The obligately outbreeding root hemiparasite Striga hermonthica (Orobanchaceae)is a serious threat to subsistence agriculture in sub-Saharan Africa. Resistance to this parasite in its crop hosts, such as rice, sorghum and maize, is not common, and the evolution of host adaptations that are able to overcome new sources of resistance is an ever-present risk. Research into host adaptation in S. hermonthica has generally sought to correlate the genetic relationship between Striga individuals with host identity; however, such approaches must be supported by lab-based evidence of host adaptation, otherwise ad hoc field sampling may result in the confounding of host identity with isolation-by-distance. Additionally, genetic variation used to reconstruct relationships is unlikely to provide an insight into relationships at functional loci underlying host adaptation. In this thesis, I use a range of new approaches to investigate several different aspects of parasite adaptation in the S. hermonthica-Sorghum bicolor pathosystem. Host adaptations, or pre-adaptations, are commonly revealed using tests for differential virulence between Striga populations and host genotypes; that is, by demonstrating population-level genotype-by-genotype interactions. Evidence for such interactions was found between three West African populations of S. hermonthica and five sorghum cultivars. These interactions were shown to be strongest at the parasite post-attachment life stage, and to depend on the parasite virulence metric used. Environmental influences on host-parasite interactions were strong and variable between years and sites. Candidate genetic loci for virulence, responding to selection in a micro-evolutionary fashion, were identified by Fst differentiation-based approaches ('outlier analyses') that aim to uncover associations between particular loci and environmental drivers, such as host identity. An AFLP outlier analysis was used on Striga plants parasitising nine sorghum cultivars in a field trial in Burkina Faso. Significant locus-specific differentiation was detected at 14 out of 1275 loci. However, predicted allele frequencies at these loci did not correlate with a field measure of Striga virulence across host-selected sub-populations. Simulation results suggested that the estimated levels of Fst at outlying loci could mean that alleles underlying host adaptation exist at intermediate frequencies in populations. A three-generation pedigree, created from a cross between S. hermonthica individuals from an East African and a West African population, enabled further insights into the genetics of adaptation. Individuals from a pseudo-backcrossed F1 (BCF1) generation, grown on two different sorghum hosts and in axenic culture, indicated significant host-related segregation distortion. Analyses of virulence in the BCF1 also provided strong evidence for epistasis, and for an effect of maternal identity. A second outlier analysis of host adaptation, investigating the East African population used in the pedigree, indicated some correspondence between outlier loci and loci found to be differentially segregating between different hosts in the BCF1 generation, and demonstrated the differing genomic extents of these phenomena. The results accumulated across these experiments provide evidence for a complex, polygenic basis to virulence in S. hermonthica.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:574069 |
| Date | January 2013 |
| Creators | Pescott, Oliver |
| Contributors | Scholes, J. D. ; Butlin, R. K. |
| Publisher | University of Sheffield |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://etheses.whiterose.ac.uk/3979/ |
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