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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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WormBook,
2006]
Spermatogenesis creates functional sperm from an initially undifferentiated germ cell. In the nematode Caenorhabditis elegans, both males and hermaphrodites engage in spermatogenesis. The hermaphrodite germ line, like that of the male, initiates spermatogenesis during the L4 larval stage. The hermaphrodite germ line differs from that of the male because it ceases spermatogenesis and switches to oogenesis during the adult stage. Each hermaphrodite stores her sperm and uses them to fertilize her oocytes. Many mutants have been identified where hermaphrodite self-fertility is disrupted. If such a self-sterile hermaphrodite is mated to a wild-type male, mutant hermaphrodites that either lack sperm or contain defective sperm will produce outcross progeny. Easily implemented tests are then applied to identify the subset of these mutants that produce defective sperm. Currently, more than 44 genes are known that are required for normal spermatogenesis. This chapter discusses the 25 best-understood genes that affect spermatogenesis and mutants are grouped based on the cellular structure or process that is affected. C. elegans spermatozoa lack an acrosome and a flagellum, which are organelles found in the spermatozoa produced by most other species. Like other nematodes, C. elegans spermatozoa move by crawling using a single pseudopod. Wild-type spermatogenesis and its defects in mutants can be studied in vivo because the animal is transparent and in vitro because a simple, chemically defined medium that supports development has been discovered. Unlike nearly all other C. elegans cells, homogeneous sperm can be obtained in sufficient quantities to permit biochemical analyses.
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[
WormBook,
2006]
Wild C. elegans and other nematodes live in dirt and eat bacteria, relying on mechanoreceptor neurons (MRNs) to detect collisions with soil particles and other animals as well as forces generated by their own movement. MRNs may also help animals detect bacterial food sources. Hermaphrodites and males have 22 putative MRNs; males have an additional 46 MRNs, most, if not all of which are needed for mating. This chapter reviews key aspects of C. elegans mechanosensation, including MRN anatomy, what is known about their contributions to behavior as well as the neural circuits linking MRNs to movement. Emerging models of the mechanisms used to convert mechanical energy into electrical signals are also discussed. Prospects for future research include expanding our understanding of the molecular basis of mechanotransduction and how activation of MRNs guides and modulates behavior.
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[
WormBook,
2007]
Great inroads into the understanding of aging have been made using C. elegans as a model system. Several genes have been identified that, when mutated, can extend lifespan. Yet, much about aging remains a mystery, and new technologies that allow the simultaneous assay of expression levels of thousands of genes have been applied to the question of how and why aging might occur. With correct experimental design and statistical analysis, differential gene expression between two or more populations can be obtained with high confidence. The ability to survey the entire genome in an unbiased way is a great asset for the study of complex biological phenomena such as aging. Aging undoubtedly involves changes in multiple genes involved in multiple processes, some of which may not yet be known. Gene expression profiling of wild type aging, and of strains with increased life spans, has provided some insight into potential mechanisms, and more can be expected in the future.
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[
WormBook,
2005]
The morphogenesis of the C. elegans embryo is largely controlled by the development of the epidermis, also known as the hypodermis, a single epithelial layer that surrounds the animal. Morphogenesis of the epidermis involves cell-cell interactions with internal tissues, such as the developing nervous system and musculature. Genetic analysis of mutants with aberrant epidermal morphology has defined multiple steps in epidermal morphogenesis. In the wild type, epidermal cells are generated on the dorsal side of the embryo among the progeny of four early embryonic blastomeres. Specification of epidermal fate is regulated by a hierarchy of transcription factors. After specification, dorsal epidermal cells rearrange, a process known as dorsal intercalation. Most epidermal cells fuse to generate multinucleate syncytia. The dorsally located epidermal sheet undergoes epiboly to enclose the rest of the embryo in a process known as ventral enclosure; this movement requires both an intact epidermal layer and substrate neuroblasts. At least three distinct types of cellular behavior underlie the enclosure of different regions of the epidermis. Following enclosure, the epidermis elongates, a process driven by coordinated cell shape changes. Epidermal actin microfilaments, microtubules, and intermediate filaments all play roles in elongation, as do body wall muscles. The final shape of the epidermis is maintained by the collagenous exoskeleton, secreted by the apical surface of the epidermis.
