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Front Genet,
2018]
In the last decade, case studies in plants and animals provided increasing insight into the molecular mechanisms of developmental plasticity. When complemented with evolutionary and ecological analyses, these studies suggest that plasticity represents a mechanism facilitating adaptive change, increasing diversity and fostering the evolution of novelty. Here, we summarize genetic, molecular and evolutionary studies on developmental plasticity of feeding structures in nematodes, focusing on the model organism <i>Pristionchus pacificus</i> and its relatives. Like its famous cousin <i>Caenorhabditis elegans</i>, <i>P. pacificus</i> reproduces as a self-fertilizing hermaphrodite and can be cultured in the laboratory on <i>E. coli</i> indefinitely with a four-day generation time. However, in contrast to <i>C. elegans</i>, <i>Pristionchus</i> worms show more complex feeding structures in adaptation to their life history. <i>Pristionchus</i> nematodes live in the soil and are reliably found in association with scarab beetles, but only reproduce after the insects' death. Insect carcasses usually exist only for a short time period and their turnover is partially unpredictable. Strikingly, <i>Pristionchus</i> worms can have two alternative mouth-forms; animals are either stenostomatous (St) with a single tooth resulting in strict bacterial feeding, or alternatively, they are eurystomatous (Eu) with two teeth allowing facultative predation. Laboratory-based studies revealed a regulatory network that controls the irreversible decision of individual worms to adopt the St or Eu form. These studies revealed that a developmental switch controls the mouth-form decision, confirming long-standing theory about the role of switch genes in developmental plasticity. Here, we describe the current understanding of <i>P. pacificus</i> mouth-form regulation. In contrast to plasticity, robustness describes the property of organisms to produce unchanged phenotypes despite environmental perturbations. While largely opposite in principle, the relationship between developmental plasticity and robustness has only rarely been tested in particular study systems. Based on a study of the Hsp90 chaperones in nematodes, we suggest that robustness and plasticity are indeed complementary concepts. Genetic switch networks regulating plasticity require robustness to produce reproducible responses to the multitude of environmental inputs and the phenotypic output requires robustness because the range of possible phenotypic outcomes is constrained. Thus, plasticity and robustness are actually not mutually exclusive, but rather complementary concepts.
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Journal of Helminthology,
1955]
Osche (1952) has recently published a sorely needed, comprehensive revision of the genus Rhabditis Dujardin [1844] (sensu lato) including detailed study of certain features of the cephalic end, especially of the stoma or mouth cavity. For some time to come his study will surely be the point of departure for morpholigical and systematic work on the group. On the basis principally of the structure of the metastom (a subdivision of the stoma) and of the esophagus, he recognizes some eight subgenera in the genus Rhabditis, which are as follows: Rhabditis Dujardin [1844] (sensu stricto), Choriorhabditis Osche, 1952, Telorhabditis Osche, 1952, Caenorhabditis Osche, 1952, Mesorhabditis Osche, 1952, Teratorhabditis Oshce, 1952, Protorhabditis Osche, 1952, and Parasitorhabditis Fuch, 1937. For all of these save the last he lists the species recognized by him. For a revision of Parasitorhabditis he refers to an unpublished manuscript by Ruhm.....
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Vitam Horm,
2022]
The round worms or nematodes are the largest phylum of animals with an estimated species number of more than one million. Nematodes have invaded all ecosystems and are known from all continents including Antarctica. Parasitic species infest plants, animals and humans often with high host-specificity. Free-living species are known from marine, fresh water and soil systems, the latter of which contain many culturable species. This includes Caenorhabditis elegans, a species that was developed as one of the most prominent model systems in modern biology since the 1960ies. Pristionchus pacificus is a second nematode model organism that can easily be cultured in the laboratory. This species shows a number of complex traits including omnivorous feeding and the capability of predation on other nematodes. Predation depends on the formation of teeth-like denticles in the mouth of P. pacificus, structures unknown from C. elegans and most other nematodes. Here, we review the current knowledge about the role of vitamin B<sub>12</sub> for the predatory behavior in P. pacificus and correlate its role with that on the physiology and development in C. elegans.
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Wiley Interdiscip Rev Dev Biol,
2014]
The Caenorhabditis elegans pharynx is a rhythmically pumping organ composed initially of 80 cells that, through fusions, amount to 62 cells in the adult worm. During the first 100 min of development, most future pharyngeal cells are born and gather into a double-plate primordium surrounded by a basal lamina. All pharyngeal cells express the transcription factor PHA-4, of which the concentration increases throughout development, triggering a sequential activation of genes with promoters responding differentially to PHA-4 protein levels. The oblong-shaped pharyngeal primordium becomes polarized, many cells taking on wedge shapes with their narrow ends toward the center, hence forming an epithelial cyst. The primordium then elongates, and reorientations of the cells at the anterior and posterior ends form the mouth and pharyngeal-intestinal openings, respectively. The 20 pharyngeal neurons establish complex but reproducible trajectories using 'fishing line' and growth cone-driven mechanisms, and the gland cells also similarly develop their processes. The genetics behind many fate decisions and morphogenetic processes are being elucidated, and reveal the pharynx to be a fruitful model for developmental biologists.
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WormBook,
2012]
C. elegans feeding depends on the action of the pharynx, a neuromuscular pump that joins the mouth to the intestine. The pharyngeal muscle captures food-bacteria-and transports it back to the intestine. It accomplishes this through a combination of two motions, pumping and isthmus peristalsis. Pumping, the most visible and best understood of the two, is a cycle of contraction and relaxation that sucks in liquid from the surrounding environment along with suspended particles, then expels the liquid, trapping the particles. Pharyngeal muscle is capable of pumping without nervous system input, but during normal rapid feeding its timing is controlled by two pharyngeal motor neuron types. Isthmus peristalsis, a posterior moving wave of contraction of the muscle of the posterior isthmus, depends on a third motor neuron type. Feeding motions are regulated by the presence and quality of food in the worm's environment. Some types of bacteria are better at supporting growth than others. Given a choice, worms are capable of identifying and seeking out higher-quality food. Food availability and quality also affect behavior in other ways. For instance, given all the high-quality food they can eat, worms eventually become satiated, stop eating and moving, and become quiescent.