Changes in the expression and localization of cohesin subunits during meiosis in a non-mammalian vertebrate, the medaka fish

Sex-specific meiosis responses to Gsdf in medaka (Oryzias latipes)

The meiotic entry of undifferentiated germ cells is sexually specific and strictly regulated by the testicular or ovarian environment. Germline stem cells with a set of abnormal sex chromosomes and associated autosomes undergo defective meiotic processes and are eventually eliminated by yet to be defined post-transcriptional modifications. Herein, we report the role of gsdf, a member of BMP/TGFβ family uniquely found in teleost, in the regulation of meiotic entry in medaka (Oryzias latipes) via analyses of gametogenesis in gsdf-deficient XX and XY gonads in comparison with their wild-type siblings. Several differentially expressed genes, including the FKB506-binding protein 7 (fkbp7), were significantly upregulated in pubertal gsdf-deficient gonads. The increase in alternative pre-mRNA isoforms of meiotic synaptonemal complex gene sycp3 was visualized using Integrative Genomics Viewer and confirmed by real-time qPCR. Nevertheless, immunofluorescence analysis showed that Sycp3 protein products reduced significantly in gsdf-deficient XY oocytes. Transmission electron microscope observations showed that normal synchronous cysts were replaced by asynchronous cysts in gsdf-deficient testis. Breeding experiments showed that the sex ratio deviation of gsdf^-/- XY gametes in a non-Mendelian manner might be due to the non-segregation of XY chromosomes. Taken together, our results suggest that gsdf plays a role in the proper execution of cytoplasmic and nuclear events through receptor Smad phosphorylation and Sycp3 dephosphorylation to coordinate medaka gametogenesis, including sex-specific mitotic divisions and meiotic recombination.

The Zebrafish Meiotic Cohesin Complex Protein Smc1b Is Required for Key Events in Meiotic Prophase I

The eukaryotic structural maintenance of chromosomes (SMC) proteins are involved in key processes of chromosome structure and dynamics. SMC1β was identified as a component of the meiotic cohesin complex in vertebrates, which aids in keeping sister chromatids together prior to segregation in meiosis II and is involved in association of homologous chromosomes in meiosis I. The role of SMC1β in meiosis has primarily been studied in mice, where mutant male and female mice are infertile due to germ cell arrest at pachytene and metaphase II stages, respectively. Here, we investigate the function of zebrafish Smc1b to understand the role of this protein more broadly in vertebrates. We found that zebrafish smc1b is necessary for fertility and has important roles in meiosis, yet has no other apparent roles in development. Therefore, smc1b functions primarily in meiosis in both fish and mammals. In zebrafish, we showed that smc1b mutant spermatocytes initiated telomere clustering in leptotene, but failed to complete this process and progress into zygotene. Furthermore, mutant spermatocytes displayed a complete failure of synapsis between homologous chromosomes and homolog pairing only occurred at chromosome ends. Interestingly, meiotic DNA double strand breaks occurred in the absence of Smc1b despite failed pairing and synapsis. Overall, our findings point to an essential role of Smc1b in the leptotene to zygotene transition during zebrafish spermatogenesis. In addition, ovarian follicles failed to form in smc1b mutants, suggesting an essential role in female meiosis as well. Our results indicate that there are some key differences in Smc1b requirement in meiosis among vertebrates: while Smc1b is not required for homolog pairing and synapsis in mice, it is essential for these processes in zebrafish.

Production of transgenic medaka fish carrying fluorescent nuclei and chromosomes

As with zebrafish, attention has focused on the teleost medaka Oryzias latipes as an experimental animal representative of non-mammalian vertebrates in various fields of biological science. To enable real-time analyses of the dynamics of nuclei and chromosomes in living medaka cells, we produced a transgenic medaka expressing a fusion protein between histone H2B and green fluorescent protein (GFP) under the control of a cytomegalovirus (CMV) promoter. Since the nuclei and chromosomes of transgenic medaka cells are labeled with GFP, their morphological changes can be instantly monitored throughout the mitotic cell cycle progression under a fluorescent microscope without any fixation and staining of samples. However, GFP-labeling of nuclei and chromosomes is not successful during early embryonic development until zygotic expression begins and during the meiotic cell cycle progression, because the CMV promoter does not work in these stages. In addition, histone H2B-GFP fusion proteins are expressed in an organ-specific manner; strong and ubiquitous expression occurs in cells comprising the gut and fin, whereas the expression is restricted to certain types of cells in the liver and brain. These findings suggest that the CMV-driven expression of the histone H2B-GFP transgene is modified depending on the integration site of the transgene in the genome. Nevertheless, easy and precise monitoring of cytological changes in nuclei and chromosomes in the majority of mitotic cells by using the transgenic medaka will greatly contribute to a better understanding of control mechanisms of nuclear and chromosomal behaviors in vertebrate cells.

