The mechanism of sex determination and the development control of the life-cycle of the gastro-intestinal parasite, <i>Strongyloides ratti</i> was investigated using a combination of genetic and parasitological techniques. Parasitological analysis investigated the effects of intra- and extra-host factors on the development of the free-living phase of the life cycle. An initial analysis of the distribution of infective stages among host faecal pellets showed that they were significantly overdispersed and well described by the negative binomial distribution. This overidspersion was found to occur over a wide range of infection intensities and to increase significantly during infection. Further investigation, coupled with an artificial selection experiment, suggested the existence of two discrete developmental switches; an intra-host sex determination switch and an extra-host free-living female/directly developing iL3 development conversion. Analysis of the effects of host immunity on these developmental switches clarified the way in which the composition of the free-living phase varies over the course of an infection. Increasing host immunity results in a greater proportion of female larvae developing into free-living females rather than into directly developing iL3s. Further evidence suggested that the increase in proportion of female larvae that develop into free-living females with increased host immunity is due to an increased temperature sensitivity of the free-living female/directly developing iL3 developmental conversion. Increasing host immunity also alters the sex ratio, resulting in a greater proportion of larvae developing into free-living males. In addition, increased parasitic female age appears to increase the proportion of larvae that develop into free-living males, but does not increase the proportion of female larvae that develop into free-living females. In conclusion, this thesis has used a combined parasitological and genetic approach to investigate the mechanism of sex determination of <i>S. ratti</i>. This understanding now allows a rational view of the <i>S. ratti </i> life-cycle to be presented. These findings are discussed in relation to previous studies of <i>Strongyloides spp.</i> and further questions raised by this work are discussed.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:652197 |
| Date | January 1999 |
| Creators | Harvey, Simon Crawford |
| Publisher | University of Edinburgh |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://hdl.handle.net/1842/10942 |
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