▪ Abstract Meiotic chromosomes have been studied for many years, in part because of the fundamental life processes they represent, but also because meiosis involves the formation of homolog pairs, ...

Meiotic chromosomes: integrating structure and function.

Meiosis is a special type of cell division that produces haploid gametes from diploid parental cells. Chromosome number is reduced during meiosis because a single round of DNA replication is followed by two rounds of chromosome segregation (Fig. 1). Fusion of two gametes during sexual reproduction restores the diploid chromosome complement. The second division of meiosis (the equational division) resembles mitosis: Sister chromatids separate and segregate. The first division, however, is unique. Reductional chromosome segregation at meiosis I differs from mitosis and meiosis II in a number of respects. First, sister chromatids remain associated with each other. Second, the two copies of the same chromosome (called homologous chromosomes or homologs) behave in a coordinate fashion, such that one chromosome moves to one pole of the spindle apparatus and its homolog moves to the opposite pole. This coordination in chromosome behavior depends on complex processes and elaborate structures that bring homologs together during meiotic prophase and hold them together until the transition between metaphase I and anaphase I. In most organisms, the relevant processes include alignment of homologs, assembly of the synaptonemal complex (SC, described below), genetic recombination, and the formation of chiasmata (stable connections between homologs formed at the sites of crossovers). These events occur during a very lengthy prophase that is divided into a series of substages based on changes in chromosome morphology (Table 1). This article reviews our current knowledge of the processes and structures that promote and maintain interactions between homologs and thereby ensure proper reductional chromosome segregation. Emphasis is placed on observations made in recent years, particularly those that have enhanced our understanding of the molecular mechanisms underlying meiotic chromosome behavior.

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Meiotic chromosomes: it takes two to tango

The Saccharomyces cerevisiae Spo11 protein catalyses DNA double-strand breaks (DSBs) that initiate meiotic recombination. The model plant Arabidopsis thaliana possesses at least three SPO11 homologues. T-DNA and ethyl-methane sulfonate mutagenesis allowed us to show that meiotic progression is altered in plants in which the AtSPO11-1 gene is disrupted. Both male and female meiocytes formed very few bivalents. Furthermore, no fully synapsed chromosomes were observed during prophase I. Later, in meiosis I, we observed that chromosomes segregated randomly, leading to the production of a large proportion of non-functional gametes. These meiotic aberrations were associated with a drastic reduction in meiotic recombination. Thus, our data show that initiation of meiotic recombination by SPO11- induced DSBs is a mechanism conserved in plants. Furthermore, unlike Drosophila and Caenorhabditis elegans, but like fungi, SPO11 is necessary for normal synapsis in plants.

https://www.embopress.org/doi/pdf/10.1093/emboj/20.3.589

AtSPO11‐1 is necessary for efficient meiotic recombination in plants

In eukaryotic cells, sister DNA molecules remain physically connected from their production at S phase until their separation during anaphase. This cohesion is essential for the separation of sister chromatids to opposite poles of the cell at mitosis. It also permits chromosome segregation to take place long after duplication has been completed. Recent work has identified a multisubunit complex called cohesin that is essential for connecting sisters. Proteolytic cleavage of one of cohesin's subunits may trigger sister separation at the onset of anaphase.

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Splitting the chromosome: cutting the ties that bind sister chromatids.

We have studied four Caenorhabditis elegans homologs of the Rad21/Scc1/Rec8 sister-chromatid cohesion protein family. Based on the RNAi phenotype and protein localization, it is concluded that one of them, W02A2.6p, is the likely worm ortholog of yeast Rec8p. The depletion of C. elegans W02A2.6p (called REC-8) by RNAi, induced univalent formation and splitting of chromosomes into sister chromatids at diakinesis. Chromosome synapsis at pachytene was defective, but primary homology recognition seemed unaffected, as a closer-than-random association of homologous fluorescence in situ hybridization (FISH) signals at leptotene/zygotene was observed. Depletion of REC-8 also induced chromosome fragmentation at diakinesis. We interpret these fragments as products of unrepaired meiotic double-stranded DNA breaks (DSBs), because fragmentation was suppressed in a spo-11 background. Thus, REC-8 seems to be required for successful repair of DSBs. The occurrence of DSBs in REC-8-depleted meiocytes suggests that DSB formation does not depend on homologous synapsis. Anti-REC-8 immunostaining decorated synaptonemal complexes (SCs) at pachytene and chromosomal axes in bivalents and univalents at diakinesis. Between metaphase I and metaphase II, REC-8 is partially lost from the chromosomes. The partial loss of REC-8 from chromosomes between metaphase I and metaphase II suggests that worm REC-8 might function similarly to yeast Rec8p. The loss of yeast Rec8p from chromosome arms at meiosis I and centromeres at meiosis II coordinates the disjunction of homologs and sister chromatids at the two meiotic divisions.

/pdf/a-caenorhabditis-elegans-cohesion-protein-with-functions-in-1c1kwqes0m.pdf

A Caenorhabditis elegans cohesion protein with functions in meiotic chromosome pairing and disjunction

The phenotype of the desynaptic (dy) mutant of maize in microsporocytes at meiotic prophase was compared with normal microsporocytes of a closely related strain and with microsporocytes of a maize inbred line (KYS) assumed to be normal. Strikingly more univalents and open arms of bivalents were found in the mutant cells than in normal cells at diakinesis, and where there was heterozygosity for a distal knob (heterochromatic region), separation was usually equational, indicating the occurrence of normal crossing-over followed by failure of chiasma maintenance in the mutant. Differences found in the mutant by electron microscopy were a statistically significant wider dimension of the synaptonemal complex central region and also less twisting of synapsed configurations at pachytene. It is suggested that these are side-effect symptoms of a defect in the synaptonemal complex (or associated substance), which is expressed later as sporadic loss of chiasma maintenance.Key words: desynaptic, chiasma maintenance, s...

The desynaptic mutant of maize as a combined defect of synaptonemal complex and chiasma maintenance.

Homozygotes for the dsy1 desynaptic mutant of maize show massive failure of chiasma maintenance during diplotene and diakinesis. Although some chiasmata persist until anaphase I in most microsporocytes expressing this mutant, homozygotes are completely or nearly completely sterile, owing apparently to disjunctive irregularities. Pachytene synaptic errors and some synaptic failure also are found, but recombination nodules are common in homologously synapsed regions, and equational separation of a heterozygous knob into univalents or open arms at diakinesis clearly demonstrates that chiasma failure occurs following crossing-over. A wider than normal synaptonemal complex central region and uniform apparent weakness of central region cross connections to spreading procedures strongly suggest the presence of a genetic lesion in a synaptonemal complex central region component. The dsy1 mutant may provide an especially important source of material for molecular studies on the nature of chiasma maintenance mechanism.

Evidence from a maize desynaptic mutant points to a probable role of synaptonemal complex central region components in provision for subsequent chiasma maintenance.

R. W. Riess

Papers

The desynaptic mutant of maize as a combined defect of synaptonemal complex and chiasma maintenance.

Evidence from a maize desynaptic mutant points to a probable role of synaptonemal complex central region components in provision for subsequent chiasma maintenance.