forms a sphere. The relationship of these observations to genome redundancy is discussed.     

Chromosomal rearrangements and the first indication of an ♀X1X1X2X2/♂X1X2Y sex chromosome system in Rineloricaria fishes (Teleostei: Siluriformes)

Rineloricaria is the most diverse genus within the freshwater fish subfamily Loricariinae, and it is widely distributed in the Neotropical region. Despite limited cytogenetic data, records from southern and south-eastern Brazil suggest a high rate of chromosomal rearrangements in this genus, mirrored in remarkable inter- and intraspecific karyotype variability. In the present work, we investigated the karyotype features of Rineloricaria teffeana, an endemic representative from northern Brazil, using both conventional and molecular cytogenetic techniques. We revealed different diploid chromosome numbers (2n) between sexes (33♂/34♀), which suggests the presence of an ♀X₁X₁X₂X₂/♂X₁X₂Y multiple sex chromosome system. The male-limited Y chromosome was the largest and the only biarmed element in the karyotype, implying Y-autosome fusion as the most probable mechanism behind its origination. C-banding revealed low amounts of constitutive heterochromatin, mostly confined to the (peri)centromeric regions of most chromosomes (including the X₂ and the Y) but also occupying the distal regions of a few chromosomal pairs. The chromosomal localization of the 18S ribosomal DNA (rDNA) clusters revealed a single site on chromosome pair 4, which was adjacent to the 5S rDNA cluster. Additional 5S rDNA loci were present on the autosome pair 8, X₁ chromosome, and in the presumed fusion point on the Y chromosome. The probe for telomeric repeat motif (TTAGGG)_n revealed signals of variable intensities at the ends of all chromosomes except for the Y chromosome, where no detectable signals were evidenced. Male-to-female comparative genomic hybridization revealed no sex-specific or sex-biased repetitive DNA accumulations, suggesting a presumably low level of neo-Y chromosome differentiation. We provide evidence that rDNA sites might have played a role in the formation of this putative multiple sex chromosome system and that chromosome fusions originate through different mechanisms among different Rineloricaria species.     

An asymmetric chromosome pair undergoes synaptic adjustment and crossover redistribution during Caenorhabditis elegans meiosis: implications for sex chromosome evolution

Heteromorphic sex chromosomes, such as the X/Y pair in mammals, differ in size and DNA sequence yet function as homologs during meiosis; this bivalent asymmetry presents special challenges for meiotic completion. In Caenorhabditis elegans males carrying mnT12, an X;IV fusion chromosome, mnT12 and IV form an asymmetric bivalent: chromosome IV sequences are capable of pairing and synapsis, while the contiguous X portion of mnT12 lacks a homologous pairing partner. Here, we investigate the meiotic behavior of this asymmetric neo-X/Y chromosome pair in C. elegans. Through immunolocalization of the axis component HIM-3, we demonstrate that the unpaired X axis has a distinct, coiled morphology while synapsed axes are linear and extended. By showing that loci at the fusion-proximal end of IV become unpaired while remaining synapsed as pachytene progresses, we directly demonstrate the occurrence of synaptic adjustment in this organism. We further demonstrate that meiotic crossover distribution is markedly altered in males with the asymmetric mnT12/+ bivalent relative to controls, resulting in greatly reduced crossover formation near the X;IV fusion point and elevated crossovers at the distal end of the bivalent. In effect, the distal end of the bivalent acts as a neo-pseudoautosomal region in these males. We discuss implications of these findings for mechanisms that ensure crossover formation during meiosis. Furthermore, we propose that redistribution of crossovers triggered by bivalent asymmetry may be an important driving force in sex chromosome evolution.     

Dosage effect of a gene with a regulating effect on anthocyanin synthesis in a trisomic Petunia hybrida

A Petunia hybrida inbred line (W 28) has white flowers with red spots on the corolla. These spots are the result of back mutations of an unstable allele of the gene Anl for anthocyanin synthesis. Among the progeny of a population of selfed plants a primary trisomic with red-spotted white flowers was found. The reversion frequency was more than twice as high as compared with disomic plants of the same family.It was found that the chromosome in triplicate was not the chromosome on which the gene Anl is localized. It can be concluded that there is an independently segregating factor which influences the frequency of back mutations of the Anl locus. Twin spots were found among the flowers of the trisomic. They consisted of two adjacent sectors, one with a spot frequency equal to that of the flowers of disomic plants, and the other with a spot frequency more than twice as high as that of the trisomic. Probably an irregular distribution of the extra chromosome resulted in one sector with the normal diploid number of chromosomes, and an adjacent sector with two extra chromosomes. The reversion frequencies in the sector suggest that the factor which affects the reversion frequency of the unstable alleles of Anl exhibits a dosage effect.     

