Extant interspecific hybridization among trematodes within the Schistosoma haematobium species complex in Nigeria

Background: Natural interspecific hybridization between the human parasite (Schistosoma haematobium [Sh]) and bovine parasites (S. bovis [Sb], S. curassoni [Sc]) is increasingly reported in Africa. We developed a multi-locus PCR DNA-Seq strategy that amplifies two unlinked nuclear (transITS, BF) and two linked organellar genome markers (CO1, NAD5) to genotype S. haematobium eggs collected from infected people in Ile Oluji/Oke Igbo, Ondo State (an agrarian community) and Kachi, Jigawa State (a pastoral community) in Southwestern and Northern Nigeria, respectively. Principal Findings: We applied this methodology against 57 isolates collected from a total of 219 participants. All patients from Jigawa state were infected with just one of two haplotypes of an S. haematobium x S. bovis hybrid based on sequences obtained at CO1, NAD5, transITS and BF markers. Whereas samples collected from Ondo state were varied. Mitonuclear discordance was observed in all 17 patients, worms possessed an Sb mitochondrial genome but one of four different haplotypes at the nuclear markers, either admixed (heterozygous between Sh x Sc or Sh x Sb) at both markers (n=10), Sh at BF and admixed at transITS (Sh x Sc) (n=5), admixed (Sh x Sc) at BF and homozygous Sc at transITS (n=1) or homozygous Sh at BF and homozygous Sc at transITS (n=1). Significance: Previous work suggested that zoonotic transmission of S. bovis in pastoral communities, where humans and animals share a common water source, is a driving factor facilitating interspecific hybridization. However, our data showed that all isolates were hybrids, with greater diversity identified in Southwestern Nigeria, a non-pastoral site. Further, one patient possessed an S. bovis mitochondrial genome but was homozygous for S. haematobium at BF and homozygous for S. curassoni at transITS supporting at least two separate backcrosses in its origin, suggesting that interspecific hybridization may be an ongoing process.