Chromosome painting in meiosis reveals pairing of specific chromosomes in polyploid <Emphasis Type="Italic">Solanum</Emphasis> species

Analysis of chromosome pairing has been an important tool to assess the genetic similarity of homologous and homoeologous chromosomes in polyploids. However, it is technically challenging to monitor the pairing of specific chromosomes in polyploid species, especially for plant species with a large number of small chromosomes. We developed oligonucleotide-based painting probes for four different potato chromosomes. We demonstrate that these probes are robust enough to monitor a single chromosome throughout the prophase I of meiosis in polyploid Solanum species. Cultivated potato (Solanum tuberosum, 2n?=?4x?=?48) is an autotetraploid. We demonstrate that the four copies of each potato chromosome pair as a quadrivalent in 66–78% of the meiotic cells at the pachytene stage. Solanum demissum (2n?=?6x?=?72) is a hexaploid and has been controversial regarding its nature as an autopolyploid or allopolyploid. Interestingly, no hexavalent pairing was observed in meiosis. Instead, we observed three independent bivalents in 83–98% of the meiotic cells at late diakinesis and early metaphase I for the four chromosomes. These results suggest that S. demissum has evolved into a cytologically stable state with predominantly bivalent pairing in meiosis.     

Molecular and cytological analyses of A and C genomes at meiosis in synthetic allotriploid <Emphasis Type="Italic">Brassica</Emphasis> hybrids (ACC) between <Emphasis Type="Italic">B.napus</Emphasis> (AACC) and <Emphasis Type="Italic">B.oleracea</Emphasis> (CC)

Success of interspecific hybridization relies mostly on the adequate similarity between the implicated genomes to ensure synapsis, pairing and recombination between appropriate chromosomes during meiosis in allopolyploid species. Allotetraploid Brassica napus (AACC) is a model of natural hybridization between Brassica rapa (AA) and Brassica oleracea (CC), which are originally derived from a common ancestor, but genomic constitution of the same chromosomes probably varied among these species through time after establishment, giving rise to cytogenetic difference in the synthetic hybrids. Herein we investigated meiotic behaviors of A and C chromosomes of synthetic allotriploid Brassica hybrids (ACC) at molecular and cytological levels, which result from the interspecific cross between natural B. napus (AACC) and B.oleracea (CC), and the results showed that meiosis course was significantly aberrant in allotriploid Brassica hybrids, and chromosomes aligned chaotically at metaphase I, chromosome bridges and lags were frequently observed from later metaphase I to anaphase II during meiosis. Simultaneously, we also noticed that meiosis-related genes were abruptly down-regulated in allotriploid Brassica hybrids, which likely accounted for irregular scenario of meiosis observed in these synthetic hybrids. Therefore, these results indicated that inter-genomic exchanges of A and C chromosomes could occur frequently in synthetic Brassica hybrids, and provided an efficient approach for genetic changes of homeologous chromosomes during meiosis in polyploid B.napus breeding program.     

Behavior of sex chromosomes at early meiosis stages in three wood mice species of the genus <Emphasis Type="Italic">Apodemus</Emphasis> (Rodentia,Muridae)

The prophase of the first meiotic division was studied in field mice of the species Apodemus (Sylvaemus) flavicollis, A. (S.) ponticus, and A. (S.) uralensis by light and electron microscopy. The karyotypes of three species were described on the base of electron microscopy of synaptonemal complexes in spermatocytes I. The axial elements of the sex chromosomes at early-middle pachytene synapse along the major portion of the Y axis; at late pachytene-early diplotene, the synapsis region shrinks; and at diakinesis-metaphase I, X and Y chromosomes associate end-to-end in all species studied. The behavior of sex chromosomes in the synapsis in the species studied was quite uniform. The results are discussed in the context of earlier data on the behavior of sex chromosomes in various rodent species in meiosis prophase I and their banding.     

Genetic Collection of Meiotic Mutants of Rye <Emphasis Type="Italic">Secale cereale</Emphasis> 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.     

