We could talk until we're blue in the face about this quiz on words for the color "blue," but we think you should take the quiz and find out if you're a whiz at these colorful terms. Words nearby exoskeleton exorcism , exorcist , exorcize , exordium , exoserosis , exoskeleton , exosmosis , exosphere , exospore , exosporium , exostosis.
Fossilized dung from a dinosaur ancestor yields a new beetle species Nikk Ogasa June 30, Science News. The Vertebrate Skeleton Sidney H. Derived forms of exoskeleton exoskeletal , adjective.
All hard parts, such as hair, teeth, and nails, that develop from the ectoderm or mesoderm in vertebrates. A hard outer structure, such as the shell of an insect, that provides protection or support for an organism. Published by Houghton Mifflin Company. Most organisms have a mechanism to fix themselves in the substrate.
Shortening the muscles then draws the posterior portion of the body forward. Although a hydrostatic skeleton is well-suited to invertebrate organisms such as earthworms and some aquatic organisms, it is not an efficient skeleton for terrestrial animals.
Figure 2. Muscles attached to the exoskeleton of the Halloween crab Gecarcinus quadratus allow it to move. An exoskeleton is an external skeleton that consists of a hard encasement on the surface of an organism. For example, the shells of crabs and insects are exoskeletons Figure 2. This skeleton type provides defence against predators, supports the body, and allows for movement through the contraction of attached muscles. As with vertebrates, muscles must cross a joint inside the exoskeleton.
Shortening of the muscle changes the relationship of the two segments of the exoskeleton. Arthropods such as crabs and lobsters have exoskeletons that consist of 30—50 percent chitin, a polysaccharide derivative of glucose that is a strong but flexible material. Chitin is secreted by the epidermal cells. The exoskeleton is further strengthened by the addition of calcium carbonate in organisms such as the lobster. Because the exoskeleton is acellular, arthropods must periodically shed their exoskeletons because the exoskeleton does not grow as the organism grows.
Figure 3. The skeletons of humans and horses are examples of endoskeletons. Comparisons between molting in nematodes and molting in arthropods would reveal, for instance, which features are invariant and presumably very constrained aspects of molting, and which are different solutions to the same problem. Second, a number of serious human diseases are caused by parasitic nematodes.
Finally, the nematode Caenorhabditis elegans is a model organism, with a sequenced genome and well-developed molecular genetic tools. These tools make it possible to carry out forward and reverse genetic screens, determine the time and place that candidate genes are expressed, investigate order of gene action, etc.
Paradoxically, until now, molting has not been subjected to such systematic scrutiny. This may be because recognition of nematodes' membership in the ecdysozoan club is recent, so molting has not been viewed as a prominent feature of their biology.
Nevertheless, a number of known features of molting in C. For instance, it has been difficult to demonstrate that molting in nematodes requires cholesterol, the precursor of steroid hormones, and C. In addition, despite encoding a bounty of nuclear receptors, the C. The research article by Frand et al. The authors used RNA interference to silence each of the 19, predicted C. This screen produced a list of genes whose interference produced molting defects in larvae.
Significantly, interference with gene function could cause arrest at different larval molts, suggesting that many of these genes are required for molting, per se, not simply for the transition between two specific stages. Importantly, the genes included nine genes that had previously been shown to be involved in C. The nine genes include nhr , the homolog of the orphan nuclear receptor ftz-f1 [ 10 ], which plays a key role in insect metamorphosis, as well as lpr-1 [ 11 ], a homolog of gpmegalin , which is involved in vitamin D a cholesterol derivative uptake in mammals.
Re-isolating these genes in a forward screen for molting defects lends further support to the wishful hypothesis that steroid hormones are involved in nematode molting, just as they are in molting in the arthropod members of this clade. Although the screen was based on a failure in the last step of the molt shedding of the old cuticle , many of the candidates are likely to play a role in the earlier stages of the molting process.
Indeed, the list of genes includes transcription factors, signaling proteins, and molecules involved in cuticle secretion and remodeling of basement membranes, as well as the expected list of proteins involved in the eventual release of the collagenous cuticle.
E A collar of old cuticle found midway along the length of the animal mostly restricts expression of the GFP-tagged mlt gene to the front half of the worm. As expected for a first description of the results of a genome-wide screen, it is not yet possible to weave all the candidate genes into a solid story.
Nevertheless, the authors do make a significant contribution to our understanding of how these genes might make the nematode change its coat by investigating in more detail eight genes representing the major functional categories. First, fusions to green fluorescent protein GFP revealed that all eight genes were expressed in epithelial cells, with a pulse of fluorescence prior to each molt Figure 1B—1D.
Thus, these genes are expressed in the right place at the right time. Second, the ability of RNA interference of molting genes to block GFP expression of the select tagged molting genes was used to order gene expression cascades. This analysis revealed, for instance, that interference of nhr encoding another orphan nuclear receptor previously implicated in molting [ 12 ] affected the expression of the eight GFP-tagged molting genes, suggesting that nhr may be generally required for the expression of molting genes.
By contrast, blocking the molting gene mlt-9 caused the expected molting defect without affecting the expression of mlt and mlt As an additional bonus from this study, the authors noted that some genotypes tended to develop constrictions in their cuticle, and that in these animals, both GFP expression and molting were confined to the anterior part of the animal Figure 1E. This result is reminiscent of the now classic experiments in insect endocrinology that demonstrated the role of circulating hormones in the control of molting [ 3 ].
Thus, Frand et al. The information gained from this screen is important in itself, since few genes involved in nematode molting had previously been identified. These genes can now be compared to the homologous genes in other ecdysozoans, and may also be useful for developing effective agents for controlling parasitic nematodes.
Furthermore, modifications of the screen that was used e. For example, little is known about the mechanism that monitors the organism's size and signals that a molt must start. In insects, we know that it is dependent on size weight , nutritional status, and time of day, but how this information is integrated and transduced is unknown for most species.
Likewise, little is known about how ecdysozoans know how many immature stages they must complete before becoming an adult. For example, we do not know how a third-instar caterpillar determines that it still has two more caterpillar stages to go through before becoming a pupa chrysalis , how a third-instar fruit fly larva knows that it has completed its larval life, and how a third-instar C. Thus, is a failure to enter metamorphosis for an insect mechanistically related to adding an extra larval instar for a worm?
The screen conducted by Frand et al. Finally, we know little of both global checkpoints that must be cleared in order for molting to progress and any signals between molting tissues. Frand et al. I thank Alison Frand and Carl Thummel for comments. I am grateful to Alison Frand for providing Figure 1. Citation: Ewer J How the ecdysozoan changed its coat.
PLoS Biol 3 10 : e
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