Segregation Of Univalents Research Articles

Crossover shortage in potato is caused by StMSH4 mutant alleles and leads to either highly uniform unreduced pollen or sterility.

The balanced segregation of homologous chromosomes during meiosis is essential for fertility and is mediated by crossovers (COs). A strong reduction of CO number leads to the unpairing of homologous chromosomes after the withdrawal of the synaptonemal complex. This results in the random segregation of univalents during meiosis I and ultimately to the production of unbalanced and sterile gametes. However, if CO shortage is combined with another meiotic alteration that restitutes the first meiotic division, then uniform and balanced unreduced male gametes, essentially composed of nonrecombinant homologs, are produced. This mitosis-like division is of interest to breeders because it transmits most of the parental heterozygosity to the gametes. In potato, CO shortage, a recessive trait previously referred to as desynapsis, was tentatively mapped to chromosome 8. In this article, we have fine-mapped the position of the CO shortage locus and identified StMSH4, an essential component of the class I CO pathway, as the most likely candidate gene. A 7 base-pair insertion in the second exon of StMSH4 was found to be associated with CO shortage in our mapping population. We also identified a second allele with a 3,820 base-pair insertion and confirmed that both alleles cannot complement each other. Such nonfunctional alleles appear to be common in potato cultivars. More than half of the varieties we tested are carriers of mutational load at the StMSH4 locus. With this new information, breeders can choose to remove alleles associated with CO shortage from their germplasm to improve fertility or to use them to produce highly uniform unreduced male gametes in alternative breeding schemes.

Bivalent individualization during chromosome territory formation in Drosophila spermatocytes by controlled condensin II protein activity and additional force generators.

Reduction of genome ploidy from diploid to haploid necessitates stable pairing of homologous chromosomes into bivalents before the start of the first meiotic division. Importantly, this chromosome pairing must avoid interlocking of non-homologous chromosomes. In spermatocytes of Drosophila melanogaster, where homolog pairing does not involve synaptonemal complex formation and crossovers, associations between non-homologous chromosomes are broken up by chromosome territory formation in early spermatocytes. Extensive non-homologous associations arise from the coalescence of the large blocks of pericentromeric heterochromatin into a chromocenter and from centromere clustering. Nevertheless, during territory formation, bivalents are moved apart into spatially separate subnuclear regions. The condensin II subunits, Cap-D3 and Cap-H2, have been implicated, but the remarkable separation of bivalents during interphase might require more than just condensin II. For further characterization of this process, we have applied time-lapse imaging using fluorescent markers of centromeres, telomeres and DNA satellites in pericentromeric heterochromatin. We describe the dynamics of the disruption of centromere clusters and the chromocenter in normal spermatocytes. Mutations in Cap-D3 and Cap-H2 abolish chromocenter disruption, resulting in excessive chromosome missegregation during M I. Chromocenter persistence in the mutants is not mediated by the special system, which conjoins homologs in compensation for the absence of crossovers in Drosophila spermatocytes. However, overexpression of Cap-H2 precluded conjunction between autosomal homologs, resulting in random segregation of univalents. Interestingly, Cap-D3 and Cap-H2 mutant spermatocytes displayed conspicuous stretching of the chromocenter, as well as occasional chromocenter disruption, suggesting that territory formation might involve forces unrelated to condensin II. While the molecular basis of these forces remains to be clarified, they are not destroyed by inhibitors of F actin and microtubules. Our results indicate that condensin II activity promotes chromosome territory formation in co-operation with additional force generators and that careful co-ordination with alternative homolog conjunction is crucial.