Evolution of karyotype,sex chromosomes,and meiosis in mygalomorph spiders (Araneae: Mygalomorphae)

Spider diversity is partitioned into three primary clades, namely Mesothelae, Mygalomorphae, and Araneomorphae. Mygalomorph cytogenetics is largely unknown. Our study revealed a remarkable karyotype diversity of mygalomorphs. Unlike araneomorphs, they show no general trend towards a decrease of 2n, as the chromosome number was reduced in some lineages and increased in others. A biarmed karyotype is a symplesiomorphy of mygalomorphs and araneomorphs. Male meiosis of some mygalomorphs is achiasmatic, or includes the diffuse stage. The sex chromosome system X₁X₂0, which is supposedly ancestral in spiders, is uncommon in mygalomorphs. Many mygalomorphs exhibit more than two (and up to 13) X chromosomes in males. The evolution of X chromosomes proceeded via the duplication of chromosomes, fissions, X–X, and X‐autosome fusions. Spiders also exhibit a homomorphic sex chromosome pair. In the germline of mygalomorph males these chromosomes are often deactivated; their deactivation and pairing is initiated already at spermatogonia. Remarkably, pairing of sex chromosomes in mygalomorph females is also initiated at gonial cells. Some mygalomorphs have two sex chromosome pairs. The second pair presumably arose in early‐diverging mygalomorphs, probably via genome duplication. The unique behaviour of spider sex chromosomes in the germline may promote meiotic pairing of homologous sex chromosomes and structural differentiation of their duplicates, as well as the establishment of polyploid genomes. © 2013 The Linnean Society of London, Biological Journal of the Linnean Society, 2013, 109 , 377–408.     

Genetics of the peroxidase isoenzymes in Petunia

Summary By starch gel electrophoresis three mobility variants of a cathodic moving doublet of bands, encoded by the structural gene prxC, were detected in all organs of flowering petunias. In root tissue two of the variants showed a lower electrophoretic mobility than in other organs. During development of flower buds the PRXc enzymes showed an increase in mobility. The gene prxC was located on chromosome IV by showing linkage to the genes An3 and Dw1, by trisomic segregation, and by the construction of triply heterozygous trisomics IV. The gene order on chromosome IV is B1-An3/Dw1-prxC. It was concluded that the temporal programming difference in the expression of the alleles prxC2 and prxC3 is caused by internal site mutation. Analysis of progeny obtained by crossing of lines to the trisomic IV with genotype prxC1/C1/C2 showed differential expression of the two prxC1 alleles of the trisomic IV.     

TRISOMICS IN SOLANUM CHACOENSE: FERTILITY AND CYTOLOGY

Twenty trisomic plants found in the progeny 3x x 2x crosses in Solatium chacoense and their F₁ trisomies obtained by 2x + 1 X 2x crosses were studied with respect to their fertility and cytology. The female transmission of the extra chromosome in the trisomics varied from 2 to 60 %. The transmission frequencies of F₁ trisomies were similar to their parent trisomies in most of the lines. The transmission through the pollen ranged from 0 to 20 %. Female and male fertility of the parent trisomies was high. They produced an average of 37 seeds per pollination as the female or as the male parent. The F₁ trisomies produced about half the seed set of their parent trisomies. The extra chromosomes of six trisomies were identified by pachytene analysis. They were isochromosomes for the long arms of chromosomes I, IV and IX and the short arms of IV, IX and XII. Chromosome morphology of the extra chromosomes in pachytene stage was described. A chromosome association of 12 II + 1 I was found in 66 % of the cells at MI. About 29 % of the cells had one trivalent and 5 % had three or five univalents. The frequency of trivalent formation was not affected by the length of the extra chromosome. The possibility of univalent shift in secondary trisomies was discussed.     

