scholarly journals A Preliminary Note on the Pollen Development of Lathyrus Odoratus

1925 ◽  
Vol 2 (2) ◽  
pp. 199-209
Author(s):  
JOAN LATTER
Keyword(s):  
Nuclear Membrane ◽  
Pollen Development ◽  
Cell Walls ◽  
Crossing Over ◽  
Haploid Number ◽  
Preliminary Note ◽  
Homologous Chromosomes ◽  
Lathyrus Odoratus ◽  
Thread Formation ◽  
Contraction Stage

1. In this paper the main points in the pollen development of Lathyrus odoratus are briefly described. At the beginning of the meiotic phase the reticulum contracts from the nuclear membrane and exhibits an entirely granular appearance. 2. During thread formation, occasional amœboid nucleoli are observed. Nucleolar "budding" is also seen. 3. Connecting strands are constantly found between the synizetic knot and the nucleolus. 4. The thread at all stages appears usually to be a continuous structure. 5. The synizetic knot is followed by a stage in which the thread is thrown into seven definite loops which radiate out from the centre of the nucleus. For this stage, which has frequently been called the second contraction stage in cytological literature, the name broxonema is proposed. 6. The haploid number of chromosomes in Lathyrus odoratus is seven. Each loop represents one pair of homologous chromosomes joined distally end to end. 7. The arms of each loop are twisted round one another at one period. This affords opportunity for exchange of segments of chromosomes, and gives a possible physical basis for crossing over in a telosynaptic form. 8. These stages are followed by typical diakinesis and heterotypic divisions. 9. During homotypic telophase evanescent cell plates occur between the daughter nuclei. Later, the cell walls are formed by furrowing. 10. The tapetum remains uninucleate throughout.

Synaptonemal complexes in the mosquito Culex quinquefasciatus

1978 ◽  
Vol 36 (2) ◽  
pp. 212-213
Author(s):  
Annelise Fiil
Keyword(s):  
Nuclear Membrane ◽  
Culex Quinquefasciatus ◽  
Meiotic Prophase ◽  
Crossing Over ◽  
Serial Sections ◽  
Homologous Chromosomes ◽  
Synaptonemal Complexes ◽  
Meiosis I

The presence of synaptonemal complexes between the paired homologous chromosomes at meiotic prophase is a prerequisite for meiotic crossing over, and it may be important for the regular disjunctions of the chromosomes at meiosis I (Moses, 1968; Westergaard and von Wettstein, 1972; Gillies, 1975). Reconstructions of nuclei during zygotene and pachytene have shown that the ends of the synaptonemal complexes in many organisms are attached to the nuclear membrane, often in a polarized fashion (Moens, 1969; Rasmussen, 1976); such a bouquet arrangement of the chromosomes is found in Culex.Materials and MethodsOvaries from Culex quinquefasciatus were fixed in glutaraldehyde, followed by 0s04, and embedded in Epon. The synaptonemal complexes were reconstructed from serial sections.Results and DiscussionCulex has 3 pairs of very long metacentric or slightly submetacentric chromosomes which during pachytene loop around the nucleus several times (Fig. 1). The centromeric regions are fused, and the synaptonemal complexes do not continue through the structure.

Effect of the Pairing Gene Ph1 and Premeiotic Colchicine Treatment on Intra- and Interchromosome Pairing of Isochromosomes in Common Wheat

Genetics ◽  
1998 ◽  
Vol 150 (3) ◽  
pp. 1199-1208 ◽  
Author(s):  
Juan M Vega ◽  
Moshe Feldman
Keyword(s):  
Common Wheat ◽  
Homologous Chromosome ◽  
Colchicine Treatment ◽  
Crossing Over ◽  
Meiotic Pairing ◽  
Factors Affecting ◽  
Homologous Chromosomes ◽  
Polyploid Wheat ◽  
Main Gene ◽  
Ph1 Gene

Abstract The analysis of the pattern of isochromosome pairing allows one to distinguish factors affecting presynaptic alignment of homologous chromosomes from those affecting synapsis and crossing-over. Because the two homologous arms in an isochromosome are invariably associated by a common centromere, the suppression of pairing between these arms (intrachromosome pairing) would indicate that synaptic or postsynaptic events were impaired. In contrast, the suppression of pairing between an isochromosome and its homologous chromosome (interchromosome pairing), without affecting intrachromosome pairing, would suggest that homologous presynaptic alignment was impaired. We used such an isochromosome system to determine which of the processes associated with chromosome pairing was affected by the Ph1 gene of common wheat—the main gene that restricts pairing to homologues. Ph1 reduced the frequency of interchromosome pairing without affecting intrachromosome pairing. In contrast, intrachromosome pairing was strongly reduced in the absence of the synaptic gene Syn-B1. Premeiotic colchicine treatment, which drastically decreased pairing of conventional chromosomes, reduced interchromosome but not intrachromosome pairing. The results support the hypothesis that premeiotic alignment is a necessary stage for the regularity of meiotic pairing and that Ph1 relaxes this alignment. We suggest that Ph1 acts on premeiotic alignment of homologues and homeologues as a means of ensuring diploid-like meiotic behavior in polyploid wheat.