Research on meiotic chromosome pairing in <Emphasis Type="Italic">Roegneria sinica</Emphasis> var. <Emphasis Type="Italic">media</Emphasis> evaluated using genomic in situ hybridization

The analysis of chromosome pairing during meiosis is important for understanding the relationships between different genomes. To evaluate the diversity of chromosome pairing behavior in the wild species of Roegneria sinica var. media Keng with St and H genomes in Triticeae (Poaceae), differences and similarities in the meiotic chromosome pairing behaviors of the two genomes in two populations of R. sinica var. media, were analyzed using genomic in situ hybridization. Chromosome pairing at meiotic metaphase I in the two populations of R. sinica var. media mainly formed bivalents, although several univalents, trivalents and quadrivalents also occurred. Chromosome pairings occurred mainly between homologous chromosomes. However, some non-homologous pairings were observed under natural conditions. No significant differences in karyotype were found between the St and H genomes. Chromosome pairing behaviors differed between and within the two populations. Genetic variation occurred mainly within populations (94.04 %), and variation was more abundant in one population than the other. The genomes St and H differed, but there was some relationship between the two genomes. These findings suggest that homoeologous pairing of chromosomes or exchanges occurred between different genomes of the wild species in Triticeae during evolution. The findings also provide conclusive cytological evidence for genetic variation within the wild species, which forms the basis of their genetic diversity.     

Inverted meiosis and its place in the evolution of sexual reproduction pathways

Inverted meiosis is observed in plants (Cyperaceae and Juncaceae) and insects (Coccoidea, Aphididae) with holocentric chromosomes, the centromeres of which occupy from 70 to 90% of the metaphase chromosome length. In the first meiotic division (meiosis I), chiasmata are formed and rodlike bivalents orient equationally, and in anaphase I, sister chromatids segregate to the poles; the diploid chromosome number is maintained. Non-sister chromatids of homologous chromosomes remain in contact during interkinesis and prophase II and segregate in anaphase II, forming haploid chromosome sets. The segregation of sister chromatids in meiosis I was demonstrated by example of three plant species that were heterozygous for chromosomal rearrangements. In these species, sister chromatids, marked with rearrangement, segregated in anaphase I. Using fluorescent antibodies, it was demonstrated that meiotic recombination enzymes Spo11 and Rad5l, typical of canonical meiosis, functioned at the meiotic prophase I of pollen mother cells of Luzula elegance and Rhynchospora pubera. Moreover, antibodies to synaptonemal complexes proteins ASY1 and ZYP1 were visualized as filamentous structures, pointing to probable formation of synaptonemal complexes. In L. elegance, chiasmata are formed by means of chromatin threads containing satellite DNA. According to the hypothesis of the author of this review, equational division of sister chromatids at meiosis I in the organisms with inverted meiosis can be explained by the absence of specific meiotic proteins (shugoshins). These proteins are able to protect cohesins of holocentric centromeres from hydrolysis by separases at meiosis I, as occurs in the organisms with monocentric chromosomes and canonical meiosis. The basic type of inverted meiosis was described in Coccoidea and Aphididae males. In their females, the variants of parthenogenesis were also observed. Until now, the methods of molecular cytogenetics were not applied for the analysis of inverted meiosis in Coccoidea and Aphididae. Evolutionary, inverted meiosis is thought to have appeared secondarily as an adaptation of the molecular mechanisms of canonical meiosis to chromosome holocentrism.     

<Emphasis Type="Italic">In Silico</Emphasis> Identification and Characterization of Meiotic DNA: <Emphasis Type="Italic">AluJb</Emphasis> Possibly Participates in the Attachment of Chromatin Loops to Synaptonemal Complex

Earlier, using bioinformatic methods, we reported the identification of repeated DNA sequences (RSs), presumably responsible for the attachment of chromatin loops to the lateral elements of synaptonemal complex in meiotic chromosomes. In the present study, consensus sequences for this class of RS were identified. It was demonstrated that at least part of these sequences belonged to the AluJb subfamily of Alu sequences. The Alu copies distribution along the major human histocompatibility complex (MHC) and their spatial separation from the sites of meiotic recombination was examined. It was demonstrated that simple sequences, like (GT/CA) _n , were flanking meiotic recombination sites. A model of the RS organization in meiotic chromosome, most efficiently linking experimental data on the meiotic recombination in MHC and the in silico data on the RS localization (the coefficient of multiple correlation, r = 0.92) is suggested.     

