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[
1987]
Ascaris and several other parasitic nematodes undergo chromatin diminution in the somatic cell precursors of the early embryo. In 1910 Boveri hypothesized that the chromatin lost might include genes essential to the function of the germ line. We have cloned a germ line-specific cDNA which codes for the major sperm protein. Using this clone as a probe we found that these genes show no loss or rearrangement of DNA in somatic cells which have undergone chromatin diminution. Actin and a-tubulin genes from Ascaris are also unchanged following diminution. Ascaris and the free-living nematode Caenorhabditis elegans differ substantially in the numbers of actin and major sperm protein genes, in spite of conservation of gene
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[
1979]
We have isolated temperature sensitive maternal effect mutants in the free-living nematode Caenorhabditis elegans. We use C. elegans for several basic reasons. It is easy to culture in the laboratory and it has a rapid life cycle. The genetics of C. elegans have been elucidated by Brenner and more recently have been refined by the lethal analysis of Herman et. al. Both embryonic and postembryonic development can be observed directly and conveniently on the living worm with Nomarski differential interference optics because egg shell and worm cuticle are transparent. The precise embryonic cell lineages of C. elegans are known from fertilization to the 200 blastomere stage. All of the postembryonic somatic cell lineages are precisely known. It ...
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Caenorhabditis elegans is a free-living soil nematode, about 1 mm in length, that is found around the world. It is currently a common laboratory model for many aspects of cellular, developmental, and molecular biology. Its popularity comes from its transparency (allowing all nuclei to be followed in living animals at all stages of development), its anatomical simplicity (1000 cells), its small genome (100 Mbp), an invariant somatic cell lineage, ease of laboratory culture, rapid generation time, and a mode of reproduction which facilitates classical genetic analysis. An interested beginner needs only a petri plate, some Escherichia coli, and a stereo dissecting microscope to begin study of this fascinating creature.
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[
WormBook,
2005]
In C. elegans, the germ line is set apart from the soma early in embryogenesis. Several important themes have emerged in specifying and guiding the development of the nascent germ line. At early stages, the germline blastomeres are maintained in a transcriptionally silent state by the transcriptional repressor PIE-1 . When this silencing is lifted, it is postulated that correct patterns of germline gene expression are controlled, at least in part, by MES-mediated regulation of chromatin state. Accompanying transcriptional regulation by PIE-1 and the MES proteins, RNA metabolism in germ cells is likely to be regulated by perinuclear RNA-rich cytoplasmic granules, termed P granules. This chapter discusses the molecular nature and possible roles of these various germline regulators, and describes a recently discovered mechanism to protect somatic cells from following a germline fate.
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[
WormBook,
2005]
In mammals, flies, and worms, sex is determined by distinctive regulatory mechanisms that cause males (XO or XY) and females (XX) to differ in their dose of X chromosomes. In each species, an essential X chromosome-wide process called dosage compensation ensures that somatic cells of either sex express equal levels of X-linked gene products. The strategies used to achieve dosage compensation are diverse, but in all cases, specialized complexes are targeted specifically to the X chromosome(s) of only one sex to regulate transcript levels. In C. elegans, this sex-specific targeting of the dosage compensation complex (DCC) is controlled by the same developmental signal that establishes sex, the ratio of X chromosomes to sets of autosomes (X:A signal). Molecular components of this chromosome counting process have been defined. Following a common step of regulation, sex determination and dosage compensation are controlled by distinct genetic pathways. C. elegans dosage compensation is implemented by a protein complex that binds both X chromosomes of hermaphrodites to reduce transcript levels by one-half. The dosage compensation complex resembles the conserved 13S condensin complex required for both mitotic and meiotic chromosome resolution and condensation, implying the recruitment of ancient proteins to the new task of regulating gene expression. Within each C. elegans somatic cell, one of the DCC components also participates in the separate mitotic/meiotic condensin complex. Other DCC components play pivotal roles in regulating the number and distribution of crossovers during meiosis. The strategy by which C. elegans X chromosomes attract the condensin-like DCC is known. Small, well-dispersed X-recognition elements act as entry sites to recruit the dosage compensation complex and to nucleate spreading of the complex to X regions that lack recruitment sites. In this manner, a repressed chromatin state is spread in cis over short or long distances, thus establishing the global, epigenetic regulation of X chromosomes that is maintained throughout the lifetime of hermaphrodites.