Meiotic hybridogenesis in triploid Misgurnus loach derived from a clonal lineage

Triploid loaches Misgurnus anguillicaudatus are derived from unreduced diploid gametes produced by an asexual clonal lineage that normally undergoes gynogenetic reproduction. Here, we have investigated the reproductive system of two types of triploids: the first type carried maternally inherited clonal diploid genomes and a paternally inherited haploid genome from the same population; the second type had the same clonal diploid genomes but a haploid genome from another, genetically divergent population. The germinal vesicles of oocytes from triploid females (3n=75) contained only 25 bivalents, that is, 50 chromosomes. Flow cytometry revealed that the majority of the progeny resulting from fertilization of eggs from triploid females with normal haploid sperm were diploid. This indicates that triploid females mainly produced haploid eggs. Microsatellite analyses of the diploid progeny of triploid females showed that one allele of the clonal genotype was not transmitted to haploid eggs. Moreover, the identity of the eliminated allele differed between the two types of triploids. Our results demonstrate that there is preferential pairing of homologous chromosomes as well as the elimination of unmatched chromosomes in the course of haploid egg formation, that is, meiotic hybridogenesis. Two distinct genomes in the clone suggest its hybrid origin.

Diploid sperm produced by artificially sex-reversed clone loaches

Clone loaches reproduce unisexually in a wild population of Hokkaido Island, Japan. These clone loaches produce genetically identical unreduced eggs which develop to diploid individuals without any genetic contribution of sperm donors. In the present study, sex reversal of clone loaches was attempted and the reproductive potential of resultant clone males was examined. Clone loaches administered 0.5 ppm of 17-alpha methyltestosterone (MT) for 30 days from 1 month after hatching differentiated into physiological males. These sex-reversed clone males produced fertile spermatozoa with a diploid DNA content. Diploid spermatozoa had significantly larger heads than normal haploid sperm, but had a normal shape showing a head, mid-piece, and tail. The motility of diploid spermatozoa was low after ambient water was added. Concentration of diploid spermatozoa per unit of sperm was lower than that of control haploid spermatozoa. Microsatellite genotyping revealed that triploid progeny from the cross between a normal diploid female and a sex-reversed clone male had two alleles specific to the diploid clone male and one allele of the mother loach. These results indicated that the sex-reversed clone males produced fertile diploid spermatozoa genetically identical to the clone lineage.

Chromosome elimination in the interspecific hybrid medaka between Oryzias latipes and O. hubbsi

An interspecific hybrid medaka (rice fish) between Oryzias latipes and O. hubbsi is embryonically lethal. To gain an insight into the cellular and molecular mechanisms that cause the abnormalities occurring in the hybrid medaka, we investigated the behavior of chromosomes and the expression patterns of proteins responsible for the chromosome behavior. The number of chromosomes in the hybrid embryos gradually decreased to nearly half, since abnormal cell division with lagging chromosomes at anaphase eliminated the chromosomes from the cells. The chromosome lagging occurred at the first cleavage and continued throughout embryogenesis even after the midblastula transition. Fluorescent in-situ hybridization analyses revealed that the chromosomes derived from O. hubbsi are preferentially eliminated in both O. latipes-hubbsi and O. hubbsi-latipes embryos. Whole-mount immunocytochemical analyses using antibodies against alpha-tubulin, gamma-tubulin, inner centromere protein, Cdc20, Mad2, phospho-histone H3 and cohesin subunits (SMC1alpha, SMC3 and Rad21) showed that the expression patterns of these proteins in the hybrid embryos are similar to those in the wild-type embryos, except for phospho-histone H3. Phospho-histone H3 present on chromosomes at metaphase was lost from normally separated chromosomes at anaphase, whereas it still existed on lagging chromosomes at anaphase, indicating that the lagging chromosomes remain in the metaphase state even when the cell has proceeded to the anaphase state. On the basis of these findings, we discuss the cellular and molecular mechanisms of chromosome elimination in the hybrid medaka.