Variation at Two Loci affecting Homoeologous Meiotic Chromosome Pairing in <Emphasis Type="Italic">Triticum aestivum</Emphasis> × <Emphasis Type="Italic">Aegilops mutica</Emphasis> Hybrids

HOMOEOLOGOUS chromosomes of the three genomes of bread wheat (Triticum aestivum 2n=6x=42) are normally prevented from pairing at meiosis by the activity of an allele at the Ph locus on chromosome 5B^L (refs. 1–4). This activity is responsible for the regular bivalent-forming meiotic behaviour and for the stable disomic inheritance of T. aestivum. If allelic variation occurs at the PA locus in nature it is extremely rare, although mutation has been induced and mutant alleles isolated^3,4.     

Identification of all homoeologous chromosomes of newly synthetic allotetraploid <Emphasis Type="Italic">Cucumis</Emphasis> × <Emphasis Type="Italic">hytivus</Emphasis> and its wild parent reveals stable subgenome structure

Allopolyploidy and homoeologous recombination are two important processes in reshaping genomes and generating evolutionary novelties. Newly formed allopolyploids usually display chromosomal perturbations as a result of pairing errors at meiosis. To understand mechanisms of stabilization of allopolyploid species derived from distant chromosome bases, we investigated mitotic stability of a synthetic Cucumis allotetraploid species in relation to meiosis chromosome behavior. The Cucumis × hytivus is an allotetraploid synthesized from interspecific hybridization between cucumber (Cucumis sativus, 2n = 14) and its wild relative Cucumis hystrix (2n = 24) followed by spontaneous chromosome doubling. In the present study, we analyzed the wild parent C. hystrix and the latest generation of C. hytivus using GISH (genomic in situ hybridization) and cross-species FISH (fluorescence in situ hybridization). The karyotype of C. hystrix was constructed with two methods using cucumber fosmid clones and repetitive sequences. Using repeat-element probe mix in two successive hybridizations allowed for routine identification of all 19 homoeologous chromosomes of allotetraploid C. hytivus. No aneuploids were identified in any C. hytivus individuals that were characterized, and no large-scale chromosomal rearrangements were identified in this synthetic allotetraploid. Meiotic irregularities, such as homoeologous pairing, were frequently observed, resulting in univalent and intergenomic multivalent formation. The relatively stable chromosome structure of the synthetic Cucumis allotetraploid may be explained by more deleterious chromosomal viable gametes compared with other allopolyploids. The knowledge of genetic and genomic information of Cucumis allotetraploid species could provide novel insights into the establishment of allopolyploids with different chromosome bases.     

