Genetic collection of meiotic mutants of rye Secale cereale L

Genetic collection of meiotic mutants of winter rye Secale cereale L. (2n = 14) was created. Mutations were detected in inbred F2 generations after self-fertilization of the F1 hybrids, obtained by individual crossing of rye plants (cultivar Vyatka) or weedy rye with plants from autofertile lines. The mutations cause partial or complete plant sterility and are maintained in collection in a heterozygous state. Genetic analysis accompanied by cytogenetic study of meiosis has revealed six mutation types. (1) Nonallelic asynaptic mutations sy1 and sy9 caused the formation of only axial chromosome elements in prophase and anaphase. The synaptonemal complexes (SCs) were absent, the formation of the chromosome "bouquet" was impaired, and all chromosomes were univalent in meiotic metaphase I in 96% (sy1) and 67% (sy2) of cells. (2) Weak asynaptic mutation sy3, which hindered complete termination of synapsis in prophase II. Subterminal asynaptic segments were always observed in the SC, and at least one pair of univalents was present in metaphase I, but the number of cells with univalents did not exceed 2%. (3) Mutations sy2, sy6, sy7, sy8, sy10, and sy19, which caused partially nonhomologous synapsis: change in pairing partners and fold-back chromosome synapsis in prophase I. In metaphase I, the number of univalents varied and multivalents were observed. (4) Mutation mei6, which causes the formation of ultrastructural protrusions on the lateral SC elements, gaps and branching of these elements. (5) Allelic mutations mei8 and mei10, which caused irregular chromatin condensation along chromosomes in prophase I, sticking and fragmentation of chromosomes in metaphase I. (6) Allelic mutations mei5 and mei10, which caused chromosome hypercondensation, defects of the division spindle formation, and random arrest of cells at different meiotic stages. However, these mutations did not affect the formation of microspore envelopes even around the cells, whose development was blocked at prophase I. Analysis of cytological pictures of meiosis in double rye mutants reveled epistatic interaction in the mutation series sy9 > sy1 > sy3 > sy19, which reflects the order of switching these genes in the course of meiosis. The expression of genes sy2 and sy19 was shown to be controlled by modifier genes. Most meiotic mutations found in rye have analogs in other plant species.     

Genetic Collection of Meiotic Mutants of Rye <Emphasis Type="Italic">Secale cereale</Emphasis> L.

Genetic collection of meiotic mutants of winter rye Secale cereale L. (2n = 14) was created. Mutations were detected in inbred F₂ generations after self-fertilization of the F₁ hybrids, obtained by individual crossing of rye plants (cultivar Vyatka) or weedy rye with plants from autofertile lines. The mutations cause partial or complete plant sterility and are maintained in collection in a heterozygous state. Genetic analysis accompanied by cytogenetic study of meiosis has revealed six mutation types. (1) Nonallelic asynaptic mutations sy1 and sy9 caused the formation of only axial chromosome elements in prophase and anaphase. The synaptonemal complexes (SCs) were absent, the formation of the chromosome “bouquet” was impaired, and all chromosomes were univalent in meiotic metaphase I in 96.8% (sy1) and 67% (sy2) of cells. (2) Weak asynaptic mutation sy3, which hindered complete termination of synapsis in prophase I. Subterminal asynaptic segments were always observed in the SC, and at least one pair of univalents was present in metaphase I, but the number of cells with 14 univalents did not exceed 2%. (3) Mutations sy2, sy6, sy7, sy8, sy10, and sy19, which caused partially nonhomologous synapsis: change in pairing partners and fold-back chromosome synapsis in prophase I. In metaphase I, the number of univalents varied and multivalents were observed. (4) Mutation mei6, which causes the formation of ultrastructural protrusions on the lateral SC elements, gaps and branching of these elements. (5) Allelic mutations mei8 and mei8-10, which caused irregular chromatin condensation along chromosomes in prophase I, sticking and fragmentation of chromosomes in metaphase I. (6) Allelic mutations mei5 and mei10, which caused chromosome hypercondensation, defects of the division spindle formation, and random arrest of cells at different meiotic stages. However, these mutations did not affect the formation of microspore envelopes even around the cells, whose development was blocked at prophase I. Analysis of cytological pictures of meiosis in double rye mutants reveled epistatic interaction in the mutation series sy9 > sy1 > sy3 > sy19, which reflects the order of switching these genes in the course of meiosis. The expression of genes sy2 and sy19 was shown to be controlled by modifier genes. Most meiotic mutations found in rye have analogs in other plant species.     

