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

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 439 individuals of globally invasive common ragweed (Ambrosia artemisiifolia L.) across native (North American) and invasive (European, Australian) ranges. By integrating whole-genome resequencing, flow cytometry and trait vs genetic differentiation comparison (QST-FST), we tested whether genome size evolution is shaped by selection, climate and life-history traits. Genome size was significantly larger in Australian genotypes, driven by increased TE and rRNA (ribosomal RNA) abundance. Crucially, trait vs genetic differentiation comparison provided evidence of divergent selection on genome size in North American and European populations, but not in Australia. Genome size was correlated with mean annual temperature (MAT) across all ranges, linking genomic traits to environmental variables. Genome size evolution during invasion can be rapid, adaptive and range-specific, with TE-driven genome expansion emerging as a potential genomic response to the demographic and environmental pressures accompanying colonisation of novel environments.