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[
WormBook,
2006]
Early cell lineages and arrangement of blastomeres in C. elegans are similar to the pattern found in Ascaris and other studied nematodes leading to the assumption that embryonic development shows little variation within the phylum Nematoda. However, analysis of a larger variety of species from various branches of the phylogenetic tree demonstrate that prominent variations in crucial steps of early embryogenesis exist among representatives of this taxon. So far, most of these variations have only been studied on a descriptive level and thus essentially nothing is known about their molecular or genetic basis. Nevertheless, it is obvious that the limited morphological diversity of the freshly hatched juvenile and the uniformity of the basic body plan contrast with the many modifications in the way a worm is generated from the egg cell. This chapter focuses on the initial phase between egg activation and gastrulation and deals with the following aspects: reproduction and diploidy, polarity, cleavage and germ line, cell lineages; cell cycles and maternal contribution, cell-cell communication and cell specification, gastrulation.
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[
1987]
Mutations in genes that control developmental patterns undoubtedly underlie evolutionary change in development. The elucidation of the precise genetic basis of evolutionary change requires the identification and genetic analysis of key genes that control normal developmental patterns of an organism ("developmental control genes"), the analysis of the precise nature of developmental differences between that organism and its related species, and the determination of what changes in these developmental control genes actually cause the observed evolutionary developmental differences. Nematodes offer an excellent opportunity to study the roles of developmental control genes in evolutionary change. The simple anatomy and rapid life cycle of the nematode Caenorhabditis elegans has allowed a detailed analysis of its wild-type development. As a result, the complete cell lineage of C. elegans has been elucidated. This lineage is nearly invariant in the wild type; each cell is formed after a defined lineage history and at a specific time during development. Thus, the developmental defects of mutants can be accurately determined at the level of the fates expressed by specific cells at specific times in development. Through genetic analyses of C. elegans developmental mutants, genes have been identified that play crucial roles in specifying and expressing the normal developmental program. If these genes code for developmental control processes common to different nematode species, then mutations of these genes might underlie interspecific developmental change. Other nematode species can be isolated from the wild and cultured in the laboratory with ease. The relatively simple cellular anatomy of nematodes allows the direct comparison of cell lineages between different species on the level of individual cells and cell divisions. If genes affecting development in C. elegans play evolutionary roles, then developmental differences between species should emerge that parallel, or even are identical to, mutationally induced changes in C. elegans. It should eventually be possible to test directly which genes are responsible for certain evolutionary differences in development by altering
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[
WormBook,
2005]
C. elegans presents a low level of molecular diversity, which may be explained by its selfing mode of reproduction. Recent work on the genetic structure of natural populations of C. elegans indeed suggests a low level of outcrossing, and little geographic differentiation because of migration. The level and pattern of molecular diversity among wild isolates of C. elegans are compared with those found after accumulation of spontaneous mutations in the laboratory. The last part of the chapter reviews phenotypic differences among wild isolates of C. elegans.
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[
2012]
Genetic divergence appears to be high among nematodes, while morphological variation is low. To better understand how this fits together and to trace the evolution of development in this phylum we started a comprehensive comparative survey of embryogenesis comprising all branches of the phylogenetic tree. We find considerable differences, in particular between basal and more derived species. This review focuses on cellular pattern formation and cell fate assignment during early development. Our data indicate that evolution of nematodes went from indeterminate early cleavage without initial polarity to invariant cell lineages with establishment of polarity before first division. Different ways to establish this polarity and the variety of taxon-specific spatial arrangements of cells require modifications with respect to cell specification processes and the underlying molecular mechanisms. We conclude that the standard pattern as found in the model system C. elegans constitutes only one of the many ways to construct a nematode and discuss the adaptive value of the observed developmental variations.
