Induction of germ cell formation by oskar.

The oskar gene directs germ plasm assembly and controls the number of germ cell precursors formed at the posterior pole of the Drosophila embryo. Mislocalization of oskar RNA to the anterior pole leads to induction of germ cells at the anterior. Of the eight genes necessary for germ cell formation at the posterior, only three, oskar, vasa and tudor, are essential at an ectopic site.     

黄边糙鸟蛤生殖腺的组织学研究

采集黄边糙鸟蛤成年个体,并进行活体解剖和显微镜观察.结果表明:性成熟个体为雌雄两性功能同体存在的同步性雌雄同体.性腺由树枝状分枝的生殖管、输精管和输卵管组成.生殖管反复分枝成生精小管、生卵小管及其末端膨大的生殖泡囊.性腺由外膜及其内生精小管和生卵小管盘绕交错排列构成.外膜含单层上皮及薄层肌肉,生殖小管和泡囊皆由位于基膜上的生殖上皮构成,生殖上皮不断向腔内增殖各阶段生殖细胞.本文详细描述了精原细胞、卵原细胞、精母细胞、卵母细胞以及精子和卵子的形态结构.此外,还报道了极少数黄边糙鸟蛤性腺存在一些雄性滤泡转化为雌性滤泡现象.     

p63 protects the female germ line during meiotic arrest

Meiosis in the female germ line of mammals is distinguished by a prolonged arrest in prophase of meiosis I between homologous chromosome recombination and ovulation. How DNA damage is detected in these arrested oocytes is poorly understood, but it is variably thought to involve p53, a central tumour suppressor in mammals. While the function of p53 in monitoring the genome of somatic cells is clear, a consensus for the importance of p53 for germ line integrity has yet to emerge. Here we show that the p53 homologue p63 (refs 5, 6), and specifically the TAp63 isoform, is constitutively expressed in female germ cells during meiotic arrest and is essential in a process of DNA damage-induced oocyte death not involving p53. We also show that DNA damage induces both the phosphorylation of p63 and its binding to p53 cognate DNA sites and that these events are linked to oocyte death. Our data support a model whereby p63 is the primordial member of the p53 family and acts in a conserved process of monitoring the integrity of the female germ line, whereas the functions of p53 are restricted to vertebrate somatic cells for tumour suppression. These findings have implications for understanding female germ line fidelity, the regulation of fertility and the evolution of tumour suppressor mechanisms.     

Unexpectedly similar rates of nucleotide substitution found in male and female hominids

In 1947, it was suggested that, in humans, the mutation rate is dramatically higher in the male germ line than in the female germ line. This hypothesis has been supported by the observation that, among primates, Y-linked genes evolved more rapidly than homologous X-linked genes. Based on these evolutionary studies, the ratio (alpha(m)) of male to female mutation rates in primates was estimated to be about 5. However, selection could have skewed sequence evolution in introns and exons. In addition, some of the X-Y gene pairs studied lie within chromosomal regions with substantially divergent nucleotide sequences. Here we directly compare human X and Y sequences within a large region with no known genes. Here the two chromosomes are 99% identical, and X-Y divergence began only three or four million years ago, during hominid evolution. In apes, homologous sequences exist only on the X chromosome. We sequenced and compared 38.6 kb of this region from human X, human Y, chimpanzee X and gorilla X chromosomes. We calculated alpha(m) to be 1.7 (95% confidence interval 1.15-2.87), significantly lower than previous estimates in primates. We infer that, in humans and their immediate ancestors, male and female mutation rates were far more similar than previously supposed.     

