Xanthium strumarium

Manzo, Eleonora & Tomasello, Salvatore, 2025, Recent Long-Distance-Dispersal Explains the Range Disjunction of the Old-Word Cockleburs (Xanthium strumarium), Journal of Biogeography (e 15104) 52 (5), pp. 1-16 : 2-3

1.1 | Xanthium strumarium

It is a thermophilic complex that is commonly found in sandy habitats, such as ditches and riverbanks ( Arcangeli 1882; Widder 1923). It is mainly distributed throughout Eurasia and Africa, and known to be native to these regions, with records in the literature dating back to the Dioscorides' De Materia Medica (First century BC) and fossil records from the last interglacial period ( Löve 1975; Chauhan 1991). However, the species has suffered a progressive decline in the last two centuries (parallel to the spreading of the non-native X. orientale ) and is nowadays difficult to find in nature in a great part of its native range (Müller-Kiefer and Tomasello 2022). Differently from its congenerics, the species did not manage to spread outside its native distribution range. The only putative stable populations found outside the Old World were in southern Brazil, initially described by Vellozo (1881) as a different species ( X. brasilicum Vell. ) and presumably originated from burs arrived from the Mediterranean basin (Widder 1923).

Xanthium strumarium distribution range is unique when compared to the other species of the genus, which are all native to America (Tomasello 2018). Even subtribe Ambrosiinae Less. and tribe Heliantheae Cass. to which Xanthium belongs are well-known to have an American origin and distribution centre ( Baldwin 2009). The subtribe, for example, is composed by about thirteen genera and over 150 species (Tomasello et al. 2019), and the only other taxon non-native to America apart from X. strumarium is Ambrosia maritima L. (Martin et al. 2018). Ambrosia maritima is supposed to be native to the Mediterranean basin and the coast of central-western Africa, although its distinctiveness from other taxa of the genus with an American origin is questionable based on phylogenetic data (Martin et al. 2018). At the tribal level, only a few examples are known of genera including species with native ranges found outside America ( Baldwin 2009), all of them from subtribe Ecliptinae Less. ( Blainvillea Cass. , Eclipta L., Indocypraea Orchard , Exomiocarpon Lawalrée , Fenixia Merr. , Hoffmanniella Schltr. ex Lawalrée , Pentalepis F.Muell. ).

The very distinctive distribution of X. strumarium in the Old World may be the results of a relatively recent long-distance dispersal event, as hypothesized also for Ambrosia maritima (Martin et al. 2018) . This could be explained by means of the dispersal strategy of Xanthium burs, that include zoochory (as above mentioned), but also hydrochory and thalassochory. Takakura and Fujii (2010) demonstrated that burs of Xanthium can be soaked in salty water for weeks and still be viable, a feature that could have favoured the arrival of Xanthium to the Old World via sea. Also, mammal and bird migration routes from America to Eurasia ( Backensto et al. 2016; Yong et al. 2021) might have facilitated the spread of Xanthium diaspores in Eurasia.

On the other hand, it is widely acknowledged that from the Miocene to the present, numerous alternations of cold and warm periods have impacted the palaeogeography of continents, leading to the cyclical connections and separations of landmasses through ice sheet formation and sea-level changes ( Brikiatis 2014; Hosner et al. 2015). These shifts resulted in the formation of different land bridges that facilitated animal and plant dispersal ( Hosner et al. 2015; Maguilla et al. 2018). When concerning connections between America and the Old World, a few important events need to be mentioned. The De Geer Land Bridge appeared during the Late Cretaceous and Early Palaeocene in two time windows around 69 million years ago (Ma) and 65.5 Ma, connecting North America with Eurasia through Greenland and Fennoscandia ( Brikiatis 2014). The Thulean Route, present from the Early Palaeocene until the Early Eocene, also connected North America with Eurasia via the British Isles and Greenland ( Brikiatis 2014). The Japan Land Bridges appeared multiple times due to sea level changes, connecting the Japanese archipelago to modern Russia during the Pleistocene (Millien-Parra and Jaeger 1999). Finally, The Beringia Land Bridge, perhaps the most well-known, connected North America to Eurasia through Russia, Alaska, and Canada and appeared multiple times during the Palaeocene and most recently during the Last Glacial Maximum circa 30–20 thousand years ago ( Brikiatis 2014; Hoffecker et al. 2016).

With the present study, we want to test if the arrival of the diaspores of the ancestor of X. strumarium in the new continent was caused by a long-distance dispersal event or if alternatively, it was the result of a progressive dispersal through a land bridge (e.g., the Beringian Bridge). Additionally, we aim at addressing a few main questions concerning the biogeographic history of Xanthium strumarium . (a) What is the temporal framework in which the ancestor of the species arrived in the Old World? (b) What have been the possible pathways followed by the species to spread all-over the Old World? To answer these questions, we used herbarium specimens and applied target enrichment of nuclear genes to infer the phylogeography of the species. We used Bayesian methods to estimate its divergence time and ancestral range reconstruction to test different possible scenarios of the pathways followed by the genus Xanthium to reach and colonise the Old World.