S. P. Sosnikhina

Meiotic mutants of rye Secale cereale L. : I. Synaptic mutant sy-1.

SummaryA mutant form of weedy rye characterized by male and female sterility and having a hereditary block in the chromosome synapsis has been found and described. Genetic analysis has shown the synapsis block to be determined by the recessive allele of a gene designated as sy-1. Electron microscopy of surface-spread microsporocyte nuclei revealed the complete absence of the synaptonemal complex over the whole meiotic prophase I, although the axial cores were perfectly formed by each chromosome. Only univalents were observed at metaphase I, their average number ranging from 13.1 to 14.0 per cell. A precocious distribution of univalents at the poles is observed at metaphase I. All of the later stages of meiosis were irregular and resulted in the formation of abnormal microspores. Thus, the mutant proves to be asynaptic because of the blocked initiation of synapses at prophase 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.

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 96.8% (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.

Genetic Control of Chromosome Synapsis at Meiosis in Rye Secale cereale L.: The sy19 Gene Controlling 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 doublesy1sy19 mutants indicates that thesy19 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.

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 were 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 meiocyte. Nonhomologous synapsis leads to the appearance of univalents at metaphase I (on average 4.16 ± 0.002 per meiocyte) and multivalents (on average 0.12 ± 0.007 per meiocyte). The presence of multivalents in 12% 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.

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.

Molecular genetic mapping of the sy1 and sy9 asynaptic genes in rye (Secale cereale L.) using microsatellite and isozyme markers

Studies of phenotypical expression of synaptic mutations in combination with the localization of corresponding genes on a genetic map permit individual stages of the meiotic process to be differentiated. Two rye asynaptic genes, sy1 and sy9, were mapped with the use of microsatellite markers (SSR) in the pericentromeric regions of the long chromosome arms 7R and 2R, respectively. The sy9 gene cosegregated with two SSR markers Xscm43 and Xgwm132. The asynaptic gene sy1 was mapped within the interval between the isozyme locus Aat2 and two cosegregating loci Xrems1188 and Xrems1135 that are located at a distance of 0.4 cM proximally and 0.1 cM distally with respect to the gene lous. Possible evolutionary relationships of the mapped genes with homeological loci of the Triticeae species and more distant cereal species, such as maize and rice, are discussed.

Expression and Inheritance of a desynaptic phenotype with impaired homologous synapsis in rye

The cytological phenotype was studied in a desynaptic form isolated from a population of rye cultivar Vyatka. The primary defect of desynaptic plants was identified as nonhomologous (heterologous) chromosome synapsis, which was observed by electron microscopy of synaptonemal complexes (SCs) in meiotic prophase I. Synapsis defects involved switches of synapsing axial elements to nonhomologous partners, asynapsis in the switching region, and foldbacks formed by the SC lateral elements. Defective bivalent formation was observed at later stages: the univalent number varied and multivalent chromosome associations were observed in single cells in metaphase I. The desynaptic phenotype was controlled by two recessive genes, sy8a and sy8b, which acted and were inherited independently. In a hybrid combination with line Ku-2/63, the desynaptic phenotype was suppressed by the dominant allele of a third gene for inhibitor I; the segregation in hybrid families corresponded to 57:7.

Impairment of homologous chromosome synapsis in meiosis in rye Secale cereale L. Caused by a recessive mutation of the sy18 gene

Expression and inheritance of the sy18 mutation causing impairment of synapsis homology were studied. It was established that the abnormal phenotype is determined by a recessive allele of the sy18 gene. Univalents and multivalents are observed in homozygotes for this mutant allele. According to the electron microscopic analysis of synaptonemal complexes in mutants, homologous synapsis occurs together with nonhomologous synapsis. The sy18 gene was found to have no allelism with asynaptic genes sy1 and sy9 and with genes sy10 and sy19 causing, like sy18, disturbances in synapsis homology.

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.