Maternal inhibition of hepatitis B surface antigen gene expression in transgenic mice correlates with de novo methylation

Differential modifications of the genome during gametogenesis result in a functional difference between the paternal and maternal genomes at the moment of fertilization. A possible cause of this imprinting is the methylation of DNA. The insertion of foreign DNA into transgenic mice allows the tagging of regions that are differentially methylated during gametogenesis. We describe here a transgenic mouse strain in which the expression of the hepatitis B surface antigen gene is irreversibly repressed following its passage through the female germ line. This inhibition is accompanied by the methylation of all the HpaII and HhaI sites within the foreign gene, which we have shown to be integrated into a site on chromosome 13. The irreversibility reported here contrasts with what is found with other transgenic mice sequences which are reversibly methylated after passage through the male or female germ line, though in both cases methylation appears to be important in the imprinting process.     

白蜡虫雄性生殖系统的构造及其发育的初步研究

报道了白蜡虫雄虫的生殖系统的构造及其发育情况.     

Germline transmission of genetically modified primordial germ cells

Primordial germ cells (PGCs) are the precursors of sperm and eggs. In most animals, segregation of the germ line from the somatic lineages is one of the earliest events in development; in avian embryos, PGCs are first identified in an extra-embryonic region, the germinal crescent, after approximately 18 h of incubation. After 50-55 h of development, PGCs migrate to the gonad and subsequently produce functional sperm and oocytes. So far, cultures of PGCs that remain restricted to the germ line have not been reported in any species. Here we show that chicken PGCs can be isolated, cultured and genetically modified while maintaining their commitment to the germ line. Furthermore, we show that chicken PGCs can be induced in vitro to differentiate into embryonic germ cells that contribute to somatic tissues. Retention of the commitment of PGCs to the germ line after extended periods in culture and after genetic modification combined with their capacity to acquire somatic competence in vitro provides a new model for developmental biology. The utility of the model is enhanced by the accessibility of the avian embryo, which facilitates access to the earliest stages of development and supplies a facile route for the reintroduction of PGCs into the embryonic vasculature. In addition, these attributes create new opportunities to manipulate the genome of chickens for agricultural and pharmaceutical applications.     

Control of plant germline proliferation by SCF(FBL17) degradation of cell cycle inhibitors

Flowering plants possess a unique reproductive strategy, involving double fertilization by twin sperm cells. Unlike animal germ lines, the male germ cell lineage in plants only forms after meiosis and involves asymmetric division of haploid microspores, to produce a large, non-germline vegetative cell and a germ cell that undergoes one further division to produce the twin sperm cells. Although this switch in cell cycle control is critical for sperm cell production and delivery, the underlying molecular mechanisms are unknown. Here we identify a novel F-box protein of Arabidopsis thaliana, designated FBL17 (F-box-like 17), that enables this switch by targeting the degradation of cyclin-dependent kinase A;1 inhibitors specifically in male germ cells. We show that FBL17 is transiently expressed in the male germ line after asymmetric division and forms an SKP1-Cullin1-F-box protein (SCF) E3 ubiquitin ligase complex (SCF(FBL17)) that targets the cyclin-dependent kinase inhibitors KRP6 and KRP7 for proteasome-dependent degradation. Accordingly, the loss of FBL17 function leads to the stabilization of KRP6 and inhibition of germ cell cycle progression. Our results identify SCF(FBL17) as an essential male germ cell proliferation complex that promotes twin sperm cell production and double fertilization in flowering plants.     

Control of the sperm-oocyte switch in Caenorhabditis elegans hermaphrodites by the fem-3 3' untranslated region.

