Evolution in a recently arisen species complex: Phylogeography of Manihot esculenta Crantz (Euphorbiaceae)

The nature of gene flow within and between plant species makes plants interesting candidates for studying population divergence. This dissertation examines the population genetic structure and population history of the Neotropical species Manihot esculenta Crantz (Euphorbiaceae) and the closely related, potentially hybridizing species M. pruinosa Pohl. Two primary research questions are addressed. The first concerns the evolutionary and geographical origins of the root crop cassava (M. esculenta subsp. esculenta). Genetic variation from three low-copy nuclear genes and five microsatellite loci was used to examine the relationship of the crop to wild M. esculenta populations and to M. pruinosa. Results indicate the following: (1) the genetic diversity of cassava is a subset of that found in its conspecific wild relative, M. esculenta subsp. flabellifolia ; (2) the crop is most closely related to flabellifolia populations occurring along the southern border of the Amazon basin; and (3) cassava does not appear to be derived from M. pruinosa . Taken together, these findings suggest that the crop was domesticated directly from flabellifolia populations in southern Amazonia and is not of hybrid origin, as traditionally proposed. The second research question focuses on the phylogeographic structure of the natural populations in the study system. Inferences about population structure and population history were made using two approaches, one based on allele frequencies alone, and one incorporating information on genealogical relationships among alleles (haplotypes). These analyses indicate the following: (1) after an initial range expansion, flabellifolia populations became differentiated through recurrent, restricted gene flow, establishing an overall pattern of genetic isolation-by-distance; (2) northeastern and western flabellifolia populations are genetically more similar than would be predicted by the current species distribution and appear to have become isolated from each other in the recent past; this fragmentation may reflect post-Pleistocene habitat shifts in the Amazon basin. At the interspecific level, all data indicate a very close genetic relationship between flabellifolia and pruinosa. The two species share microsatellite alleles and DNA sequence haplotypes, and genealogical relationships among haplotypes indicate incomplete lineage sorting. These patterns suggest a recent divergence between the two species and probably some occasional interspecific introgression.