Meiosis reveals the early steps in the evolution of a neo-XY sex chromosome pair in the African pygmy mouse Mus minutoides.

Sex chromosomes of eutherian mammals are highly different in size and gene content, and share only a small region of homology (pseudoautosomal region, PAR). They are thought to have evolved through an addition-attrition cycle involving the addition of autosomal segments to sex chromosomes and their subsequent differentiation. The events that drive this process are difficult to investigate because sex chromosomes in almost all mammals are at a very advanced stage of differentiation.

Meiotic behavior of a complex hexavalent in heterozygous mice for Robertsonian translocations: insights for synapsis dynamics.

Natural populations of the house mouse Mus musculus domesticus show great diversity in chromosomal number due to the presence of chromosomal rearrangements, mainly Robertsonian translocations. Breeding between two populations with different chromosomal configurations generates subfertile or sterile hybrid individuals due to impaired meiotic development. In this study, we have analyzed prophase-I spermatocytes of hybrids formed by crossing mice from Vulcano and Lipari island populations.

Transition from a meiotic to a somatic-like DNA damage response during the pachytene stage in mouse meiosis.

Homologous recombination (HR) is the principal mechanism of DNA repair acting during meiosis and is fundamental for the segregation of chromosomes and the increase of genetic diversity. Nevertheless, non-homologous end joining (NHEJ) mechanisms can also act during meiosis, mainly in response to exogenously-induced DNA damage in late stages of first meiotic prophase. In order to better understand the relationship between these two repair pathways, we studied the response to DNA damage during male mouse meiosis after gamma radiation.

Do Exogenous DNA Double-Strand Breaks Change Incomplete Synapsis and Chiasma Localization in the Grasshopper Stethophyma grossum?

Meiotic recombination occurs as a programmed event that initiates by the formation of DNA double-strand breaks (DSBs) that give rise to the formation of crossovers that are observed as chiasmata. Chiasmata are essential for the accurate chromosome segregation and the generation of new combinations of parental alleles. Some treatments that provoke exogenous DSBs also lead to alterations in the recombination pattern of some species in which full homologous synapsis is achieved at pachytene.

Transcription reactivation during the first meiotic prophase in bugs is not dependent on synapsis.

During meiosis, transcription is precisely regulated in relation to the process of chromosome synapsis. In mammals, transcription is very low until the completion of synapsis in early pachytene, and then reactivates during mid pachytene, up to the end of diplotene. Moreover, chromosomes or chromosomal regions that do not achieve synapsis undergo a specific process of inactivation called meiotic silencing of unpaired chromatin (MSUC).

Chromatin organization and remodeling of interstitial telomeric sites during meiosis in the Mongolian gerbil (Meriones unguiculatus).

Telomeric DNA repeats are key features of chromosomes that allow the maintenance of integrity and stability in the telomeres. However, interstitial telomere sites (ITSs) can also be found along the chromosomes, especially near the centromere, where they may appear following chromosomal rearrangements like Robertsonian translocations. There is no defined role for ITSs, but they are linked to DNA damage-prone sites.

B1 was the ancestor B chromosome variant in the western Mediterranean area in the grasshopper Eyprepocnemis plorans.

We analyzed the distribution of 2 repetitive DNAs, i.e. ribosomal DNA (rDNA) and a satellite DNA (satDNA), on the B chromosomes found in 17 natural populations of the grasshopper Eyprepocnemis ploransplorans sampled around the western Mediterranean region, including the Iberian Peninsula, Balearic Islands, Sicily, and Tunisia.

Dynamics of cohesin subunits in grasshopper meiotic divisions.

The cohesin complex plays a key role for the maintenance of sister chromatid cohesion and faithful chromosome segregation in both mitosis and meiosis. This complex is formed by two structural maintenance of chromosomes protein family (SMC) subunits and two non-SMC subunits: an α-kleisin subunit SCC1/RAD21/REC8 and an SCC3-like protein. Several studies carried out in different species have revealed that the distribution of the cohesin subunits along the chromosomes during meiotic prophase I is not regular and that some subunits are distinctly incorporated at different cell stages.