Pushing the (nuclear) envelope into meiosis

A recent study shows that a short isoform of a mammalian nuclear lamin is important for homologous chromosome interactions during meiotic prophase in mice.Meiosis is the specialized cell division process required for sexual reproduction. As cells enter meiotic prophase, a relatively long period preceding the two chromosome divisions, nuclei and chromosomes undergo remodeling to promote interactions between homologous chromosomes. Each chromosome must find and identify its unique partner within the volume of the nucleus, a process that obviously involves large-scale chromosome movements.Over 100 years ago, cytological analysis of meiotic cells revealed a unique chromosome configuration termed the meiotic ''bouquet'', in which chromosome ends seem to be attached to the nuclear periphery, frequently in a tight cluster. The presence of the bouquet was found to coincide with the stage during which homologous chromosomes undergo pairing and synapsis. This was the first indication that interactions between the chromosomes and the nuclear envelope might be important for meiotic pairing. More recent analysis in diverse model systems has revealed that the bouquet is a consequence of interactions between chromosomes and cytoskeletal elements - microtubules or actin cables - via a protein bridge that spans the nuclear envelope. A study recently published in PLOS Genetics [1] has shed further light on the role of the nuclear lamina in meiotic progression by studying the role of a meiosis-specific isoform of a nuclear lamin protein.In metazoans the nuclear envelope is fortified by the nuclear lamina, a meshwork of intermediate filament proteins (lamins) and associated proteins that underlies the inner nuclear membrane. The lamina confers structural rigidity to nuclei and also interacts with a wide variety of nucleoplasmic, transmembrane and chromosome-associated proteins. The composition of the lamina in metazoans shows tissue-specific variability and developmental regulation. Most differentiated mammalian cells express both A-type lamins (lamins A and C, which are generated by alternative splicing of the LMNA gene) and B-type lamins (encoded by two different genes), whereas some invertebrates express only a single lamin protein. Stem cells typically lack A-type lamins, which are also dispensable for early development in mice.Among the nuclear envelope components that interact with lamins are LINC (linker of nucleoskeleton and cytoskeleton) complexes. These versatile networks involve a pair of SUN/KASH proteins that bridge both membranes of the nuclear envelope. SUN domain proteins traverse the inner membrane, with their amino termini projecting into the nucleus and their SUN domains in the lumen between the two membranes. Their partners have membrane-spanning regions adjacent to their carboxy-terminal KASH domains, short peptides that bind to the SUN domains. Using a variety of interaction modules, LINC complexes create connections between nuclear structures such as the lamina or chromosomes and cytoskeletal elements such as actin filaments or microtubules. Throughout the eukaryotes, they have essential roles in diverse processes, including the positioning and migration of nuclei within cells and anchorage of centrosomes to the nuclear envelope. During meiosis, specific LINC complexes are recruited to interact with chromosomes through the expression of meiosis-specific proteins that bind to telomeres or, less frequently, to other specialized loci [2]. These connections, probably in conjunction with meiosis-specific modifications to the cytoskeleton and motor proteins, lead to large-scale chromosome motions that facilitate homologous chromosome pairing. These movements involve dramatic motion of the LINC proteins within the nuclear membrane, sometimes involving movements of up to several micrometers that occur within a few seconds [3]. This stands in sharp contrast to the behavior of some of the same protein complexes in somatic or premeiotic cells, in which they show highly constrained motion and minimal turnover [3].In the new PLOS Genetics study [1], groups led by Manfred Alsheimer and Ricardo Benavente, both of the University of Würzburg, have now engineered a disruption of an exon in the mouse LMNA gene that is specific to the meiotic isoform lamin C2 to generate C2-deficient mice (C2^-/- mice). These collaborators have previously provided important insights into the regulation and functions of cell-type specific lamin isoforms, particularly during meiosis. Using antibodies, they characterized the lamin isoforms present in rat spermatocytes [4]. Immunolocalization revealed that a truncated isoform of lamin C (lamin C2) was localized in a patchy pattern along the nuclear envelope, along with a short B-type lamin (lamin B3) [4]. Because these short isoforms lack domains implicated in interactions between lamin subunits, they and others