LIN-41 inactivation leads to delayed centrosome elimination and abnormal chromosome behavior during female meiosis in Caenorhabditis elegans

During oogenesis, two successive meiotic cell divisions occur without functional centrosomes because of the inactivation and subsequent elimination of maternal centrosomes during the diplotene stage of meiosis I. Despite being a conserved phenomenon in most metazoans, the means by which this centrosome behavior is controlled during female meiosis remain elusive. Here, we conducted a targeted RNAi screening in the Caenorhabditis elegans gonad to identify novel regulators of centrosome behavior during oogenesis. We screened 513 genes known to be essential for embryo production and directly visualized GFP–γ-tubulin to monitor centrosome behavior at all stages of oogenesis. In the screening, we found that RNAi-mediated inactivation of 33 genes delayed the elimination of GFP–γ-tubulin at centrosomes during oogenesis, whereas inactivation of nine genes accelerated the process. Depletion of the TRIM-NHL protein LIN-41 led to a significant delay in centrosome elimination and to the separation and reactivation of centrosomes during oogenesis. Upon LIN-41 depletion, meiotic chromosomes were abnormally condensed and pulled toward one of the two spindle poles around late pachytene even though the spindle microtubules emanated from both centrosomes. Overall, our work provides new insights into the regulation of centrosome behavior to ensure critical meiotic events and the generation of intact oocytes.     

A spontaneous derivative of abnormal chromosome 10 in maize

Summary A new type of abnormal chromosome 10 has been found among maize plants grown from seeds sent by Dr. Y. C. Ting of Harvard University. This chromosome deviates in its morphology from the orthodox abnormal chromosome 10 described by Rhoades (1952) and from the one described by Ting (1958b). It produces a low degree of neo-centric activity.Cytological observations of plants heterozygous for the new abnormal chromosome 10 and either an orthodox abnormal chromosome 10 or a normal one, have suggested that the new type was derived from an orthodox abnormal 10 through spontaneous breakage and loss of an important piece of its long arm. The delection involved the distal part of the long arm of orthodox abnormal chromosome 10, proximally limited by the third most distal dissimilar and prominent chromomere. This corresponds approximately to the extra segment at the end of orthodox abnormal chromosome 10 which remains unpaired in heterozygotes with the normal 10. It bears a large heterochromatic knob. The missing piece is a part of the larger fraction of the long arm of orthodox abnormal chromosome 10 that remains unaffected by crossingover in a heteromorphic bivalent having a normal chromosome 10 (telo-segment). The telo-segment has its proximal limit at the left of the most proximal of the 3 dissimilar chromomeres, probably between the R and Sr ₂ loci. It has been proposed that a factor or factors responsible for neo-centric activity are located in the portion of the telosegment between its proximal limit and the third most distal dissimilar chromomere (3 dissimilar chromomere region).Since the telo-segment of the orthodox abnormal 10 also bears a large knob in its distal half, it has been suggested that this segment has a dual role in neo-centric activity. The factor or factors located in the proximal piece of the telo-segment would stimulate over-abundance of fiber-forming substance, whereas local production of chromosomal fibers would depend ultimately on the knob's activity.If the large knob is absent, its role in neo-centric activity would be transferred to the next smaller and distally located hetero-chromatic mass, such as the knob-like body near the end of the new abnormal 10 which results from the fusion of the two most proximal prominent chromomeres of the telo-segment.This work has been partly done in the United States, under an I.C.A. — National Academy of Sciences fellowship.     

The relationship of abnormal chromosome 10 to B-chromosomes in maize

Abnormal chromosome 10 (K10) is known to increase recombination in maize and to induce preferential segregation in knobbed heterozygotes during megasporogenesis. In spite of the considerable interest generated by these findings, the origin of the K10 chromosome is unknown. It has been postulated that the extra segment of K10 arose by simple translocation between normal 10 and a B-chromosome. This hypothesis was tested by comparing meiosis in haploids with either K10 or the normal 10 and carrying a single B-chromosome. The frequency of bivalent configurations was found to be similar in the two types of haploids suggesting that the K10 and B-chromosomes do not share homologies that lead to chiasma formation. These results lend no support to the hypothesis that the K10 chromosome came from a B type. The implications of these results to the action of K10 at meiosis are also discussed.     

