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[
2008]
In this chapter, selected aspects of the early embryogenesis of five representatives from different branches of the phylogenetic tree are compared with C. elegans and the impact of the observed differences for evolutionary considerations are discussed. Following a brief reference to phylogeny, basic features of early embryogenesis of C. elegans will be summarised to aid in appreciating the data from other nematodes reported subsequently.
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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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[
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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[
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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[
1979]
In many invertebrates, cell lineages are apparently invariant from individual to individual. A given precursor cell follows a specific pattern of cell divisions, and its descendants follow fates that correspond to their respective positions in the lineage tree. Such a reproducible sequence of events provides an excellent system for studying how cells come to pursue particular fates during development. We have been interested to know if a cell's fate is specified by factors intrinsic to the cell, or if it is influenced by interactions between the cell and its environment. C. elegans is a particularly suitable organism for lineage studies because it is transparent throughout its life cycle, and because it consists of relatively few cells. Furthermore, C. elegans is a favorable organism for genetics, so the control of cell lineages can be studied by characterizing mutations that are defective in known lineages. The cell lineages of C. elegans have been described in the embryo to the 182 cell stage and after hatching. Approximately 50 cells resume divisions post-embyronically. In the somatic tissues, the number of cells (or nuclei) is increased from about 550 to about 950 in hermaphrodites and to about 1025 in males. These post-embryonic lineages are essentially invariant from worm to worm. As the worm enlarges and matures sexually, cells (or nuclei) are added to previously existing tissues (hypodermis, muscle, gut, and nervous system), and structures necessary for reproduction are elaborated. The latter include a gonad in both sexes, a vulva in hermaphrodites, and a tail specialized for copulation in males. This paper summarizes the results of laser ablation experiments performed on cells in the post-embryonic lineages of C. elegans. In particular, we focus on those experiments that demonstrate a regulative capacity in the cells of this predominantly invariant system. The post-embyronic lineages have the practical advantage for these studies that they can be traced by direct observation of the cells as they divide and assume their final fate. The regulative response, therefore, can be described at a level of cellular detail that has not been possible in other deletion studies. Our aim in performing these experiments is to infer how cells are controlled during normal development from their behavior in