Abstract
Background: Schistosomes cause schistosomiasis, the world's second most important parasitic disease of public health importance after malaria. A peculiar feature of schistosomes is their ability to hybridize. So far, individual hybrid schistosomes are considered as such when they display incongruent genetic signatures at a nuclear (ITS-2) and a mitochondrial (COI) gene. Nevertheless, the introgression levels of any Schistosoma hybrid have never been assessed at the whole genome level. Here we have analysed both the morphological characteristics and the genomic composition of the schistosome that infected over one hundred humans in Corsica during summer 2013. This parasite was determined as a hybrid between the ruminant-infective (Schistosoma bovis) and the human-infective (Schistosoma haematobium) schistosomes. Characterizing precisely the genomic introgression of these hybrids is essential to better understand their life history traits, and the disease dynamics and epidemiology in natura. Results: For this work, we first performed genome assembly improvement and annotation of S. bovis. Whole genome sequencing of the European hybrid revealed that this parasite in strongly introgressed and composed of 77% of sequences of S. haematobium origin, and 23% of S. bovis origin. We also present the morphological characterization of adult hybrids, and compatibility with their potential vector snails. Conclusions: For the first time since the discovery of hybrid schistosomes, these results reveal at the whole genomic level a complex admixture of parental genomes highlighting (i) the high permeability of schistosomes to other species' alleles, and (ii) the importance of hybrid formation for pushing species boundaries not only conceptionally but also geographically.