In the Caenorhabditis elegans hermaphrodite germ line, sperm and then oocytes are made from a common pool of germ-cell precursors. The decision to differentiate as a sperm or an oocyte is regulated by the sex-determining gene, fem-3. Expression of fem-3 in the hermaphrodite germ line directs spermatogenesis and must be negatively regulated to allow the switch to oogenesis. In adult hermaphrodites (which are producing oocytes), most fem-3 RNA is found in the germ line, consistent with both the requirement for fem-3 in hermaphrodite spermatogenesis and the maternal effects of fem-3 on embryonic sex determination. Whereas loss-of-function mutants in fem-3 produce only oocytes, hermaphrodites carrying any of nine fem-3 gain-of-function (gf) mutations make none; instead sperm are produced continuously and in vast excess over wild-type amounts. Genetic analyses suggest that fem-3(gf) mutations have escaped a negative control required for the switch to oogenesis. Here we report that all nine fem-3(gf) mutants carry sequence alterations in the fem-3 3' untranslated region (3' UTR). There is no increase in the steady-state level of fem-3(gf) RNA over wild-type, but there is an increase in the polyadenylation of fem-3(gf) RNA that is coincident with the unregulated fem-3 activity. Results of a titration experiment support the hypothesis that a regulatory factor may bind the fem-3 3' UTR. We speculate that fem-3 RNA is regulated through its 3' UTR by binding a factor that inhibits translation, and discuss the idea that this control may be part of a more general regulation of maternal RNAs.     

Exclusion of germ plasm proteins from somatic lineages by cullin-dependent degradation

In many animals, establishment of the germ line depends on segregation of a specialized cytoplasm, or 'germ plasm', to a small number of germline precursor cells during early embryogenesis. Germ plasm asymmetry involves targeting of RNAs and proteins to a specific region of the oocyte and/or embryo. Here we demonstrate that germ plasm asymmetry also depends on degradation of germline proteins in non-germline (somatic) cells. We show that five CCCH finger proteins, components of the Caenorhabditis elegans germ plasm, are targeted for degradation by the novel CCCH-finger-binding protein ZIF-1. ZIF-1 is a SOCS-box protein that interacts with the E3 ubiquitin ligase subunit elongin C. Elongin C, the cullin CUL-2, the ring finger protein RBX-1 and the E2 ubiquitin conjugation enzyme UBC5 (also known as LET-70) are all required in vivo for CCCH finger protein degradation. Degradation is activated in somatic cells by the redundant CCCH finger proteins MEX-5 and MEX-6, which are counteracted in the germ line by the PAR-1 kinase. We propose that segregation of the germ plasm involves both stabilization of germline proteins in the germ line and cullin-dependent degradation in the soma.     

DNA指纹图在小鼠ES细胞嵌合体鉴定中的应用

尝试应用多位点DNA指纹技术,检测经过胚胎干细胞(ES细胞)途径所获得的嵌合体小鼠中ES细胞在各种脏器中的嵌合情况、检测ES细胞在嵌合体小鼠中是否实现种系传递。结果表明：采用新型的人工合成的寡核苷酸多聚体探针(JL-02探针)的多位点DNA指纹图谱,具有足够的多态性和很好的稳定性。适用于鉴定ES细胞在嵌合体小鼠各种组织器官中的嵌合情况,也适用于鉴定ES细胞是否具有种系嵌合能力,尤其适用于在嵌合体研究中,最适宜的品种组合具有相同的毛色或相同的酶型。     

Germline stem cells and follicular renewal in the postnatal mammalian ovary

A basic doctrine of reproductive biology is that most mammalian females lose the capacity for germ-cell renewal during fetal life, such that a fixed reserve of germ cells (oocytes) enclosed within follicles is endowed at birth. Here we show that juvenile and adult mouse ovaries possess mitotically active germ cells that, based on rates of oocyte degeneration (atresia) and clearance, are needed to continuously replenish the follicle pool. Consistent with this, treatment of prepubertal female mice with the mitotic germ-cell toxicant busulphan eliminates the primordial follicle reserve by early adulthood without inducing atresia. Furthermore, we demonstrate cells expressing the meiotic entry marker synaptonemal complex protein 3 in juvenile and adult mouse ovaries. Wild-type ovaries grafted into transgenic female mice with ubiquitous expression of green fluorescent protein (GFP) become infiltrated with GFP-positive germ cells that form follicles. Collectively, these data establish the existence of proliferative germ cells that sustain oocyte and follicle production in the postnatal mammalian ovary.     

