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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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[
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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[
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]
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]
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,
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.