-
[
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.
-
[
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.
-
[
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.
-
[
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.
-
[
Parasitology,
2006]
The size and fecundity of parasitic nematodes are constrained by the host immune response. For the parasitic nematode of rats, Strongyloides ratti, parasitic females infecting immunized rats are smaller and less fecund than those infecting naive rats. Here, we investigated whether these constraints on size and fecundity are life-long. This was done by comparison of worms from different immunization and immunosuppression regimes. It was found that the per capita fecundity of parasitic females of S. ratti is fully reversed, but that their size is only partially reversed, if previously immunized hosts are subsequently immunosuppressed, suggesting that fecundity is not subject to life-long constraints. The host immune response also resulted in allometric changes in the parasitic females. The significance of these results with respect to the growth and control of nematode fecundity are discussed.
-
[
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.
-
[
Dev Growth Differ,
2003]
Dauer larvae of Caenorhabditis elegans are formed when young larvae experience conditions of low food availability and high conspecific population density; non-dauer, third stage larvae are formed in conditions of plenty. This developmental response to environmental conditions is an example of phenotypic plasticity; that is, an environmentally induced change in phenotype and, as such, a manifestation of a genotype-environment interaction. Extensive variation was found in reaction norms of phenotypic plasticity of dauer formation among wild lines of C. elegans. Recombinant-inbred lines were constructed from parental lines with very different reaction norms of dauer formation. These recombinant-inbred lines had a wide range of reaction norms, of a range greater than that set by the parental lines. The natural variation in reaction norms of dauer formation in C. elegans is, presumably, an adaptation to enhance fitness under the lines' different natural prevailing conditions. The genetic basis of this variation, as well as its phenotypic consequences, are now ripe for further investigation.
-
[
Int J Parasitol,
2002]
We describe a strategy for the mutagenesis of the free-living adult generation of Strongyloides ratti and selection of worms carrying new mutations in the subsequent F2 generation of infective larvae. We demonstrate that this strategy is successful via the selection of infective larvae that are resistant to the anthelmintic ivermectin at a concentration of 10 ng/ml. The majority of these larvae were unable to give rise to patent infections when used to infect parasite naive rats, implying that the majority of the ivermectin resistance mutations confer pleiotropic defects on parasitic, but not on free-living, development.
-
[
Worm,
2014]
Mutually exclusive selection of one exon in a cluster of exons is a rare form of alternative pre-mRNA splicing, yet suggests strict regulation. However, the repertoires of regulation mechanisms for the mutually exclusive (ME) splicing in vivo are still unknown. Here, we experimentally explore putative ME exons in C. elegans to demonstrate that 29 ME exon clusters in 27 genes are actually selected in a mutually exclusive manner. Twenty-two of the clusters consist of homologous ME exons. Five clusters have too short intervening introns to be excised between the ME exons. Fidelity of ME splicing relies at least in part on nonsense-mediated mRNA decay for 14 clusters. These results thus characterize all the repertoires of ME splicing in this organism.
-
[
Parasitology,
2002]
Nematode infections are subject to density-dependent effects on their establishment, survivorship and fecundity within a host. These effects act to regulate and stabilize the size of nematode populations. Understanding how these density-dependent effects occur is important to guide the development of control strategies against parasitic nematodes and the diseases that they cause. These density-dependent effects have been hypothesized to result from intraspecific competition between parasites for limited resources or from the action of host immune responses. However, no specific evidence exists to distinguish between these two hypotheses. We find that in nematode (Strongyloides ratti) infections, density-dependent effects on parasite establishment, survivorship and fecundity are mediated by the host immune response. These density-dependent effects are only observed late in primary infections and no density-dependent effects are observed in infections in immuno-compromised animals. We find no evidence for intraspecific competition between parasites in experimental infections over a range of doses that encompasses all that is observed in natural infections. We conclude that density-dependent effects due to the immune response will act to regulate S. ratti infections before competition for space or nutrients within the host gut ever occurs.