Bovine papillomavirus genome elicits skin tumours in transgenic mice

Transmission of the bovine papillomavirus-1 (BPV-1) genome through the mouse germ line results in the heritable formation of fibropapillomas of the skin, a tissue-specific phenotype analogous to that observed in natural BPV-1 infection of cattle. Oncogenesis is slow, with tumours first arising at 8-9 months of age, usually in areas prone to wounding. Extrachromosomal BPV-1 DNA is detected in all tumours, whereas normal tissues show only integrated DNA.     

MRT-2 checkpoint protein is required for germline immortality and telomere replication in C. elegans

The germ line is an immortal cell lineage that is passed indefinitely from one generation to the next. To identify the genes that are required for germline immortality, we isolated Caenorhabditis elegans mutants with mortal germ lines--worms that can reproduce for several healthy generations but eventually become sterile. One of these mortal germline (mrt) mutants, mrt-2, exhibits progressive telomere shortening and accumulates end-to-end chromosome fusions in later generations, indicating that the MRT-2 protein is required for telomere replication. In addition, the germ line of mrt-2 is hypersensitive to X-rays and to transposon activity. Therefore, mrt-2 has defects in responding both to damaged DNA and to normal double-strand breaks present at telomeres. mrt-2 encodes a homologue of a checkpoint gene that is required to sense DNA damage in yeast. These results indicate that telomeres may be identified as a type of DNA damage and then repaired by the telomere-replication enzyme telomerase.     

A kappa-immunoglobulin gene is formed by site-specific recombination without further somatic mutation.

The active gene for a kappa light chain is formed by a somatic recombination event that joins one of several hundred variable region genes to one of a series of recombination sites (J-segments) encoded close to the kappa constant region gene. The nucleotide sequences of cloned germ line and somatically recombined genes define the precise organisation of these genetic segments and the site and nature of the recombination event that joined them. Apart from somatic recombination, no further alteration of ther germ line sequence has occurred. The J-segment is of special interest as it encodes signals for both DNA and RNA splicing and provides a means of generating further immunoglobulin gene diversity.     

A gradient of sex linkage in the pseudoautosomal region of the human sex chromosomes

Three independent pseudoautosomal loci are linked to sex determination at frequencies which define a gradient of linkage. The segregation patterns of these loci indicate that X/Y recombination results from a single obligatory meiotic crossing-over in the pseudoautosomal region. Recombination in male germ cells in the terminal regions of the short arms of the X and Y chromosomes in 10-fold greater than between the same regions of the X chromosomes in female germ cells.     

Activation of a transposable element in the germ line but not the soma of Caenorhabditis elegans

The genetic activity of transposable elements is tightly controlled in many species. Transposons that are relatively quiescent under certain circumstances can excise or transpose at greatly increased rates under other circumstances. For example, 'genomic shock' can activate quiescent maize transposons, 'cytotype' and tissue-specific splicing regulate Drosophila P factors, copy number controls Tn5 transposition in bacteria, and developmental timing affects the production of transposon-like intracisternal A-particles in mouse embryos. The Caenorhabditis elegans transposable element Tc1 is subject to both strain-specific and tissue-specific control. Multiple copies of Tc1 are present in the genome of all C. elegans strains collected from nature. However, these elements are genetically active in only certain isolates. For example, in C. elegans variety Bristol transposition and excision of Tc1 are undetectable, but in variety Bergerac transposition and excision are frequent. Moreover, in variety Bergerac, Tc1 is about 1,000-fold more active in somatic cells than in germ cells. We have investigated the genetic basis for the germ/soma regulation of Tc1 activity. We have isolated mutants that exhibit increased frequencies of Tc1 excision in the germ line. The frequencies of Tc1 excision in the soma are unaltered in these mutants. These mutants also exhibit high frequencies of Tc1 germ-line transposition, and this results in a mutator phenotype. Nearly all mutator-induced mutations are caused by insertion of Tc1.