Format: ORAL

Sushil Dahal1 Carolina M. Siniscalchi2 Ryan A. Folk1

1 Department of Biological Sciences, 295 Lee Boulevard, Mississippi State, MS 39762, USA. 2 Mississippi State University Libraries, 395 Hardy Road, Mississippi State, MS 39762, USA.

Biotic disjunctions, the occurrences of related organisms in disconnected areas of the Earth, have attracted scientific attention for past 200 years. Despite being represented in many familiar plants such as bald cypress, flowering dogwood, sweetgum, partridgeberry, etc., the eastern North American (ENA)Mexican (M) disjunction remains poorly understood. Major outstanding questions include the divergence times of taxa exhibiting the disjunction and environmental/geological processes that may underlie the disjunction. Symphyotrichum, one of the most diverse genera in the eastern US, extends across the Americas, the Caribbean, and Eurasia, with several examples of disjunct ENA-M taxa. We performed molecular work using herbarium specimens to sequence accessions representing most of the species-level diversity of broader Symphyotrichum. We performed a sequence capture approach on DNA libraries using the Angiosperms353 baitset and used HybPiper to assemble the sequence data. Our phylogenetic analyses used both concatenation in RAxML-NG and coalescence in ASTRAL-III. We used MCMCTREE to perform divergence time estimation and R package BioGeoBEARS to infer ancestral regions and biogeographic transitions between North America and Mexico.Finally, we used an ancestral niche reconstruction approach in Utremi to test for a role of historical aridification in generating the disjunction. Our molecular data suggest a recent radiation of Symphyotrichum at the Plio-Pleistocene boundary (~2.5 mya), with early connections to Mexico in ancestral lineages that closed off shortly after. Several subsequent dispersals to Mexico in the mid-Pleistocene were followed by vicariance across this region. Except for some present-day broadly distributed species, there is a complete lack of movement between ENA and M after 1 mya. A reconstructed disjunct distribution of suitable habitat in Pleistocene climatic models corroborate results from biogeographic modeling and confirm a role for aridification in the North American interior in creating the ENA-M disjunction.