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[
1990]
Three DNA repair systems (photoreactivation, excision repair and post-replication repair) exist in most organisms. Their status has been examined in the nematode C. elegans. This metazoan is deficient in photoreactivation but possesses efficient excision and post-replication repair systems. The stage-specific variations in hypersensitivity displayed by radiation-sensitive (rad) mutants, as well as the stage-specific excision-repair deficiency displayed by
rad-3, indicate that DNA repair is developmentally regulated in this popular model system. In addition, various data suggest that C. elegans may possess novel
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[
1997]
Caenorhabditis elegans aquatic toxicity assays were standardized with five common reference toxicants: CdCl2, NaCl, KCl, sodium lauryl sulfate (SLS), and sodium pentachlorophenate (PCP). Aquatic toxicity testing was conducted in 3 media: a standard C. elegans medium; EPA moderately hard reconstituted water; and EPA moderately hard mineral water. Test duration in each medium was 24h without a food source, and 24h and 48h with Escherichia coli strain OP50 as a food source. Each test was replicated three times with each replicate having 6 wells per concentration, 10 worms per well. LC50 values were calculated using probit analysis. The average LC50s for each set of replicants were compared to assess sensitivity and reproducibility of the data, identifying expected variation between replicate tests. These reference toxicants increase the database for C. elegans and provide a benchmark for further application.
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[
1998]
In this study initial data were generated to develop laboratory control charts for aquatic toxicity testing using the nematode Caenorhabditis elegans. Tests were performed using two reference toxicants: CdCl2 and CuCl2. All tests were performed for 24 h without a food source and for 48 h with a food source in a commonly used nematode aquatic medium. Each test was replicated 6 times with each replicate having 6 wells per concentration with 10 +/- 1 worms per well. Probit analysis was used to estimate LC50 values for each test. The data were used to construct a mean laboratory control chart for each reference toxicant. The coefficient of variation (CV) for three of the four reference toxicant tests was less than 20%, which demonstrates an excellent degree of reproducibility. These CV values are well within suggested standards for determination of organism sensitivity and overall test system credibility. A standardized procedure for performing 24 h and 48 h aquatic toxicity studies with C. elegans is
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[
1987]
Nematode sperm are crawling cells that exhibit a type of locomotion characteristic of an entire class of protozoa as well as numerous embryonic, differentiated, and transformed metazoan cells. Despite considerable variation in morphology and speed of locomotion expressed by these various types of crawling, or amoeboid, cells, there is general agreement that in all cases locomotion is propelled by cytoplasmic contraction involving myosin-induced sliding of actin filaments and regulated, in ways that are not fully understood, by a spectrum of actin-binding proteins. We began to study the motility of sperm of Caenorhabditis elegans hoping to exploit the mutability of this cell in order to analyze the molecular basis of amoeboid movement genetically. Much to our surprise, we discovered that sperm motility is not driven by an actin-based mechanism. Subsequent work, however, has shown that nematode sperm do share many fundamental properties with other amoeboid cells. As a consequence, sperm continue to serve as a profitable model for understanding how cells crawl and, at the same time, have allowed us to examine a new type of cellular motor.
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[
1999]
Caenorhabditis elegans is a free-living soil nematode that is commonly used as a biological model. Recently, much work has been done using the nematode as a toxicological model as well. Much of the work involving C. elegans has been performed in aquatic media, since it lives in the interstitial water of soil. However, testing in soil would be expected to more accurately reproduce the organism's normal environment and may take into consideration other factors not available in an aquatic test, i.e., toxicant availability effects due to sorption, various chemical interactions, etc. This study used a modification of a previous experimental protocol to determine 24h LC50 values for Cu in a Cecil series soil mixture, and examined the use of CuCl2 as a reference toxicant for soil toxicity testing with C. elegans. Three different methods of determining percent lethality were used, each dependent on how the number of worms missing after the recovery process was used in the lethality calculations. Only tests having >/= 80% worm recovery and >/= 90% control survival were used in determining the LC50S, by Probit analysis. The replicate LC50 values generated a control chart for each method of calculating percent lethality. The coefficient of variation (CV) for each of the three methods was </= 14%. The control charts and the protocol outlined in this study are intended to be used to assess test organism health and to monitor precision of future soil toxicity tests with C.
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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.