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WormBook,
2005]
Synaptogenesis is a process involving the formation of a neurotransmitter release site in the presynaptic neuron and a receptive field at the postsynaptic partners, and the precise alignment of pre- and post-synaptic specializations. In C. elegans synapses are found as en passant axonal swellings along the nerve processes. Genetic screens using a synaptic vesicle-associated GFP marker have identified key players in synaptic target recognition and organization of the presynaptic terminals. Importantly, the functions of most genes are evolutionarily conserved. Further studies using a combination of genetic modifier screens and reverse genetics have begun to reveal the underlying signaling pathways.
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WormBook,
2006]
Through genetic analyses, the function of genes is investigated by studying organisms where gene function is altered. In classical forward genetic screening, individuals are treated with mutagens to induce DNA lesions and mutants with a phenotype of interest are sought. After a mutant is found, the gene mutated is identified through standard molecular techniques. Detailed studies of the mutant phenotype coupled with molecular analyses of the gene allows elucidation of the gene's function. Forward genetics has been responsible for our understanding of many biological processes and is an excellent method for identifying genes that function in a particular process.In reverse genetics, the functional study of a gene starts with the gene sequence rather than a mutant phenotype. Using various techniques, a gene's function is altered and the effect on the development or behaviour of the organism is analysed. Reverse genetics is an important complement to forward genetics. For example, using reverse genetics, one can investigate the function of all genes in a gene family, something not easily done with forward genetics. Further, one can study the function of a gene found to be involved in a process of interest in another organism, but for which no forward genetic mutants have yet been identified. Finally, the vast majority of genes have not yet been mutated in most organisms and reverse genetics allows their study. The availability of complete genome sequences combined with reverse genetics can allow every gene to be studied.This chapter gives detailed protocols for the two main methods of perturbing gene function in C. elegans: RNA interference and the creation of deletion mutants. Either technique can be applied to the study of individual genes. With less than a day of actual work, RNAi creates a knockdown of gene function without altering the organism's DNA (see below). In contrast, with about a month of work, a deletion mutation permanently removes all gene function. Deciding which technique to use will depend on the nature of the experiment. The techniques can also be combined, where RNAi is used for rapid screening of loss of function phenotypes and then deletion mutants are made to study genes of particular interest. RNAi can also be carried out on a global scale, where knockdown of (nearly) every gene is tested for inducing a phenotype of interest. In this case, the reverse genetics technique of RNAi can be thought of as a forward genetic screening tool.
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WormBook,
2007]
Strongyloides is a genus of parasitic nematodes, which, unusually, has a free-living adult generation. Here we introduce the biology of this genus, especially the fascinating, but complex, life-cycle together with an overview of the taxonomy, morphology, genetics and genomics of this genus.
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WormBook,
2010]
The nervous system represents the most complex tissue of C. elegans both in terms of numbers (302 neurons and 56 glial cells = 37% of the somatic cells in a hermaphrodite) and diversity (118 morphologically distinct neuron classes). The lineage and morphology of each neuron type has been described in detail and neuronal fate markers exists for virtually all neurons in the form of fluorescent reporter genes. The ability to "phenotype" neurons at high resolution combined with the amenability of C. elegans to genetic mutant analysis make the C. elegans nervous system a prime model system to elucidate the nature of the gene regulatory programs that build a nervous system-a central question of developmental neurobiology. Discussing a number of regulatory genes involved in neuronal lineage determination and neuronal differentiation, I will try to carve out in this review a few general principles of neuronal development in C. elegans. These principles may be conserved across phylogeny.
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WormBook,
2005]
A genetic enhancer is a mutation in one gene that intensifies the phenotype caused by a mutation in another gene. The phenotype of the double mutant is much stronger than the summation of the single mutant phenotypes. The isolation of enhancers can lead to the identification of interacting genes, including genes that act redundantly with respect to each other. Examples in Caenorhabditis elegans of dominant enhancers are presented first, followed by a review of recessive enhancers of null mutations. In some of these cases, the interacting genes are related in structure and function, but in other cases, the interacting genes are nonhomologous. Recessive enhancers of non-null mutations can also be useful. A powerful advance for the identification of recessive enhancers is genome-wide screening based on RNA interference.