Reverse breeding: A modern plant breeding approach for hybrid recreation

Reverse breeding is a modern plant breeding method for producing complementing parental lines for any heterozygous plant through achaismatic meiosis (meiosis without crossovers). The achiasmatic meiosis leads to univalent segregation at meiotic metaphase-I and the generation of aneuploid gametes. These gametes are then regenerated as doubled-haploid (DH) plants. Each DH carries combinations of its parental chromosomes, and complementing DH pairs can be crossed to reconstitute the initial hybrid. In reverse breeding, the suppression of meiotic crossovers in a hybrid ensures the transmission of non-recombinant chromosomes to haploid gametes. The PAIR2 gene is required for homologous chromosome synapsis at meiosis-I in plants. An insertional mutation in the rice PAIR2 gene, the ortholog of Arabidopsis thaliana ASY1, results in a defect in homologous chromosome pairing during meiosis, display univalents at metaphase-I. Essentially, reverse breeding follows an approach akin to the generation of a DH population from an F1 hybrid, carrying a dominant-acting transgene that down-regulates the expression of Disrupted Meiotic cDNA1 (DMC1), resulting in inhibition of crossover recombination and thereby enabling intact-chromosome inheritance. In earlier reports on reverse breeding in A. thaliana, a hybrid was constructed of using two of its natural ecotypes (Col-0 and Laer-0), carrying an RNAi transgene targeting the meiotic recombinase (RecA homolog) DMC1 that prevented the formation of meiotic crossover recombination. This method mainly included steps: (i) the generation and selection of RNAi: DMC1 transformed lines; (ii) the generation of achiasmatic hybrids; (iii) the crossing of achiasmatic hybrids to GFP-tailswap to generate haploid chromosome substitution lines (CSLs); (iv) the generation of DHs by spontaneous doubling of haploid CSLs; and (v) the crossing of complementing CSLs to recreate the initial hybrid. The scope of reverse breeding could be envisioned for the improvement of agricultural crops, as it may enable the generation of parental breeding lines for the recreation of hybrid.

Segregation of the amphitelically attached univalent X chromosome in the spittlebug Philaenus spumarius.

In meiosis I, homologous chromosomes combine to form bivalents, which align on the metaphase plate. Homologous chromosomes then separate in anaphase I. Univalent sex chromosomes, on the other hand, are unable to segregate in the same way as homologous chromosomes of bivalents due to their lack of a homologous pairing partner in meiosis I. Here, we studied univalent segregation in a Hemipteran insect: the spittlebug Philaenus spumarius. We determined the chromosome number and sex determination mechanism in our population of P. spumarius and showed that, in male meiosis I, there is a univalent X chromosome. We discovered that the univalent X chromosome in primary spermatocytes forms an amphitelic attachment to the spindle and aligns on the metaphase plate with the autosomes. Interestingly, the X chromosome remains at spindle midzone long after the autosomes have separated. In late anaphase I, the X chromosome initiates movement towards one spindle pole. This movement appears to be correlated with a loss of microtubule connections between the kinetochore of one chromatid and its associated spindle pole.

Separase Is Required for Homolog and Sister Disjunction during Drosophila melanogaster Male Meiosis, but Not for Biorientation of Sister Centromeres.

Spatially controlled release of sister chromatid cohesion during progression through the meiotic divisions is of paramount importance for error-free chromosome segregation during meiosis. Cohesion is mediated by the cohesin protein complex and cleavage of one of its subunits by the endoprotease separase removes cohesin first from chromosome arms during exit from meiosis I and later from the pericentromeric region during exit from meiosis II. At the onset of the meiotic divisions, cohesin has also been proposed to be present within the centromeric region for the unification of sister centromeres into a single functional entity, allowing bipolar orientation of paired homologs within the meiosis I spindle. Separase-mediated removal of centromeric cohesin during exit from meiosis I might explain sister centromere individualization which is essential for subsequent biorientation of sister centromeres during meiosis II. To characterize a potential involvement of separase in sister centromere individualization before meiosis II, we have studied meiosis in Drosophila melanogaster males where homologs are not paired in the canonical manner. Meiosis does not include meiotic recombination and synaptonemal complex formation in these males. Instead, an alternative homolog conjunction system keeps homologous chromosomes in pairs. Using independent strategies for spermatocyte-specific depletion of separase complex subunits in combination with time-lapse imaging, we demonstrate that separase is required for the inactivation of this alternative conjunction at anaphase I onset. Mutations that abolish alternative homolog conjunction therefore result in random segregation of univalents during meiosis I also after separase depletion. Interestingly, these univalents become bioriented during meiosis II, suggesting that sister centromere individualization before meiosis II does not require separase.

