Evolutionary Relationships in Polygoneae Rchb. (Polygonaceae) with an Emphasis on the Amphi-Pacific Muehlenbeckia Meisn
Polygoneae is one of five tribes included in Polygonoideae that is in turn one of the three subfamilies recognized in the buckwheat family Polygonaceae. Representatives of the other two subfamilies, Eriogonoideae and the monotypic Symmerioideae, are included in this study as outgroups in addition to some species of the Plumbaginaceae. Taxon sampling was based on previous broad-scale studies in Polygonaceae and expanded for the eight genera thought to comprise Polygoneae. These include Atraphaxis, Fallopia, Knorringia, Muehlenbeckia, Oxygonum, Polygonum, Polygonella and Reynoutria . The following questions were addressed based on this taxon sampling: 1) What are the evolutionary relationships among the genera included in Polygoneae? 2) Is Muehlenbeckia a monophyletic group? 3) Is Polygonella included in Polygonum as morphological studies suggest? 4) Does the morphologically distinct Oxygonum that is restricted to Africa and for which there were no sequence data available prior to this study belong to Polygoneae? In addition, a species level study of Muehlenbeckia that sampled 22 of the 30 recognized species was done because Muehlenbeckia is unique in Polygonaceae based on its amphi-Pacific, Southern Hemisphere distribution in a predominantly North Temperate family. Therefore, the historical biogeography of Muehlenbeckia is of interest and in this study it is attempted to pinpoint geographic areas the most recent common ancestor of Muehlenbeckia occupied and to estimate divergence times within Muehlenbeckia using a fossil calibration point. Furthermore, because Muehlenbeckia shows great morphological variability and its members are not easily divided into discrete units, understanding evolutionary relationships in this group would benefit from a molecular phylogenetic approach. To this end, Maximum Likelihood, Maximum Parsimony and Bayesian Inference methods were used on two data sets. One matrix of 145 species was generated to answer large scale questions in Polygoneae. This matrix included data for the gene regions nrITS, matK, ndhF and trnL-trnF. A second data set included 87 species and 7 gene regions (matK, ndhF, 3'rps16-5'trnK, trnL-trnF and 3'trnV-ndhC, the second intron of LEAFY and nrITS) to address species-level relationships within Muehlenbeckia. Results of this study indicate that Polygoneae is composed of two clades where Atraphaxis, Duma + Polygonum (ADP clade) form a sister pair with Reynoutria, Muehlenbeckia + Fallopia (RMF clade). Muehlenbeckia is monophyletic when three Australian species are segregated to the newly described genus Duma that belongs to the ADP clade and is not closely related to the main clade formed by all other species of Muehlenbeckia sampled. Because Polygonella is nested within Polygonum section Duravia , all eleven species of Polygonella are transferred to Polygonum based on this study. Similarly, because all species of Koenigia sampled are nested in Aconogonon, the latter were transferred to Koenigia (priority of name). Oxygonum was shown not to belong to Polygoneae, though determination of its exact placement awaits more data. Within the Muehlenbeckia clade, evolutionary relationships are best explained by a strong geographic signal. Ecological traits are also important with respect to the phylogeny, since most of the Australian species that form a clade are fire ephemerals. Estimates of divergence times in Muehlenbeckia indicate that it is likely a relatively young group with a minimum age of 14.5 million years. Because this post-dates the isolation of South America and Australia/New Guinea/New Zealand from Gondwana, the disjunction of the Australasian and Central and South American taxa vicariant speciation is ruled out. Speciation of Muehlenbeckia in the Southern Hemisphere may have involved stepping-stone dispersal via New Guinea from Asia where Fallopia that is the closest relative of Muehlenbeckia is native. Importantly, all three analyses methods (Bayesian Inference, Maximum Likelihood and Maximum Parsimony) recover M. platyclada that is native to New Guinea as sister to all other species of Muehlenbeckia . In addition, the diversification within the Muehlenbeckia clade is best explained by events of long distance dispersal between New Zealand and Australia and South America because New Zealand is the most likely ancestral area for most species of extant Muehlenbeckia.