Molecular perspectives of chromosome pairing at meiosis

Ideas about the mechanisms that regulate chromosome pairing, recombination, and segregation during meiosis have gained in molecular detail over the last few years. The purpose of this article is to survey briefly the shifts in paradigms and experiments that have generated new perspectives. It has never been very clear what it is that brings together the homologous chromosomes at meiotic prophase. For a while it appeared that the synaptonemal complex might be the nuclear organelle responsible for synapsis, but the supporting evidence has not been entirely convincing. Whatever the mechanism, it has always been assumed that homologous synapsis creates the opportunity for homologous DNA sequences to initiate recombination. At present, alternative ideas are developing. Attractive is the concept that double strand DNA repair mechanisms, that find and use the undamaged homologue for repair, have evolved into a meiotic mechanism for the recognition and pairing of homologous sequences. Subsequent intimate synapsis of homologous chromosomes in the context of the synaptonemal complex may serve later functions in the regulation of interference and segregation at first anaphase. A number of areas that are being tested at present and some that may be investigated in the future are discussed at the end of the review.     

Evidence for a role of the synaptonemal complex in provision for normal chromosome disjunction at meiosis II in maize

The meiotic behavior of heterozygotes from three different maize pericentric inversion stocks was quantitatively observed at a variety of stages throughout meiosis I and II. With heterozygosity for either of two of these inversions, the usual mode of pairing observed at pachytene involved synapsis of the centromere containing inverted region, and synaptic failure of the centromere region was rarely found. Abnormal chromosome behavior at subsequent meiotic stages was rare in these cases. With heterozygosity for the third inversion, however, homologous synapsis was generally found in the distal regions of the chromosome involved, the inverted region was often non-homologously synapsed, and a substantial frequency of cells apparently showed synaptic failure in the centromere containing inverted region. A substantial frequency of cells at anaphase II in this case contained two lagging monads in the plate region of the spindle. Where cells could be identified as sisters, sister cells showed identical behavior at anaphase II. Findings seem to be most simply explained by the supposition that pachytene synapsis of the centromere region is important to provision for sister centromere association until anaphase II.     

Four loci on abnormal chromosome 10 contribute to meiotic drive in maize

We provide a genetic analysis of the meiotic drive system on maize abnormal chromosome 10 (Ab10) that causes preferential segregation of specific chromosomal regions to the reproductive megaspore. The data indicate that at least four chromosomal regions contribute to meiotic drive, each providing distinct functions that can be differentiated from each other genetically and/or phenotypically. Previous reports established that meiotic drive requires neocentromere activity at specific tandem repeat arrays (knobs) and that two regions on Ab10 are involved in trans-activating neocentromeres. Here we confirm and extend data suggesting that only one of the neocentromere-activating regions is sufficient to move many knobs. We also confirm the localization of a locus/loci on Ab10, thought to be a prerequisite for meiotic drive, which promotes recombination in structural heterozygotes. In addition, we identified two new and independent functions required for meiotic drive. One was identified through the characterization of a deletion derivative of Ab10 [Df(L)] and another as a newly identified meiotic drive mutation (suppressor of meiotic drive 3). In the absence of either function, meiotic drive is abolished but neocentromere activity and the recombination effect typical of Ab10 are unaffected. These results demonstrate that neocentromere activity and increased recombination are not the only events required for meiotic drive.     

Peculiarities of chromosome organization in meiosis

Meiotic and mitotic chromosomes have a complex of differences. (1) At the early prophase I of meiosis, chromosomes acquire protein axial elements (AEs) that were absent in mitosis; in addition to somatic cohesins, AEs contain the meiosis-specific cohesins REC8, SMC1β, and STAG3. (2) At the middle prophase I, protein lateral elements (LEs) of synaptonemal complexes (SCs) are formed on the basis of AEs. The LE proteins are not conserved, but in Saccharomyces cerevisiae and Arabidopsis thaliana they contain functional domains with conserved secondary structures. Among the almost 679 thousand proteins of primitive eukaryotes that we studied by bioinformatics methods, in green and brown algae, some lower fungi, and Coelenterata, we revealed proteins or functional domains similar to SC proteins. (3) During the pachytene and diplotene stages of meiosis, chromosomes of spermatocytes and mother pollen cells acquire a general structure resembling the structure of amphibian and avian lampbrush chromosomes in miniature. Lateral chromatin loops with sizes of 90, 160, and even over 480 Kb were observed in human spermatocytes during the diplotene stage. In combination, all these observations confirm the considerable conservation of the scheme of molecular and ultrastructural organization of meiotic chromosomes in a large variety of eukaryotic organisms.     

Confocal analysis of chromosome behavior in wheat x maize zygotes.