MEIOSIS IN THE GRASSHOPPER. II. THE PRELEPTOTENE SPIRAL STAGE DURING OOGENESIS AND SPERMATOGENESIS IN MELANOPLUS FEMUR-RUBRUM

1972 ◽  
Vol 14 (2) ◽  
pp. 397-401 ◽  
Author(s):  
Kathleen Church
Keyword(s):  
Dna Synthesis ◽  
Crossing Over ◽  
Chromosome Behaviour ◽  
Homologous Chromosomes ◽  
Diploid Complement

Chromosome behaviour occurring from premeiotic DNA synthesis to leptotene of meiosis is described for both males (spermatogenesis) and females (oogenesis) in the grasshopper Melanoplus femur-rubrum. These events include a period of chromosome spiralization and contraction following premeiotic DNA synthesis and prior to leptotene. The diploid complement of chromosomes becomes visible in both sexes. No pairing between homologous chromosomes or chiasmata are observed in either sex. The results suggest that synapsis and crossing over must occur following preleptotene spiralization during spermatogenesis and oogenesis in this grasshopper.

scholarly journals The Arabidopsis HOP2 Gene Has a Role in Preventing illegitimate Exchanges between Nonhomologous Chromosomes

2021 ◽  
Author(s):  
YIsell Farahani-Tafreshi ◽  
Chun Wei ◽  
Peilu Gan ◽  
Jenya Daradur ◽  
C. Daniel Riggs ◽  
...  
Keyword(s):  
Active Role ◽  
Crossing Over ◽  
Wild Type ◽  
Positive Role ◽  
Homologous Chromosomes ◽  
Chromosome Synapsis ◽  
Radiation Induced ◽  
Nonhomologous Chromosome ◽  
Chromosome Exchanges

Meiotic homologous chromosomes pair up and undergo crossing over. In many eukaryotes both intimate pairing and crossing over require the induction of double stranded breaks (DSBs) and subsequent repair via Homologous Recombination (HR). In these organisms, two key proteins are the recombinases RAD51 and DMC1. Recombinase-modulators HOP2 and MND1 have been identified as proteins that assist RAD51 and DMC1 and are needed to promote stabilized pairing. We have probed the nature of the genetic lesions seen in hop2 mutants and looked at the role of HOP2 in the fidelity of genetic exchanges. Using γH2AX as a marker for unrepaired DSBs we found that hop2-1 and mnd1 mutants have different appearance/disappearance for DSBs than wild type, but all DSBs are repaired by mid-late pachytene. Therefore, the bridges and fragments seen from metaphase I onward are due to mis-repaired DSBs, not unrepaired ones. Studying Arabidopsis haploid meiocytes we found that wild type haploids produced the expected five univalents, but hop2-1 haploids suffered many illegitimate exchanges that were stable enough to produce bridged chromosomes during segregation. Our results suggest that HOP2 has a significant active role in preventing nonhomologous associations. We also found evidence that HOP2 plays a role in preventing illegitimate exchanges during repair of radiation-induced DSBs in rapidly dividing petal cells. Thus, HOP2 plays both a positive role in promoting homologous chromosome synapsis and a separable role in preventing nonhomologous chromosome exchanges. Possible mechanisms for this second important role are discussed.

Tapetum character states: analytical keys for tapetum types and activities

1997 ◽  
Vol 75 (9) ◽  
pp. 1448-1459 ◽  
Author(s):  
E. Pacini
Keyword(s):  
Pollen Development ◽  
Cell Walls ◽  
Pollen Grains ◽  
Cell Types ◽  
Plastid Differentiation ◽  
Pollen Coat ◽  
Tapetal Cells ◽  
Different Types ◽  
Compound Pollen ◽  
Pollen Formation

The different types of tapetum found in the spermatophyta are described, along with associated characters. The characters (taken singly, pairwise, or in multiple combinations) are (i) tapetum types; (ii) cell walls, tapetum types, and loculus; (iii) tapetal cells individually, tapetum types, and loculus; (iv) number of pollen grains enveloped by tapetal cells and type of pollen dispersing unit; (v) cell types and tapetum types; (vi) number of nuclei per cell and tapetum type; (vii) cycles of hyperactivity; (viii) exine formation; (ix) orbicles; (x) peritapetal membrane; (xi) plastid differentiation; (xii) stage of pollen development in which tapetal cells degenerate and type of pollen coat; (xiii) storage vacuoles; (xiv) sporophytic proteins; and (xv) devices of tapetal origin responsible for compound pollen formation and pollination. Examples are given and an analytical key of structural and functional diversity is provided as a helpful approach to the study of the tapetum. Key words: tapetum types, activities, pollen dispersing units.

