Format: ORAL

Brian J. Sanderson*1,8,10, Diksha Ghambir1, Guanqiao Feng1, Nan Hu1, Quentin C. Cronk2, Diana M. Percy2, Francisco Molina Freaner3, Matthew G. Johnson1, Lawrence B. Smart4, Ken Keefover-Ring5, Tongming Yin6, Tao Ma7, Stephen P. DiFazio8, Jianquan Liu7,9,

1 Department of Biological Sciences, Texas Tech University, Lubbock, TX 79409-3131 USA 2 Department of Botany, University of British Columbia, Vancouver, BC, V6T 1Z4 Canada 3 Universidad Nacional Automoa de Mexico, Hermosilla, Mexico 4 Horticulture Section, School of Integrative Plant Science, Cornell University, Cornell AgriTech, Geneva, New York 14456 USA 5 Departments of Botany and Geography, University of Wisconsin-Madison, Madison, WI 53706, USA 6 Key Laboratory of Tree Genetics and Biotechnology of Jiangsu Province and Education Department of China, Nanjing Forestry University, Nanjing, China 7 Key Laboratory of Bio-Resource and Eco-Environment of Ministry of Education & College of Life Sciences, Sichuan University, Chengdu 610065, China 8 Department of Biology, West Virginia University, Morgantown, WV, 26506 USA 9 State Key Laboratory of Grassland Agro-Ecosystem, Institute of Innovation Ecology & College of Life Sciences, Lanzhou University, Lanzhou 730000, China 10 Current Address: Department of Molecular Biosciences, University of Kansas, Lawrence, KS 66045, USA

Understanding divergence times in the Salicaceae (poplars, willows, and relatives) are crucial for understanding environmental impacts on trait evolution such as sex chromosomes, pollination type, stature, and cold hardiness. Two factors underlie the challenges to estimation of divergence times: 1) the impact of hybridization on estimation of the species tree and 2) accurate placement of fossils onto the tree for calibrating the molecular clock. We used sequence capture protocols to investigate the intricate gene histories in bothPopulusandSalix. These histories often conflict with the overall species tree of the Salicaceae family. The diverse gene histories are linked to evidence of gene flow resulting from hybridization, dating back to the earliest divergence events in these genera. Current coalescent dating methods demand extensive computational resources and are feasible only when a limited number of genes are used. We show that variations in gene tree histories can significantly affect estimates of divergence times. Furthermore, despite the abundance of Salicaceae fossils, their limited morphological characteristics make it challenging to confidently place them within the family tree. Our findings demonstrate that calibrating the molecular clock using the most reliable fossils of the origins of subgenera within Salix and Populus leads to a substantial alteration in estimated divergence times compared to calibrating the clock based on the most reliable assessment of the Populus+Salix clades origin time.