Genetic Features of Triticale–Wheat Hybrids with Vaviloid-Type Spike Branching

Vaviloid spike branching, also called sham ramification, is a typical trait of Triticum vavilovii Jakubz. and is characterized by a lengthening of the spikelet axis. In this article, we present the results of a study of three triticale–wheat hybrid lines with differences in terms of the manifestation of the vaviloid spike branching. Lines were obtained by crossing triticale with hexaploid wheat, T. aestivum var. velutinum. The parental triticale is a hybrid of synthetic wheat (T. durum × Ae. tauschii var. meyrei) with rye, S. cereale ssp. segetale. Line 857 has a karyotype corresponding to hexaploid wheat and has a spike morphology closest to normal, whereas Lines 808/1 and 844/4 are characterized by the greatest manifestation of vaviloid spike branching. In Lines 808/1 and 844/4, we found the substitution 2RL(2DL). The karyotypes of the latter lines differ in that a pair of telocentric chromosomes 2DS is detected in Line 808/1, and these telocentrics are fused into one unpaired chromosome in Line 844/4. Using molecular genetic analysis, we found a deletion of the wheat domestication gene Q located on 5AL in the three studied hybrid lines. The deletion is local since an analysis of the adjacent gene B1 showed the presence of this gene. We assume that the manifestation of vaviloid spike branching in two lines (808/1 and 844/4) is associated with a disturbance in the joint action of genes Q and AP2L2-2D, which is another important gene that determines spike morphology and is located on 2DL.

Male meiotic spindle features that efficiently segregate paired and lagging chromosomes

Chromosome segregation during male meiosis is tailored to rapidly generate multitudes of sperm. Little is known about mechanisms that efficiently partition chromosomes to produce sperm. Using live imaging and tomographic reconstructions of spermatocyte meiotic spindles in Caenorhabditis elegans, we find the lagging X chromosome, a distinctive feature of anaphase I in C. elegans males, is due to lack of chromosome pairing. The unpaired chromosome remains tethered to centrosomes by lengthening kinetochore microtubules, which are under tension, suggesting that a ‘tug of war’ reliably resolves lagging. We find spermatocytes exhibit simultaneous pole-to-chromosome shortening (anaphase A) and pole-to-pole elongation (anaphase B). Electron tomography unexpectedly revealed spermatocyte anaphase A does not stem solely from kinetochore microtubule shortening. Instead, movement of autosomes is largely driven by distance change between chromosomes, microtubules, and centrosomes upon tension release during anaphase. Overall, we define novel features that segregate both lagging and paired chromosomes for optimal sperm production.

Male meiotic spindle features that efficiently segregate paired and lagging chromosomes

Chromosome segregation during male meiosis is tailored to rapidly generate multitudes of sperm. Little, however, is known about the mechanisms that efficiently segregate chromosomes to produce sperm. Using live imaging in Caenorhabditis elegans, we find that spermatocytes exhibit simultaneous pole-to-chromosome shortening (anaphase A) and pole-to-pole elongation (anaphase B). Electron tomography unexpectedly revealed that spermatocyte anaphase A does not stem from kinetochore microtubule shortening. Instead, movement is driven by changes in distance between chromosomes, microtubules, and centrosomes upon tension release at anaphase onset. We also find that the lagging X chromosome, a distinctive feature of anaphase I in C. elegans males, is due to lack of chromosome pairing. The unpaired chromosome remains tethered to centrosomes by continuously lengthening kinetochore microtubules which are under tension, suggesting a ‘tug of war’ that can reliably resolve chromosome lagging. Overall, we define features that partition both paired and lagging chromosomes for optimal sperm production.

Spatial Regulation of Polo-like Kinase Activity during C. elegans Meiosis by the Nucleoplasmic HAL-2/HAL-3 Complex

Proper partitioning of homologous chromosomes during meiosis relies on the coordinated execution of multiple interconnected events: Homologs must locate, recognize and align with their correct pairing partners. Further, homolog pairing must be coupled to assembly of the synaptonemal complex (SC), a meiosis-specific tripartite structure that maintains stable associations between the axes of aligned homologs and regulates formation of crossovers between their DNA molecules to create linkages that enable their segregation. Here we identify HAL-3 (Homolog Alignment 3) as an important player in coordinating these key events during C. elegans meiosis. HAL-3 and the previously-identified HAL-2 are interacting and interdependent components of a protein complex that localizes to the nucleoplasm of germ cells. hal-3 (or hal-2) mutants exhibit multiple meiotic prophase defects including failure to establish homolog pairing, inappropriate loading of SC subunits onto unpaired chromosome axes, and premature loss of synapsis checkpoint protein PCH-2. Further, loss of hal function results in misregulation of the subcellular localization and activity of polo-like kinases (PLK-1 and PLK-2), which dynamically localize to different defined subnuclear sites during wild-type prophase progression to regulate distinct cellular events. Moreover, loss of PLK-2 activity partially restores tripartite SC structure in a hal mutant background, suggesting that the defect in pairwise SC assembly in hal mutants reflects inappropriate PLK activity. Together our data support a model in which the nucleoplasmic HAL-2/HAL-3 protein complex constrains both localization and activity of meiotic Polo-like kinases, thereby preventing premature interaction with stage-inappropriate targets.