proposed that these proteins might form a more flexible network. This idea was supported by experiments in which meiosis-specific lamin C2 was ectopically expressed in fibroblasts and found to be more mobile within the nuclear envelope than full-length lamin C [5]. Expression of lamin C2 also resulted in aberrant localization of Sun1 in these cells. The collaborators also demonstrated that spermatogenesis was disrupted in Lmna^-/- mice, although oocyte meiosis was not obviously perturbed [6]. Although defects in meiosis-specific processes were observed in the knockout mice, it was not possible to rule out an indirect effect of lamin depletion in somatic cells on meiosis in spermatocytes, prior to the new study.An important feature of the new research [1] is that the C2^-/- mice show normal expression of all other A-type lamins. The C2^-/- males recapitulate the meiotic failure seen in Lmna^-/- mice. Nevertheless, their chromosomes frequently fail to synapse and they engage in heterologous associations or show aberrant telomere-telomere interactions; all of these defects are rare in wild-type spermatocytes. As a result of extensive apoptosis and failure of sperm maturation, the males are completely infertile. However, females are fertile, despite some evidence for pairing defects in C2^-/- oocytes.These sex-specific differences in the effects of lamin C2 loss are somewhat surprising. They could in part reflect differential implementation of meiotic checkpoints, which cull defective spermatocytes more ruthlessly than oocytes [7]. However, analysis of homologous pairing and synapsis in the C2^-/- mutant mice also revealed more severe defects in males. Both male and female mice lacking Sun1 protein are completely sterile and show synaptic failure during meiotic prophase [8]. This suggests that LINC-mediated chromosome dynamics are essential for homolog interactions during meiosis in both sexes. The milder defects caused by loss of lamin C2 in both male and female meiosis suggest that it has a less direct role in mediating chromosome movement than Sun1. This is consistent with the idea that expression of short lamin isoforms during meiosis acts primarily to increase the mobility of proteins within the nuclear envelope, relative to somatic cells. It seems likely that the dynamics of pairing, synapsis and recombination differ dramatically between spermatocytes, which are produced continually during the adult life of the male, and oocytes, which undergo meiotic prophase during fetal development. Such differences might render male meiosis more sensitive to changes in nuclear envelope organization or dynamics.The modifications made to the mouse nuclear envelope during meiosis are likely to be conserved in concept, if not in detail, in other taxa. As mentioned above, the isoforms and expression patterns of lamin proteins have diverged rapidly among the metazoa, as have the structures and functions of LINC complexes. For example, amphibians lack lamin C (and lamin C2), suggesting that its meiotic role in mammals is a recent innovation. Furthermore, the mouse Sun1 protein has a C2H2 zinc finger lacking in primate orthologs, which might suggest that it has evolved a distinct way to connect with meiotic chromosomes. It is thus not currently clear which aspects of meiotic lamina remodeling in mice can be extrapolated to other species.In Caenorhabditis elegans, meiotic chromosome dynamics are probably mediated by post-translational modification of the amino-terminal (nucleoplasmic) domain of sun-1 [9]. It is not yet known how this modification contributes to the function of the meiotic LINC complex. Direct observation has indicated that the motion of LINC complexes within the nuclear envelope becomes much less constrained as cells enter meiosis [3]. Phosphorylation of sun-1 may weaken interactions between the LINC complexes and the lamina to increase their mobility within the nuclear envelope, and/or promote interactions between LINC complexes to create high load-bearing aggregates of these proteins necessary to drive chromosome movement. It is not currently known whether the lamina itself is modified in C. elegans meiotic nuclei, but it is easy to imagine that phosphorylation could also be used to tweak protein-protein interactions within the lamina to optimize its properties during meiosis and other specialized cellular processes. It is likely that metazoans have evolved a wide range of mechanisms to modify their nuclear envelopes to meet the special demands of meiotic prophase.Homologous chromosome pairing remains one of the most mysterious aspects of meiosis. This new work in mice [1] adds an important piece of the puzzle by illuminating how the nuclear lamina can be modified to facilitate meiotic chromosome dynamics. To understand this process will clearly require looking beyond the chromosomes, and even beyond the nucleus, to the cellular networks connected by LINC complexes.     

