Authors
F Chevalier1; W Le Clec'h1; P T LoVerde2; B Gourbal3; G Mitta3; T J Anderson1; 1 Texas Biomedical Research Institute, United States; 2 University of Texas Health San Antonio, United States; 3 Université de Perpignan Via Domitia, France Discussion
Interactions between parasitic trematodes and their aquatic snail hosts provide a classical example of gene-for-gene co-evolution. Trematode infections typically castrate snails, leading to selection of costly defense mechanisms, while parasites evolve to circumvent these defenses. We used a genetic approach to identify the parasite genes involved in overcoming snail defenses in the Biomphalaria glabrata - Schistosoma mansoni system. We performed genetic crosses between two schistosome populations (SmBRE and SmLE) with distinctive patterns of host specificity: while both parasite populations infect BgBRE snails, only SmLE can infect the BgBS90 snail population. The F1 parasite progeny from our crosses were unable to infect BgBS90, while ability to infect BgBS90 snails was recovered in some F2 progeny, consistent with recessive inheritance. To identify the regions of the S. mansoni genome involved in snail host specificity we used a bulk segregant analysis (or extreme QTL (X-QTL)) approach. We (i) allowed pooled F2 progeny to infect either BgBRE or BgBS90 snails, (ii) collected the cercariae emerging from each of these snail populations and (iii) quantitatively genotyped pools of selected and unselected schistosome progeny to measure allele frequencies exome-wide. Two genome regions (on chromosome 2 and 3) showed dramatic enrichment in parasites genotypes able to infect BgBS90, and clearly underlie host specificity. These two genome regions span multiple genes: the challenge now is to identify the specific genes and mutations involved. Our long-term aim is to identify interacting genes in both parasite and snail to understand host-parasite interaction and evolution at the molecular level. 
