Genetic Structure Along Temperature Gradient in a Recent Invader Cardamine hirsuta in Eastern Japan

Invasive species often spread rapidly, which appears discrepant because species distribution is often limited by environmental conditions. Even in widely distributed species, there are differences among genetic clusters along environmental gradients, suggesting that the environmental range of one genetic group is limited but extended distribution is possible through genetic differentiation. Cardamine hirsuta is native to Europe, invaded Japan in the 1970s, and is now found throughout Japan. To understand how C. hirsuta has rapidly spread across a climatic gradient in eastern Japan, where there is a large temperature gradient along the latitude, we characterized its genetic structure and addressed three hypotheses: (1) a genetic cluster is widely distributed because it favors a broad range of environmental conditions, (2) different genetic clusters are distributed across distinct environmental conditions because of different environmental preferences, and (3) admixed plants between different genetic clusters fill gaps where original genetic clusters are less adapted. We analyzed 82 individuals collected from 29 sites, and ADMIXTURE analysis showed the lowest cross-validation error at K = 4. Cluster 2 was widely distributed in eastern Japan, whereas Cluster 3 was found in northern areas and Cluster 1 was found in southern areas. Individuals with mixed ancestry were observed, particularly in regions where the distributions of genetic clusters overlapped. The potential distributions of different clusters, which were associated mainly with the temperature gradient, overlapped without obvious gaps, providing no evidence that individuals with mixed ancestry preferentially occurred in gaps between the potential distributions of the original genetic clusters. We conclude that genetic clusters within an invasive species can show contrasting climatic associations, while some clusters may occur across a comparatively broad climatic range. Considering such intraspecific genetic structure may provide additional information for assessing potential range expansion and contribute to more effective monitoring of ongoing invasions.