Speciation of Eversmann and Mongolian hamsters (<Emphasis Type="Italic">Allocricetulus</Emphasis>, Cricetinae): Experimental hybridization

Contrary to the pre-existing concepts based on morphological and karyological data on the impossibility of hybridization between hamster species of the genus Allocricetulus (A. curtatus and A. eversmanni), F1 hybrids in both combinations, and backcrosses, were obtained for the first time. Some restrictions of hybridization between females of A. eversmanni and males of A. curtatus were detected. Analysis of the synaptonemal complex in the hybrids produced by crossing a female of A. curtatus with a male of A. eversmanni demonstarted that a significant portion of the cells was subject to arrest and selection at the stage of meiosis prophase I. Nevertheless, some spermatocytes still generate viable spermatozoa. Thus, the possibility of hybridization in the laboratory between two Allocricetulus species with different numbers of chromosomes may testify to a relatively low divergence between them.     

Defects in meiotic recombination delay progression through pachytene in <Emphasis Type="Italic">Tex19.1</Emphasis><Superscript><Emphasis Type="Italic">−/−</Emphasis></Superscript> mouse spermatocytes

Recombination, synapsis, chromosome segregation and gene expression are co-ordinately regulated during meiosis to ensure successful execution of this specialised cell division. Studies with multiple mutant mouse lines have shown that mouse spermatocytes possess quality control checkpoints that eliminate cells with persistent defects in chromosome synapsis. In addition, studies on Trip13^mod/mod mice suggest that pachytene spermatocytes that successfully complete chromosome synapsis can undergo meiotic arrest in response to defects in recombination. Here, we present additional support for a meiotic recombination-dependent checkpoint using a different mutant mouse line, Tex19.1^?/?. The appearance of early recombination foci is delayed in Tex19.1^?/? spermatocytes during leptotene/zygotene, but some Tex19.1^?/? spermatocytes still successfully synapse their chromosomes and we show that these spermatocytes are enriched for early recombination foci. Furthermore, we show that patterns of axis elongation, chromatin modifications and histone H1t expression are also all co-ordinately skewed towards earlier substages of pachytene in these autosomally synapsed Tex19.1^?/? spermatocytes. We also show that this skew towards earlier pachytene substages occurs in the absence of elevated spermatocyte death in the population, that spermatocytes with features of early pachytene are present in late stage Tex19.1^?/? testis tubules and that the delay in histone H1t expression in response to loss of Tex19.1 does not occur in a Spo11 mutant background. Taken together, these data suggest that a recombination-dependent checkpoint may be able to modulate pachytene progression in mouse spermatocytes to accommodate some types of recombination defect.     

Usefulness of the Non-conventional <Emphasis Type="Italic">Caenorhabditis elegans</Emphasis> Model to Assess <Emphasis Type="Italic">Candida</Emphasis> Virulence

Invasive candidiasis is caused mainly by Candida albicans, but other Candida species have increasing etiologies. These species show different virulence and susceptibility levels to antifungal drugs. The aims of this study were to evaluate the usefulness of the non-conventional model Caenorhabditis elegans to assess the in vivo virulence of seven different Candida species and to compare the virulence in vivo with the in vitro production of proteinases and phospholipases, hemolytic activity and biofilm development capacity. One culture collection strain of each of seven Candida species (C. albicans, Candida dubliniensis, Candida glabrata, Candida krusei, Candida metapsilosis, Candida orthopsilosis and Candida parapsilosis) was studied. A double mutant C. elegans AU37 strain (glp-4;sek-1) was infected with Candida by ingestion, and the analysis of nematode survival was performed in liquid medium every 24 h until 120 h. Candida establishes a persistent lethal infection in the C. elegans intestinal tract. C. albicans and C. krusei were the most pathogenic species, whereas C. dubliniensis infection showed the lowest mortality. C. albicans was the only species with phospholipase activity, was the greatest producer of aspartyl proteinase and had a higher hemolytic activity. C. albicans and C. krusei caused higher mortality than the rest of the Candida species studied in the C. elegans model of candidiasis.     

Homoeologous recombination in the presence of <Emphasis Type="Italic">Ph1</Emphasis> gene in wheat

A crossover (CO) and its cytological signature, the chiasma, are major features of eukaryotic meiosis. The formation of at least one CO/chiasma between homologous chromosome pairs is essential for accurate chromosome segregation at the first meiotic division and genetic recombination. Polyploid organisms with multiple sets of homoeologous chromosomes have evolved additional mechanisms for the regulation of CO/chiasma. In hexaploid wheat (2n = 6× = 42), this is accomplished by pairing homoeologous (Ph) genes, with Ph1 having the strongest effect on suppressing homoeologous recombination and homoeologous COs. In this study, we observed homoeologous COs between chromosome 5M^g of Aegilops geniculata and 5D of wheat in plants where Ph1 was fully active, indicating that chromosome 5M^g harbors a homoeologous recombination promoter factor(s). Further cytogenetic analysis, with different 5M^g/5D recombinants, showed that the homoeologous recombination promoting factor(s) may be located in proximal regions of 5M^g. In addition, we observed a higher frequency of homoeologous COs in the pericentromeric region between chromosome combination of rec5M^g#2S·5M^g#2L and 5D compared to 5M^g#1/5D, which may be caused by a small terminal region of 5DL homology present in chromosome rec5M^g#2. The genetic stocks reported here will be useful for analyzing the mechanism of Ph1 action and the nature of homoeologous COs.     