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Embryogenesis of the nematode Caenorhabditis elegans is determinate and virtually invariant from individual to individual. The fertilized egg develops into an anatomically relatively simple juvenile animal having a constant number of only 550 cells (or nuclei) at hatching. The complete embryonic cell lineage up to the 220-cell stage has been described previously, and some lineages have been followed considerably further. During postembryonic development, the cell number increases to about 950 in mature hermaphrodites (including the 143 of the somatic gonad structures) and to about 1025 in males. These cells arise from about 50 blast cells which resume division after hatching. The lineages of all these cells have been described.
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[
WormBook,
2008]
Germline apoptosis shares with somatic apoptosis a reliance on key components of the core apoptotic machinery, including CED-3 and CED-4. However, germline apoptosis differs from somatic apoptosis in its regulation. Whereas somatic apoptosis is developmentally programmed by cell lineage, germline apoptosis occurs as part of an oogenesis program. One category of germline apoptosis, dubbed "physiological" germline apoptosis, reduces the number of cells that complete oogenesis, and is independent of the BH3-only apoptosis effecter EGL-1. A second category, termed "stress-induced" germline apoptosis, is triggered by a genomic integrity checkpoint. Some mechanisms that are monitored by this DNA-damage checkpoint are also involved in germ cell "immortality," or preservation of a continuous germ cell lineage over successive generations. In addition, exposure to certain environmental insults or pathogens induces germ cell apoptosis. Here we will review the mechanisms that control each of the pathways leading to germ cell apoptosis and discuss their functional significance. Germline apoptosis is an integral part of oogenesis in many animals, including humans. Because many of the regulators of C. elegans germline apoptosis are conserved, we suggest that this nematode provides a valuable model for understanding controls of germline apoptosis more broadly.
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[
WormBook,
2005]
The C. elegans germ line proliferates from one primordial germ cell (PGC) set aside in the early embryo to over a thousand cells in the adult. Most germline proliferation is controlled by the somatic distal tip cell, which provides a stem cell niche at the distal end of the adult gonad. The distal tip cell signals to the germ line via the Notch signaling pathway, which in turn controls a network of RNA regulators. The FBF-1 and FBF-2 RNA-binding proteins promote continued mitoses in germ cells located close to the distal tip cell, while the GLD-1 , GLD-2 , GLD-3 , and NOS-3 RNA regulators promote entry into meiosis as germ cells leave the stem cell niche. In addition to these key regulators, many other genes affect germline proliferation.
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[
1979]
Recombinant DNA methods have been used to characterize the genome of Caenorhabditis elegans. To determine if DNA rearrangements occur during somatic differentiation, fifteen randomly cloned Bam H1 fragments of somatic DNA were hybridized to Bam H1 digests of germ and somatic DNA's on Southern filters. In this way, 50 fragments representing 0.3% of the genome were compared and no size differences were detected. The DNA's of two interbreeding strains of C. elegans were also compared to determine the degree of evolutionary divergence. Fifteen percent of the fragments differed between the two strains. However, no differences could be found between the rDNA's. The DNA's of C. elegans and C. briggsae were compared and very little homology could be detected even though these species are morphologically very similar. The fragments that differ in size between the two interbreeding strains are being genetically mapped. These experiments suggest that non-random segregation of chromosomes might be occurring in
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[
WormBook,
2005]
The features that differentiate the C. elegans male from the hermaphrodite arise during postembryonic development. The major male mating structures, consisting of the blunt tail with fan and rays, the hook, the spicules and proctodeum, and the thin body, form just before the last larval molt. Male and hermaphrodite embryogenesis are similar but some essential male cell fates are already established at hatching. The male mating structures arise from three important sets of male-specific blast cells. These cells generate a total of 205 male-specific somatic cells, including 89 neurons, 36 neuronal support cells, 41 muscles, 23 cells involved in differentiating the hindgut, and 16 hypodermal cells associated with mating structures. Genetic and molecular studies have identified many genes required for male development, most of which also function in the hermaphrodite. Cell-cell interactions play a role in patterning all three of the generative tissues. Male-specific neurons, including sensory neurons of the rays, hook, post-cloacal sensilla, and spicules, differentiate at the end of the last larval stage and send out axons to make connections into the existing neuropil, greatly enlarging the posterior ganglia. The hindgut is highly differentiated to accommodate the spicules and the joining of the reproductive tract to the cloaca. A complex male-specific program generates many new muscles for copulation. The cell lineage and genetic program that gives rise to the one-armed male gonad appears to be a variation on that of the hermaphrodite.