Synaptonemal complex analysis of domestic sheep (Ovis aries) with Robertsonian translocations. II. Trivalent and pairing abnormalities in Massey I and Massey II heterozygotes

Zygotene and pachytene spermatocytes from Massey I (t1 5;26) and Massey II (t2 8;11) translocation heterozygotes each contained one trivalent, often delayed in pairing, while cells from double Massey translocation heterozygotes had two such trivalents. As meiosis progressed, trivalents became fully paired, with acrocentric axes in a cis configuration. Abnormal pairing configurations often resulted from interactions between unpaired chromosome axes or segments. However, when two Massey trivalents were present in the same nucleus, there was no pairing interaction between them. In different Massey translocation heterozygotes, trivalent-involved pairing abnormalities occurred in 14–28% of cells, with XY–trivalent and XY–bivalent–trivalent associations being as high as 7.1–23.1%. In spermatocytes from single and double Massey translocation heterozygotes with normal-sized testes, the total SC abnormality frequency was 34.4% for the t1 heterozygotes, 27.1% for the t2 heterozygotes, and 21.4% for the double heterozygote. One Massey II heterozygote with one normal and one small testis had significantly higher SC abnormality frequency (54%) than normal rams. A trisomic cell was recorded in one ram and two hyperdiploid cells in another ram, but these were unrelated to the translocations. It is suggested that resolution of pairing abnormalities by synaptic adjustment is important in reducing the effects on fertility of the translocations.Key words: sheep, Robertsonian translocation, trivalent, abnormal pairing configuration.

Evolution and the meaning of metaphase

We used an evolutionary test to ask whether the congression of chromosomes to the spindle equator is important in itself or just a mitotic happenstance. If congression matters, then it might evolve if absent initially. Previous workers established that newly made trivalents, meiotic units of three chromosomes, generally do not congress to the spindle equator. Instead, these young trivalents lie close to the pole to which two of the three chromosomes are oriented. We studied ancient sex-chromosome trivalents that arose hundreds of thousands to several million years ago in several species of praying mantids and one grasshopper. All these old trivalents lie near the spindle equator at metaphase; some of them congress as precisely to the equator as the ordinary chromosomes in the same cells. We conclude that congression evolved independently two or three times in the materials studied. Therefore, the metaphase position of chromosomes midway between the poles appears to matter, but why? In the praying mantids, the evident answer is that metaphase is a quality-control checkpoint. Sometimes the three chromosomes are not associated in a trivalent but rather are present as a bivalent plus an unpaired chromosome, which lies near one pole. Earlier workers showed that such cells are blocked in metaphase and eventually degenerate; this prevents the formation of sperm with abnormal combinations of sex chromosomes. We suggest that the quality-control system would have trouble distinguishing an unpaired chromosome from an uncongressed, newly arisen trivalent, both of which would lie near a spindle pole. If so, the confused quality-control system would block anaphase imprudently, causing a loss of cells that would have produced normal sperm. Hence, we conclude that the congression of the trivalent to the equator probably evolved along with the metaphase quality-control checkpoint. The mechanism of congression in old trivalents is uncertain, but probably involves an interesting force-sensitive regulation of the motors associated with particular chromosomes. We also examined the congression of two newly made quadrivalents when they orient with three kinetochores to one pole and one to the other. As others have described, one of these quadrivalents does not congress, while the other quadrivalent comes closer than expected to the spindle equator. Such variation in the extent of congression may provide materials on which natural selection can act, leading to the evolution of congression. The trivalents of praying mantids are attractive materials for further studies of the mechanism of congression and of the idea that metaphase is a checkpoint for progression through the cell cycle.

Meiosis in Saccharomyces cerevisiae mutants lacking the centromere-binding protein CP1.

CP1 (encoded by the CEP1 gene) is a centromere binding protein of Saccharomyces cerevisiae that binds to the conserved DNA element I (CDEI) of yeast centromeres. To investigate the function of CP1 in yeast meiosis, we analyzed the meiotic segregation of CEN plasmids, nonessential chromosome fragments (CFs) and chromosomes in cep1 null mutants. Plasmids and CFs missegregated in 10-20% of meioses with the most frequent type of aberrant event being precocious sister segregation at the first meiotic division; paired and unpaired CFs behaved similarly. An unpaired chromosome I homolog (2N + 1) also missegregated at high frequency in the cep1 mutant (7.6%); however, missegregation of other chromosomes was not detected by tetrad analysis. Spore viability of cep1 tetrads was significantly reduced, and the pattern of spore death was nonrandom. The inviability could not be explained solely by chromosome missegregation and is probably a pleiotropic effect of cep1. Mitotic chromosome loss in cep1 strains was also analyzed. Both simple loss (1:0 segregation) and nondisjunction (2:0 segregation) were increased, but the majority of loss events resulted from nondisjunction. We interpret the results to suggest that CP1 generally promotes chromatid-kinetochore adhesion.

Chromosome pairing in the Triticum monococcum complex: evidence for pairing control genes

The occurrence and behavior of pairing control genes at the diploid level were analyzed by using the models and equations developed by Jackson and co-workers. It appears that all pairing control genes are codominant and they are detectable only as heterozygotes in diploids. The phenotypic expressions of such genes are the production of univalents, and this is positively correlated with the occurrence of unpaired chromosome segments at pachytene. Analyses of a large number of accessions of the Triticum monococcum complex and various hybrid combinations have also shown that pairing control gene alleles occur within accessions of the same diploid species and in intraspecific and interspecific hybrids. Strength differences among the pairing control alleles are indicated by different frequencies of univalents at the diploid level, and the occurrence of pairing control genes is not correlated with taxonomic units.Key words: mathematical models, meiotic analyses, pairing control gene, heterozygotes, pachytene, Triticum diploids.