Karyotype Variation and Environmental Adaptation in the Invasive Alien Freshwater Flatworm Girardia in China

Freshwater planarians of the genus Girardia demonstrate a remarkable ability to colonize new regions through anthropogenic transport and subsequent natural dispersal. In this study, we conducted a comprehensive field investigation and integrative analysis of Girardia species in invaded regions of China. Our results reveal that G. tigrina and G. sinensis are the principal invasive species in China. Long-term monitoring indicates a predominance of asexual reproduction, with no evidence of sexual reproduction. Karyological analysis showed a basic haploid complement of 8 metacentric chromosomes in each invasive Girardia population. Notably, we identified triploid, mixoploid, and aneuploid individuals-representing the first documentation of such karyotypic diversity in Girardia species from China. This variation is consistent with the observed reproductive strategies and may reflect chromosomal instability following introduction. Furthermore, invasive Girardia species demonstrate high ecological plasticity, enabling successful colonization and persistence in unfavorable environments, for example, affected by pollution, high temperature, human intervention. Meanwhile they may impact native communities through resource competition and structural shifts. Therefore, the Girardia may represent a notable example of how climate change drives the geographic expansion of invasive species. These findings provide crucial baseline data for reconstructing invasion pathways and demographic history, and offer valuable insights for future monitoring, management, and conservation efforts in freshwater ecosystems, as global temperatures rise.