One of the most challenging problems in systematics today is understanding the relationships between closely related species, especially those that have been subject to processes such as hybridization and polyploidy, the doubling of an organism's genome. This research investigates the effects of these processes on the relationships among species of Sorghum sect. Sorghum (Poaceae). Sorghum sect. Sorghum includes cultivated sorghum [Sorghum bicolor (L.) Moench] and johnsongrass [Sorghum halepense (L.) Pers.], one of the world's most noxious weeds. S. halepense is a polyploid, thought to be derived from S. bicolor. S. bicolor and S. halepense are not genetically isolated and may hybridize, creating complex patterns of relationship. DNA sequence data for multiple sections of the plants' nuclear and chloroplast genomes from individuals of this species complex will be used to reconstruct evolutionary relationships. These relationships can be examined for the presence of patterns, which may be correlated with geography, plant appearance, invasiveness, or other factors.
In light of the economic importance of this species complex in Sorghum, a better understanding of their relationships is essential. Cultivated sorghum is the fifth most important cereal crop in the world and a staple grain in the semi-arid tropics. It has a small genome size relative to that of maize, making it an ideal model organism for comparative genomics studies in the grasses. S. halepense is one of the ten most noxious weeds worldwide. Invasive species rank second behind only habitat destruction as the largest threat to biodiversity worldwide and cost billions of dollars each year in reductions in crop yields. This research also will contribute to a greater understanding of the process of polyploidy and its impact on plant evolution. The process of polyploidy has been linked to extensive genomic changes. Current estimates suggest that at least 50-70% of angiosperms have undergone genome duplication at some point in their evolutionary past, making polyploidy a significant factor in plant evolution. Polyploidy has also shaped the genomes of many other organisms, such as vertebrates, including humans, and plays a role in some cancers. Polyploidy and hybridization both have the potential to increase genetic variability within a species, and possibly invasive ability. A greater understanding of these processes and their effects on the evolution of these economically important species may be beneficial to a variety of fields, including plant breeding and weed control efforts.
