Pairing Competition between Identical and Homologous Chromosomes in Autotetraploid Rye. I. Submetacentric Chromosomes

Meiotic pairing preferences between identical and homologous but not identical chromosomes were analyzed in ten induced tetraploid/diploid chimaeral rye plants (Secale cereale) heterozygous for telomeric heerochromatin C-bands in both arms of chromosome 1R. These plants were the progeny of two crosses between only one plant of cv. Petkus, used as male, and two plants of the inbred lines E and R, respectively. Different pairing preferences for chromosome 1R were found: (1) between plants, (2) between chromosome arms within the same plant and (3) between bivalents and multivalents within the same plant. The possible influence in the preferences of several factors such as differences in C-heterochromatin content in the chromosomes analyzed, specific genetic control and independence in pairing behavior between both arms and partner exchange is discussed.     

Pairing Competition between Identical and Homologous Chromosomes in Rye and Grasshoppers

Meiotic pairing preferences between identical and homologous but not identical chromosomes were analyzed in spontaneous tetraploid/diploid chimeras of three male grasshoppers (Eyprepocnemis plorans) whose chromosome pair 11 were heterozygous for C-banding pattern and in four induced tetraploid/diploid chimaeral rye plants (Secale cereale) heterozygous for telomeric heterochromatin C-bands in chromosomes 1R and 2R. In the grasshoppers, a preference for identical over homologous pairing was observed, whereas in rye both a preference for homologous rather than identical pairing and random pairing between the four chromosomes of the set was found. From the results in rye, it can be deduced that pairing preferences do not depend exclusively on the similarities between chromosomes involved. It is suggested that genotypic or cryptic structural differences between the homologous chromosomes of each pair analyzed might be responsible for the pairing preferences found. This hypothesis can also explain the results obtained in grasshoppers, although the possibility of premeiotic association cannot be excluded in this material.     

Linkage of Genes Controlling Morphological Traits with Isozyme Markers of Rye Chromosomes

Data on linkage of 12 rye genes controlling morphological traits (el, Vs, ln, w, np, ct2, Hs, Ddw, cb, mn, vi1, mp) with one or several isozyme markers of individual rye chromosomes (2R–7R) are presented. Linkage of the following gene pairs was established: chromosome 2R: Est3/5–el, el–-Glu, Sod2–el, Sod2–Vs; chromosome 3R: ln–Got4; chromosome 4R: w–Got1, np–Got1; chromosome 5R: Est4–ct2, Est6/9–ct2, ct2–Est2, ct2–Aco2, Est2–Hs, Aco2–Hs, Est2–Ddw, Aco2–Ddw; chromosome 6R:Lap2–cb, cb–Aco1, Est10–mn; chromosome 7R: Acph2/3–vi1, Got2–vi1, mp–Acph2/3. The reasons for mapping a very small number of genes in rye in spite of high intraspecific variability of this species are discussed. An approach is suggested to improve this situation by simultaneous identification and mapping of all diverse spontaneous mutations maintained in heterozygous state in various rye cultivars.     

Evolution of Genomic Structures on Mammalian Sex Chromosomes

Throughout mammalian evolution, recombination between the two sex chromosomes was suppressed in a stepwise manner. It is thought that the suppression of recombination led to an accumulation of deleterious mutations and frequent genomic rearrangements on the Y chromosome. In this article, we review three evolutionary aspects related to genomic rearrangements and structures, such as inverted repeats (IRs) and palindromes (PDs), on the mammalian sex chromosomes. First, we describe the stepwise manner in which recombination between the X and Y chromosomes was suppressed in placental mammals and discuss a genomic rearrangement that might have led to the formation of present pseudoautosomal boundaries (PAB). Second, we describe ectopic gene conversion between the X and Y chromosomes, and propose possible molecular causes. Third, we focus on the evolutionary mode and timing of PD formation on the X and Y chromosomes. The sequence of the chimpanzee Y chromosome was recently published by two groups. Both groups suggest that rapid evolution of genomic structure occurred on the Y chromosome. Our re-analysis of the sequences confirmed the species-specific mode of human and chimpanzee Y chromosomal evolution. Finally, we present a general outlook regarding the rapid evolution of mammalian sex chromosomes.     

