Investigating the role of hybridization and polyploidy in the evolution of semi domesticated legume tree crop Leucaena using DNA sequencing and AFLP finger printing

Leucaena is a New World mimosoid legume genus consisting of 17 diploid and five polyploid species that include a number of important semi-domesticated taxa in south-central Mexico as well as a pantropically cultivated forage species that is widely naturalized and invasive. The incipient domestication of diploid Leucaena over thousands of years in south central Mexico has been hypothesized to have influenced the origins of one or more widely cultivated polyploid species. The prevalence of hybridization and polyploidy facilitated by human translocation have resulted in the reticulate evolutionary patterns complicating the inferences on species relationships. The precise origin of polyploids within Leucaena remains largely unclear.;In order to understand the origins of polyploids and elucidate the influence of indigenous domestication on the evolution of the genus a combination of phylogenetic and population genetic studies are employed. The research presented in this dissertation strategically establishes diploid level relationships and evaluation of species limits among diploids followed by the phylogenetic investigation of all species within the genus to understand the origin of polyploids.;The phylogenetic analyses of the data derived from four linked chloroplast sequences and seven independent SCAR-based nuclear encoded markers from multiple accessions of all diploid taxa within the genus provided a diploid level framework. Phylogenetic analyses distinguished all the diploid taxa into three major clades, and monophyly of each diploid species is highly supported except L. lanceolata. Exclusive test on species limits through population level studies employing AFLP analyses among diploids identified two distinct lineages within L. lanceolata.;The phylogenetic analyses of multilocus sequence data for all 22 species of Leucaena have suggested allopolyploid origins for all five polyploid species involving divergent diploid parental species. Furthermore, the results from multiple independent gene trees and population level studies have clarified more specific origins for L. leucocephala a widely cultivated species in Mexico and economically important tropical forage species. The evidence for the involvement of divergent diploid L. trichandra as a parental species in the origins of four of the five polyploid species is particularly interesting in the context of human translocation and origin of species. Finally, this dissertation has provided better understanding of the species relationships within Leucaena that can be directly applicable in selective breeding programs for the improvement of economically important species within Leucaena for agro forestry.