Landeryou, Toby (2015) Investigation of species barriers with African schistosomes. (PhD thesis), Kingston University, .
Abstract
Schistosomes are dioecious parasitic flatworms, which live in the vasculature of their mammalian definitive hosts. They are the causative agent of schistosomaiasis, a disease of considerable medical and veterinary importance in tropical and subtropical regions. Many species of 'Schistosoma' can interbreed with each other and produce viable young with several hosts (both human and animal) being shown to be infected with hybrid offspring. This indicates a breakdown of speciation boundaries and illustrates a lack of genetic barriers that would prevent successful hybridization. In this study a two-pronged investigation into the strength of species barriers between African species of Schistosoma looked into the evolution of reproductive proteins in 'Schistosoma'. The second looked into the potential role of the Z chromosome in the speciation. Genes coding for sperm proteins associated with fertilization of the egg and sperm flagella locomotion diverge rapidly in most invertebrate species, creating important species barriers. In the first study three sperm protein associated genes; beta 1,4 galactotransferase, sperm nuclear basic protein and antigenic sperm protein 1. These were identifies using the 'Schistosoma mansoni' genome resource and then sequenced across several closely related species from both the 'S.mansoni' and 'S.haematobium' groups. Low levels of genetic diversity were observed between species within the groups but large amounts of variation occurred between the S. mansoni and S. haematobium groups. A high level of nonsynonymous mutations was recorded leading to marked differences in the properties of the sperm proteins. A high level of nonsynonymous mutations was recorded leading to marked differences in the properties of the sperm proteins. In the second study the investigation of genes across the Z chromosome sought to find potential areas of divergence in 'Schistosoma'. Using the 'S.haematobium' and 'S.mansoni' species groups, genes were sequenced across different locations of the Z chromosome to see any indication possible divergence across chromosomes. Areas of the chromosome did show substantial divergence between species, with the presence of the start of evolutionary strata on the Z chromosome. Both sets of results indicate a recent, rapid divergence of the 'Schistosoma' species group but does question the apparent fragility of speciation barriers within closely related species.
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