A synaptonemal complex-derived mechanism for meiotic segregation precedes the evolutionary loss of homology between sex chromosomes in arvicolid mammals.

Synapsis and reciprocal recombination between sex chromosomes are restricted to the pseudoautosomal region. In some animal species, sex chromosomes do not present this region, although they utilize alternative mechanisms that ensure meiotic pairing and segregation. The subfamily Arvicolinae (Rodentia, Cricetidae) includes numerous species with achiasmate sex chromosomes.

Inactivation or non-reactivation: what accounts better for the silence of sex chromosomes during mammalian male meiosis?

During the first meiotic prophase in male mammals, sex chromosomes undergo a program of transcriptional silencing called meiotic sex chromosome inactivation (MSCI). MSCI is triggered by accumulation of proteins like BRCA1, ATR, and γH2AX on unsynapsed chromosomes, followed by local changes on the sex chromatin, including histone modifications, incorporation of specific histone variants, non-histone proteins, and RNAs. It is generally thought that MSCI represents the transition of unsynapsed chromatin from a transcriptionally active state to a repressed state.

Incomplete synapsis and chiasma localization: the chicken or the egg?

In the present study, and as a sincere tribute from the Cytogenetics teams from Madrid to Professor Máximo Drets on his 80th birthday, we have analyzed and compared 3 different grasshopper species with different synaptic patterns, a standard pattern, a second pattern with synapsis restricted to the proximal regions, and a third pattern with synapsis restricted to the distal regions. In the 3 species we have thoroughly analyzed the relationships among cohesin axis morphogenesis, formation of double strand breaks (DSBs) and recombination initiation. Our results demonstrate that in every case recombination initiation precedes synapsis, and that there is a direct relationship between the absence of meiotic recombination and the existence of particular unsynapsed chromosomal regions during prophase I.

A high incidence of meiotic silencing of unsynapsed chromatin is not associated with substantial pachytene loss in heterozygous male mice carrying multiple simple robertsonian translocations.

Meiosis is a complex type of cell division that involves homologous chromosome pairing, synapsis, recombination, and segregation. When any of these processes is altered, cellular checkpoints arrest meiosis progression and induce cell elimination. Meiotic impairment is particularly frequent in organisms bearing chromosomal translocations.

Cohesin axis maturation and presence of RAD51 during first meiotic prophase in a true bug.

We have analyzed in a true bug, Graphosoma italicum (Pentatomidae, Hemiptera), the temporal and functional relationships between recombination events, synapsis progression, and SMC1alpha and SMC3 cohesin axis maturation throughout the male first meiotic prophase. The localization of the histone variant histone H3 trimethylated at lysine 9 at chromosome ends has allowed us to determine the association of these heterochromatic domains through prophase I stages. Results highlighted that cohesins provide to be good markers for synapsis progression since the formation, morphology, and development of the SMC1alpha and SMC3 cohesin axes resemble the synaptonemal complex dynamics and, also, that in this species the initiation of recombination precedes synapsis.

CDK2 is required for proper homologous pairing, recombination and sex-body formation during male mouse meiosis.

Cyclin-dependent kinase 2 (CDK2) was assumed to be essential in the mammalian cell cycle both at the G1-S transition and throughout the S phase. Interestingly, ablation of Cdk2 in mice does not have substantial consequences for embryonic or postnatal development, but both males and females are infertile. In the present study, we have analysed the meiotic alterations leading to infertility in Cdk2-/- male mice.

Sequential assembly of centromeric proteins in male mouse meiosis.

The assembly of the mitotic centromere has been extensively studied in recent years, revealing the sequence and regulation of protein loading to this chromosome domain. However, few studies have analyzed centromere assembly during mammalian meiosis. This study specifically targets this approach on mouse spermatocytes.

Relationship between incomplete synapsis and chiasma localization.

One of the subjects within the meiotic field that has been actively investigated in the recent years is the temporal and functional relationships between meiotic recombination, cohesin loading and synaptonemal complex (SC) assembly. Although the study of meiotic mutants has shed some light, many questions remain to be answered. Here, we have studied this topic in the orthopteran Paratettix meridionalis, a species with telocentric chromosomes, which shows two unusual cytological features: pairing and synapsis of homologues during prophase I are restricted to the non-centromeric distal regions and extremely distal chiasma localization in metaphase I bivalents.