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.     

Meiotic mutations in rye Secale cereale L

Spontaneous meiotic mutations of winter rye Secale cereale L. (2n = 14) were revealed in inbred F2 progenies, which were obtained by self-pollination of F1 hybrids resulting from crosses of individual plants of cultivar Vyatka or weedy rye with plants of self-fertile inbred lines. The mutations cause partial or complete sterility, and are maintained in heterozygote condition. Six types of mutations were distinguished as the result of cytological analysis of meiosis and genetic analysis. (1) Plants with nonallelic asynaptic mutations sy1 and sy9 lacked bivalents in 96.8 and 67.0% metaphase I cells, respectively, formed only axial elements but not the mature synaptonemal complex (SC), and had defects in telomere clustering in early prophase I. (2) Weak asynaptic mutant sy3 showed incomplete synapsis at the start of SC degradation at diplotene and lower chiasma number; yet only 2% meiocytes lacked bivalents in MI. (3) Mutations sy2, sy6, sy7, sy8, sy10, and sy19 caused nonhomologous synapsis; i.e., a varying number of univalents and occasional multivalents were observed in MI, which was preceded by switches of pairing partners and fold-back synapsis at mid-prophase I. (4) Mutation mei6 led to the formation of protrusions and minor branched structures of the SC lateral elements. (5) Allelic mutations mei8 and mei8-10 caused irregular chromatin condensation along the chromosome length in prophase I, which was accompanied by chromosome sticking and fragmentation in MI. (6) Allelic mutations mei5 and mei10 determined chromosome supercondensation, caused the disturbance of meiotic spindle assembly, arrested meiosis at various stages but did not affect formation of the pollen wall, thus arrested meiocytes got covered with the pollen wall. Analysis of double mutants revealed recessive epistatic interactions for some mutations; the epistatic group was sy9 > sy1 > sy3 > sy19. This reflects the sequence of meiotic events controlled by the corresponding genes. The expression of sy2 and sy19 proved to be modified by additional genes. Most meiotic mutations found in rye have analogs in other plants.     

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.     

Functional genomics identifies monopolin: a kinetochore protein required for segregation of homologs during meiosis i

The orderly reduction in chromosome number that occurs during meiosis depends on two aspects of chromosome behavior specific to the first meiotic division. These are the retention of cohesion between sister centromeres and their attachment to microtubules that extend to the same pole (monopolar attachment). By deleting genes that are upregulated during meiosis, we identified in Saccharomyces cerevisiae a kinetochore associated protein, Mam1 (Monopolin), which is essential for monopolar attachment. We also show that the meiosis-specific cohesin, Rec8, is essential for maintaining cohesion between sister centromeres but not for monopolar attachment. We conclude that monopolar attachment during meiosis I requires at least one meiosis-specific protein and is independent of the process that protects sister centromere cohesion.     

Identification of yeast meiosis-specific genes by differential display

Meiosis, a specialized cell division process, occurs in all sexually reproducing organisms. During this process a diploid cell undergoes a single round of DNA replication followed by two rounds of nuclear division to produce four haploid gametes. In yeast, the meiotic products are packaged into four spores that are enclosed in a sac known as an ascus. To enhance our understanding of the meiotic developmental pathway and spore formation, we followed differential expression of genes in meiotic versus vegetatively growing cells in the yeast Saccharomyces cerevisiae. Such comparative analyses have identified five different classes of genes that are expressed at different stages of the sporulation program. We identified several meiosis-specific genes including some already known to be induced during meiosis. Here we describe one of these previously uncharacterized genes, SSP1, which plays an essential role in meiosis and spore formation. SSP1 is induced midway through meiosis, and the homozygous mutant-diploid cells fail to sporulate. In ssp1 cells, meiosis is delayed, nuclei fragment after meiosis II, and viability declines rapidly. The ssp1 defect is not related to a microtubule-cytoskeletal-dependent event and is independent of two rounds of meiotic divisions. Our results suggest that Ssp1 is likely to function in a pathway that controls meiotic nuclear divisions and coordinates meiosis and spore formation. Functional analysis of other uncharacterized genes is underway.     

