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[
WormBook,
2007]
Strongyloides is a genus of parasitic nematodes, which, unusually, has a free-living adult generation. Here we introduce the biology of this genus, especially the fascinating, but complex, life-cycle together with an overview of the taxonomy, morphology, genetics and genomics of this genus.
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Worm Breeder's Gazette,
1994]
The beginning Strongyloides is an obligate parasitic nematode. It is the parasitic nematode taxonomically closest to Caenorhabditis. Its life-cycle (see below) consists of two distinct adult generations; one female and parasitic and the other dioecious and free-living. The free-living adult generation (known as heterogonic development) can be omitted and instead larvae can develop to infectiveL3 sdirectly (homogonic development). Cytological studies have concluded that the parasitic generation reproduces by mitotic parthenogenesis, and the free-living generation by pseudogamy (maternal inheritance only)(1). Here I describe a genetic analysis of the life-cycle of S. ratti. Doing genetics with Strongyloides Clones A rat can be infected with a singleL3 ,which gives rise to a single parasitic female. Larvae produced by this worm are able to develop both directly and indirectly. We refer to such infections (and the population derived from these) as clones, but they should probably be more correctly called isofemale lines. Controlled matings Virgin free-living males and females can be grown by collecting early stage larvae and maintaining them individually until they are mature. Virgin males and females from different clones can be brought together so that mating can occur. The progeny can be cloned back into rats or can be analyzed directly. Genetics of the free-living generation Crosses have been made between individual virgin males and virgin females of different clones (the parental clones). The resulting progeny of such crosses have also been cloned (the progeny clones). Parental and progeny clones have been analyzed by minisatellite fingerprinting (Jeffreys' probe 33.15)(2). The fingerprints showed that inheritance was bi-parental, and thus that pseudogamy did not occur. The inheritance patterns are fully consistent with the occurrence of "normal" sexual reproduction(3). These results were incompatible with the earlier cytological observations. Genetics of the parasitic generation Parthenogenesis can occur by a number of cytological mechanisms. In some cases, progeny are identical to each other and to their mother (mitotic and some forms of meiotic parthenogenesis). In other cases the progeny differ from each other and from their mother (other forms of meiotic parthenogenesis). In view of the quite different conclusions drawn from the cytological and genetic data, I have also analyzed the progeny of single parasitic females with respect to the functional difference (mitotic or meiotic) in parthenogenetic reproduction. Larval progeny of individual parasitic females have been analyzed for a RFLP in a PCR fragment of C. elegans actin 4 (4). Alleles of this locus segregated in a Mendelian manner in crosses of the free-living generation. Parasitic females that were heterozygous for this marker produced progeny all of which were heterozygous. Thus, it was concluded that the parasitic female was functionaIly mitotic. The genetic and cytological studies were not in conflict for this stage. The future Strongyloides is a parasitic nematode with a life-cycle that is amenable to genetic manipulation. Admittedly it isn't quite as easy to maintain or work with as C. elegans, but it is the most accessible of the parasitic nematodes. I would be happy to supply further details of the methods used or the work itself, if anyone is interested. References 1. Bolla & Roberts, 1968, J. Parasit., 54, 849-855. 2. Jeffreys, et al., 1985, Nature, 314, 67-73. 3. Viney, et al., 1993, Proc. R. Soc. Lond. B, 254, 213-219. 4. Krause, et al., 1989, J. Mol. Biol., 208, 381-392.
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Parasitology,
1994]
Strongyloides ratti has a complex life-cycle with two adult generations, one free-living and dioecious and one parasitic and female only. The parasitic females reproduce by parthenogenesis, but it is unclear whether this is mitotic or meiotic in nature. This question has been addressed genetically by analysing the progeny of parasitic females that were heterozygous at an actin locus for evidence of allelic segregation. Such progeny were similarly heterozygous showing that segregation had not occurred. It was therefore concluded that reproduction in the parasitic female of S. ratti is functionally mitotic.
