Urban spatial heterogeneity shapes the evolution of an antiherbivore defense trait and its genes in white clover

Abstract

Urbanization is a global threat to biodiversity due to its large impact on environmental changes. Recently, urban environmental change has been shown to impact the evolution of many species. However, much remains unknown about how urban environments influence evolutionary processes and outcomes due to the non‐linearity and discontinuity of environmental variables along urban–rural gradients. Here, we focused on the evolution of hydrogen cyanide (HCN) production and its components (presence/absence of cyanogenic glycosides and the hydrolytic enzyme linamarase) in the herbaceous plant white clover Trifolium repens, which thrive in both urban and rural areas. To comprehensively elucidate how plants evolve and adapt to heterogenous urban environments, we collected 3299 white clover plants from 122 populations throughout Sapporo, Japan. We examined the spatial variation in environmental factors, such as herbivory, sky openness, impervious surface cover, snow depth, and temperature, and how variation in these factors was related to the production of HCN, cyanogenic glycosides, and linamarase. Environmental factors showed complex spatial variation due to the heterogeneity of the urban landscape. Among these factors, herbivory, sky openness, and impervious surface cover were highly related to the frequency of plants producing HCN in populations. We also found that impervious surface cover was related to the frequency of plants producing cyanogenic glycosides, while herbivory pressure was not. As a result, the cyanogenic glycoside frequency showed a clearer trend along urban–rural gradient rather than HCN frequency, and thus, the predicted spatial distributions of HCN and cyanogenic glycosides were inconsistent. These results suggest that urban landscape heterogeneity and trait multifunctionality determines mosaic‐like spatial distribution of evolutionary traits.