Karyology and karyotype analysis of diploid freshwater planarian populations of the Dugesia gonocephala group (Platyhelminthes,Tricladida) found in Sardinia

Karyology and karyotype analysis were carried out on freshwater planarian populations of the Dugesia gonocephala group. The strains studied were all diploid with chromosomic number 2n = 16; n = 8. They came from 12 sites mainly localized on the west of the island of Sardinia. Three karyotypes indicated with the letters A, B and C were found in which eight homomorphic pairs of chromosomes were easily identified. In karyotype A all chromosomes are metacentric. Ten populations of the twelve examined showed this karyotype which appears to be the most common. In karyotype B the seventh pair of chromosomes is submetacentric. This karyotype is quite common having been previously found in another eight Sardinian localities. Karyotype C differs from the others in having submetacentric third and seventh pairs of the chromosome complement. It was found in only one locality. The differences observed between these three karyotypes could be interpreted either as sign of differentiation at species level, or as an intraspecific variation due to chromosome mutations (pericentric inversions). This revised version was published online in July 2006 with corrections to the Cover Date.     

A unique cytogenetic system in monotremes

All 3 extant genera of monotremes show a unique kind of cytogenetic system involving the formation of a structurally heterozygous chain multiple apparently coupled with a system of complementary gametic elimination. In the echidna Tachyglossus there are 63 chromosomes in the male and 64 in the female. This is associated with an X₁X₂Y/X₁X₁X₂X₂ sex chromosome system. Additionally in both sexes there are 6 mitotic chromosomes (a-f) which have no obvious homologous partners. At male meiosis these are included with the 3 sex chromosomes in a chain multiple of nine which has the constitution X₁·Y·X₂·f·e·d·c·b·a. This shows convergent orientation at first metaphase leading to the production of two kinds of sperm, namely X₁X₂ eca and Yfdb. Since no individual of either sex has been found homozygous for any of the a-f elements it follows that only gametes carrying different combinations of the three unpaired elements give rise to viable offspring. Whether this depends on gametic selection or on zygotic lethality is not known. An apparently identical system operates in Zaglossus. In the platypus Ornithorhynchus, on the other hand, there are 52 chromosomes in both males and females associated with an XY/XX sex chromosome mechanism and the presence of 4 consistently unpaired elements (a-d) at mitosis. A chain multiple of 10 forms at male meiosis involving the 2 sex chromosomes, the 4 unpaired elements and two of the small pairs of autosomes. Additionally the six longest autosome pairs in Tachyglossus and the X₁ show a polymorphism for size which in heterozygous combination leads to the formation of unequal bivalents at male meiosis.     

The fate of recombinant chromosomes and genome interaction in Nicotiana asymmetric somatic hybrids and their sexual progeny

Genomic in-situ hybridization (GISH) was used to monitor the behaviour of parental genomes, and the fate of intergenomic chromosome translocations, through meiosis of plants regenerated from asymmetric somatic hybrids between Nicotiana sylvestris and N. plumbaginifolia. Meiotic pairing in the regenerants was exclusively between chromosomes or chromosome segments derived from the same species. Translocation (recombinant) chromosomes contained chromosome segments from both parental species, and were detected at all stages of meiosis. They occasionally paired with respectively homologous segments of N. sylvestris or N. plumbaginifolia chromosomes. Within hybrid nuclei, the meiotic division of N. plumbaginifolia lagged behind that of N. sylvestris. However, normal and recombinant chromosomes were eventually incorporated into dyads and tetrads, and the regenerants were partially pollen fertile. Recombinant chromosomes were transmitted through either male or female gametes, and were detected by GISH in sexual progeny obtained on selfing or backcrossing the regenerants to N. sylvestris. A new recombinant chromosome in one plant of the first backcross generation provided evidence of further chromosome rearrangements occurring at, or following, meiosis in the original regenerants. This study demonstrates the stable incorporation of chromosome segments from one parental genome of an asymmetric somatic hybrid into another, via intergenomic translocation, and reveals their transmission to subsequent sexual progeny.     

Telomere repeats within the neo-Y-chromosome of Drosophila miranda

The clone Dmir1098, isolated from a genomic lambda library of Drosophila miranda labels exclusively the tips of the giant chromosomes in the highly polytenized nuclei of the female larval salivary glands. However, the in situ hybridizations to male metaphase plates, using the same probe, reveal a massive labeling block within the neo-Y-chromosome in addition to the labeling blocks at both chromosome ends. From the comparison with the Y chromosome labeling pattern of D. pseudoobscura, a sibling species to D. miranda, an end-to-end fusion mechanism involving the telomere repeats would be the most straightforward explanation for the karyotype change in D. miranda.