Intraspecific leaf trait variation, genetic structure and adaptive strategies in forest endemics: a case study from the genus Aegonychon (Boraginaceae)

The Italian flora includes approximately 1700 endemic plant species, around 130 of which are found in woodlands and forests of the South and the Islands (1). Most of these endemics are herbaceous understory species, which account for up to 80% of the biodiversity in temperate forests and play key ecosystem roles, such as the cycling of carbon and other elements. Several historical, ecological, or biological factors may contribute to the emergence and development of endemic taxa, including a relatively recent origin, a narrow ecological niche, limited dispersal ability, low adaptive phenotypic plasticity, or genetic variability. As part of a project on Italian forest endemics, we aimed to assess the role of these factors using a model system consisting of a species pair with one narrow-range endemic and one closely related species with a broad continental distribution. These species are Aegonychon calabrum (Ten.) Holub and A. purpurocaeruleum (L.) Holub, belonging to the tribe Lithospermeae in family Boraginaceae (2). The former occurs with scattered populations in mountain forests of southern Italy, whereas the latter is widespread in the understory of thermophilic forests across many European countries and in all Italian regions, except Sardinia. We analysed representative population samples of both species to evaluate potential differences in the range and patterns of intraspecific variation in key leaf traits associated with resource-use attitudes (LA, SLA, LDMC), as well as in Grime’s ecological strategies and genetic structure. We also compared genetic parameters and phenotypic traits to evaluate the roles of genetic drift and natural selection at the intraspecific level. LA and LDMC showed significant differences, with higher values in A. purpurocaeruleum. Notably, we detected significant differences in LA even among populations of A. calabrum. However, the two species did not differ in their Grime’s ecological strategies, showing the prevalence of stress-tolerant and ruderal components. Genetic diversity values were comparable between the two species (He = 0.295 in A. calabrum and He = 0.311 in A. purpurocaeruleum), indicating no evidence of genetic erosion in the endemic species. The comparison between genetic and phenotypic distances suggests that directional selection is the main evolutionary force shaping variation in both species. Nevertheless, two of the three A. calabrum populations exhibited niche conservatism, likely influenced by stabilizing selection. The result of this study challenges the widespread assumption that endemic species necessarily display low genetic variability and highlight the existence of different selective pressures among populations of narrow-ranged endemic species