scholarly journals Mechanistic insight into crossing over during mouse meiosis

2019 ◽  
Author(s):  
Shaun E. Peterson ◽  
Scott Keeney ◽  
Maria Jasin
Keyword(s):  
Inbred Strains ◽  
Strand Break ◽  
Crossing Over ◽  
Recombination Suppression ◽  
Heteroduplex Dna ◽  
Homologous Chromosomes ◽  
Mismatch Correction ◽  
D Loop ◽  
Loop Migration ◽  
Dna Ends

SUMMARYCharacteristics of heteroduplex DNA illuminate how strands exchange during homologous recombination, but mismatch correction can obscure them. To investigate recombination mechanisms, meiotic crossover products were analyzed at two hotspots in Msh2–/– mice containing homologous chromosomes derived from inbred strains. Recombination frequencies were unchanged in the mutant, implying that MSH2-dependent recombination suppression does not occur at this level of diversity. However, a substantial fraction of crossover products retained heteroduplex DNA in the absence of MSH2, and some also had multiple switches between parental markers suggestive of MSH2-independent correction. Recombinants appeared to reflect a biased orientation of crossover resolution, possibly stemming from asymmetry at DNA ends established in earlier intermediates. Many crossover products showed no evidence of heteroduplex DNA, suggesting dismantling by D-loop migration. Unlike the complexity of crossovers in yeast, these two modifications of the original double-strand break repair model may be sufficient to explain most meiotic crossing over in mice.

scholarly journals A rigorous measure of genome-wide genetic shuffling that takes into account crossover positions and Mendel’s second law

2017 ◽  
Author(s):  
Carl Veller ◽  
Nancy Kleckner ◽  
Martin A. Nowak
Keyword(s):  
Evolutionary Genetics ◽  
Selective Advantage ◽  
Second Law ◽  
Crossing Over ◽  
Homologous Chromosomes ◽  
Crossover Interference ◽  
Independent Assortment ◽  
Human Male ◽  
Genome Wide ◽  
High Level

AbstractComparative studies in evolutionary genetics rely critically on evaluation of the total amount of genetic shuffling that occurs during gamete production. However, such studies have been ham-pered by the fact that there has been no direct measure of this quantity. Existing measures consider crossing over by simply counting the average number of crossovers per meiosis. This is qualitatively inadequate because the positions of crossovers along a chromosome are also critical: a crossover towards the middle of a chromosome causes more shuffling than a crossover towards the tip. More-over, traditional measures fail to consider shuffling from independent assortment of homologous chromosomes (Mendel’s second law). Here, we present a rigorous measure of genome-wide shuffling that does not suffer from these limitations. We define the parameter r̅ as the probability that the alleles at two randomly chosen loci will be shuffled in the production of a gamete. This measure can be decomposed into separate contributions from crossover number and position and from independent assortment. Intrinsic implications of this metric include the fact that r̅ is larger when crossovers are more evenly spaced, which suggests a novel selective advantage of crossover interference. Utilization of r̅ is enabled by powerful emergent methods for determining crossover positions, either cytologically or by DNA sequencing. Application of our analysis to such data from human male and female reveals that: (i) r̅ in humans is close to its maximum possible value of 1/2, (ii) this high level of shuffling is due almost entirely to independent assortment, whose contribution is ~30 times greater than that of crossovers.

scholarly journals ZYP1 is required for obligate cross-over formation and cross-over interference in Arabidopsis

2021 ◽  
Vol 118 (14) ◽  
pp. e2021671118
Author(s):  
Martin G. France ◽  
Janina Enderle ◽  
Sarah Röhrig ◽  
Holger Puchta ◽  
F. Chris H. Franklin ◽  
...  
Keyword(s):  
Genetic Analysis ◽  
Synaptonemal Complex ◽  
Class Ii ◽  
Class I ◽  
Crossing Over ◽  
Wild Type ◽  
Homologous Chromosomes ◽  
Virtual Absence ◽  
Meiotic Progression ◽  
Null Mutants

The synaptonemal complex is a tripartite proteinaceous ultrastructure that forms between homologous chromosomes during prophase I of meiosis in the majority of eukaryotes. It is characterized by the coordinated installation of transverse filament proteins between two lateral elements and is required for wild-type levels of crossing over and meiotic progression. We have generated null mutants of the duplicated Arabidopsis transverse filament genes zyp1a and zyp1b using a combination of T-DNA insertional mutants and targeted CRISPR/Cas mutagenesis. Cytological and genetic analysis of the zyp1 null mutants reveals loss of the obligate chiasma, an increase in recombination map length by 1.3- to 1.7-fold and a virtual absence of cross-over (CO) interference, determined by a significant increase in the number of double COs. At diplotene, the numbers of HEI10 foci, a marker for Class I interference-sensitive COs, are twofold greater in the zyp1 mutant compared to wild type. The increase in recombination in zyp1 does not appear to be due to the Class II interference-insensitive COs as chiasmata were reduced by ∼52% in msh5/zyp1 compared to msh5. These data suggest that ZYP1 limits the formation of closely spaced Class I COs in Arabidopsis. Our data indicate that installation of ZYP1 occurs at ASY1-labeled axial bridges and that loss of the protein disrupts progressive coalignment of the chromosome axes.

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