Studies of the evolution of parasitic plants in the genus Striga, using systematic, population genetics, and genomic approaches
Estep, Matthew Charles
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Understanding the evolutionary histories of groups of organisms is a major focus of evolutionary biology. Three powerful molecular approaches, phylogenetics, population genetics and genomics, can be used to reveal evolutionary relationships at the gene, genome and population levels. These tools were used to investigate Striga, a genus of parasitic plants. Six chloroplast loci were employed to construct a hypothesis of evolutionary relationships within the genus. The resulting hypothesis depicts three distinct clades, with the only non-grass parasite (S. gesneriodies) sharing a very recent common ancestor with the grass parasite S. aspera. Sample sequencing of the nuclear genome revealed fourteen repetitive elements that are influencing genome size and arrangement in Striga. These include a DNA transposon, three satellite repeats, and ten retroelements. Genome size values suggest that polyploidization is also contributing to genome evolution within Striga. Twelve microsatellite markers were developed to investigate population structure within S. hermonthica, one of the few allogamous species within the genus and the greatest threat to agriculture. These markers were then applied to accessions collected in Mali to reveal a large amount of genetic diversity that is broadly distributed across populations with little genetic differentiation and large amounts of gene flow. Some population structure was apparent, but could not be attributed to “isolation by distance” or host species, suggesting other geo-ecological variables may be acting to differentiate Northern populations from Southern populations. Together these analyses offer a valuable understanding of species evolution, genome evolution, and population structure within this clade of agriculturally important parasites and will help direct future work in combating witchweeds.