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要旨:
The nematode genus Strongyloides consists of parasites that live as parthenogenetic females in the small intestines of their hosts. In addition to producing parasitic offspring, they can also form a facultative free-living generation with males and females (for review see Viney and Lok, 2007). While some species of Strongyloides, like S. ratti, and the human parasite S. stercoralis, employ an XX/XO sex determining system, S. papillosus, a parasite of farm ruminants, contains no true X chromosome. For this species it has been suggested that males are the product of a sex specific chromatin diminution event which leads to the elimination of a large portion of one of the homologues of a chromosomal pair during the single mitotic oocyte maturation division (for review see Streit, 2008). Contrary to earlier reports that were based on cytological observations, males of S. ratti (Viney et al., 1993) and S. papillosus (Eberhardt et al., 2007) do contribute genetic information to the next generation. We have generated molecular genetic markers for S. ratti and S. papillosus and we are analyzing their inheritance and linkage both genetically and molecularly (FISH). We have shown that in both species recombination within chromosomes occurs. All this indicates that these two species undergo standard sexual reproduction and are therefore amenable to classical genetic analysis and mapping. One of our goals is to complement the ongoing S. ratti whole genome sequencing with a genetic linkage map for this species. We are particularly interested in genetic differences between the two species, which relate to the different sex determining systems. The ability to reliably distinguish between different species is an indispensable prerequisite for comparative studies. Generally all Strongyloides found in farm ruminants were considered to belong to the species S. papillosus, first described as a parasite of sheep. We have analyzed the sequences of the 18S rDNA and of three protein-encoding genes from Strongyloides individuals isolated from sheep and cattle from Germany, Mali and the USA. Our data clearly indicate that there exist at least two different, genetically isolated, sympatric populations that therefore should be considered different, relatively closely related, species. One species was the predominant Strongyloides in cattle and was found in this host only. The other species was the only species we found in sheep but did also occur in cattle at low numbers.