The effect of wheat-rye substitution on chromosome elimination: An analysis of univalents’ behavior in wheat meiosis with dimonosomy and tetramonosomy

The effect of wheat-rye chromosome 1Rv/1A, 2R/2D and 6R/6A substitutions characterized by differences in the expression of the equational division of sister centromeres in anaphase I on segregation and the elimination of wheat and rye univalents was investigated. To determine the individual effect of each of the studied chromosomal pairs, a comparative analysis of the univalent behavior in the meiosis of double monosomics 1Rv-1A, 2R-2D, 6R-6A and tetramonosomics 1Rv-2R-1A-2D, 1Rv-6R-1A-6A, 2R-6R-2D-6A was conducted. 2R/2D substitution was experimentally demonstrated to suppress an equational univalents division, while 6R/6A substitution resulted in high frequency chromosomes' elimination, especially in the meiosis of 2R-6R-2D-6A tetramonosomics. Other meiotic mechanisms, together with the sister chromatids separation at anaphase I, may affect the elimination of the final products of univalent segregation. It was demonstrated that the number and pattern of univalent chromosomal behavior affected hybrid plant fertility.

An Analysis of Univalent Segregation in Meiotic Mutants ofArabidopsis thaliana: A Possible Role for Synaptonemal Complex

During first meiotic prophase, homologous chromosomes are normally kept together by both crossovers and synaptonemal complexes (SC). In most eukaryotes, the SC disassembles at diplotene, leaving chromosomes joined by chiasmata. The correct co-orientation of bivalents at metaphase I and the reductional segregation at anaphase I are facilitated by chiasmata and sister-chromatid cohesion. In the absence of meiotic reciprocal recombination, homologs are expected to segregate randomly at anaphase I. Here, we have analyzed the segregation of homologous chromosomes at anaphase I in four meiotic mutants of Arabidopsis thaliana, spo11-1-3, dsy1, mpa1, and asy1, which show a high frequency of univalents at diplotene. The segregation pattern of chromosomes 2, 4, and 5 was different in each mutant. Homologous univalents segregated randomly in spo11-1-3, whereas they did not in dsy1 and mpa1. An intermediate situation was observed in asy1. Also, we have found a parallelism between this behavior and the synaptic pattern displayed by each mutant. Thus, whereas spo11-1-3 and asy1 showed low amounts of SC stretches, dsy1 and mpa1 showed full synapsis. These findings suggest that in Arabidopsis there is a system, depending on the SC formation, that would facilitate regular disjunction of homologous univalents to opposite poles at anaphase I.

Mechanisms of Meiotic Restitution and Their Genetic Regulation in Wheat–Rye Polyhaploids

Meiosis has been studied in partially fertile wheat-rye F1 hybrids yielded by crosses Triticum aestivum (Saratovskaya 29 variety) x Secale cereale L. (Onokhoiskaya variety) (4x = 28). Hybrid self-fertility proved to be caused by formation of restituted nuclei, which appear after equational segregation of univalent chromosome in AI and sister chromatid non-separation in AII of meiosis, as well as after AI blockage in three different ways. Both types of meiotic restitution were found in each hybrid plant. Expression of the "meiotic restitution" trait varied significantly in polyhaploids of the same genotype (ears of the same plants, anthers of the same ear, microsporocytes of the same anther). Chromatin condensation in prophase proved to be related to the division type and univalent segregation in AI. During reduction segregation of univalents in AI, sister chromatid cohesion and chromosome supercondensation remained unchanged. The results obtained suggest that in the remote hybrids with haploid karyotype of the parental origin (polyhaploids), the program of two-stage meiosis may be fundamentally transformed to ensure one instead of two divisions. We propose that meiotic restitution is a result of special genetic regulation of the kinetochore organization (both structural and functional) and chromatin condensation, i.e. of major meiotic mechanisms.

How meiotic cells deal with non-exchange chromosomes.

The chromosomes which segregate in anaphase I of meiosis are usually physically bound together through chiasmata. This association is necessary for proper segregation, since univalents sort independently from one another in the first meiotic division and this frequently leads to genetically unbalanced offspring. There are, however, a number of species where genetic exchanges in the form of meiotic cross-overs, the prerequisite of the formation of chiasmata, are routinely missing in one sex or between specific chromosomes. These species nevertheless manage to segregate these non-exchange chromosomes. There are four direct modes for associating achiasmatic chromosomes: (a) modified SC, (b) adhesion of chromatids comparable to somatic pairing, (c) 'stickiness' of heterochromatin or (d) specific 'segregation bodies', consisting of material structurally different from chromatin. There is also the possibility that the spindle-possibly joining forces with the kinetochores--carries out the faithful segregation of univalents which are not directly physically attached to one another. Finally, amphitelic orientation of univalents in metaphase I and pairing of the chromatids in meiosis II appear to ensure correct segregation as well.

