Characteristics of the completed chloroplast genome sequence of Xanthium spinosum: comparative analyses, identification of mutational hotspots and phylogenetic implications.

BACKGROUND: The invasive species Xanthium spinosum has been used as a traditional Chinese medicine for many years. Unfortunately, no extensive molecular studies of this plant have been conducted. RESULTS: Here, the complete chloroplast (cp) genome sequence of X. spinosum was assembled and analyzed. The cp genome of X. spinosum was 152,422 base pairs (bp) in length, with a quadripartite circular structure. The cp genome contained 115 unique genes, including 80 PCGs, 31 tRNA genes, and 4 rRNA genes. Comparative analyses revealed that X. spinosum contains a large number of repeats (999 repeats) and 701 SSRs in its cp genome. Fourteen divergences (Π > 0.03) were found in the intergenic spacer regions. Phylogenetic analyses revealed that Parthenium is a sister clade to both Xanthium and Ambrosia and an early-diverging lineage of subtribe Ambrosiinae, although this finding was supported with a very weak bootstrap value. CONCLUSION: The identified hotspot regions could be used as molecular markers for resolving phylogenetic relationships and species identification in the genus Xanthium.

Ragweeds and relatives: Molecular phylogenetics of Ambrosiinae (Asteraceae).

Ambrosiinae are one of the most distinct subtribes in the Heliantheae alliance (Asteraceae), mainly due to specialization toward wind pollination. Taxa of the subtribe are principally native to the Americas, although some species have attained a cosmopolitan distribution. Members of subtribe Engelmanniinae are considered close to Ambrosiinae, due to shared morphological traits. However, the placement of Ambrosiinae within the Heliantheae alliance has not yet been corroborated by phylogenetic analyses. In the present study, we test the circumscription of subtribe Ambrosiinae and examine relationships among its genera. We used sequence information from three plastid (psbA-trnH, trnQ-rps16 and trnL-F) and two nuclear (ITS and D35) marker regions. Phylogenetic inference analyses were conducted, applying Bayesian Inference (BI) and Maximum Likelihood (ML). Subtribe Ambrosiinae is found monophyletic or nearly so in all analyses. The genera Dugesia and Rojasianthe (previously considered part of subtribe Engelmanniinae) in some cases cluster together with Ambrosiinae; these genera are clearly not part of Engelmanniinae. Within Ambrosiinae, the genera Parthenium and Parthenice occupy basal positions, whereas members of the genus Ambrosia are the most derived representatives of the subtribe. Previous subdivision of Ambrosiinae into "Iveae" (members having androgynous capitula and free achenes) and "Ambrosieae" (genera with unisexual heads and achenes enclosed in burs) is not corroborated. Results also allow consideration of relationships among species and subgeneric groups within Parthenium, Iva, and Ambrosia.

How many names for a beloved genus? - Coalescent-based species delimitation in Xanthium L. (Ambrosiinae, Asteraceae).

The species category is the fundamental unit in biology. In the last decades, several studies have been carried out, using sequence information and phylogenies to resolve issues in taxonomically problematic groups. The multispecies coalescent theory, and the species-delimitation methods developed in the last years based on that, offer powerful and objective tools to determine species boundaries using sequence data. The genus Xanthium is a morphologically variable complex with several local forms, nowadays become cosmopolitan due to human-mediated dispersal. Past taxonomic treatments of the genus were based essentially on the burr morphology. They varied considerably in number of recognized species, depending on the importance that the different authors gave to burr traits such as burr size, pubescence, number and length of spines in the burrs, and the degree to which those are hooked. We used sequence information from two plastid regions (the intergenic spacer regions psbA-trnH and trnQ-rps16) and two nuclear ones (ITS1-5.8 S-ITS2 rDNA, and the single-copy nuclear marker D35). Two of the most advanced coalescent-based species delimitation methods (BP&P and STACEY) were applied, in order to objectively determine species boundaries in Xanthium. Results from the species-delimitation methods strongly support scenarios with a reduced number of species. Prior on the effective population size parameter (θ) had a strong influence on BP&P results. Analyses with large and small θ prior supported species delimitation scenarios with four and five species, respectively. STACEY recognized five cluster in the dataset, all supported by high posterior probability values. Five species are identified in Xanthium, in a great extent corresponding to the infrageneric classification given for the genus in past taxonomic revisions.