黑麦染色体的银染研究

在对黑麦染色体银染过程的盐酸解离条件进行探索的同时,对黑麦染色体的银染正反应区进行了研究,首次发现经短时间空气干燥(4-24h)的黑麦染色体制片,随着盐酸解离强度的递增,分别出现了核仁组织区(NOR)、NOR和端粒以及NOR和着丝点的银染正反应,就此现象讨论了端粒和着丝点的银染机理。     

Genomic Subtraction Recovers Rye-Specific DNA Elements Enriched in the Rye Genome

Repetitive DNA sequence families have been identified in methylated relic DNAs of rye. This study sought to isolate rye genome-specific repetitive elements regardless of the level of methylation, using a genomic subtraction method. The total genomic DNAs of rye-chromosome-addition-wheat lines were cleaved to short fragments with a methylation-insensitive 4-bp cutter, MboI, and then common DNA sequences between rye and wheat were subtracted by annealing with excess wheat genomic DNA. Four classes of rye-specific repetitive elements were successfully isolated from both the methylated and non-methylated regions of the genome. Annealing of the DNA mixture at a ratio of the enzyme-restricted fragments:the sonicated fragments (1:3–1:5) was key to this success. Two classes of repetitive elements identified here belong to representative repetitive families: the tandem 350-family and the dispersed R173 family. Southern blot hybridization patterns of the two repetitive elements showed distinct fragments in methylation-insensitive EcoO109I digests, but continuous smear signals in the methylation-sensitive PstI and SalI digests, indicating that both of the known families are contained in the methylated regions. The subtelomeric tandem 350-family is organized by multimers of a 380-bp-core unit defined by the restriction enzyme EcoO109I. The other two repetitive element classes had new DNA sequences (444, 89 bp) and different core-unit sizes, as defined by methylation-sensitive enzymes. The EcoO109I recognition sites consisting of PyCCNGGPu-multi sequences existed with high frequency in the four types of rye repetitive families and might be a useful tool for studying the genomic organization and differentiation of this species.     

The Study of Joint Inheritance of Mutations Impairing the Structure of Meiotic Chromosomes in Rye Secale cereale L.

Genetic analysis has demonstrated that meiotic mutations mei8 (irregular condensation and fragmentation of meiotic chromosomes) andmei10 (chromosome overcompaction) are nonallelic. Mutation mei10 exhibits digenic inheritance (with a segregation ratio of 13 : 3) in the combinations of crosses studied. It is assumed that the phenotypic expression of mutation mei10is suppressed by the effect of recessive genelch1 or lch2 (long chromosomes), both of which have been revealed in one of the parental lines (Mc10). These genes determine weak condensation of meiotic chromosomes. In double mutantsmei8 mei10, the mutations are expressed independently of each other. Gene mei10 is linked with gene mei8(r^ = 36.8 ± 5.38%); genes lch1 and lch2 are not linked either with them or with each other. Taking into account the data on the linkage between genes mei10and sy10 and between mei8andsy10, the order of genes in the linkage group is shown to be the following: mei8–sy10–mei10.     

Abnormal Condensation of Meiotic Chromosomes Caused by the mei8 Mutation in Rye Secale cereale L.

Inheritance of two spontaneous meiosis-specific mutations with similar cytologic phenotype was studied. Both mutations were independently obtained from two rye populations (Vyatka variety and weedy rye). Both mutations are recessive, allelic, and monogenically inherited; the corresponding gene is designated mei8. The mutant alleles of the gene cause abnormal meiotic chromosome structure expressed as irregular compaction along the chromosome length, chromatin stickiness at all stages of meiosis, and chromosome fragmentation in anaphase I.     

Formation and Expression of Pseudogenes on the B Chromosome of Rye

B chromosomes (Bs) are dispensable components of the genomes of numerous species. In contrast with the prevalent view that Bs do not harbor genes, our recent sequence analysis revealed that Bs of rye (Secale cereale) are rich in gene-derived sequences. We compared these gene-like fragments of the rye B with their ancestral A-located counterparts and confirmed an A chromosomal origin and the pseudogenization of B-located gene-like fragments. About 15% of the pseudogene-like fragments on Bs are transcribed in a tissue-type and genotype-specific manner. In addition, B-located sequences can cause in trans down- or upregulation of A chromosome–encoded genic fragments. Phenotypes and effects associated with the presence of Bs might be explained by the activity of B-located pseudogenes. We propose a model for the evolution of B-located pseudogenes.     

The Structure of Mature Rye Endosperm

Endosperm tissue of mature kernels of rye (Secale cereale L.)cv. Dominant was examined by light and transmission electronmicroscopy. It was found that storage protein in sub-aleuronecells occupies up to 35 per cent of the cell volume and formsa continuous matrix in which starch grains and cytoplasmic remnantsare embedded. In the prismatic endosperm, the storage proteinis present as a fine network interspersed between the numeroustype A and B starch grains. Protein bodies are not found inthe prismatic endosperm; only a few, less than 1 µm indiameter, are observed in pockets of disorganized cytoplasmin the sub-aleurone tissue. Thick cell walls and intercellularmaterial may contribute to the high pentosan content of ryeendosperm. Secale cereale L., rye, endosperm, protein matrix, ultrastructure     

Genetic Analysis of Mutation sy2 which Causes Nonhomologous Meiotic Synapsis in Chromosomes of Diploid Rye Secale cereale L.