Meiosis in Mesostoma ehrenbergii ehrenbergii (Turbellaria, Rhabdocoela). III. Univalent chromosome segregation during the first meiotic division in spermatocytes.

At metaphase I of meiosis in spermatocytes of Mesostoma ehrenbergii ehrenbergii [2n = 10] three bivalents and four univalents form. The same two chromosome pairs always form the univalents. Analysis of metaphase I, anaphase I and metaphase II configurations in fixed testis material suggested that the distribution of the four univalents is not a random process but the correct segregation of one member of each pair to each pole is actively achieved before the end of metaphase I. In live preparations of testis material univalents were observed to move between the poles of metaphase I cells, eventually reaching the correct segregation. All cells observed to enter anaphase I had the correct segregation of univalents. It is proposed that the univalent movement during metaphase I is directed towards obtaining the correct segregation of univalents before the cells enter anaphase.

Time sequence of in vitro maturation and chromosomal normality in metaphase I and metaphase II of the squirrel monkey (Saimiri sciureus) oocyte.

The in vitro maturation sequence of squirrel monkey oocytes and their chromosomal normality was studied. Follicular growth was induced by a hormonal regimen of follicle-stimulating hormone (FSH) and human chorionic gonadotropin (hCG) and oocytes were collected laparoscopically 15–16 h after hCG administration from 55 squirrel monkeys. A total of 666 oocytes were prepared and analyzed for maturational stage and the presence of chromosomal aberrations. At recovery, 84.7% of the aspirated oocytes were in the genninal vesicle or prophase I stage and the numbers of atretic and mature ova were low (15.3% and 8.3%, respectively). The highest rate of in vitro matured oocytes (75.2%) was found after 31 h of incubation. In the chromosomal analysis studies of 366 oocytes, 31(8.5%) metaphase 1, 14 (3.8%) anaphase I and 181 (49.5%) metaphase II oocytes were examined. Of 28 countable metaphase I plates, 21 (75.0%) consisted of 22 bivalents, however meiotic univalents were also observed in 7 (25.0%) cases. Of these, 2 showed a secondary association. Of 181 oocytes, 145 (80.1%) were analyzable numerically. Of these, 88 (60.7%) had the normal haploid 22 chromosome number, whereas 7 (4.8%) showed aneuploidy (4 hyperploidy and 3 hypoploidy). An additional 50 (34.5%) oocytes were found to be at the metaphase II stage with first polar body chromosomes. Of these, 43 had a normal haploid number of chromosomes (22/22). In 7 (4.8%), aneuploidy was observed (6, 21/23 and 1, 20/24). The unequal chromosome number was assumed to be due to a random segregation of univalents at the anaphase I stage. Additionally there was no significant difference in maturation rate among 3 sizes of follicles (1–2 mm, 2–3 mm, >3 mm). A significant correlation was found for the maturation rate between oocytes with and without cumulus cells after 21 h of culture.

Segregation of univalents on mini spindles.

MANY authors have reported that the behaviour of univalent chromosomes at meiosis is irregular. At metaphase they tend to lie off the metaphase plate and closer to the poles than the bivalents; and it has been suggested that this is because the univalents move to the poles earlier. In the triploid species Leucopogon juniperinus univalents are close to the poles at metaphase, but serial sections show that they are not in the same plane as the spindle on which the bivalents are positioned (Fig. 1). Closer examination of the metaphase cells shows that the univalents are attached to a separate, much reduced spindle, which is attached to the same pole as the “bivalent” spindle but is formed in a different plane (Fig. 1, cells 2 and 3).

Studies on Ambrosia: III. Pistillate Ambrosia elatior x A. trifida and Its Bearing on Matroclinic Sex Inheritance

1. Pistillate plants of A. elatior always yield a mixed progeny of monoecious plants, intergrades, and pistillate individuals, irrespective of the genotype of the pollen parent. 2. Pistillate plants were crossed with A. trifida, which is strictly monoecious in nature. Seven hybrids were secured: two pistillate, four intergrades, and one normal monoecious. 3. The hybrids had approximately 30 somatic chromosomes (A. trifida n = 12, A. elatior n = 18). The first meiotic division was extremely irregular and involved primarily a separation of autosynaptic mates and a random segregation of univalents. The second division was regular and equational.