BACKGROUND

is a snail intermediate host for , a trematode responsible for human schistosomiasis. BS90 is one of the most well studied strains of owing to its high resistance to infection by most strains of . An F2 mapping study from 1999 identified two RAPD markers that associated with what appeared to be single-locus, dominant resistance by the BS90 population relative to the susceptible M-line population. One marker cannot be mapped, but the other, , maps to within 5 Mb of , a region we recently showed has a very large effect on resistance within another snail population challenged by the same strain of parasite (PR1). Here we tested the hypothesis that the region contains the causal gene/s that explain the iconic resistance of BS90 snails.

METHODS

We used marker-assisted backcrossing to drive the BS90 version of the PTC2 region (+/-~1 Mb on either side) into an M-line (susceptible strain) genetic background, and the M-line version into a BS90 genetic background. We challenged the offspring with PR1-strain schistosomes and tested for effects of allelic variation in the region in a common genetic background.

RESULTS

Relative to M-line haplotypes, the BS90 haplotype actually confers enhanced susceptibility. So we reject our original hypothesis. One possible explanation for our result was that the causal gene linked to is near, but not in the block that we introgressed into each line. So we used an F2 cross to independently test the effects of the and regions in a randomized genetic background. We confirmed that the BS90 haplotype confers increased susceptibility, and we see a similar, although non-significant effect at . We discuss possible reasons why our results differed so dramatically from those of the 1999 study. We also present Pacbio assemblies of the and flanking region in BS90 and M-line, compare with previously published haplotypes, and discuss candidate genes that might be behind the enhanced susceptibility of the BS90 haplotype.