Discussion

With discovery of snails as vectors of trematode parasites first in the 1880s, distribution of snail hosts was recognized as critical aspect of epidemiology of trematode-caused infectious diseases, making snails a logical target for integrated control efforts. In depth studies, mostly of the Biomphalaria snail/Schistosoma mansoni parasite combination, showed additional complexity in transmission patterns due to variable host competence among and within vector snail species caused by genetic variation in snail (immune)biology. Motivation to gain broader, more comprehensive understanding of vector biology led to characterization of the genome of the snail B. glabrata in 2017. Genome annotation yielded unprecedented details of snail communication, neuroendocrinology, immunity, reproduction and regulation of gene expression. This information helps to interpret distribution and persistence of snail vectors in the field, as well as determinants of host competence to S. mansoni. The genomic data continue to provide access into B. glabrata biology, enabling large scale proteomics, linkage studies for anti-parasite resistance, and characterization of (novel) immune genes. Continued genome mining, also supported by long read sequencing of select BAC clones has improved description and assembly of major anti-trematode immune lectins, belonging to the gene family of VIgLs (Variable Immunoglobin and Lectin domain containing molecules). Genome analyses also disclosed that B. glabrata snails, employing hemoglobin as oxygen carrier, retain an evolutionary relic of a gene for ancestral respiratory hemocyanin named Hcl-1 (hemocyanin-like 1). Functional transcriptomics indicate that Hcl-1 has undergone neofunctionalization toward influencing the reproductive success of B. glabrata: RNAi knockdown of Hcl-1 negatively impacts viability of snail eggs. The value of genome analyses for exploring B. glabrata snail vector biology increases with novel (future) genome assemblies of additional B. glabrata strains, and of other pulmonate and prosobranch snail vectors of trematode parasites. Comparative genome mining will identify unique and shared biological features of snails that govern snail distribution and transmission of particular trematodes, providing leads that may be targeted for vector- or transmission control. Integrative, synthetic consideration of such findings will inform parasitology by revealing general determinants of trematode virulence and snail host competence.