Abstract
Stable resistance to the anthelmintic hycanthone can be produced in the human blood fluke Schistosoma mansoni by exposing immature parasites in mice to the drug. Within a single generation, genomic rearrangements, detected as rRNA-encoding DNA restriction fragment length polymorphisms (RFLPs), accompany the appearance of resistance in this model. One of these RFLPs, an approximately 3.6-kilobase BamHI fragment, was shown previously to associate consistently with resistance in independent generations of the JHU strain of S. mansoni. To characterize the genetic changes responsible for this RFLP, the fragment was cloned and sequenced. A comparison of the cloned fragment with a normal 18S rRNA gene demonstrated that the drug resistance-associated RFLP fragment arises through the addition of 732 base pairs into an 18S rRNA gene, 134 base pairs downstream of the junction of the intergenic spacer and the mature 18S rRNA gene. The mutation is nonrandom, targets one, or a few only, of the 100 or so copies of the ribosomal genes, and may represent the incomplete duplication of the gene since the inserted element is identical in sequence to the region contiguous to it. The sequence spanning the junction of the insertion and the original 18S rRNA gene was used as a specific primer for the BamHI RFLP in PCR experiments. The analysis conclusively demonstrated that the mutation is induced rather than selected by the drug since the junctional sequence was not detectable in the drug-sensitive parent population of schistosomes. In addition, analysis of four, independently derived, resistant lines indicated that the same region of the gene was mutated each time. Together, these data demonstrate that reproducible changes are induced during the acquisition of resistance in schistosomes and suggest that the resistant phenotype is induced rather than selected from preexisting forms.