Changes in chlorophyll content and genome size in synanthropic and natural populations of Achillea millefolium agg. (Asteraceae) along an elevational gradient in the Karkonosze Mts (Central Europe)

Despite extensive research on the effects of elevation on photosynthetic performance and genome characteristics in alpine plants, little is known about how synanthropic, lowland-origin species respond genetically and physiologically when colonizing high-elevation environments. In particular, the combined responses of genome size variation, ploidy structure, and photosynthetic pigment content along elevational gradients remain poorly understood for polyploid plant complexes exposed to strong anthropogenic pressure. Achillea millefolium is widespread in the Karkonosze Mountains across a wide elevational range, occurring in both natural and synanthropic habitats. We collected samples along a 1,000-m elevational gradient to assess how genome size and chlorophyll content vary with elevation and to explore genetic variation between populations. We conducted single nucleotide polymorphisms analysis using Diversity Arrays Technology Sequencing (DArT-seq) and measured genome size and chlorophyll content from the same individuals. Our results show that both genome size and chlorophyll content decrease with increasing altitude. Crucially, the presence of A. millefolium was closely associated with tourist infrastructure, suggesting that human-mediated corridors facilitate its upward migration. At higher elevations, the dominance of hexaploids suggests that higher ploidy levels may support survival in harsh climates. This study identifies polyploidy and genome downsizing, alongside the plastic modulation of photosynthetic pigment levels, as key mechanisms enabling the persistence and potential adaptation of this synanthropic species in harsh mountain environments. These findings demonstrate how such colonizing populations navigate environmental pressures at high altitudes through integrated genomic and physiological shifts, providing a foundation for understanding how plants may adapt to and thrive in new ecological niches, potentially guiding ecological management in other mountainous regions.