Phylogeny and biogeography of a mint subfamily Lamioideae with emphasis on New World and Hawaiian diversifications
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To understand the mechanisms, which have led to the present-day biodiversity across the globe, we need to closely examine the speciation and diversification processes among key lineages within the tree of life. The focus of my dissertation research has been to investigate evolutionary origins and relationships within one such globally distributed and ecologically and economically important group, the plant family Lamiaceae (the "mint" family). Within Lamiaceae, its second largest subfamily, Lamioideae, is a species-rich, distinct, and widespread lineage, representing an excellent model system for the study of ecological and evolutionary questions. Although the Old World (OW) forms the center of diversity for most members of Lamioideae, only two of its lineages, Stachydeae and Synandreae, migrated and diversified via separate dispersal events into the New World (NW). Stachydeae, which also forms the largest tribe within Lamioideae, is a taxonomically complex lineage exhibiting remarkable morphological, ecological, biogeographical, and chromosomal diversity. Apart from its largest member, the genus Stachys , another major component of Stachydeae are the Hawaiian native mints (∼60 species), representing one of the largest plant radiations in the Hawaiian Islands. The origin and evolution of the Hawaiian mints from the NW Stachys warrants further exploration, since information gleaned from such studies may provide us with a better understanding of angiosperm evolution during prehistoric times, as dispersal to volcanic islands involve colonization of newly formed habitats, and early angiosperms may have experienced similar environmental instabilities. My study plays a crucial role in elucidating the evolutionary origin, relationships, and biogeographical history of the various lamioid members including the NW groups Stachydeae and Synandreae. Comparative studies of such NW flowering plant lineages may increase our understanding of processes involved in the origin, establishment and diversification of angiosperms in the different parts of the world, including colonization into the Americas. Plant molecular phylogenetic studies including members of Lamioideae have often relied on investigating evolutionary relationships based on data from chloroplast (cpDNA) and nuclear ribosomal (rDNA) DNA markers. However, cpDNA is known to be inherited through the maternal parent only, and ribosomal DNA markers may have undergone homogenization towards either the paternal or the maternal parent, and therefore retained only one parental copy in species of hybrid origins. Hence, these markers, although commonly used, may not give us a complete estimate of the evolutionary history. Hence, the overall goal of my dissertation research has been to incorporate data from independently inherited low-copy nuclear markers, comparing the results with those from cpDNA and rDNA, along with formal analyses of historical biogeography to get an in-depth understanding of speciation and diversification patterns among lamioid mints. In particular, I have emphasized on the two NW lineages Stachydeae and Synandreae that provide excellent parallel systems, in which to investigate the colonization of angiosperm lineages into the New World. The first chapter of my dissertation is aimed at broadly characterizing inter-relationships among tribes within the Lamioideae based on data from both cpDNA and a low-copy nuclear locus ( PPR ), along with various bioinformatics methods, including ancestral area reconstructions and fossil-based molecular dating. The second chapter of my dissertation investigates phylogenetic relationships, divergence timings and dispersal routes within tribe Stachydeae, with emphasis on the origin of the Hawaiian mints from their closest NW Stachys relatives, through broad sampling of rDNA and cpDNA sequence data. In the third chapter of my dissertation, I have expanded on this study by analyzing sequence data from five low-copy nuclear genes to provide further insight into the complex origin and evolutionary relationships between the New World Stachys and the Hawaiian mints. The fourth and final chapter of my dissertation investigates the phylogenetic relationships within the NW tribe Synandreae based on low-copy nuclear markers with extensive sampling of its five genera. This study also examines the evolutionary patterns between the two NW lamioid tribes Stachydeae and Synandreae.