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Bioessays,
2009]
Nematodes are important parasites of humans and other animals. Nematode parasitism is thought to have evolved by free-living, facultatively developing, arrested larvae becoming associated with animals, ultimately becoming parasites. The formation of free-living arrested larvae of the nematode Caenorhabditis elegans is controlled by the environment, and involves dafachronic acid (DA) and transforming growth factor (TGF)-beta signalling. Recent data have shown that DA acid signalling plays a conserved role in controlling larval development in both free-living and parasitic species. In contrast, TGF-beta signalling does not seem to be conserved; this difference perhaps points to how nematode parasitism did evolve.
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Med Microbiol Immunol,
2006]
Parasitic nematodes are widespread and important pathogens of humans and other animals. The parasitic nematodes Strongyloides have an unusual life cycle in which there is a facultative free-living generation in addition to the obligate parasitic generation. The genomes of many species of parasitic nematodes, including Strongyloides ratti and Strongyloides stercoralis, have been investigated, principally by expressed sequence tag (EST) analyses. These have discovered very many genes from these parasites but, in so doing, have also revealed how different these species are from each other and from other organisms. Understanding the role and function of these newly discovered genes is now the challenge, made more difficult by the parasitic lifestyle. The genomic information available for parasitic nematodes is allowing new approaches for the control of parasitic nematodes to be considered.
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[
International C. elegans Meeting,
1995]
S. ratti is a rhabditid parasitic nematode which has a free-living generation outside of the host. This can develop in one of two ways. In heterogonic development larvae develop into free-living adults, these mate and their progeny develop into infective L3s. In homogonic development larvae develop directly into infective L3s. These routes may have analogies with the alternative (dauer) development of C. elegans. In Strongyloides which developmental path is followed depends on many factors. Here I will describe the development of isofemale lines of S. ratti under different conditions.
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Proc Biol Sci,
1996]
Strongyloides ratti is a nematode parasite of rats. It is able to undergo two types of development outside the host: heterogonic (free-living adults and sexual reproduction) and homogonic (direct larval development). Homogonic development has a number of similarities with the development of the dauer stage of free-living nematodes, including Caenorhabditis elegans. Using isofemale lines of the parasite, factors that control this developmental choice have been investigated. Isofemale lines can be selected for both heterogonic and homogonic development, but are still able to respond to environmental conditions. By using temperature shift experiments it has been possible to determine when larvae become developmentally committed. All larvae are developmentally committed after 24 h at 19 degrees C.
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[
Parasitol Today,
1999]
The nematode Strongyloides ratti has a remarkable life cycle, which has both a parasitic and a free-living phase. The free-living phase includes a choice between two developmental routes. Here, Mark Viney discusses recent advances in understanding the biology of this developmental switch and shows how the life cycle of this nematode can be used to explore the lifestyle transitions common to all parasitic nematodes, as well as to address other basic biological questions.
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[
Naturwissenschaften,
2004]
Animals respond to signals and cues in their environment. The difference between a signal (e.g. a pheromone) and a cue (e.g. a waste product) is that the information content of a signal is subject to natural selection, whereas that of a cue is not. The model free-living nematode Caenorhabditis elegans forms an alternative developmental morph (the dauer larva) in response to a so-called 'dauer pheromone', produced by all worms. We suggest that the production of 'dauer pheromone' has no fitness advantage for an individual worm and therefore we propose that 'dauer pheromone' is not a signal, but a cue. Thus, it should not be called a pheromone.
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[
Int J Parasitol,
2005]
The many similarities between arrested dauer larvae of free-living nematodes and infective L3 of parasitic nematodes has led to suggestions that they are analogous lifecycle stages. The control of the formation of dauer larvae in Caenorhabditis elegans is well understood, with a TGF-beta-superfamily growth factor playing a central role. Recent analyses of the expression of homologous TGF-beta genes in parasitic nematodes has allowed this analogy to be tested; but the results so far do not support it. Rather, the results imply that in the evolution of animal parasitism, parasitic nematodes have taken signalling pathways and molecules from their free-living ancestors and used them in different ways in the evolution of their parasitic lifestyles.