Some features of meiosis key events in rye and its synaptic mutants

The Peterhof Collection of spontaneous meiotic mutants of rye was used as a model to study the genetic control of meiosis key events in an organism with a large genome. A combination of methods, which included fluorescence in situ DNA-DNA hybridization, sequencing of recombinogenic proteins, and immunocytochemical analysis of meiosis proteins, clearly showed that mutation sy1 affects recombination events, asynapsis in mutant sy9 is connected with defects of the assembly of synaptonemal complex axial cores, and that synapsis defects in mutant sy10 are coupled with the presence of protein Zyp1 in the core region. The assembly of proteins Asy1 and Zyp1 on the axes of meiotic chromosomes was shown to occur separately, which is a specific feature of rye, as compared to arabidopsis.     

The expression of mutation sy2 causing nonhomologous synapsis in meiosis of diploid rye Secale cereale L

The cytological expression of spontaneous mutation sy2 isolated from a population of weedy rye was examined. It was demonstrated that the primary defect of meiosis in the mutant plants is nonhomologous synapsis, which occurs simultaneously with the homologous one. An electron microscope study of the synaptonemal complex (SC) at prophase I showed synaptic abnormalities that manifested as "switches" of synapting axial elements to the nonhomologous partner and the formation of foldbacks of lateral SC elements. The sy2 mutants are characterized by one to two such events per meiosis. Nonhomologous synapsis leads to the appearance of univalents at metaphase I (on average 4.16 +/- 0.022 per meiocyte) and multivalents (on average 0.12 +/- 0.007 per meiocyte). The presence of multivalents in 12.0% of meiocytes at metaphase I may result from recombination in ectopic regions of homology. It is suggested that the sy2 mutation impairs a component of the system that limits synapsis in meiocytes to only homologous chromosome pairs.     

Genetic Control of Meiosis in Plants

The genes controlling meiotic progression in plants and not affecting mitotic progression are most widely studied in maize Zea mays and cruciferous plant Arabidopsis thaliana. These include the genes controlling the differentiation of somatic cells into sporogenous ones and meiosis-initiating genes, genes encoding meiosis-specific proteins of chromosomes and synaptonemal complexes, genes of mediator proteins and enzymes of meiotic DNA recombination and crossover, and genes controlling meiosis-specific behavior of centromeres and the course of two meiotic divisions. A large number of such genes have been cloned and studied at the molecular level. The studies of meiotic genes in rice Oriza sativa are actively developing, while studies of corresponding genes in barley Hordeum vulgare, rye Secale cereale, tomato Solanum lycopersicum, and hexaploid wheat Triticum aestivum are less advanced. To identify meiotic genes, chemical and insertional mutagenesis, genetic and cytological analysis, genomic and proteomic studies, methods of reverse genetics, and bioinformatics are used.     

Chemical inactivation of cdc7 kinase in budding yeast results in a reversible arrest that allows efficient cell synchronization prior to meiotic recombination

Genetic studies in budding yeast have provided many fundamental insights into the specialized cell division of meiosis, including the identification of evolutionarily conserved meiosis-specific genes and an understanding of the molecular basis for recombination. Biochemical studies have lagged behind, however, due to the difficulty in obtaining highly synchronized populations of yeast cells. A chemical genetic approach was used to create a novel conditional allele of the highly conserved protein kinase Cdc7 (cdc7-as3) that enables cells to be synchronized immediately prior to recombination. When Cdc7-as3 is inactivated by addition of inhibitor to sporulation medium, cells undergo a delayed premeiotic S phase, then arrest in prophase before double-strand break (DSB) formation. The arrest is easily reversed by removal of the inhibitor, after which cells rapidly and synchronously proceed through recombination and meiosis I. Using the synchrony resulting from the cdc7-as3 system, DSB-dependent phosphorylation of the meiosis-specific chromosomal core protein, Hop1, was shown to occur after DSBs. The cdc7-as3 mutant therefore provides a valuable tool not only for understanding the role of Cdc7 in meiosis, but also for facilitating biochemical and cytological studies of recombination.     

Meiotic genes and proteins in cereals

We review the current status of our understanding and knowledge of the genes and proteins controlling meiosis in five major cereals, rye, wheat, barley, rice and maize. For each crop, we describe the genetic and genomic infrastructure available to investigators, before considering the inventory of genes and proteins that have roles to play in this process. Emphasis is given throughout as to how translational genomic and proteomic approaches have enabled us to circumvent some of the intractable features of this important group of plants.     

