Selection shapes the evolution of genome size in a globally invasive plant

Summary Biological invasions provide powerful natural experiments for understanding how genome architecture responds to novel climatic environments. Transposable elements (TEs) can rapidly restructure genomes, yet their role in adaptive genome size evolution during invasion remains poorly understood. Here, we examined genome size and TE abundance in 440 individuals of the globally invasive common ragweed ( Ambrosia artemisiifolia L.) across native (North American) and invasive (European, Australian) ranges. By integrating whole-genome resequencing, flow cytometry, and P ST –F ST comparisons, we tested whether genome size evolution is shaped by selection, climate, and life-history traits. Genome size was significantly larger in Australian populations, driven by increased TE and rRNA (ribosomal RNA) abundance. Crucially, P ST –F ST analyses revealed divergent selection on genome size in North American and European populations, but not in Australia. Furthermore, genome size correlated with mean annual temperature (MAT), in North America, linking genomic traits to environmental variables, while its effect on flowering time was significant but weak and largely overlapping with the polygenic background. Genome size evolution during invasion can be rapid, adaptive, and range-specific. TE-driven genome expansion challenges the assumption of a genome size reduction during invasion, demonstrating that selection and environmental context shape genome architecture during range expansion.