Abstract
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.
Highlights
Cosmopolitan species provide opportunities to test hypotheses on dispersal processes and with modern phylogenetic tools, to clarify the status of speciesDiversity 2017, 9, 20; doi:10.3390/d9020020 www.mdpi.com/journal/diversityDiversity 2017, 9, 20 as native or alien to an area
Our global sampling of P. nodiflora (n = 160 populations) included 35 populations from the Americas where we found 10 ribotypes, a discovery level of 29%; for Australia we sampled
NE) in a flood plain disturbed by cattle. This isolated occurrence in Australia (Figure 2), location and clade position (Figure 1) and network position (Figure 3) accords with Bean’s [15]) criteria for characteristics associated with an alien species (C1b, C2b, C5b, C6b, C9b, C10b) and we suggest that this ribotype has recently arrived in Australia from the Americas
Summary
Cosmopolitan species provide opportunities to test hypotheses on dispersal processes (vicariance versus long distance dispersal) and with modern phylogenetic tools, to clarify the status of speciesDiversity 2017, 9, 20; doi:10.3390/d9020020 www.mdpi.com/journal/diversityDiversity 2017, 9, 20 as native or alien to an area. Strong evidence suggests that their wide distribution is due to non-human dispersal mechanisms In other species it is clear that humans have aided the movement of plants (e.g., sweet potato, [4]) and there is a third class in which the distribution of the species is cryptogenic [5]—the basis to the biogeography is puzzling and yet to be resolved. In the Americas P. nodiflora occurs from lower North America (including Florida) to northern South America [6] with a disjunct occurrence in Brazil [6,7]. In Australia, P. nodiflora occurs in tropical and sub-tropical regions with several disjunct temperate occurrences [8]
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