Differential expression of anti-Müllerian hormone (amh) and anti-Müllerian hormone receptor type II (amhrII) in the teleost medaka

In mammals, the anti-Müllerian hormone (Amh) is responsible for the regression of the Müllerian ducts; therefore, Amh is an important factor of male sex differentiation. The amh gene has been cloned in various vertebrates, as well as in several teleost species. To date, all described species show a sexually dimorphic expression of amh during sex differentiation or at least in differentiated juvenile gonads. We have identified the medaka amh ortholog and examined its expression pattern. Medaka amh shows no sexually dimorphic expression pattern. It is expressed in both developing XY male and XX female gonads. In adult testes, amh is expressed in the Sertoli cells and in adult ovaries in granulosa cells surrounding the oocytes, like in mammals. To better understand the function of amh, we cloned the anti-Müllerian hormone receptor type II (amhrII) ortholog and compared its expression pattern with amh, aromatase (cyp19a1), and scp3. During gonad development, amhrII is coexpressed with medaka amh in somatic cells of the gonads and shows no sexually dimorphic expression. Only the expression level of the Amh type II receptor gene was decreased noticeably in adult female gonads. These results suggest that medaka Amh and AmhrII are involved in gonad formation and maintenance in both sexes.

Synaptonemal complex protein SYCP3: Conserved polymerization properties among vertebrates

Synaptonemal complexes (SCs) are meiosis-specific, nuclear structures that are critically involved in synapsis, recombination and segregation of homologous chromosomes. Although the SC structure is conserved in evolution this is not the case for its protein components. To provide information on SC proteins which would be important for our understanding of the conserved SC structure and function, here we compared ortholog SYCP3 proteins of two evolutionary distant vertebrate species, namely rat and medaka fish. To this end we have investigated the polymerization properties of both proteins by immunocytochemistry, electron microscopy and cell fractionation. We found that despite of the sequence differences that have accumulated over the last 450 million years mammalian and fish SYCP3 have similar properties that allow them to co-assemble higher order structures under experimental conditions. We also provide a likely explanation as to how heterozygous mutations in the SYCP3 gene can lead to a defective meiosis.

Similarity of the domain structure of proteins as a basis for the conservation of meiosis

Meiosis is conserved in all eucaryotic kingdoms, and homologous rows of variability are revealed for the cytological traits of meiosis. To find the nature of these phenomenons, we reviewed the most-studied meiosis-specific proteins and studied them with the methods of bioinformatics. We found that synaptonemal complex proteins have no homology of amino-acid sequence, but are similar in the domain organization and three-dimensional (3D) structure of functionally important domains in budding yeast, nematode, Drosophila, Arabidopsis, and human. Recombination proteins of Rad51/Dmc1 family are conserved to the extent which permits them to make filamentous single-strand deoxyribonucleic acid (ssDNA)-protein intermediates of meiotic recombination. The same structural principles are valid for conservation of the ultrastructure of kinetochores, cell gap contacts, and nuclear pore complexes, such as in the cases when ultrastructure 3D parameters are important for the function. We suggest that self-assembly of protein molecules plays a significant role in building-up of all biological structures mentioned.

Knock-down of DMY initiates female pathway in the genetic male medaka, Oryzias latipes

DMY is the second vertebrate sex-determining gene identified from the fish, Oryzias latipes. In this study, we used two different ways of sex reversal, DMY knock-down and estradiol-17beta (E2) treatment, to determine the possible function of DMY during early gonadal sex differentiation in XY medaka. Our findings revealed that the mitotic and meiotic activities of the germ cells in the 0 day after hatching (dah) DMY knock-down XY larvae were identical to those of the normal XX larvae, suggesting the microenvironment of these XY gonads to be similar to that of the normal XX gonad, where DMY is naturally absent. Conversely, E2 treatment failed to initiate mitosis in the XY gonad, possibly due to an active DMY, even though it could initiate meiosis. Present study is the first to prove that the germ cells in the XY gonad can resume the mitotic activity, if DMY was knocked down.

Structural components of the synaptonemal complex, SYCP1 and SYCP3, in the medaka fish Oryzias latipes

The synaptonemal complex (SC) is a meiosis-specific structure essential for synapsis of homologous chromosomes. For the first time in any non-mammalian vertebrates, we have isolated cDNA clones encoding two structural components of the SC, SYCP1 and SYCP3, in the medaka, and investigated their protein expression during gametogenesis. As in the case of mammals, medaka SYCP1 and SYCP3 are expressed solely in meiotically dividing cells. In the diplotene stage, SYCP1 is diminished at desynaptic regions of chromosomes and completely lost on the chromosomes at later stages. SYCP3 is localized along the arm and centromeric regions of chromosomes at metaphase I, and its existence on the whole chromosomes persists up to anaphase I, a situation different from that reported in the mouse, in which SYCP3 is confined to the centromeric regions but lost on the arm regions at metaphase I. Thus, the expression patterns of SC components are different in mammals and fish despite the resemblance in morphological structure of the SC, suggesting divergence in the function of the SC in regulation of meiosis-specific chromosomal behavior. Since the antibody against medaka SYCP3 is cross-reactive to other fishes, it should be generally useful for a meiosis-specific marker in fish germ cells.