Inverted meiosis: the true bugs as a model to study.

Most of the meiotic literature is based on species with monocentric chromosomes, however meiosis in protoctist, plant and animal species with holocentric chromosomes is less characterized. In some cases, an inverted meiotic sequence is claimed to occur, in which segregation of homologs is postponed until the second meiotic division. Additionally, other features also deserve interest, namely: (i) the different behavior of sex chromosomes if compared to that of the autosomes; (ii) the absence of a canonical kinetochore structure; (iii) the restriction of the kinetic activity to the chromosomal ends; (iv) the variations in the orientation of bivalents at the division plate, and (v) the possible occurrence of chiasma terminalization.

Meiotic pairing and segregation of achiasmate sex chromosomes in eutherian mammals: the role of SYCP3 protein.

In most eutherian mammals, sex chromosomes synapse and recombine during male meiosis in a small region called pseudoautosomal region. However in some species sex chromosomes do not synapse, and how these chromosomes manage to ensure their proper segregation is under discussion. Here we present a study of the meiotic structure and behavior of sex chromosomes in one of these species, the Mongolian gerbil (Meriones unguiculatus).

Condensin I reveals new insights on mouse meiotic chromosome structure and dynamics.

Chromosome shaping and individualization are necessary requisites to warrant the correct segregation of genomes in either mitotic or meiotic cell divisions. These processes are mainly prompted in vertebrates by three multiprotein complexes termed cohesin and condensin I and II. In the present study we have analyzed by immunostaining the appearance and subcellular distribution of condensin I in mouse mitotic and meiotic chromosomes.

Sequential loading of cohesin subunits during the first meiotic prophase of grasshoppers.

The cohesin complexes play a key role in chromosome segregation during both mitosis and meiosis. They establish sister chromatid cohesion between duplicating DNA molecules during S-phase, but they also have an important role during postreplicative double-strand break repair in mitosis, as well as during recombination between homologous chromosomes in meiosis. An additional function in meiosis is related to the sister kinetochore cohesion, so they can be pulled by microtubules to the same pole at anaphase I.

Mammalian SGO2 appears at the inner centromere domain and redistributes depending on tension across centromeres during meiosis II and mitosis.

Shugoshin (SGO) is a family of proteins that protect centromeric cohesin complexes from release during mitotic prophase and from degradation during meiosis I. Two mammalian SGO paralogues - SGO1 and SGO2 - have been identified, but their distribution and function during mammalian meiosis have not been reported. Here, we analysed the expression of SGO2 during male mouse meiosis and mitosis.

Involvement of synaptonemal complex proteins in sex chromosome segregation during marsupial male meiosis.

Marsupial sex chromosomes break the rule that recombination during first meiotic prophase is necessary to ensure reductional segregation during first meiotic division. It is widely accepted that in marsupials X and Y chromosomes do not share homologous regions, and during male first meiotic prophase the synaptonemal complex is absent between them. Although these sex chromosomes do not recombine, they segregate reductionally in anaphase I.

A perikinetochoric ring defined by MCAK and Aurora-B as a novel centromere domain.

Mitotic Centromere-Associated Kinesin (MCAK) is a member of the kinesin-13 subfamily of kinesin-related proteins. In mitosis, this microtubule-depolymerising kinesin seems to be implicated in chromosome segregation and in the correction of improper kinetochore-microtubule interactions, and its activity is regulated by the Aurora-B kinase. However, there are no published data on its behaviour and function during mammalian meiosis.

Sex chromosomes, synapsis, and cohesins: a complex affair.

During first meiotic prophase, homologous chromosomes are held together by the synaptonemal complex, a tripartite proteinaceous structure that extends along the entire length of meiotic bivalents. While this feature is applicable for autosomes, sex chromosomes often escape from this rule. Many species present sex chromosomes that differ between them in their morphology, length, and gene content.