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Patterns of long-distance dispersal in Tiquilia subg. Tiquilia (Boraginaceae): implications for the origins of amphitropical disjuncts and Galápagos Islands endemics¹

²Section of Integrative Biology and Institute of Cellular and Molecular Biology, University of Texas at Austin, 1 University Station A6700, Austin, Texas 78712 USA; ³Department of Botany, Charles Darwin Research Station, Puerto Ayora, Isla Santa Cruz, Galápagos, Ecuador

ABSTRACT

Plant biogeographers have long argued whether plant disjunctions result from vicariance or dispersal. One of the classic patterns of plant disjunction involves New World amphitropical disjuncts, as exemplified by Tiquilia subg. Tiquilia (Boraginaceae). Subgenus Tiquilia forms a heterogeneous group of ~20 species that is amphitropically distributed in the deserts of North and South America, with four taxa endemic to the Galápagos Islands. The current study reconstructs the biogeographic history of subg. Tiquilia in order to explore the origins of New World amphitropical disjunction and of Galápagos endemism. A strongly supported phylogeny of the subgenus is estimated using sequence data from matK, ndhF, rps16, ITS, and waxy. Biogeographic analyses using combined and individual marker data sets reveal a complex history of long-distance dispersal in subg. Tiquilia. Biogeographic reconstructions imply a North American origin of the subgenus and its three major lineages and require at least four long-distance dispersal events to explain its current distribution. The South American taxa of subg. Tiquilia result from three independent and nonsimultaneous colonization events, while the monophyly and continental origins of the Galápagos endemics are unresolved. This study contributes to a growing body of evidence that intercontinental dispersal has been more common than previously realized.

Key Words: amphitropical disjunction • Boraginaceae • Galápagos Islands • long-distance dispersal • molecular phylogeny • North America • South America • Tiquilia