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[
WormBook,
2006]
In the last decade, nematodes other than C. elegans have been studied intensively in evolutionary developmental biology. A few species have been developed as satellite systems for more detailed genetic and molecular studies. One such satellite species is the diplogastrid nematode Pristionchus pacificus. Here, I provide an overview about the biology, phylogeny, ecology, genetics and genomics of P. pacificus.
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[
1984]
Developmental fates of blastomeres in early C. elegans embryos appear to be governed by internally segregating, cell-autonomous determinants. To ascertain whether previously described gut-lineage dterminants are nuclear or cytoplasmic, laser microsurgery was used to show that exposing the nucleus of a non-gut-precursor cell to gut-precursor cytoplasm can cause the progeny of the resulting hybrid cell to express gut-specific differentiation markers, supporting the view that the determinants are cytoplasmic. In attempts to obtain molecular probes for such determinants, a library of monoclonal antibodies to early embryonic antigens was generated and screened by immunofluorescence microscopy for antibodies reacting with lineage-specific components. Three of the antibodies react with cytoplasmic granules (P granules) that segregate specifically with the germ line in early cleavages and are found uniquely in germ-line cells throughout the life cycle. Experiments on unfertilized eggs, on mutant embryos with defects in early cleavage, and on normal embryos treated with various cytoskeletal inhibitors indicate that P-granule segregation depends upon fertilization and requires the function of actin microfilaments, but is independent of spindle and microtubule functions. Work on the biochemical nature and function of the P granules is in progress.
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[
1984]
Germ cells in a wide variety of invertebrate and vertebrate species contain distinctive cytoplasmic organelles that have been visualized by electron microscopy. The ubiliquity of such structures suggests that they play some role in germ-line determination or differentiation, or both. However, the nature and function of these structures remain unknown. We describe experiments with two types of immunologic probes, rabbit sera and mouse monoclonal antibodies, directed against ctyoplamsic granules that are unique to germ-line cells in the nematode, Caenorhabditis elegans, and that may correspond to the germ-line-specific structures seen by electron microscopy in C. elegans embryos. The antibodies have been used to follow the granules, termed P granules, during early embryonic cleavage stages and throughout larval and adult development. P granules become progressively localized to the germ-line precursor cells during early embryogenesis. We are using conditionally lethal maternal-effect mutations to study this localization process. In addition to providing a rapid assay for P granules in wild-type, mutant, and experimentally maipulated embryos, the antibodies also promise to be useful in biochemically characterizing the granules and in investigating their
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[
WormBook,
2005]
Asymmetric cell divisions play an important role in generating diversity during metazoan development. In the early C. elegans embryo, a series of asymmetric divisions are crucial for establishing the three principal axes of the body plan (AP, DV, LR) and for segregating determinants that specify cell fates. In this review, we focus on events in the one-cell embryo that result in the establishment of the AP axis and the first asymmetric division. We first describe how the sperm-derived centrosome initiates movements of the cortical actomyosin network that result in the polarized distribution of PAR proteins. We then briefly discuss how components acting downstream of the PAR proteins mediate unequal segregation of cell fate determinants to the anterior blastomere AB and the posterior blastomere P 1 . We also review how a heterotrimeric G protein pathway generates cortically based pulling forces acting on astral microtubules, thus mediating centrosome and spindle positioning in response to AP polarity cues. In addition, we briefly highlight events involved in establishing the DV and LR axes. The DV axis is established at the four-cell stage, following specific cell-cell interactions that occur between P 2 and EMS , the two daughters of P 1 , as well as between P 2 and ABp , a daughter of AB . The LR axis is established shortly thereafter by the division pattern of ABa and ABp . We conclude by mentioning how findings made in early C. elegans embryos are relevant to understanding asymmetric cell division and pattern formation across metazoan evolution.
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Applications of, and investigations on lectins in nematology reflect the existing classification of nematodes according to their life-styles, i.e. free-living, plant-parasitic and animal-parasitic. In animal-parasitic nematodes, lectins have predominately been used to study the cuticle and its interaction between nematode and host. In plant-parasitic nematodes, investigations on the cuticle and amphid exudates have been predominant. Nematode-plant interactions on the other hand have attracted only minor attention. Ironically, however, the free-living nematodes in general, and the widely used model system Caenorhabditis elegans in particular, have been used very little for study of lectins, in spite of the many advantages offered by this organism as a genetic and an experimental model system.
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[
Methods Cell Biol,
1995]
Both the localization and distribution of nucleic acid sequences in genomes and in cells can be visualized by hybridization of labeled probe DNAs to cytological preparations of chromosomes or tissues. With the introduction of nonisotopically labeled nucleotides that could be incorporated into cloned DNAs by enzymatic methods in vitro, it became possible to detect the site of hybridization quickly using antibodies that recognized the modifying group on the nucleotides incorporated into the probe DNA. More recently, nucleotides labeled with a fluorescent molecule have been incorporated into probes by invitro enzymatic reactions and the site of hybridization can then be visualized directly. As fluorescence in situ hybridization provides a rapid and high-resolution method for mapping genes, it is being sued increasingly for mapping cloned DNAs to chromosomes and for the ordering of clones in large-scale genome projects. On the other hand, physically mapped clones can also be used to label chromosomes for analysis of such biological processes as chromosome segregation, pairing in meiosis, and interphase nuclear order. Nonisotopic methods of hybridization are also ideally suited to visualization of mRNA distributions in tissues, because the signal can be detected in thick specimens, in contrast to isotopic methods that require thin specimens for detection by autoradiography...
