Authors

Discussion

Gene flow between species is an important source of novel traits in free-living organisms. For example, introgressed genes from Neanderthals play a significant role in evolution of modern humans. Schistosoma haematobium causes urogenital schistosomiasis in over 112 million people across Africa, with the highest burden in Nigeria. We sequenced whole genomes from 120 S. haematobium miracidia from patients in 10 states in Nigeria. In addition, we sequenced 38 S. bovis recovered from cattle from 4 states. We identified S. bovis mitochondria in our S. haematobium samples as previously observed in multiple West African studies suggesting hybridization. However, two features of our results are consistent with hybridization being ancient: (i) S. bovis-like mtDNA found in S. haematobium or S. bovis fall into distinct clusters: if hybridization was common we would expect mtDNA trees to be intermingled. (ii) A suite of analytic approaches (Principal Components Analysis, Admixture, phylogenetics, etc.) demonstrate that the nuclear genomes of these species are well differentiated, suggesting strong barriers to gene flow. The large number of S. bovis sequences further reveal that introgression is unidirectional (S. bovis -> S. haematobium). We conclude that the chimeric S. haematobium genomes found in West Africa result from rare hybridization, followed by adaptive introgression of S. bovis genes, and that gene exchange between these species occurs on an evolutionary rather than an epidemiological timescale.