Molecular systematics of Tiquilia (Boraginaceae): age, origin, dispersal history, and gypsophilic evolution
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Tiquilia Pers. is a genus of approximately 30 species of subshrubs that grow in all deserts of North and South America, as well as in the Galápagos Islands. The genus is divided into two subgenera, subg. Tiquilia and subg. Eddya, that differ in morphology, chromosome number, habitat, and geographic distribution. Three distinct evolutionary themes, including the origins of North American desert plant lineages, the origins of North/South American amphitropical disjunctions, and the evolution of obligate gypsophily in plants of the Chihuahuan Desert Region (CDR) are explored with a molecular phylogenetic approach using Tiquilia as a case study. DNA sequence data from ndhF, matK, rps16, ITS, and waxy were collected for 28 species of Tiquilia and three outgroups. Phylogenetic analyses strongly support the monophyly of both subgenera as well as seven major lineages within Tiquilia, and indicate that the genus, subgenera, and major lineages evolved in North America. Molecular dating analyse suggest that Tiquilia arose in the Eocene, that both subgenera and all major lineages diverged in the Miocene, and that species diversity within each major lineage is of late Tertiary age. The phylogenetically isolated position of Tiquilia within the Boraginales in conjunction with these diversification dates supports the hypotheses of earlier researchers concerning the age and origins of the characteristic North American desert flora. Phylogeographic analyses of subg. Tiquilia require at least three separate long-distance dispersal events, all originating in North America, to explain the distribution of subg. Tiquilia in North and South America, contributing to a growing body of evidence that intercontinental dispersal has been more common than previously realized. The continental origins of the four Galápagos endemic taxa of subg. Tiquilia are unresolved. Phylogenetic analyses of subg. Eddya imply two origins of obligate gypsophily in the subgenus. The widespread obligate gypsophile T. hispidissima possesses the highest level of intraspecific sequence diversity in the genus, supporting the relatively great age of obligate gypsophily in this species and in the CDR. The pattern of geographic variation in this species correlates well with geographic variation in other CDR gypsophilic plant groups, suggesting the existence of broader phylogeographic patterns among CDR gypsophiles.