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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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[
1987]
Vitellogenins of many insects, vertebrates, nematodes and sea urchins are very similar in size and amino acid composition. We have determined the nucleotide sequences of the genes that encode vitellogenins in nematodes (C. elegans) and sea urchins (S. purpuratus), and compared the deduced amino acid sequences to the published sequences of two vertebrate vitellogenins (X. laevis and G. gallus). This comparison demonstrated unequivocally that the nematode and vertebrate proteins are encoded by distant members of a single gene family. The less extensive sequence data available for the sea urchin gene indicates that this, too, may be a member of this family of genes, as may the vitellogenin genes of locust. On the other hand, we were unable to detect any similarity between these genes and the D. melanogaster yolk protein genes. Thus it appears that while nematodes, vertebrates, sea urchins and at least some insects utilize the same family of genes to encode vitellogenins, Drosophila uses a different gene family. All of the vitellogenin genes are regulated in a tissue-specific manner. They are expressed in the intestine in nematodes, in the liver in vertebrates, in the fat body in insects, and in the intestine and gonad in sea urchins. Their production is limited to adult females in all species except sea urchins, in which they are expressed by adults of both sexes. In nematodes we have identified two heptameric sequence elements repeated multiple times in all eleven of the vitellogenin genes sequenced. One of these elements is also present in the vertebrate promoters and has recently been shown to be required for transcriptional activation. All of the 5' ends of the vitellogenin mRNAs of nematodes, vertebrates and locust can be folded into potentially-stable secondary structures. We present evidence that these structures have been strongly selected for and presumably perform some function in regulation of vitellogenin production.
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Genetic analysis of C. elegans development has focused on developmental events that take place after hatching, during postembryonic development. After hatching with 558 cells, about 10% of these are blast cells that undergo further cell divisions (Fig. 1) to generate a total of 959 neurons, muscles, intestinal and hypodermal cells in the hermaphrodite and 1031 cells in the male. Like embryonic development (se Edgar, Chap. 19 this Vol.), the pattern of cell division and differentiation during C. elegans postembryonic development is nearly invariant and has been completely described. The cell lineage of wild-type, mutant, or laser-ablated animals can be determined by direct observation of development using Normarski optics. Because most cells during C. elegans postembryonic development generate unique patterns of descendents (though symmetries in the lineage exist), the cell lineage produced by a particular blast cell during development is a signature of that cell's identity. Any changes in cell identity, induce, for example, by laser ablation or neighboring cells or by mutation, can be recognized by a change in the lineage produced by that cell. By laser ablation, it has been shown that in many cases, that patterns of cell lineage executed by particular cells do not depend on their neighbors and instead reflect some intrinsic developmental program. On the other hand, the lineages of particular blast cells, for example, those that generate the hermaphrodite vulva, have been shown by laser ablation experiments to depend on interactions with their neighbors. Thus the pattern of cell divisions and differentiations that normally occur during C. elegans development depends on the ancestry of cells in some cases on their neighbors or positional signals in other cases. Two major goals of developmental genetic analysis in C. elegans have been to explain how genes couple cell lineage information to cell identity and to explain how genes control and mediate cell-cell interactions. As described below, this analysis has revealed molecular mechanisms for the generation of lineage asymmetry and for intercellular signaling that are general to perhaps all metazoans.
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[
WormBook,
2006]
There are two sexes in C. elegans, hermaphrodite and male. While there are many sex-specific differences between males and hermaphrodites that affect most tissues, the basic body plan and many of its structures are identical. However, most structures required for mating or reproduction are sexually dimorphic and are generated by sex-specific cell lineages. Thus to understand cell fate specification in hermaphrodites, one must consider how the body plan, which is specified during embryogenesis, influences the fates individual cells. One possible mechanism may involve the asymmetric distribution of POP-1 /Tcf, the sole C. elegans Tcf homolog, to anterior-posterior sister cells. Another mechanism that functions to specify cell fates along the anterior-posterior body axis in both hermaphrodites and males are the Hox genes. Since most of the cell fate specifications that occur in hermaphrodites also occur in males, the focus of this chapter will be on those that only occur in hermaphrodites. This will include the cell fate decisions that affect the HSN neurons, ventral hypodermal P cells, lateral hypodermal cells V5 , V6 , and T ; as well as the mesodermal M, Z1 , and Z4 cells and the intestinal cells. Both cell lineage-based and cell-signaling mechanisms of cell fate specification will be discussed. Only two direct targets of the sex determination pathway that influence cell fate specification to produce hermaphrodite-specific cell fates have been identified. Thus a major challenge will be to learn additional mechanisms by which the sex determination pathway interacts with signaling pathways and other cell fate specification genes to generate hermaphrodite-specific cell fates.