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[
WormBook,
2006]
Although several Caenorhabditis species are now studied in laboratories in great detail, the knowledge of the ecology of most Caenorhabditis species is scarce. In this chapter we present data on the habitat, animal associations, and geographical distribution of the eighteen described and five undescribed Caenorhabditis species currently known to science. The habitats of these species are very diverse, ranging from rotting cactus tissue to inflamed auditory canals of zebu cattle. Some species, including C. elegans , have only been isolated from anthropogenic habitats. Consequently, their natural habitat is unknown. All Caenorhabditis species are colonizers of nutrient- and bacteria-rich substrates and none of them is a true soil nematode. Dauer juveniles of many Caenorhabditis species were shown to be associated with terrestrial arthropods or gastropods. An association with invertebrates is also likely for the remaining species. The type of association is either phoresy (for transport to a new habitat) or necromeny (to secure the body of the associated animal as a future food source). There are also some records of Caenorhabditis species associated with vertebrates. The Caenorhabditis stem species was probably a colonizer of nutrient-rich substrates and was phoretic on arthropods. Some evolutionary trends within the taxon are discussed.
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[
2006]
In the development and use of animal models of cognitive dysfunction, it is important to develop complementary models to exploit the unique advantages of the different species. Nonmammalian vertebrates such as fish provide the opportunity to directly observe neurodevelopmental processes and determine the impact of developmental permutations on learning and memory. Zebrafish in particular are valuable because of the availability of morpholine techniques to transiently suppress specific parts of genomic expression. Invertebrate models such as C. elegans and drosophila provide other advantages, particularly the elegant genetic manipulations available. The simple nervous systems in these models are useful in determining mechanisms of cognitive function. The development of new methods for high-throughput tests of cognitive function for fish can provide a means for rapid screening of potential toxic agents as well as promising therapeutic agents. It is equally important to develop specific tests of various aspects of cognitive function, including habituation, associative learning, memory, and attention as well as to be able to differentiate changes in sensorimotor function from cognition. Key in the use of nonmammalian models is the determination of which mechanisms of cognitive function are similar to mammals and which are different. Nonmammalian models can be used in concert with classic mammalian models to determine the neural bases of cognitive function and to aid in the discovery of toxicants and potential therapeutic agents.
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The world of modern biology is unified by genetics. Genetic approaches have the ability to transcend species and provide cross-links between fields for several reasons. First, is the fact that all species are evolutionarily related. Thus, distinct species have similar gene function, and DNA sequence homology can be found between even distantly related species. Indeed, DNA sequence homology is used as a metric device to determine evolutionary relationships among species. Second, molecular genetic manipulation changes both the genotype and phenotype of an organism. Such manipulations represent an extremely fine-scale tool for dissection of the underlying biochemistry, physiology, anatomy, and development of an individual species. Because virtually any gene can be manipulated at will in many species, a dedicated approach can lead to an unraveling of the relationship between genotype and phenotype for almost any gene in these species.....
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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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[
WormBook,
2005]
The knowledge about C. elegans provides a paradigm for comparative studies. Nematodes are very attractive in evolutionary developmental biology given the species richness of the phylum and the easiness with which several of these species can be cultured under laboratory conditions. Embryonic, gonad, vulva and male tail development were studied and compared in nematodes of five different families, providing a detailed picture of evolutionary changes in development. In particular, vulva development has been studied in great detail and substantial differences in the cellular, genetic and molecular mechanisms have been observed between C. elegans and other nematodes. For example, vulva induction relies on the single anchor cell in C. elegans, whereas a variety of different cellular mechanisms are used in related species. In recent years, a few species have been developed as satellite systems for detailed genetic and molecular studies, such as Oscheius tipulae and Pristionchus pacificus.
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[
WormBook,
2005]
C. elegans is a member of a group of nematodes called rhabditids, which encompasses a large number of ecologically and genetically diverse species. A new, preliminary phylogenetic analysis is presented for concatenated sequences of three nuclear genes for 48 rhabditid and diplogastrid species (including 10 Caenorhabditis species), as well as four species representing the outgroup. Although many relationships are well-resolved, more data are still needed to resolve some key relationships, particularly near the base of the rhabditid tree. There is high confidence for two major clades: (1) a clade comprising Mesorhabditis Parasitorhabditis, Pelodera, Teratorhabditis plus a few other species; (2) a large clade (Eurhabditis) comprising most of the remaining rhabditid genera, including Caenorhabditis and its sistergroup Protorhabditis-Prodontorhabditis-Diploscapter. Eurhabditis also contains the parasitic strongylids, the entomopathogenic Heterorhabditis, and the monophyletic group Oscheius which includes the satellite model organism O. tipulae. The relationships within Caenorhabditis are well resolved. The analysis also suggests that rhabditids include diplogastrids, to which the second satellite model organism Pristionchus pacificus belongs. Genetic disparity within Caenorhabditis is as great as that across vertebrates, suggesting Caenorhabditis lineages are quickly evolving, ancient, or both. The phylogenetic tree can be used to reconstruct evolutionary events within rhabditids. For instance, the reproductive mode changed multiple times from gonochorism to hermaphroditism, but only once from hermaphroditism to gonochorism. Complete retraction of the male tail tip, leading to a blunt, peloderan tail, evolved at least once. Reversions to unretracted tail tips occurred within both major rhabditid groups. The phylogeny also provides a guide to species which would be good candidates for future genome projects and comparative studies.
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[
1983]
Nematodes are a very large group of animals. The estimated 500,000 species represent an independent phylum, and a very successful one, since they are found, with the exception of the pelagic and aerial habitats, in every type of environment. The great majority of nematodes are free-living and inhabit in large numbers the top few centimeters of the ocean's bed, fresh water muds, and a variety of soils. In the soil, where it has been measured, their biomass is comparable to that of insects. A few hundred species are extremely important in human health and agriculture because of their parasitic relationship to plants and animals. In humans, parasitic nematodes can cause very severe diseases, such as filariasis and river blindness (Oncocercus)...
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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