Format: ORAL

Eleonora Manzo1, Justus Mller-Kiefer1, Sebastian Nillies1, Diego G. Gutirrez2, Salvatore Tomasello1

1. Georg-August University Göttingen, Germany 2. Argentine Museum of Natural Sciences (MACN), Buenos Aires, Argentina

Xanthium is a peculiar genus of the Asteraceae family, identifiable by its spiny female capitula (burs) and wind-pollination. Features of the burs are the primary morphological characteristics used for identifying taxa within the genus. The high degree of their morphological variability has led to inconsistent taxonomic treatment in the past, resulting in numerous names given by different authors. The genus can be divided into two sections: sect. Xanthium, which includes all taxa with unarmed stems; and sect. Acantoxanthium comprising plants with stems with trifurcate spines. The taxa of sect. Acanthoxanthium are native of South America, but due to human-mediated dispersal, some of its lineages have now reached a cosmopolitan distribution.  Apart from the widespread X. spinosum, several other taxa have been described within this section. The aim of the present study is to determine species boundaries in Xanthium sect. Acanthoxanthium. The study employed phylogenomics, including target enrichment of hundreds of nuclear loci and complete plastomes, and geometric morphometrics, alongside coalescent-based species delimitation approaches. To ensure a comprehensive range of morphological variations in the section, including types and original material for some taxa, we surveyed key herbaria such as P, B, WU, and BA. Analysis of herbarium specimens that are nearly 200 years old, comprising type material, along with modern extraction techniques and integrative taxonomy analyses, have enabled us to identify distinct evolutionary lineages within the section. This further confirms the existence of two previously acknowledged species, X. spinosum and X. ambrosioides (Tomasello 2018). Two further independent lineages are inferred as species, the neglected X. argentum, initially described by Widder (1923) based on a single herbarium voucher from the province of "Nubles" in Chile and nowadays probably extremely rare; and X. catharticum, differentiated from X. spinosum, is its vicariant in the high-altitude environments of the Andes.