Population genetics of the parasitic nematode, Strongyloides ratti

• Main Author: Fisher, Matthew C.
• Published: University of Edinburgh 1997
• Subjects: 576.58
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.650929

This study investigates the genetic structure of natural populations of the nematode parasite, <I>Strongyloides ratti. S. ratti </I>has an unusual life-cycle that may be wholly asexual, or include a free-living adult phase where conventional sexual reproduction occurs. The population genetic structure of <I>S. ratti</I> was investigated i) in relation to the partitioning of parasites within and between hosts, within and between difference geographical locations and ii) in relation to the intensity of infection of different hosts. In this manner, the extent of population subdivision was characterised and the breeding structure measured. <I>S. ratti</I> was sampled from 11 colonies of rats in England and Scotland, there being 123 rats in total. 76 were infected giving an overall prevalence of infection of 62%. Small numbers of rats were also sampled from sites within Germany, 16% of which were infected. 1472 infective larvae were collected from these rats and the genotypes determined for each polymorphic locus. Analyses of variance and F-statistics were used to ensure the distribution of genetic variation at the following hierarchical levels - within parasites, within hosts, between hosts and between samples from spatially separated geographical areas. Mixed-genotype infections were common with 76% of rats containing two or more parasite genotypes. A large proportion of the total genetic diversity was found within single rats (96%). Rats with high-intensity infections tended to obtain genetically more diverse parasite populations. A small amount (0 - 1.4%) of the total genetic diversity was attributable to variation between sampling sites, showing that limited population differentiation occurs. The frequency of sexual reproduction was low within the parasite populations studied but appeared to be adequate to establish Hardy-Weinberg equilibria proportions of genotypes within most sample sites. However, some populations show deviations from Hardy-Weinberg proportions. In particular one population had a significant excess of heterozygotes. This is taken as evidence of limited allelic segregation as a consequence of the low levels of sexual reproduction within this parasite population.