Partially nonhomologous (heterologous) synapsis of meiotic chromosomes in a spontaneous desynaptic mutant form of rye is determined by two recessive genes, sy2a and sy2b, that have independent expression and inheritance. The third gene, dominant inhibitor suppressing the mutant phenotype, has been revealed in hybrid combinations between sy2 mutants and lines segregating other meiotic mutants: sy10 (heterologous synapsis), sy1, and sy9(asynapsis). All three genes determining desynapsis (sy2a, sy2b, and I) were shown to be nonallelic to monogenic mutations sy10, sy1, and sy9, inherited independently of them and expressed at later stages of prophase I than the sy10 gene. The possibility of modifying monogenic segregation of mutation sy2 by gametophyte selection for a locus linked to the gene expressed as sy2 at particular frequencies of recombination between this gene and selected locus is discussed.     

Single Origin of Sex Chromosomes and Multiple Origins of B Chromosomes in Fish Genus Characidium

Chromosome painting with DNA probes obtained from supernumerary (B) and sex chromosomes in three species of fish genus Characidium (C. gomesi, C. pterostictum and C. oiticicai) showed a close resemblance in repetitive DNA content between B and sex chromosomes in C. gomesi and C. pterostictum. This suggests an intraspecific origin for B chromosomes in these two species, probably deriving from sex chromosomes. In C. oiticicai, however, a DNA probe obtained from its B chromosome hybridized with the B but not with the A chromosomes, suggesting that the B chromosome in this species could have arisen interspecifically, although this hypothesis needs further investigation. A molecular phylogenetic analysis performed on nine Characidium species, with two mtDNA genes, showed that the presence of heteromorphic sex chromosomes in these species is a derived condition, and that their origin could have been unique, a conclusion also supported by interspecific chromosome painting with a CgW probe derived from the W chromosome in C. gomesi. Summing up, our results indicate that whereas heteromorphic sex chromosomes in the genus Characidium appear to have had a common and unique origin, B chromosomes may have had independent origins in different species. Our results also show that molecular phylogenetic analysis is an excellent complement for cytogenetic studies by unveiling the direction of evolutionary chromosome changes.     

Nondisjunction in Favor of a Chromosome: The Mechanism of Rye B Chromosome Drive during Pollen Mitosis

B chromosomes (Bs) are supernumerary components of the genome and do not confer any advantages on the organisms that harbor them. The maintenance of Bs in natural populations is possible by their transmission at higher than Mendelian frequencies. Although drive is the key for understanding B chromosomes, the mechanism is largely unknown. We provide direct insights into the cellular mechanism of B chromosome drive in the male gametophyte of rye (Secale cereale). We found that nondisjunction of Bs is accompanied by centromere activity and is likely caused by extended cohesion of the B sister chromatids. The B centromere originated from an A centromere, which accumulated B-specific repeats and rearrangements. Because of unequal spindle formation at the first pollen mitosis, nondisjoined B chromatids preferentially become located toward the generative pole. The failure to resolve pericentromeric cohesion is under the control of the B-specific nondisjunction control region. Hence, a combination of nondisjunction and unequal spindle formation at first pollen mitosis results in the accumulation of Bs in the generative nucleus and therefore ensures their transmission at a higher than expected rate to the next generation.     

Determinants of Human Cyclin B1 Association with Mitotic Chromosomes

Cyclin B1–CDK1 activity is essential for mitotic entry, but questions remain regarding how the activity of this kinase is spatially regulated. Previous studies showed that the cyclin B1 subunit localizes to several compartments of a mitotic cell, including the centrosomes, mitotic spindle, kinetochores and chromosomes via distinct sequence elements. Mitotic chromosome association occurs through the unstructured N-terminal domain of cyclin B1 and is independent of CDK1 binding. Here, we use live cell imaging of human cyclin B1 fused to GFP to precisely define the sequence elements within cyclin B1 that mediate its association with condensed mitotic chromosomes. We find that a short, evolutionarily conserved N-terminal motif is required for cyclin B1 to localize to mitotic chromosomes. We further reveal a role for arginine residues within and near the destruction box sequence in the chromosome association of cyclin B1. Additionally, our data suggest that sequences further downstream in cyclin B1, such as the cytoplasmic retention sequence and the cyclin box, may negatively modulate chromosome association. Because multiple basic residues are required for cyclin B1 association with mitotic chromosomes, electrostatic interactions with DNA may facilitate cyclin B1 localization to chromosomes.     

B染色体起源的研究进展

B染色体(简写为Bs)起源的传统观点是认为它起源于携带者所在物种的基因组.目前又发现了许多新的证据来推测Bs的起源.它可能具有两种起源:种内起源和种间起源(起源于另一物种的基因组),且有证据表明同一物种的Bs可能是多次起源,同时对Bs的起源机制也作了总结.认为B8起源的研究已取得了一定的进展,但要给Bs起源下一个确切的结论仍需要大量的实验证据.