A phylogenomic inventory of meiotic genes; evidence for sex in Giardia and an early eukaryotic origin of meiosis

Sexual reproduction in eukaryotes is accomplished by meiosis, a complex and specialized process of cell division that results in haploid cells (e.g., gametes). The stereotypical reductive division in meiosis is a major evolutionary innovation in eukaryotic cells, and delineating its history is key to understanding the evolution of sex. Meiosis arose early in eukaryotic evolution, but when and how meiosis arose and whether all eukaryotes have meiosis remain open questions. The known phylogenetic distribution of meiosis comprises plants, animals, fungi, and numerous protists. Diplomonads including Giardia intestinalis (syn. G. lamblia) are not known to have a sexual cycle; these protists may be an early-diverging lineage and could represent a premeiotic stage in eukaryotic evolution. We surveyed the ongoing G. intestinalis genome project data and have identified, verified, and analyzed a core set of putative meiotic genes-including five meiosis-specific genes-that are widely present among sexual eukaryotes. The presence of these genes indicates that: (1) Giardia is capable of meiosis and, thus, sexual reproduction, (2) the evolution of meiosis occurred early in eukaryotic evolution, and (3) the conserved meiotic machinery comprises a large set of genes that encode a variety of component proteins, including those involved in meiotic recombination.     

Conserved Meiotic Genes Point to Sex in the Choanoflagellates

ABSTRACT. The choanoflagellates are a widespread group of heterotrophic aquatic nanoflagellates, which have recently been confirmed as the sister-group to Metazoa. Asexual reproduction is the only mode of cell division that has been observed within the group; at present the range of reproductive modes, as well as the ploidy level, within choanoflagellates are unknown. The recent discovery of long terminal repeat retrotransposons within the genome of Monosiga brevicollis suggests that this species also has sexual stages in its life cycle because asexual organisms cannot tolerate retrotransposons due to the rapid accumulation of deleterious mutations caused by their transposition. We screened the M. brevicollis genome for known eukaryotic meiotic genes, using a recently established "meiosis detection toolkit" of 19 genes. Eighteen of these genes were identified, none of which appears to be a pseudogene. Four of the genes were also identified in expressed sequence tag data from the distantly related Monosiga ovata . The presence of these meiosis-specific genes provides evidence for meiosis, and by implication sex, within this important group of protists.     

Genetic control of chromosomal synapsis in Secale cereale L. rye meiosis: sy19 gene, causing heterologous synapsis

Analysis of manifestation and inheritance of a new mutation inducing irregular synapsis in rye showed that abnormal phenotype is determined by a recessive allele of the sy19 gene. In the homozygotes for this mutation, even at the light microscopic level, abnormal formation of bivalents is already observed at pachytene-diakinesis. At metaphase I, the univalent frequency varies from 0 to 14; in a few cells, multivalent associations of chromosomes, which are not clearly oriented in the spindle, are detected. Electron microscopy of synaptonemal complexes revealed both homologous and heterologous synapsis in homozygotes for sy19, namely partial loss of the ability to stringent homology search. Analysis of joint inheritance of sy19 and asynaptic sy1 mutations showed that they are nonallelic, inherited independently, and interact by recessive epistasis. The phenotype of double sy1sy19 mutants indicates that the sy19 gene conditioning heterologous synapsis operates at meiosis later than the synaptic gene sy1. The epistatic group of mutations, sy9 > sy1 > sy19 and sy3, was determined.     

Molecular and genetic analysis of the yeast early meiotic recombination genes REC102 and REC107/MER2.

By selecting for mutations which could rescue the meiotic lethality of a rad52 spo13 strain, we isolated several new Rec genes required relatively early in the meiotic recombination process. This paper presents data to confirm that two of them, REC102 and REC107, are general, meiosis-specific recombination genes that have no detectable role during mitosis. Sequence analysis and genetic complementation indicate that REC107 is identical to the MER2 gene. No sequences related to REC102 have been found in the GenBank or EMBL collections. REC102 is expressed only in meiosis, prior to the reductional division, at about the time that genetic recombination occurs. Examination of the REC102 sequence indicates the presence of several sequences which may play a role in the regulation of its expression; however, the URS1 sequence commonly found in genes expressed early in meiosis is not present.