Herein, we profile the first embryonic mitosis in a hybrid of wheat and maize by using a whole-mount genomic in situ hybridization method and immunofluorescence staining with a tubulin-specific antibody. We have successfully captured the dynamics of each set of parental chromosomes in the first zygotic division of the hybrid embryo 24-28 h after crossing. During the first zygotic metaphase, although both sets of parental chromosomes congressed into the equatorial plate of the zygote, the maize chromosomes tended to lag in comparison with the wheat chromosomes. During anaphase, each parental chromosome separated into its sister chromosomes; however, some of the maize chromosomes lagged around the metaphase plate as segregants. The maize sister chromosomes that did move toward the pole showed delayed and asymmetric movement as compared with the wheat ones. Immunological staining of tubulin revealed a bipolar spindle structure in the first zygotic metaphase. The kinetochores of the maize chromosomes that lagged around the metaphase plate did not attach to the spindle microtubules. These results suggest that factors on the kinetochores of maize chromosomes that are required to control chromosome movement are deficient in the zygotic cell cycle.     

The meiotic drive system on maize abnormal chromosome 10 contains few essential genes

In maize, a distal portion of abnormal chromosome 10 (Ab10) causes the meiotic drive of itself as well as many unlinked heterochromatic regions known as knobs. The Ab10 drive system, which encodes trans- as well as cis-acting components, occupies a large region of chromosome 10L equivalent to 3% of the genome. Here we describe five new structural mutations of Ab10 (five deletions and a duplication) that arose from a screen for meiotic drive mutants. The high frequency of breakage events, detected both genetically and cytologically, suggest that the chromosome may be especially unstable. Very large deletions within the drive system are female-transmissible and plants homozygous for deficiencies lacking much of this interval can be grown to maturity. The data suggest that few genes required for normal growth and development lie within the portion of Ab10 responsible for meiotic drive. These and other published data suggest that meiotic drive systems tend to evolve in gene-sparse or otherwise information-poor regions of the genome where they are less likely to negatively affect individual fitness.     

Differential chromosome pairing affinities at meiosis in polyploid sugarcane revealed by molecular markers

Chromosome pairing at meiosis is an essential feature in cell biology, which determines trait inheritance and species evolution. Complex polyploids may display diverse pairing affinities and offer favorable situations for studying meiosis. The genus Saccharum encompasses diverse forms of polyploids with predominantly bivalent pairing. We have focused on a modern cultivar of sugarcane, R570, and taken advantage of a particular single copy probe (BNL 12.06) revealing 11 alleles by restriction fragment length polymorphism (RFLP). As for other cultivars, R570 is highly polyploid (2n=ca. 115) and indirectly derived from interspecific hybridization between Saccharum officinarum (2n=80, x=10) and S. spontaneum (2n=40-128, x=8). Here we determined the doses of the various BNL12.06 RFLP alleles among 282 progeny of R570 and estimated the mutual pairing frequencies among the corresponding homo- or homoeologous chromosomes using a maximum likelihood method. The result is an atypical picture, with pairing frequencies ranging from 0 to 40% and differential affinities leading to the identification of several chromosome subsets. This example illustrates the unsystematic meiotic behavior in a complex polyploid. It highlights a continuous range of pairing affinities between chromosomes and pinpoints a strong role of individual chromosome features, partly related to their ancestral origin, in the determination of these affinities.     

药蒲公英减数分裂异常行为探讨

对药蒲公英减数分裂各期进行了观察,研究得出药蒲公英花蕾直径大小与花粉母细胞减数分裂各期之间的关系(花蕾直径在2-7mm时为减数分裂期)。并发现药蒲公英减数分裂中出现许多异常行为。如后期桥和落后染色体;药蒲公英花粉粒空瘪,这些异常行为的原因是减数分裂过程中有倒位和重复缺失等染色体结构变异出现以至形成双着丝点染色体。减数分裂过程的异常行为也说明药蒲公英是多倍体。     

Chemically induced premature chromosome condensation in short-term cultured human peripheral lymphocytes: applications to biodosimetry

We describe here the chemical induction of premature condensed chromosomes in human peripheral lymphocytes after culture for 6 h. Many have attempted this induction without culture or with short-term culture, because this technique permits prompt cytogenetic biodosimetry of radiation accidents. Lymphocytes were separated from blood, incubated in the presence of phytohemagglutinin, ATP, and p34^cdc2/cyclin B kinase, then treated with calyculin A during the last hour. The culture medium was supplemented with a lower concentration of fetal calf serum than conventionally used to minimize its possible interference with the effects of these drugs. We obtained, rarely, a suitable morphology of premature chromosome condensation in short-term cultured lymphocytes for conventional chromosome aberration analysis.