End-to-End Association of <Emphasis Type="Italic">X</Emphasis> and <Emphasis Type="Italic">Y</Emphasis> Chromosomes in Mouse Meiosis

ACCORDING to the hypothesis of Crew and Koller¹ and Koller and Darlington², there are homologous segments in the X and Y chromosomes of the mouse and other mammals. The homologous regions in the mouse were believed to be localized in the extremely short arms proximal to the kinetochores. The end-to-end association at meiosis was thought to be the result of the formation of a chiasma between these homologous regions³. Electron microscopy revealed a short synaptonemal complex in mouse meiotic cells⁴. However, partial sex linkage has never been demonstrated in the mouse⁵ and other authors^6–10 believe that the X and Y chromosomes associate only by connexion between the chromosome ends furthest from the centromeres.     

Comparative cytogenetic analysis of four species of <Emphasis Type="Italic">Dendropsophus</Emphasis> (Hylinae) from the Brazilian Atlantic forest

We conducted a cytogenetic study of four hyline frog species (Dendropsophus elegans, D. microps, D. minutus and D. werneri) from southern Brazil. All species had 2n = 30 chromosomes, with interspecific and intraspecific variation in the numbers of metacentric, submetacentric, subtelocentric and telocentric chromosomes. C-banding and fluorochrome staining revealed conservative GC-rich heterochromatin localized in the pericentromeric regions of all species. The location of the nucleolus organizer regions, as confirmed by fluorescent in situ hybridization, differed between species. Telomeric probes detected sites that were restricted to the terminal regions of all chromosomes and no interstitial or centromeric signals were observed. Our study corroborates the generic synapomorphy of 2n = 30 chromosomes for Dendropsophus and adds data that may become useful for future taxonomic revisions and a broader understanding of chromosomal evolution among hylids.     

Centromeric and non-centromeric satellite DNA organisation differs in holocentric <Emphasis Type="Italic">Rhynchospora</Emphasis> species

Satellite DNA repeats (or satDNA) are fast-evolving sequences usually associated with condensed heterochromatin. To test whether the chromosomal organisation of centromeric and non-centromeric satDNA differs in species with holocentric chromosomes, we identified and characterised the major satDNA families in the holocentric Cyperaceae species Rhynchospora ciliata (2n = 10), R. globosa (2n = 50) and R. tenuis (2n = 2x = 4 and 2n = 4x = 8). While conserved centromeric repeats (present in R. ciliata and R. tenuis) revealed linear signals at both chromatids, non-centromeric, species-specific satDNAs formed distinct clusters along the chromosomes. Colocalisation of both repeat types resulted in a ladder-like hybridisation pattern at mitotic chromosomes. In interphase, the centromeric satDNA was dispersed while non-centromeric satDNA clustered and partly colocalised to chromocentres. Despite the banding-like hybridisation patterns of the clustered satDNA, the identification of chromosome pairs was impaired due to the irregular hybridisation patterns of the homologues in R. tenuis and R. ciliata. These differences are probably caused by restricted or impaired meiotic recombination as reported for R. tenuis, or alternatively by complex chromosome rearrangements or unequal condensation of homologous metaphase chromosomes. Thus, holocentricity influences the chromosomal organisation leading to differences in the distribution patterns and condensation dynamics of centromeric and non-centromeric satDNA.     

Chromosome nondisjunction in <Emphasis Type="Italic">Drosophila</Emphasis> strains mutant for tumor suppressor Merlin

The effect of mutation for gene Merlin on chromosome disjunction in Drosophila during meiosis was genetically studied. Chromosome nondisjunction was not registered in females heterozygous for this mutation and containing structurally normal X chromosomes. In cases when these females additionally contained inversion in one of chromosomes X, a tendency toward the appearance of nondisjunction events was observed in individuals containing mutation in the heterozygote. The genetic construct was obtained allowing the overexpression of protein corresponding to a sterile allele Mer ³ in the germ cell line. This construct relieves the lethal effect of Mer ⁴ mutation. The ectopic expression of this mutant protein leads to chromosome nondisjunction in male meiosis.