The Phylogeny and Biogeography of Phyla nodiflora (Verbenaceae) Reveals Native and Invasive Lineages throughout the World

Phyla nodiflora is an herbaceous perennial and an enigmatic species. It is indigenous to the Americas but is considered a natural component of the flora in many areas and a weed in others. Our aim was to circumscribe the native range of P. nodiflora, to explore dispersal mechanisms and routes and to test the hypothesis that P. nodiflora is native outside of the Americas. Determining whether distributions are natural or human-induced has implications for decisions regarding weed control or conservation. We undertook phylogenetic analyses using sequence variation in nuclear DNA marker ITS (Internal Transcribed Spacer) for a global sample of 160 populations of P. nodiflora sourced from Asia, Australia, central America, the Mediterranean, southern North America, South America and Africa. Analyses included maximum likelihood, maximum parsimony, a Bayesian estimation of phylogeny and a parsimony network analysis which provided a genealogical reconstruction of ribotypes. We evaluated phylogenies against extensive historical and biogeographical data. Based on the sequences, 64 ribotypes were identified worldwide within P. nodiflora and considerable geographic structure was evident with five clades: one unsupported and the remaining weakly supported (bootstrap support ranging from 52% to 71%). Populations from central and southern North America formed the core area in the indigenous range and we have detected at least three native lineages outside of this range. Within Australia P. nodiflora is represented by at least one native lineage and several post-European introductions. Phyla nodiflora is one of the few species in the family Verbenaceae to have a pan-tropical native distribution, probably resulting from natural dispersal from America to Africa then to Australasia. However, it has also undergone human-mediated dispersal, which has obscured the native-origin of some ribotypes. These introductions present a risk of diluting the pan-tropical structure evident in this species and therefore they have important conservation implications.