Flat worms

The body of flatworms (phylum Platyhehninthes) consists of two external cell layers (endoderm and ectoderm) with a third layer between. A distinct excretory system is present. In addition to a nerve net resembling that of the Cnidaria, there are a cerebral ganglion and distinct eyes. One large group of flat-worms, the planaiians (typically about 15 mm in length, Fig. 1-14), inhabit freshwater streams. They are said to be the simplest creatures in which behavior can be studied. 

The evolutionary fate of parasitic flat-worms lies intimately intertwined with the taxa they currently utilize, the taxa they can but have not yet utilized and the selective pressures driving speciation within and between parasites and their hosts. As with any other biological system, if time and conditions permit, natural selection will maintain diversity. Host switching, host speciation, vicariant events affecting parasites and/or hosts, an increase in host population size and dispersal and drug resistance are just some of the factors that will maintain or even promote the success of parasites and subsequently drive diversity in the hosts. 

A review of platyhelminth genomes written just 10 years ago would have painted a very different picture to the one which we can write today and we would never then have predicted that we would be in the position that we currently find ourselves in, with full genome sequencing projects underway for several flat-worm species. Even if one s scientific interest falls outside the currently targeted species, these efforts should not just be viewed as stamp collecting they are revealing fundamental information of potential interest and value to the entire flatworm research community. Moreover, the methods of analysis are... 

If this succeeds, schistosomes may be close to becoming a model for parasitic trema-todes. This would allow researchers to further develop and optimize techniques for the genetic modification of other parasitic flat-worms of medical or economic relevance. The combination of modern molecular approaches such as genomics, proteomics and transgenics, with classical disciplines such as systematics,... 

Initial proteomic studies in parasitic flat-worms have been undertaken using pooled samples. However, proteomic maps can be derived from individual F. hepatica worms (Fig. 17.2), although individual variation... 

PTMs, such as methylation or acetylation or addition of complex N- or O-linked glyco-sylations, are covalently bound modifications to one or several amino acid residues. This can drastically affect the outcome of the altered protein (Mann and Jensen, 2003). Genomic information, at present, cannot predict the processes of PTM and cannot be studied with genomic techniques. With protein modification, being an important area of parasitic flat-worm proteomics, the most effective method to research them is with mass spectrometry (Mann et al., 2001). 

Proteases have played a pivotal role in the development of parasitism. By the time flat-worms had emerged, members of all the major families of proteases had evolved such that parasites exhibit a complete profile of exo- and endoproteases. The best characterized of these proteases, including cathepsins B, L, C, D and LAP, are vital in the complex process of nutrient uptake from the host by sequentially degrading blood tissue proteins such as haemoglobin to free amino acids. However, other proteases are involved in separate essen-... 

Briefly, the flatworm brain is most commonly a ring-like or semicircular structure formed by the connection of two concentrations of nerve axons and nerve cell bodies by a well-developed commissure or commissures. These bilobed accumulations of nerves form the cerebral ganglia that occupy the lateral margins of the brain and comprise a core of nerve axons that are enveloped by a loose gathering of the associated cell bodies at least some flat-worm brains appear to be further surrounded by multilayer sheaths of mesenchyme that represent primitive glia-like cells (Sukhdeo and Sukhdeo, 1994). 

Unlike the situation in nematodes and arthropods, where hundreds of neuropeptides have been elucidated using biochemical, physiological and molecular techniques, flatworm neuropeptide discovery is still in its infancy. This is despite the characterization of the first flat-worm neuropeptide almost 15 years ago (Maule et al., 1991). Progress has been hindered by the inability to obtain large quantities of flatworm neuronal tissues and the absence, until recently, of a significant body of genomic and/or EST data for flatworms. Nevertheless, the available evidence does provide a snapshot of what appear to be the most abundant and widespread flatworm neuropeptides. 

Day, T.A., Haithcock, J., Kimber, M. and Maule, A.G. (2000) Functional ryanodine receptor channels in flat-worm muscle fibres. Parasitology 120, 417 122. 

Very little is known about lipid metabolism in parasites, although in several aspects, it is quite different from that of the mammalian host. Lipids, like phospholipids, triacylglycerols and cholesterol, are not synthesized de novo by parasitic flatworms, but are obtained directly from the host (cholesterol) or are synthesized from building blocks obtained from the host (fatty acids and in case of phospholipids, also the head groups). A schematic and partly hypothetical overview of the main pathways of lipid metabolism in parasitic flat-worms is shown in Fig. 20.3. Fatty acids are... 

Purine and pyrimidine nucleotides are essential components of many biochemical molecules, from DNA and RNA to ATP and NAD. In recent years, the pyrimidine and especially the purine metabolism of parasitic helminths have been investigated extensively, mainly because they are different from the pathways in the mammalian host such that they have potential as targets for chemotherapeutic attack. For a review of purine and pyrimidine pathways in parasitic helminths and protozoa, see Berens et al. (1995). Although parasitic helminths do not synthesize purines de novo, but obtain them from the host, they do possess elaborate purine salvage pathways for a more economical management of this resource. Pyrimidines, on the other hand, are synthesized de novo by all parasitic flat-worms studied so far and, as with mammalian... 

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